Development of cardiovascular function in the horse fetus
1 Department of Physiology, University of Cambridge, Downing Street, Cambridge CB2 3EG, UK
Abstract
In mammals, the mechanisms regulating an increase in fetal arterial blood pressure with advancing gestational age remain unidentified. In all species studied to date, the prepartum increase in fetal plasma cortisol has an important role in the maturation of physiological systems essential for neonatal survival. In the horse, the prepartum elevation in fetal cortisol and arterial blood pressure are delayed relative to other species. Hence, the mechanisms governing the ontogenic increase in arterial blood pressure in the horse fetus may mature much closer to term than in other fetal animals. In the chronically instrumented pony mare and fetus, this study investigated how changes in fetal peripheral vascular resistance, in plasma concentrations of noradrenaline, adrenaline and vasopressin, and in the maternal-to-fetal plasma concentration gradient of oxygen and glucose relate to the ontogenic changes in fetal arterial blood pressure and fetal plasma cortisol concentration as term approaches. The data show that, towards term in the horse fetus, the increase in arterial blood pressure occurs together with reductions in metatarsal vascular resistance, elevations in plasma concentrations of cortisol, vasopressin, adrenaline and noradrenaline, and falls in the fetal : maternal ratio of blood Pa,O2 and glucose concentration. Correlation analysis revealed that arterial blood pressure was positively related with plasma concentrations of vasopressin and noradrenaline, but not adrenaline in the fetus, and inversely related to the fetal : maternal ratio of blood Pa,O2, but not glucose, concentration. This suggests that increasing vasopressinergic and noradrenergic influences as well as changes in oxygen availability to the fetus and uteroplacental tissues may contribute to the ontogenic increase in fetal arterial blood pressure towards term in the horse.
, http://www.100md.com
Introduction
Towards term there are maturational changes in a number of physiological systems in the fetus, which ensure survival both in utero and at birth. For instance, arterial blood pressure increases in the fetus with advancing gestational age in a number of species, including the horse (Reeves et al. 1972; Boddy et al. 1974; Dawes et al. 1980; Macdonald et al. 1983; Kitanaka et al. 1989; Forhead et al. 2000a). This increases the perfusion pressure of the fetal vascular tree, and maintains placental blood flow and the delivery of oxygen and nutrients to the fetal tissues as their demands for nutrients rise with increased growth in late gestation. However, the cardiovascular causes of this ontogenic increase in fetal arterial blood pressure remain unclear. Increased cardiac output does not appear to be a contributing factor in fetal sheep, as the combined ventricular output expressed per kilogram of body weight does not increase towards term (Rudolph & Heymann, 1970). Even less is known about the contributions made by developmental changes in peripheral vascular resistance, as there have been no measurements of total peripheral vascular resistance in the fetus with increasing gestational age towards term in any species.
, http://www.100md.com
Like many of the maturational changes essential for neonatal survival (Liggins, 1994; Fowden et al. 1998), the ontogenic rise in fetal arterial blood pressure also appears to be glucocorticoid dependent. First, a close temporal relationship exists between elevations in arterial blood pressure and plasma cortisol in the fetus close to term in a number of species (Macdonald et al. 1983; Forhead et al. 2000a,b). Second, exogenous treatment of fetal sheep with synthetic (Derks et al. 1997; Fletcher et al. 2002) or natural (Wood et al. 1987) glucocorticoids elevates fetal arterial blood pressure. Third, developmental increases in fetal arterial blood pressure can be prevented by fetal bilateral adrenalectomy and restored in adrenalectomised fetuses by cortisol replacement (Unno et al. 1999). In the horse, fetal plasma cortisol rises much closer to term than in other species (Fowden & Silver, 1995). Glucocorticoid-dependent maturation of the fetal cardiovascular system may therefore occur comparatively late in gestation in equids compared with other species. It has been reported that fetal arterial blood pressure increases late in gestation in the horse in association with the delayed surge in fetal circulating glucocorticoid concentration (Forhead et al. 2000a). Furthermore, the prepartum increase in plasma cortisol coincides with maturational changes in the activity of the renin–angiotensin system in the horse fetus, suggesting that maturing pressor actions of angiotensin II may be responsible, at least in part, for the ontogenic increase in fetal arterial blood pressure in this species (Forhead et al. 2000a). However, the relationships between plasma cortisol concentrations, arterial blood pressure and changes in plasma concentrations of other vasoconstrictor agents, such as catecholamines and arginine vasopressin, in the horse fetus remain unknown.
, 百拇医药
The aim of the present study was to identify, in the chronically instrumented horse fetus, mechanisms contributing to the ontogenic increase in arterial blood pressure as the fetus approaches term, by comparing cardiovascular function between 0.6 and 0.9 of gestation. The objectives were to determine how changes in fetal peripheral vascular resistance, and in plasma concentrations of noradrenaline, adrenaline and vasopressin, relate to changes in fetal arterial blood pressure and plasma cortisol concentration with advancing gestational age.
, 百拇医药
Methods
Animals
Welsh Pony mares (n = 25) carrying fetuses of predicted gestational ages between 0.6 and 0.9 of gestation were used (Table 1). The ponies were housed in individual stables and were fed 500 g concentrates (Horse Stud Mix; Moulton's Feed Supplies, Lincolnshire, UK) twice a day with access to hay and water ad libitum. On the day preceding surgery, mares were moved into an indoor horsebox within the main animal facility. Food, but not water, was withdrawn 18 h prior to surgery, and the cyclooxygenase inhibitor, meclofenamic acid (2 mg kg–1; Arquel, Pharmacia and Upjohn, Sussex, UK) was given orally the night before surgery, and again the morning after surgery to reduce endogenous prostaglandin production associated with fasting and surgery in this species (Silver et al. 1979).
, http://www.100md.com
Surgical preparation
All procedures were performed under the UK Animals (Scientific Procedures) Act 1986. Gestational age was, first, estimated by ultrasound scan and measurement of aortic diameter and/or heart rate, and then confirmed at delivery. Surgery was performed between day 143 and 328 of predicted gestation (term is approx 330–350 days, Table 1), using techniques previously described (Taylor et al. 2001). In brief, on the morning of surgery, mares were premedicated with 20 μg kg–1 acepromazine (ACP; C-Vet, Leyland, UK), 20 μg kg–1 butorphanol (Torbugesic; Fort Dodge, Southampton, UK) and 10 μg kg–1 detomidine (Domosedan; Pfizer, Sandwich, UK) via a jugular catheter (Vygon intraflon 2 14G). Thirty minutes later, anaesthesia was induced with a further 10 μg kg–1 detomidine I.V., followed by 2 mg kg–1 ketamine I.V. (Vetalar; Pharmacia and Upjohn, Corby, UK). The trachea was intubated with a cuffed tube (Portex Ltd.; Hythe, Kent, UK) as soon as the mare became recumbent, and 100% oxygen was supplied via a to-and-fro rebreathing system (Fluotec; Ohmeda, Hatfield, UK). General anaesthesia was maintained in the recumbent position with i.v. infusion of propofol (Rapinovert; Schering Plough, Harefield, UK) at 200 μg kg–1 min–1 for the first 60 min, and then at a reduced rate of 180 and 130 μg kg–1 min–1 as required for the remaining period of surgery. After 10 min of propofol infusion, a ketamine infusion was commenced via a separate jugular catheter at 40 μg kg–1 min–1 for 60 min, and then reduced to 20–30 μg kg–1 min–1 as required (Baker et al. 1999; Taylor et al. 2001). Maternal arterial blood pressure via the facial artery was recorded throughout anaesthesia.
, http://www.100md.com
Under general anaesthesia, catheters were inserted into the maternal dorsal aorta via the circumflex artery, a main uterine vein via a small branch and the umbilical vein, and into the fetal dorsal aorta and caudal vena cava via peripheral hind limb vessels. A further catheter was secured to the fetal hind limb to measure amniotic fluid pressure. In 13 of the pregnancies, an ultrasonic flow transducer (2RS or 3RS Transonic Systems Inc., NY, USA) was implanted around one of the fetal metatarsal arteries, to measure peripheral blood flow. The fetal membranes were tied tightly and the uterine incision was closed in layers. Following fetal I.V. administration of ampicillin (25 mg (kg estimated bodyweight)–1; Penbritin; Beecham Animal Health, Brentford) and gentamycin (5 mg (kg estimated bodyweight)–1; Frangen-100; Biovet Ltd, Mullingar), all catheters were filled with heparinised saline (50 i.u. heparin ml–1 in 0.9% NaCl), plugged with brass pins, and were then exteriorised with the flow probe lead through a key hole incision in the maternal flank. The maternal abdominal and skin incisions were closed. Fetal catheters were maintained patent via a slow infusion of heparinised saline (100 i.u. heparin ml–1) via battery-operated mini pumps (MS16A syringe driver, Graseby Ltd, Watford, UK). Pumps, catheters and cables were kept in a plastic pouch sutured onto the maternal flank. Ampicillin (1 g, I.V.) was administered to the mare at surgery and for a further 3 days.
, 百拇医药
Experimental protocol
Commencing 5 days after surgery, cardiovascular, metabolic and endocrine measurements were made every other day at 10 : 00 for the duration of the experimental period, at gestational ages ranging from 148 to 341 days. On each measurement day, fetal arterial blood pressure, fetal heart rate and mean metatarsal blood flow were recorded continually at 1 s intervals for 2 h using a computerised Data Acquisition System. Fetal arterial blood pressure was corrected for amniotic pressure and fetal heart rate was triggered via a tachometer from either the arterial blood pressure or the metatarsal blood flow pulsatility. Whenever possible, simultaneous samples of maternal arterial, uterine venous, umbilical venous and fetal arterial blood (5 ml each) were taken. Values for pH (pHa), and partial pressures of oxygen and carbon dioxide (PO2 and PCO2), were obtained using a blood gas analyser (ABL 5; Radiometer, Copenhagen, Denmark), corrected to 39.5°C for fetal blood and 38°C for maternal blood. Blood haemoglobin concentration [Hb] and percentage saturation of haemoglobin with oxygen (Sat.Hb) were determined using a haemoximeter (OSM2; Radiometer). Blood glucose and lactate concentrations were determined with an automated analyser (Yellow Springs 2300 Stat Plus Glucose/Lactate Analyser; YSI Ltd, Farnborough, UK). The remainder of the maternal and fetal blood (4 ml) was transferred into appropriately treated chilled tubes, for measurement of plasma levels of cortisol, vasopressin and catecholamines. All tubes were centrifuged (4 min, 4000 r.p.m. at 4°C), and plasma aliquots transferred to PVC tubes, which were maintained at –70°C until biochemical analysis.
, http://www.100md.com
Biochemical analyses
Plasma concentrations of cortisol were measured by radioimmunoassay validated for equine plasma (RIA; Rossdale et al. 1982; Silver & Fowden, 1984). The minimum detectable quantity of cortisol in the assay was 1 ng ml–1. The intra-assay coefficient of variation was 2.7% for a mean value of 29.5 ng ml–1. The interassay coefficients of variation for two plasma pools (mean concentrations: 10.7 and 29.5 ng ml–1) were 9.4 and 7.8%, respectively. The cross-reactivities of the antiserum at 50% binding with other cortisol-related compounds were: 0.5% cortisone; 2.3% corticosterone; 0.3% progesterone; 4.6% deoxycortisol.
, http://www.100md.com
Plasma arginine vasopressin (AVP) concentrations were measured using a commercially available double-antibody RIA kit (Nichols Institute Diagnostics ltd, Saffron Walden, UK) validated for equine plasma. The lower limit of detection of the assay was 1.3 pg ml–1. The intra-assay coefficients of variation for four control plasma pools (mean concentrations: 3.2, 9.9, 12.2 and 28.9 pg ml–1) were 10.0, 6.7, 3.7 and 4.6%, respectively. The interassay coefficients of variation for two plasma samples (2.71 and 5.55 pg ml–1 AVP) were 4.1 and 9.8%, respectively. The anti-AVP antiserum was highly specific for [Arg8]vasopressin, with cross-reactivities of <0.05% against oxytocin, [Lys8]vasopressin and [Arg8]vasotocin.
, 百拇医药
Plasma noradrenaline and adrenaline concentrations were measured by high pressure liquid chromatography with electrochemical detection (Silver & Fowden, 1995). Samples were prepared by absorption of 250 μl of plasma onto acid-washed alumina, and 20 μl aliquots of the 100 μl perchloric acid eluates were injected onto the column. Dihydroxybenzylamine was added as the internal standard to each plasma sample before absorption. Recovery ranged from 63 to 97%, and all catecholamine values were corrected for their respective recovery. The interassay coefficients of variation for noradrenaline and adrenaline were 6.2% and 7.3%, respectively, and the minimum detectable dose was 10 pg ml–1.
, 百拇医药
Calculations
Fetal arterial blood pressure was corrected for amniotic pressure, which was used as zero reference. Fetal peripheral vascular resistance was calculated by dividing supra-amniotic mean arterial blood pressure by mean metatarsal blood flow. The fetal rate pressure product, an index of cardiac work, was calculated by multiplying fetal arterial blood pressure by fetal heart rate. Arterial blood oxygen content (Ca,O2), oxygen capacity (O2,cap), fetal peripheral arterial oxygen delivery (O2,del), fetal peripheral arterial glucose delivery (Glucosedel), maternal oxygen extraction (O2,maternal ext), fetal oxygen extraction (O2,fetal ext), maternal glucose extraction (Glucosematernal ext), and fetal glucose extraction (Glucosefetal ext) were calculated using eqns (1)–(8), respectively:
, 百拇医药
where one molecule of haemoglobin (MW = 64 450) binds four molecules of oxygen. The contribution of oxygen dissolved in plasma was regarded as being negligible. MA, maternal descending aorta; MV, uterine vein; FA, fetal descending aorta; FUV, umbilical vein.
Data and statistical analyses
Recordings of arterial blood pressure and heart rate were obtained from 17 of the 25 horse fetuses. In 2 of the 13 fetuses, the implanted tarsal flow probe developed an acoustic error, leaving 11 of 25 horse fetuses with successful peripheral blood flow recording. Measurements of blood gases, metabolic status and plasma cortisol concentrations were made in all pregnancies. Simultaneous blood samples were taken from the maternal descending aorta, uterine vein, umbilical vein and fetal descending aorta in 18 of the 25 pregnancies. Measurements of plasma catecholamine and vasopressin concentrations were made in 14 fetuses.
, http://www.100md.com
Variables are presented either as individual daily values with respect to gestational age (mean ± S.E.M.) for pregnancies <250 days (range: 148–228 days; 193.8 ± 2.7 days; 0.6 of gestation; n = 9), or for pregnancies >250 days (range: 265–341 days; 309.6 ± 2.4 days; 0.9 of gestation; n = 16). All data were assessed for normality of distribution using the Kolmogorov–Smirnov test. Data were normally distributed and were assessed for significance using parametric statistical tests. Significant differences between variables at 0.6 and 0.9 of gestation were determined by the Student's t test for unpaired data. Pearson product moment and partial correlation analyses were used to determine the relationships between cardiovascular, metabolic and endocrine variables with gestational age or with fetal plasma cortisol levels (Jandel SigmaStat 2.0 and Statview 4.02). Significance was accepted when P < 0.05.
, 百拇医药
Results
Cardiovascular function
Horse fetuses at 0.9 of gestation had significantly greater arterial blood pressure, rate–pressure product and peripheral blood flow, but significantly lower heart rate and peripheral vascular resistance, than horse fetuses at 0.6 of gestation (P < 0.05; Fig. 1).
Values are either mean ± S.E.M. at 0.6 (193.8 ± 2.7 days, range: 148–228 days; n = 9) and 0.9 (309.6 ± 2.4 days; range: 283–341 days, n = 16) of gestation, or values from individual horse fetuses in relation to gestational age for arterial blood pressure, heart rate, the rate–pressure product, peripheral blood flow and peripheral vascular resistance. r, Pearson product moment correlation coefficient; n, number of observations; *P < 0.05, 0.6 vs 0.9 gestation.
, 百拇医药
Correlation of fetal cardiovascular variables with gestational age and with the prevailing fetal plasma cortisol concentration showed significant positive relationships between fetal arterial blood pressure, rate–pressure product and peripheral blood flow, and both gestational age and fetal plasma cortisol (Figs 1 and 2), and showed significant negative relationships between peripheral vascular resistance and both gestational age and plasma cortisol (Figs 1 and 2), and a significant negative relationship between heart rate and gestational age (Fig. 1). Partial correlation analyses further revealed that gestational age and fetal plasma cortisol equally determined fetal arterial blood pressure and fetal peripheral blood flow (arterial blood pressure x gestation: 0.78, n = 28, P < 0.01; arterial blood pressure x cortisol: 0.65, n = 28, P < 0.01; peripheral blood flow x gestation: 0.79, n = 51, P < 0.01; peripheral blood flow x cortisol: 0.78, n = 44, P < 0.01). However, gestational age was a greater determinant than plasma cortisol of fetal peripheral vascular resistance (peripheral vascular resistance x gestation: –0.76, n = 19, P < 0.01; peripheral vascular resistance x cortisol: –0.07, n = 19, P = NS). Conversely, plasma cortisol concentration better determined rate–pressure product than gestational age (rate–pressure product x cortisol: 0.57, n = 18, P < 0.05; rate–pressure product x gestation: 0.46, n = 18, P = NS).
, 百拇医药
Association between plasma cortisol concentration and the values of arterial blood pressure, the rate–pressure product, peripheral blood flow and peripheral vascular resistance in all individual horse fetuses studied. r, Pearson product moment correlation coefficient; n, number of observations.
Hormone concentrations
Horse fetuses at 0.9 of gestation had significantly greater plasma concentrations of vasopressin, adrenaline, noradrenaline and cortisol than horse fetuses at 0.6 of gestation (Fig. 3). Fetal plasma concentrations of all hormones were low for most of the gestational period studied. However, at 290 days of gestation, abrupt elevations in the plasma concentrations of vasopressin, adrenaline and noradrenaline occurred, in close temporal association with the prepartum surge in fetal plasma cortisol (Fig. 3).
, 百拇医药
Values are either mean ± S.E.M. at 0.6 (193.8 ± 2.7 days, range: 148–228 days; n = 9) and 0.9 (309.6 ± 2.4 days; range: 283–341 days, n = 16) of gestation, or values from individual horse fetuses in relation to advancing gestational age for fetal plasma concentrations of vasopressin, adrenaline, noradrenaline and cortisol. r, Pearson product moment correlation coefficient; n, number of observations. *P < 0.05, 0.6 vs 0.9 gestation.
Correlation of the plasma concentration of vasopressin, adrenaline and noradrenaline with gestational age in individual horse fetuses showed significant positive relationships between all hormones and gestational age (Fig. 3). However, significant positive relationships were only seen between fetal plasma vasopressin and noradrenaline with cortisol (Fig. 4), but not between adrenaline and cortisol (0.36, n = 14, P = NS). Partial correlation analysis revealed that cortisol was a greater determinant than gestational age of vasopressin and noradrenaline concentrations in fetal plasma (cortisol x vasopressin: 0.90, n = 27, P < 0.01; cortisol x noradrenaline: 0.90, n = 14, P < 0.01; gestation x vasopressin: 0.09, n = 27, P = NS; gestation x noradrenaline: 0.29, n = 14, P = NS). Significant positive relationships were obtained between fetal plasma vasopressin and noradrenaline with fetal arterial blood pressure (Fig. 4), but not between adrenaline and fetal arterial blood pressure (0.34, n = 11, P = NS).
, 百拇医药
Association between plasma vasopressin and noradrenaline concentrations with either the prevailing plasma cortisol concentration or the arterial blood pressure in all individual horse fetuses studied. r, Pearson product moment correlation coefficient; n, number of observations.
Maternal plasma concentrations of vasopressin, noradrenaline, adrenaline and cortisol were similar at 0.6 and 0.9 of gestation (Table 2). No significant relationships were observed between any of the maternal hormone concentrations and gestational age.
, 百拇医药
Arterial blood gas and metabolic status
Maternal basal blood gas and metabolic status did not change significantly from 0.6 to 0.9 of gestation (Table 2). In contrast, horse fetuses at 0.6 of gestation had higher basal values for Pa,O2 and blood glucose concentration than horse fetuses at 0.9 of gestation (P < 0.05; Table 3). Therefore, a decrease in the fetal : maternal ratio of Pa,O2 (0.26 ± 0.01–0.24 ± 0.01) and blood glucose concentration (0.5 ± 0–0.36 ± 0.01) occurred from 0.6 to 0.9 of gestation (P < 0.05). Fetuses at 0.9 of gestation had significantly greater concentrations of haemoglobin than those at 0.6 of gestation. Consequently, despite the difference in Pa,O2, basal oxygen content and oxygen capacity in the equine fetus were greater at 0.9 than at 0.6 of gestation (P < 0.05; Table 2). Greater values for peripheral blood flow contributed to greater delivery of oxygen and glucose to the peripheral circulation in horse fetuses at 0.9 than at 0.6 of gestation (Table 3).
, 百拇医药
When individual values for blood gas and metabolic status were correlated with gestational age or with the prevailing fetal plasma cortisol concentration in all horse pregnancies studied, significant negative relationships were observed between gestational age and the following factors: fetal Pa,O2, fetal blood glucose concentration and the fetal: maternal ratio of Pa,O2 and blood glucose (P < 0.05; Table 4). Conversely, there were significant positive relationships between gestational age and haemoglobin concentration, oxygen content and oxygen capacity in fetal blood (P < 0.05; Table 4). Partial correlation analysis revealed that delivery of oxygen and glucose to the fetal peripheral vasculature was equally related both to gestational age and to fetal plasma cortisol (Table 4).
, http://www.100md.com
When individual values for blood gas and metabolic status were correlated with fetal arterial blood pressure, neither fetal Pa,O2 nor blood glucose were significantly related to blood pressure, although the correlation of fetal Pa,O2 with fetal arterial blood pressure just fell outside significance (Pa,O2 x fetal arterial blood pressure: –0.40, n = 25, P = 0.06; glucose x fetal arterial blood pressure: –0.03, n = 16, P = 0.91). When the fetal : maternal ratio of Pa,O2 and blood glucose concentration were correlated to fetal arterial blood pressure, negative relationships were obtained, which reached significance only between the fetal : maternal ratio of Pa,O2 and fetal arterial blood pressure (–0.68, n = 18, P < 0.01), but not between the fetal : maternal ratio of glucose and fetal arterial blood pressure (–0.41, n = 16, P = NS).
, 百拇医药
Discussion
The data in this study show that arterial blood pressure, rate–pressure product, peripheral blood flow, and plasma concentrations of noradrenaline, adrenaline, vasopressin and cortisol increased with advancing gestational age up to term in the equine fetus. In contrast, heart rate, peripheral vascular resistance and the fetal : maternal ratio of Pa,O2 and blood glucose concentration decreased towards term. While most cardiovascular and endocrine changes in the horse fetus showed significant associations with both gestational age and fetal plasma cortisol concentrations, partial correlation analyses revealed that gestational age and fetal plasma cortisol contributed equally to the ontogenic increase in fetal arterial blood pressure. In contrast, gestational age is a greater determinant than cortisol of fetal heart rate and peripheral vascular resistance, while cortisol is a greater determinant than gestational age of the rate–pressure product, and plasma concentrations of noradrenaline and vasopressin. Fetal Pa,O2 and blood glucose, and the fetal : maternal ratio of Pa,O2 and blood glucose concentration, were only related to advancing gestational age and not to cortisol. Arterial blood pressure was positively related with plasma concentrations of vasopressin and noradrenaline, but not adrenaline in the fetus, and inversely related to the fetal : maternal ratio of blood Pa,O2, but not glucose, concentration.
, 百拇医药
In the simplest terms, mean arterial blood pressure in the fetus is determined by the combined ventricular output and the total peripheral vascular resistance. Therefore, ontogenic increases in arterial blood pressure in the fetus could be accounted for by ontogenic increases in either or both of these variables. The most comprehensive study of changes in regional blood flow during fetal development is that of Rudolph & Heymann (1970) who measured the combined ventricular output and organ blood flow distribution in fetal sheep from 60 days of gestation to term. They reported that the combined ventricular output of the sheep fetus increased with advancing gestational age up to term in absolute terms, but not when expressed per kg fetal body weight. These observations suggest that increases in ventricular output towards term occur in parallel with the growing volume of the fetal vasculature and, hence, are unlikely to make a major contribution to the ontogenic increase in fetal blood pressure. Little is known about changes in total peripheral vascular resistance towards term in fetal sheep, although the resistance of several vascular beds is known to be responsive to adverse intrauterine conditions during late gestation (e.g. Cohn et al. 1974; Rudolph, 1984; Jensen & Berger, 1991; Giussani et al. 1993). In the present study, there was a pronounced fall in the vascular resistance of the metatarsal circulation of the fetal horse during late gestation, which was due to a greater ontogenic increase in hind limb blood flow than fetal arterial blood pressure. Since resistance of the metatarsal circulation was more closely related to gestational age than fetal plasma cortisol, the ontogenic fall in peripheral vascular resistance is more likely to reflect angiogenesis and an increase in the cross-sectional area of the vascular beds in the growing fetus than maturational changes in vasoreactivity of the peripheral vasculature. Indeed, when peripheral blood flow was expressed per kg body weight, there was no change in metatarsal blood flow with increasing age in the fetal horse (data not shown). These observations suggest that the ontogenic increase in fetal arterial blood pressure in the horse is not due to changes in total peripheral vascular resistance, although vascular resistance in the metatarsal circulation may not be the best index of overall changes in total peripheral vascular resistance in this species. In the absence of direct changes in either fetal cardiac output or peripheral vascular resistance, an alternative explanation for the ontogenic rise in fetal blood pressure is that placental vascular resistance rises relative to the total vascular resistance of the fetus, either by a greater rise in placental resistance or a greater fall in fetal total vascular resistance as term approaches. In this context, it has been reported that weight-normalised umbilico-placental blood flow decreases with advancing gestation in the horse (Fowden et al. 2000a) and in the sheep (Hedriana et al. 1995), but changes little in the human (Sutton et al. 1990). However, since a similar progression of mean arterial blood pressure, as seen in the fetus, also occurs after birth (Louey et al. 2000), a role for changes in placental vascular resistance in contributing to progressive changes in arterial blood pressure during the fetal period is not supported.
, 百拇医药
In the present study, there was a fall in the heart rate of the horse fetus with advancing gestational age, in common with other species (Reeves et al. 1972; Boddy et al. 1974; Dawes, 1980; Macdonald et al. 1983; Kitanaka et al. 1989; Forhead et al. 2000a). Several studies have shown that rate–pressure product is a useful marker of cardiac work, correlating well with changes in myocardial oxygen consumption (Jorgensen et al. 1973; Nelson et al. 1974). The increasing metabolic demand of the equine fetal heart with advancing gestation may reflect not only changes in cardiac growth and stroke volume (Machida et al. 1988), but also in increased cardiac work secondary to the imposed changes in cardiac afterload as a result of the ontogenic increase in fetal arterial blood pressure.
, 百拇医药
Previous studies in horses and other species have shown that developmental changes in fetal arterial blood pressure correlate strongly with ontogenic changes in the renin–angiotensin system and adrenocortical cortisol secretion (Forhead et al. 2000a,b). In the present study, abrupt elevations in the fetal concentrations of plasma vasopressin, adrenaline and noradrenaline occurred in close temporal association with the prepartum surge in plasma cortisol in the horse fetus. While significant correlations were observed between gestational age or fetal plasma cortisol concentration and fetal vasopressin, adrenaline and noradrenaline, only fetal plasma concentrations of noradrenaline and vasopressin were correlated with fetal plasma cortisol and fetal arterial blood pressure. These results suggest that ontogenic changes in fetal plasma noradrenaline and vasopressin, but not adrenaline, are also involved in mediating the ontogenic increase in fetal arterial blood pressure in the horse, presumably by constricting circulations other than those represented by the metatarsal vascular bed. Whether these ontogenic changes in plasma noradrenaline and vasopressin are matched by similar changes in the density of the respective receptor population with advancing gestational age remains unknown in the fetal horse. In fetal rats, expression of vasopressin V1 receptors is developmentally regulated (Ostrowski et al. 1993) and, in fetal baboons, the maximum vasoconstrictor response to noradrenaline of small branches of the femoral artery also increases with advancing gestastional age (Anwar et al. 2001). However, in vivo treatment of immature fetal sheep with cortisol did not enhance their pressor response to exogenous noradrenaline (Tangalakis et al. 1992), and in vitro vasoconstrictor responses to exogenous noradrenaline of peripheral vessels obtained from immature sheep fetuses treated with betamethasone were also not enhanced (Anwar et al. 1999).
, http://www.100md.com
In the present study, a fall in basal values of Pa,O2 and blood glucose concentration occurred with advancing gestational age in fetal but not maternal blood. Consequently, there were decreases in the fetal: maternal ratios of Pa,O2 and blood glucose concentration between 0.6 and 0.9 of gestation, consistent with previous observations in pregnant rabbits (Gilbert et al. 1984), sheep (Hay, 1995) and horses (Fowden et al. 2000a,b). The falls in fetal Pa,O2 and blood glucose concentration, together with changes in the rates of oxygen and glucose consumption by the placenta, help to maintain the net maternal-to-fetal plasma concentration gradients for glucose molecules needed to meet the increasing nutrient demands of the growing fetus (Meschia et al. 1980; Bell et al. 1987; Molina et al. 1991; Fowden et al. 2000a,b). The current finding that the fetal: maternal ratios of Pa,O2 and blood glucose concentration were significantly related to gestational age but not to fetal plasma cortisol concentrations supports the suggestion that these metabolic changes reflect fetal growth rather than fetal maturation in the horse. The relative fetal hypoxaemia and hypoglycaemia observed at 0.9 of gestation may contribute, in part, to the ontogenic increase in fetal arterial blood pressure, by triggering cardiovascular chemoreflex and neuroendocrine mechanisms that ensure delivery of sufficient oxygen and glucose to the fetal target organs to meet the increasing demands for growth (see Giussani et al. 1994 for review). This suggestion is supported by the inverse relationship observed between fetal blood pressure and the fetal : maternal ratios of Pa,O2 and blood glucose concentration, although only the relationship of fetal : maternal Pa,O2 reached statistical significance. It is of interest that umbilical venous and arterial PO2 also fall progressively with advancing gestation in the human fetus (Soothill et al. 1986).
, 百拇医药
In conclusion, the present data show that the increase in arterial blood pressure towards term in the horse fetus occurs in association with ontogenic reductions in metatarsal vascular resistance, elevations in plasma concentrations of vasopressin and noradrenaline, and a decrease in the fetal : maternal ratio of Pa,O2. This suggests that increasing vasopressinergic and noradrenergic influences as well as changes in oxygen uteroplacental and fetal metabolism may contribute to the mechanisms that drive fetal arterial blood pressure up with advancing gestational age in equine pregnancy.
, 百拇医药
References
Anwar MA, Ju K, Docherty CC, Poston L & Nathanielsz PW (2001). Developmental changes in reactivity of small femoral arteries in the fetal and postnatal baboon. Am J Obstet Gynecol 184, 707–712.
Anwar MA, Schwab M, Poston L & Nathanielsz PW (1999). Betamethasone-mediated vascular dysfunction and changes in hematological profile in the ovine fetus. Am J Physiol Heart Circ Physiol 276, H1137–H1143.
, 百拇医药
Baker KL, Taylor PM, Fowden AL & Bloomfield M (1999). Propofol-ketamine anaesthesia in late gestation ponies. J Vet Anaesthesia 26, 45–50.
Bell AW, Battaglia FC & Meschia G (1987). Relation between metabolic rate and body size in the ovine fetus. J Nutr 117, 1181–1186.
Boddy K, Dawes GS, Fisher R, Pinter S & Robinson JS (1974). Foetal respiratory movements electrocortical and cardiovascular responses to hypoxaemia and hypercapnia in sheep. J Physiol 243, 599–618.
, http://www.100md.com
Cohn HE, Sacks EJ, Heymann MA & Rudolph AM (1974). Cardiovascular responses to hypoxemia and acidemia in fetal lambs. Am J Obstet Gynecol 120, 817–824.
Dawes GS, Johnston BM & Walker DW (1980). Relationship of arterial pressure and heart rate in fetal new-born and adult sheep. J Physiol 309, 405–417.
Derks JB, Giussani DA, Jenkins SJ, Wentworth RA, Visser GHA, Padbury JF & Nathanielsz PW (1997). A comparative study of cardiovascular endocrine and behavioural effects of betamethasone and dexamethasone administration to fetal sheep. J Physiol 499, 217–226.
, 百拇医药
Fletcher AJW, McGarrigle HG, Edwards CMB, Fowden AL & Giussani DA (2002). Effects of low dose dexamethasone treatment on basal cardiovascular and endocrine function in fetal sheep during late gestation. J Physiol 545, 649–660.
Forhead AJ, Broughton Pipkin F & Fowden AL (2000b). Effect of cortisol on blood pressure and the renin-angiotensin system in fetal sheep during late gestation. J Physiol 526, 167–176.
Forhead AJ, Broughton Pipkin F, Taylor PM, Baker K, Balouzet V, Giussani DA & Fowden AL (2000a). Developmental changes in blood pressure and the renin-angiotensin system in Pony fetuses during the second half of gestation. J Reprod Fertil (Suppl.) 56, 693–703.
, 百拇医药
Fowden AL, Forhead AJ, White KL & Taylor PM (2000a). Equine uteroplacental metabolism at mid- and late gestation. Exp Physiol 85, 539–545.
Fowden AL, Li J & Forhead AJ (1998). Glucocorticoids and the preparation for life after birth: are there long-term consequences of the life insurance Proc Nutr Soc 57, 113–122.
Fowden AL & Silver M (1995). Comparative development of the pituitary-adrenal axis in the fetal foal and lamb. Reprod Domest Anim 30, 170–177.
, http://www.100md.com
Fowden AL, Taylor PM, White KL & Forhead AJ (2000b). Ontogenic and nutritionally induced changes in fetal metabolism in the horse. J Physiol 528, 209–219.
Gilbert M, Hay WW Jr, Johnson RL & Battaglia FC (1984). Some aspects of maternal metabolism throughout pregnancy in the conscious rabbit. Pediatr Res 18, 854–859.
Giussani DA, Spencer JAD & Hanson MA (1994). Fetal cardiovascular reflex responses to hypoxia. Fet Mat Med Rev 6, 17–37.
, http://www.100md.com
Giussani DA, Spencer JAD, Moore PJ, Bennet L & Hanson MA (1993). Afferent and efferent components of the cardiovascular reflex responses to acute hypoxia in term fetal sheep. J Physiol 461, 431–449.
Hay WW Jr (1995). Regulation of placental metabolism by glucose supply. Reprod Fertil Dev 7, 365–375.
Hedriana HL, Brace RA & Gilbert WM (1995). Changes in blood flow to the ovine chorion and amnion across gestation. J Soc Gynecol Invest 2, 727–734.
, http://www.100md.com
Jensen A & Berger R (1991). Fetal circulatory responses to oxygen lack. J Dev Physiol 16, 181–207.
Jorgensen CR, Wang K, Wang Y, Gobel FL, Nelson RR & Taylor H (1973). Effect of propranolol on myocardial oxygen consumption and its hemodynamic correlates during upright exercise. Circulation 48, 1173–1182.
Kitanaka T, Alonso JG, Gilbert RD, Siu BL, Clemons GK & Longo LD (1989). Fetal responses to long-term hypoxemia in sheep. Am J Physiol Regul Integr Comp Physiol 256, R1348–R1354.
, http://www.100md.com
Liggins GC (1994). The role of cortisol in preparing the fetus for birth. Reprod Fertil Dev 6, 141–150.
Louey S, Cock ML, Stevenson KM & Harding R (2000). Placental insufficiency and fetal growth restriction lead to postnatal hypotension and altered postnatal growth in sheep. Pediatr Res 48, 808–814.
Macdonald AA, Colenbrander B & Wensing CJG (1983). The effects of gestational age and chronic fetal decapitation on arterial blood pressure in the fetus. Eur J Obstet Gynaecol Reprod Biol 16, 63–70.
, 百拇医药
Machida N, Yasuda J & Too K (1988). A morphometric study of foetal and newborn cardiac growth in the horse. Equine Vet J 20, 261–267.
Meschia G, Battaglia FC, Hay WW & Sparks JW (1980). Utilization of substrates by the ovine placenta in vivo. Fed Proc 39, 245–249.
Molina RD, Meschia G, Battaglia FC & Hay WW Jr (1991). Gestational maturation of placental glucose transfer capacity in sheep. Am J Physiol Regul Integr Comp Physiol 261, R697–R704.
, http://www.100md.com
Nelson RR, Gobel FL, Jorgensen CR, Wang K, Wang Y & Taylor HL (1974). Hemodynamic predictors of myocardial oxygen consumption during static and dynamic exercise. Circulation 50, 1179–1189.
Ostrowski NL, Young WS, Knepper MA & Lolait SJ (1993). Expression of vasopressin V1a and V2 receptor messenger ribonucleic acid in the liver and kidney of embryonic developing and adult rats. Endocrinology 133, 1849–1859.
Reeves JT, Daoud FS & Gentry M (1972). Growth of the fetal calf and its arterial pressure blood gases and hematologic data. J Appl Physiol 32, 240–244.
, http://www.100md.com
Rossdale PD, Silver M, Ellis L & Frauenfelder H (1982). Response of the adrenal cortex to tetracosactrin (ACTH 1–24) in the premature and full term foal. J Reprod Fertil (Suppl.) 32, 545–553.
Rudolph AM (1984). The fetal circulation and its response to stress. J Dev Physiol 6, 11–19.
Rudolph AM & Heymann MA (1970). Circulatory changes during growth in the fetal lamb. Circ Res 26, 289–299.
, http://www.100md.com
Silver M, Barnes RJ, Comline RS, Fowden AL & Mitchell MD (1979). Prostaglandins in maternal and fetal plasma and in allantoic fluid during the second half of gestation in the mare. J Reprod Fertil (Suppl.) 27, 531–539.
Silver M & Fowden AL (1984). Prepartum adrenocortical maturation in the fetal foal: responses to ACTH1–24. J Endocrinol 142, 417–425.
Silver M & Fowden AL (1995). Sympathoadrenal and other endocrine and metabolic responses to hypoglycaemia in the fetal foal during late gestation. Exp Physiol 80, 651–662.
, 百拇医药
Soothill PW, Nicolaides KH, Rodeck CH & Campbell S (1986). Effect of gestational age on fetal and intervillous blood gas and acid-base values in human pregnancy. Fetal Ther 1, 168–175.
Sutton MS, Theard MA, Bhatia SJ, Plappert T, Saltzman DH & Doubilet P (1990). Changes in placental blood flow in the normal human fetus with gestational age. Pediatr Res 28, 383–387.
Tangalakis K, Lumbers ER, Moritz KM, Towstoless MK & Wintour EM (1992). Effect of cortisol on blood pressure and vascular reactivity in the ovine fetus. Exp Physiol 77, 709–717.
, 百拇医药
Taylor PM, White KL, Fowden AL, Giussani DA, Bloomfield M & Sear JW (2001). Propofol anaesthesia for surgery in late gestation pony mares. Vet Anaesth Analg 28, 177–187.
Unno N, Wong CH, Jenkins SJ, Wentworth RA, Ding XY, Li C, Robertson SS, Smotherman WP & Nathanielsz PW (1999). Blood pressure and heart rate in the ovine fetus: ontogenic changes and effects of fetal adrenalectomy. Am J Physiol Heart Circ Physiol 276, H248–H256.
Wood CE, Cheung CY & Brace RA (1987). Fetal heart rate arterial pressure and blood Volume responses to cortisol infusion. Am J Physiol Regul Integr Comp Physiol 253, R904–R909., 百拇医药(Dino A Giussani, Alison J)
Abstract
In mammals, the mechanisms regulating an increase in fetal arterial blood pressure with advancing gestational age remain unidentified. In all species studied to date, the prepartum increase in fetal plasma cortisol has an important role in the maturation of physiological systems essential for neonatal survival. In the horse, the prepartum elevation in fetal cortisol and arterial blood pressure are delayed relative to other species. Hence, the mechanisms governing the ontogenic increase in arterial blood pressure in the horse fetus may mature much closer to term than in other fetal animals. In the chronically instrumented pony mare and fetus, this study investigated how changes in fetal peripheral vascular resistance, in plasma concentrations of noradrenaline, adrenaline and vasopressin, and in the maternal-to-fetal plasma concentration gradient of oxygen and glucose relate to the ontogenic changes in fetal arterial blood pressure and fetal plasma cortisol concentration as term approaches. The data show that, towards term in the horse fetus, the increase in arterial blood pressure occurs together with reductions in metatarsal vascular resistance, elevations in plasma concentrations of cortisol, vasopressin, adrenaline and noradrenaline, and falls in the fetal : maternal ratio of blood Pa,O2 and glucose concentration. Correlation analysis revealed that arterial blood pressure was positively related with plasma concentrations of vasopressin and noradrenaline, but not adrenaline in the fetus, and inversely related to the fetal : maternal ratio of blood Pa,O2, but not glucose, concentration. This suggests that increasing vasopressinergic and noradrenergic influences as well as changes in oxygen availability to the fetus and uteroplacental tissues may contribute to the ontogenic increase in fetal arterial blood pressure towards term in the horse.
, http://www.100md.com
Introduction
Towards term there are maturational changes in a number of physiological systems in the fetus, which ensure survival both in utero and at birth. For instance, arterial blood pressure increases in the fetus with advancing gestational age in a number of species, including the horse (Reeves et al. 1972; Boddy et al. 1974; Dawes et al. 1980; Macdonald et al. 1983; Kitanaka et al. 1989; Forhead et al. 2000a). This increases the perfusion pressure of the fetal vascular tree, and maintains placental blood flow and the delivery of oxygen and nutrients to the fetal tissues as their demands for nutrients rise with increased growth in late gestation. However, the cardiovascular causes of this ontogenic increase in fetal arterial blood pressure remain unclear. Increased cardiac output does not appear to be a contributing factor in fetal sheep, as the combined ventricular output expressed per kilogram of body weight does not increase towards term (Rudolph & Heymann, 1970). Even less is known about the contributions made by developmental changes in peripheral vascular resistance, as there have been no measurements of total peripheral vascular resistance in the fetus with increasing gestational age towards term in any species.
, http://www.100md.com
Like many of the maturational changes essential for neonatal survival (Liggins, 1994; Fowden et al. 1998), the ontogenic rise in fetal arterial blood pressure also appears to be glucocorticoid dependent. First, a close temporal relationship exists between elevations in arterial blood pressure and plasma cortisol in the fetus close to term in a number of species (Macdonald et al. 1983; Forhead et al. 2000a,b). Second, exogenous treatment of fetal sheep with synthetic (Derks et al. 1997; Fletcher et al. 2002) or natural (Wood et al. 1987) glucocorticoids elevates fetal arterial blood pressure. Third, developmental increases in fetal arterial blood pressure can be prevented by fetal bilateral adrenalectomy and restored in adrenalectomised fetuses by cortisol replacement (Unno et al. 1999). In the horse, fetal plasma cortisol rises much closer to term than in other species (Fowden & Silver, 1995). Glucocorticoid-dependent maturation of the fetal cardiovascular system may therefore occur comparatively late in gestation in equids compared with other species. It has been reported that fetal arterial blood pressure increases late in gestation in the horse in association with the delayed surge in fetal circulating glucocorticoid concentration (Forhead et al. 2000a). Furthermore, the prepartum increase in plasma cortisol coincides with maturational changes in the activity of the renin–angiotensin system in the horse fetus, suggesting that maturing pressor actions of angiotensin II may be responsible, at least in part, for the ontogenic increase in fetal arterial blood pressure in this species (Forhead et al. 2000a). However, the relationships between plasma cortisol concentrations, arterial blood pressure and changes in plasma concentrations of other vasoconstrictor agents, such as catecholamines and arginine vasopressin, in the horse fetus remain unknown.
, 百拇医药
The aim of the present study was to identify, in the chronically instrumented horse fetus, mechanisms contributing to the ontogenic increase in arterial blood pressure as the fetus approaches term, by comparing cardiovascular function between 0.6 and 0.9 of gestation. The objectives were to determine how changes in fetal peripheral vascular resistance, and in plasma concentrations of noradrenaline, adrenaline and vasopressin, relate to changes in fetal arterial blood pressure and plasma cortisol concentration with advancing gestational age.
, 百拇医药
Methods
Animals
Welsh Pony mares (n = 25) carrying fetuses of predicted gestational ages between 0.6 and 0.9 of gestation were used (Table 1). The ponies were housed in individual stables and were fed 500 g concentrates (Horse Stud Mix; Moulton's Feed Supplies, Lincolnshire, UK) twice a day with access to hay and water ad libitum. On the day preceding surgery, mares were moved into an indoor horsebox within the main animal facility. Food, but not water, was withdrawn 18 h prior to surgery, and the cyclooxygenase inhibitor, meclofenamic acid (2 mg kg–1; Arquel, Pharmacia and Upjohn, Sussex, UK) was given orally the night before surgery, and again the morning after surgery to reduce endogenous prostaglandin production associated with fasting and surgery in this species (Silver et al. 1979).
, http://www.100md.com
Surgical preparation
All procedures were performed under the UK Animals (Scientific Procedures) Act 1986. Gestational age was, first, estimated by ultrasound scan and measurement of aortic diameter and/or heart rate, and then confirmed at delivery. Surgery was performed between day 143 and 328 of predicted gestation (term is approx 330–350 days, Table 1), using techniques previously described (Taylor et al. 2001). In brief, on the morning of surgery, mares were premedicated with 20 μg kg–1 acepromazine (ACP; C-Vet, Leyland, UK), 20 μg kg–1 butorphanol (Torbugesic; Fort Dodge, Southampton, UK) and 10 μg kg–1 detomidine (Domosedan; Pfizer, Sandwich, UK) via a jugular catheter (Vygon intraflon 2 14G). Thirty minutes later, anaesthesia was induced with a further 10 μg kg–1 detomidine I.V., followed by 2 mg kg–1 ketamine I.V. (Vetalar; Pharmacia and Upjohn, Corby, UK). The trachea was intubated with a cuffed tube (Portex Ltd.; Hythe, Kent, UK) as soon as the mare became recumbent, and 100% oxygen was supplied via a to-and-fro rebreathing system (Fluotec; Ohmeda, Hatfield, UK). General anaesthesia was maintained in the recumbent position with i.v. infusion of propofol (Rapinovert; Schering Plough, Harefield, UK) at 200 μg kg–1 min–1 for the first 60 min, and then at a reduced rate of 180 and 130 μg kg–1 min–1 as required for the remaining period of surgery. After 10 min of propofol infusion, a ketamine infusion was commenced via a separate jugular catheter at 40 μg kg–1 min–1 for 60 min, and then reduced to 20–30 μg kg–1 min–1 as required (Baker et al. 1999; Taylor et al. 2001). Maternal arterial blood pressure via the facial artery was recorded throughout anaesthesia.
, http://www.100md.com
Under general anaesthesia, catheters were inserted into the maternal dorsal aorta via the circumflex artery, a main uterine vein via a small branch and the umbilical vein, and into the fetal dorsal aorta and caudal vena cava via peripheral hind limb vessels. A further catheter was secured to the fetal hind limb to measure amniotic fluid pressure. In 13 of the pregnancies, an ultrasonic flow transducer (2RS or 3RS Transonic Systems Inc., NY, USA) was implanted around one of the fetal metatarsal arteries, to measure peripheral blood flow. The fetal membranes were tied tightly and the uterine incision was closed in layers. Following fetal I.V. administration of ampicillin (25 mg (kg estimated bodyweight)–1; Penbritin; Beecham Animal Health, Brentford) and gentamycin (5 mg (kg estimated bodyweight)–1; Frangen-100; Biovet Ltd, Mullingar), all catheters were filled with heparinised saline (50 i.u. heparin ml–1 in 0.9% NaCl), plugged with brass pins, and were then exteriorised with the flow probe lead through a key hole incision in the maternal flank. The maternal abdominal and skin incisions were closed. Fetal catheters were maintained patent via a slow infusion of heparinised saline (100 i.u. heparin ml–1) via battery-operated mini pumps (MS16A syringe driver, Graseby Ltd, Watford, UK). Pumps, catheters and cables were kept in a plastic pouch sutured onto the maternal flank. Ampicillin (1 g, I.V.) was administered to the mare at surgery and for a further 3 days.
, 百拇医药
Experimental protocol
Commencing 5 days after surgery, cardiovascular, metabolic and endocrine measurements were made every other day at 10 : 00 for the duration of the experimental period, at gestational ages ranging from 148 to 341 days. On each measurement day, fetal arterial blood pressure, fetal heart rate and mean metatarsal blood flow were recorded continually at 1 s intervals for 2 h using a computerised Data Acquisition System. Fetal arterial blood pressure was corrected for amniotic pressure and fetal heart rate was triggered via a tachometer from either the arterial blood pressure or the metatarsal blood flow pulsatility. Whenever possible, simultaneous samples of maternal arterial, uterine venous, umbilical venous and fetal arterial blood (5 ml each) were taken. Values for pH (pHa), and partial pressures of oxygen and carbon dioxide (PO2 and PCO2), were obtained using a blood gas analyser (ABL 5; Radiometer, Copenhagen, Denmark), corrected to 39.5°C for fetal blood and 38°C for maternal blood. Blood haemoglobin concentration [Hb] and percentage saturation of haemoglobin with oxygen (Sat.Hb) were determined using a haemoximeter (OSM2; Radiometer). Blood glucose and lactate concentrations were determined with an automated analyser (Yellow Springs 2300 Stat Plus Glucose/Lactate Analyser; YSI Ltd, Farnborough, UK). The remainder of the maternal and fetal blood (4 ml) was transferred into appropriately treated chilled tubes, for measurement of plasma levels of cortisol, vasopressin and catecholamines. All tubes were centrifuged (4 min, 4000 r.p.m. at 4°C), and plasma aliquots transferred to PVC tubes, which were maintained at –70°C until biochemical analysis.
, http://www.100md.com
Biochemical analyses
Plasma concentrations of cortisol were measured by radioimmunoassay validated for equine plasma (RIA; Rossdale et al. 1982; Silver & Fowden, 1984). The minimum detectable quantity of cortisol in the assay was 1 ng ml–1. The intra-assay coefficient of variation was 2.7% for a mean value of 29.5 ng ml–1. The interassay coefficients of variation for two plasma pools (mean concentrations: 10.7 and 29.5 ng ml–1) were 9.4 and 7.8%, respectively. The cross-reactivities of the antiserum at 50% binding with other cortisol-related compounds were: 0.5% cortisone; 2.3% corticosterone; 0.3% progesterone; 4.6% deoxycortisol.
, http://www.100md.com
Plasma arginine vasopressin (AVP) concentrations were measured using a commercially available double-antibody RIA kit (Nichols Institute Diagnostics ltd, Saffron Walden, UK) validated for equine plasma. The lower limit of detection of the assay was 1.3 pg ml–1. The intra-assay coefficients of variation for four control plasma pools (mean concentrations: 3.2, 9.9, 12.2 and 28.9 pg ml–1) were 10.0, 6.7, 3.7 and 4.6%, respectively. The interassay coefficients of variation for two plasma samples (2.71 and 5.55 pg ml–1 AVP) were 4.1 and 9.8%, respectively. The anti-AVP antiserum was highly specific for [Arg8]vasopressin, with cross-reactivities of <0.05% against oxytocin, [Lys8]vasopressin and [Arg8]vasotocin.
, 百拇医药
Plasma noradrenaline and adrenaline concentrations were measured by high pressure liquid chromatography with electrochemical detection (Silver & Fowden, 1995). Samples were prepared by absorption of 250 μl of plasma onto acid-washed alumina, and 20 μl aliquots of the 100 μl perchloric acid eluates were injected onto the column. Dihydroxybenzylamine was added as the internal standard to each plasma sample before absorption. Recovery ranged from 63 to 97%, and all catecholamine values were corrected for their respective recovery. The interassay coefficients of variation for noradrenaline and adrenaline were 6.2% and 7.3%, respectively, and the minimum detectable dose was 10 pg ml–1.
, 百拇医药
Calculations
Fetal arterial blood pressure was corrected for amniotic pressure, which was used as zero reference. Fetal peripheral vascular resistance was calculated by dividing supra-amniotic mean arterial blood pressure by mean metatarsal blood flow. The fetal rate pressure product, an index of cardiac work, was calculated by multiplying fetal arterial blood pressure by fetal heart rate. Arterial blood oxygen content (Ca,O2), oxygen capacity (O2,cap), fetal peripheral arterial oxygen delivery (O2,del), fetal peripheral arterial glucose delivery (Glucosedel), maternal oxygen extraction (O2,maternal ext), fetal oxygen extraction (O2,fetal ext), maternal glucose extraction (Glucosematernal ext), and fetal glucose extraction (Glucosefetal ext) were calculated using eqns (1)–(8), respectively:
, 百拇医药
where one molecule of haemoglobin (MW = 64 450) binds four molecules of oxygen. The contribution of oxygen dissolved in plasma was regarded as being negligible. MA, maternal descending aorta; MV, uterine vein; FA, fetal descending aorta; FUV, umbilical vein.
Data and statistical analyses
Recordings of arterial blood pressure and heart rate were obtained from 17 of the 25 horse fetuses. In 2 of the 13 fetuses, the implanted tarsal flow probe developed an acoustic error, leaving 11 of 25 horse fetuses with successful peripheral blood flow recording. Measurements of blood gases, metabolic status and plasma cortisol concentrations were made in all pregnancies. Simultaneous blood samples were taken from the maternal descending aorta, uterine vein, umbilical vein and fetal descending aorta in 18 of the 25 pregnancies. Measurements of plasma catecholamine and vasopressin concentrations were made in 14 fetuses.
, http://www.100md.com
Variables are presented either as individual daily values with respect to gestational age (mean ± S.E.M.) for pregnancies <250 days (range: 148–228 days; 193.8 ± 2.7 days; 0.6 of gestation; n = 9), or for pregnancies >250 days (range: 265–341 days; 309.6 ± 2.4 days; 0.9 of gestation; n = 16). All data were assessed for normality of distribution using the Kolmogorov–Smirnov test. Data were normally distributed and were assessed for significance using parametric statistical tests. Significant differences between variables at 0.6 and 0.9 of gestation were determined by the Student's t test for unpaired data. Pearson product moment and partial correlation analyses were used to determine the relationships between cardiovascular, metabolic and endocrine variables with gestational age or with fetal plasma cortisol levels (Jandel SigmaStat 2.0 and Statview 4.02). Significance was accepted when P < 0.05.
, 百拇医药
Results
Cardiovascular function
Horse fetuses at 0.9 of gestation had significantly greater arterial blood pressure, rate–pressure product and peripheral blood flow, but significantly lower heart rate and peripheral vascular resistance, than horse fetuses at 0.6 of gestation (P < 0.05; Fig. 1).
Values are either mean ± S.E.M. at 0.6 (193.8 ± 2.7 days, range: 148–228 days; n = 9) and 0.9 (309.6 ± 2.4 days; range: 283–341 days, n = 16) of gestation, or values from individual horse fetuses in relation to gestational age for arterial blood pressure, heart rate, the rate–pressure product, peripheral blood flow and peripheral vascular resistance. r, Pearson product moment correlation coefficient; n, number of observations; *P < 0.05, 0.6 vs 0.9 gestation.
, 百拇医药
Correlation of fetal cardiovascular variables with gestational age and with the prevailing fetal plasma cortisol concentration showed significant positive relationships between fetal arterial blood pressure, rate–pressure product and peripheral blood flow, and both gestational age and fetal plasma cortisol (Figs 1 and 2), and showed significant negative relationships between peripheral vascular resistance and both gestational age and plasma cortisol (Figs 1 and 2), and a significant negative relationship between heart rate and gestational age (Fig. 1). Partial correlation analyses further revealed that gestational age and fetal plasma cortisol equally determined fetal arterial blood pressure and fetal peripheral blood flow (arterial blood pressure x gestation: 0.78, n = 28, P < 0.01; arterial blood pressure x cortisol: 0.65, n = 28, P < 0.01; peripheral blood flow x gestation: 0.79, n = 51, P < 0.01; peripheral blood flow x cortisol: 0.78, n = 44, P < 0.01). However, gestational age was a greater determinant than plasma cortisol of fetal peripheral vascular resistance (peripheral vascular resistance x gestation: –0.76, n = 19, P < 0.01; peripheral vascular resistance x cortisol: –0.07, n = 19, P = NS). Conversely, plasma cortisol concentration better determined rate–pressure product than gestational age (rate–pressure product x cortisol: 0.57, n = 18, P < 0.05; rate–pressure product x gestation: 0.46, n = 18, P = NS).
, 百拇医药
Association between plasma cortisol concentration and the values of arterial blood pressure, the rate–pressure product, peripheral blood flow and peripheral vascular resistance in all individual horse fetuses studied. r, Pearson product moment correlation coefficient; n, number of observations.
Hormone concentrations
Horse fetuses at 0.9 of gestation had significantly greater plasma concentrations of vasopressin, adrenaline, noradrenaline and cortisol than horse fetuses at 0.6 of gestation (Fig. 3). Fetal plasma concentrations of all hormones were low for most of the gestational period studied. However, at 290 days of gestation, abrupt elevations in the plasma concentrations of vasopressin, adrenaline and noradrenaline occurred, in close temporal association with the prepartum surge in fetal plasma cortisol (Fig. 3).
, 百拇医药
Values are either mean ± S.E.M. at 0.6 (193.8 ± 2.7 days, range: 148–228 days; n = 9) and 0.9 (309.6 ± 2.4 days; range: 283–341 days, n = 16) of gestation, or values from individual horse fetuses in relation to advancing gestational age for fetal plasma concentrations of vasopressin, adrenaline, noradrenaline and cortisol. r, Pearson product moment correlation coefficient; n, number of observations. *P < 0.05, 0.6 vs 0.9 gestation.
Correlation of the plasma concentration of vasopressin, adrenaline and noradrenaline with gestational age in individual horse fetuses showed significant positive relationships between all hormones and gestational age (Fig. 3). However, significant positive relationships were only seen between fetal plasma vasopressin and noradrenaline with cortisol (Fig. 4), but not between adrenaline and cortisol (0.36, n = 14, P = NS). Partial correlation analysis revealed that cortisol was a greater determinant than gestational age of vasopressin and noradrenaline concentrations in fetal plasma (cortisol x vasopressin: 0.90, n = 27, P < 0.01; cortisol x noradrenaline: 0.90, n = 14, P < 0.01; gestation x vasopressin: 0.09, n = 27, P = NS; gestation x noradrenaline: 0.29, n = 14, P = NS). Significant positive relationships were obtained between fetal plasma vasopressin and noradrenaline with fetal arterial blood pressure (Fig. 4), but not between adrenaline and fetal arterial blood pressure (0.34, n = 11, P = NS).
, 百拇医药
Association between plasma vasopressin and noradrenaline concentrations with either the prevailing plasma cortisol concentration or the arterial blood pressure in all individual horse fetuses studied. r, Pearson product moment correlation coefficient; n, number of observations.
Maternal plasma concentrations of vasopressin, noradrenaline, adrenaline and cortisol were similar at 0.6 and 0.9 of gestation (Table 2). No significant relationships were observed between any of the maternal hormone concentrations and gestational age.
, 百拇医药
Arterial blood gas and metabolic status
Maternal basal blood gas and metabolic status did not change significantly from 0.6 to 0.9 of gestation (Table 2). In contrast, horse fetuses at 0.6 of gestation had higher basal values for Pa,O2 and blood glucose concentration than horse fetuses at 0.9 of gestation (P < 0.05; Table 3). Therefore, a decrease in the fetal : maternal ratio of Pa,O2 (0.26 ± 0.01–0.24 ± 0.01) and blood glucose concentration (0.5 ± 0–0.36 ± 0.01) occurred from 0.6 to 0.9 of gestation (P < 0.05). Fetuses at 0.9 of gestation had significantly greater concentrations of haemoglobin than those at 0.6 of gestation. Consequently, despite the difference in Pa,O2, basal oxygen content and oxygen capacity in the equine fetus were greater at 0.9 than at 0.6 of gestation (P < 0.05; Table 2). Greater values for peripheral blood flow contributed to greater delivery of oxygen and glucose to the peripheral circulation in horse fetuses at 0.9 than at 0.6 of gestation (Table 3).
, 百拇医药
When individual values for blood gas and metabolic status were correlated with gestational age or with the prevailing fetal plasma cortisol concentration in all horse pregnancies studied, significant negative relationships were observed between gestational age and the following factors: fetal Pa,O2, fetal blood glucose concentration and the fetal: maternal ratio of Pa,O2 and blood glucose (P < 0.05; Table 4). Conversely, there were significant positive relationships between gestational age and haemoglobin concentration, oxygen content and oxygen capacity in fetal blood (P < 0.05; Table 4). Partial correlation analysis revealed that delivery of oxygen and glucose to the fetal peripheral vasculature was equally related both to gestational age and to fetal plasma cortisol (Table 4).
, http://www.100md.com
When individual values for blood gas and metabolic status were correlated with fetal arterial blood pressure, neither fetal Pa,O2 nor blood glucose were significantly related to blood pressure, although the correlation of fetal Pa,O2 with fetal arterial blood pressure just fell outside significance (Pa,O2 x fetal arterial blood pressure: –0.40, n = 25, P = 0.06; glucose x fetal arterial blood pressure: –0.03, n = 16, P = 0.91). When the fetal : maternal ratio of Pa,O2 and blood glucose concentration were correlated to fetal arterial blood pressure, negative relationships were obtained, which reached significance only between the fetal : maternal ratio of Pa,O2 and fetal arterial blood pressure (–0.68, n = 18, P < 0.01), but not between the fetal : maternal ratio of glucose and fetal arterial blood pressure (–0.41, n = 16, P = NS).
, 百拇医药
Discussion
The data in this study show that arterial blood pressure, rate–pressure product, peripheral blood flow, and plasma concentrations of noradrenaline, adrenaline, vasopressin and cortisol increased with advancing gestational age up to term in the equine fetus. In contrast, heart rate, peripheral vascular resistance and the fetal : maternal ratio of Pa,O2 and blood glucose concentration decreased towards term. While most cardiovascular and endocrine changes in the horse fetus showed significant associations with both gestational age and fetal plasma cortisol concentrations, partial correlation analyses revealed that gestational age and fetal plasma cortisol contributed equally to the ontogenic increase in fetal arterial blood pressure. In contrast, gestational age is a greater determinant than cortisol of fetal heart rate and peripheral vascular resistance, while cortisol is a greater determinant than gestational age of the rate–pressure product, and plasma concentrations of noradrenaline and vasopressin. Fetal Pa,O2 and blood glucose, and the fetal : maternal ratio of Pa,O2 and blood glucose concentration, were only related to advancing gestational age and not to cortisol. Arterial blood pressure was positively related with plasma concentrations of vasopressin and noradrenaline, but not adrenaline in the fetus, and inversely related to the fetal : maternal ratio of blood Pa,O2, but not glucose, concentration.
, 百拇医药
In the simplest terms, mean arterial blood pressure in the fetus is determined by the combined ventricular output and the total peripheral vascular resistance. Therefore, ontogenic increases in arterial blood pressure in the fetus could be accounted for by ontogenic increases in either or both of these variables. The most comprehensive study of changes in regional blood flow during fetal development is that of Rudolph & Heymann (1970) who measured the combined ventricular output and organ blood flow distribution in fetal sheep from 60 days of gestation to term. They reported that the combined ventricular output of the sheep fetus increased with advancing gestational age up to term in absolute terms, but not when expressed per kg fetal body weight. These observations suggest that increases in ventricular output towards term occur in parallel with the growing volume of the fetal vasculature and, hence, are unlikely to make a major contribution to the ontogenic increase in fetal blood pressure. Little is known about changes in total peripheral vascular resistance towards term in fetal sheep, although the resistance of several vascular beds is known to be responsive to adverse intrauterine conditions during late gestation (e.g. Cohn et al. 1974; Rudolph, 1984; Jensen & Berger, 1991; Giussani et al. 1993). In the present study, there was a pronounced fall in the vascular resistance of the metatarsal circulation of the fetal horse during late gestation, which was due to a greater ontogenic increase in hind limb blood flow than fetal arterial blood pressure. Since resistance of the metatarsal circulation was more closely related to gestational age than fetal plasma cortisol, the ontogenic fall in peripheral vascular resistance is more likely to reflect angiogenesis and an increase in the cross-sectional area of the vascular beds in the growing fetus than maturational changes in vasoreactivity of the peripheral vasculature. Indeed, when peripheral blood flow was expressed per kg body weight, there was no change in metatarsal blood flow with increasing age in the fetal horse (data not shown). These observations suggest that the ontogenic increase in fetal arterial blood pressure in the horse is not due to changes in total peripheral vascular resistance, although vascular resistance in the metatarsal circulation may not be the best index of overall changes in total peripheral vascular resistance in this species. In the absence of direct changes in either fetal cardiac output or peripheral vascular resistance, an alternative explanation for the ontogenic rise in fetal blood pressure is that placental vascular resistance rises relative to the total vascular resistance of the fetus, either by a greater rise in placental resistance or a greater fall in fetal total vascular resistance as term approaches. In this context, it has been reported that weight-normalised umbilico-placental blood flow decreases with advancing gestation in the horse (Fowden et al. 2000a) and in the sheep (Hedriana et al. 1995), but changes little in the human (Sutton et al. 1990). However, since a similar progression of mean arterial blood pressure, as seen in the fetus, also occurs after birth (Louey et al. 2000), a role for changes in placental vascular resistance in contributing to progressive changes in arterial blood pressure during the fetal period is not supported.
, 百拇医药
In the present study, there was a fall in the heart rate of the horse fetus with advancing gestational age, in common with other species (Reeves et al. 1972; Boddy et al. 1974; Dawes, 1980; Macdonald et al. 1983; Kitanaka et al. 1989; Forhead et al. 2000a). Several studies have shown that rate–pressure product is a useful marker of cardiac work, correlating well with changes in myocardial oxygen consumption (Jorgensen et al. 1973; Nelson et al. 1974). The increasing metabolic demand of the equine fetal heart with advancing gestation may reflect not only changes in cardiac growth and stroke volume (Machida et al. 1988), but also in increased cardiac work secondary to the imposed changes in cardiac afterload as a result of the ontogenic increase in fetal arterial blood pressure.
, 百拇医药
Previous studies in horses and other species have shown that developmental changes in fetal arterial blood pressure correlate strongly with ontogenic changes in the renin–angiotensin system and adrenocortical cortisol secretion (Forhead et al. 2000a,b). In the present study, abrupt elevations in the fetal concentrations of plasma vasopressin, adrenaline and noradrenaline occurred in close temporal association with the prepartum surge in plasma cortisol in the horse fetus. While significant correlations were observed between gestational age or fetal plasma cortisol concentration and fetal vasopressin, adrenaline and noradrenaline, only fetal plasma concentrations of noradrenaline and vasopressin were correlated with fetal plasma cortisol and fetal arterial blood pressure. These results suggest that ontogenic changes in fetal plasma noradrenaline and vasopressin, but not adrenaline, are also involved in mediating the ontogenic increase in fetal arterial blood pressure in the horse, presumably by constricting circulations other than those represented by the metatarsal vascular bed. Whether these ontogenic changes in plasma noradrenaline and vasopressin are matched by similar changes in the density of the respective receptor population with advancing gestational age remains unknown in the fetal horse. In fetal rats, expression of vasopressin V1 receptors is developmentally regulated (Ostrowski et al. 1993) and, in fetal baboons, the maximum vasoconstrictor response to noradrenaline of small branches of the femoral artery also increases with advancing gestastional age (Anwar et al. 2001). However, in vivo treatment of immature fetal sheep with cortisol did not enhance their pressor response to exogenous noradrenaline (Tangalakis et al. 1992), and in vitro vasoconstrictor responses to exogenous noradrenaline of peripheral vessels obtained from immature sheep fetuses treated with betamethasone were also not enhanced (Anwar et al. 1999).
, http://www.100md.com
In the present study, a fall in basal values of Pa,O2 and blood glucose concentration occurred with advancing gestational age in fetal but not maternal blood. Consequently, there were decreases in the fetal: maternal ratios of Pa,O2 and blood glucose concentration between 0.6 and 0.9 of gestation, consistent with previous observations in pregnant rabbits (Gilbert et al. 1984), sheep (Hay, 1995) and horses (Fowden et al. 2000a,b). The falls in fetal Pa,O2 and blood glucose concentration, together with changes in the rates of oxygen and glucose consumption by the placenta, help to maintain the net maternal-to-fetal plasma concentration gradients for glucose molecules needed to meet the increasing nutrient demands of the growing fetus (Meschia et al. 1980; Bell et al. 1987; Molina et al. 1991; Fowden et al. 2000a,b). The current finding that the fetal: maternal ratios of Pa,O2 and blood glucose concentration were significantly related to gestational age but not to fetal plasma cortisol concentrations supports the suggestion that these metabolic changes reflect fetal growth rather than fetal maturation in the horse. The relative fetal hypoxaemia and hypoglycaemia observed at 0.9 of gestation may contribute, in part, to the ontogenic increase in fetal arterial blood pressure, by triggering cardiovascular chemoreflex and neuroendocrine mechanisms that ensure delivery of sufficient oxygen and glucose to the fetal target organs to meet the increasing demands for growth (see Giussani et al. 1994 for review). This suggestion is supported by the inverse relationship observed between fetal blood pressure and the fetal : maternal ratios of Pa,O2 and blood glucose concentration, although only the relationship of fetal : maternal Pa,O2 reached statistical significance. It is of interest that umbilical venous and arterial PO2 also fall progressively with advancing gestation in the human fetus (Soothill et al. 1986).
, 百拇医药
In conclusion, the present data show that the increase in arterial blood pressure towards term in the horse fetus occurs in association with ontogenic reductions in metatarsal vascular resistance, elevations in plasma concentrations of vasopressin and noradrenaline, and a decrease in the fetal : maternal ratio of Pa,O2. This suggests that increasing vasopressinergic and noradrenergic influences as well as changes in oxygen uteroplacental and fetal metabolism may contribute to the mechanisms that drive fetal arterial blood pressure up with advancing gestational age in equine pregnancy.
, 百拇医药
References
Anwar MA, Ju K, Docherty CC, Poston L & Nathanielsz PW (2001). Developmental changes in reactivity of small femoral arteries in the fetal and postnatal baboon. Am J Obstet Gynecol 184, 707–712.
Anwar MA, Schwab M, Poston L & Nathanielsz PW (1999). Betamethasone-mediated vascular dysfunction and changes in hematological profile in the ovine fetus. Am J Physiol Heart Circ Physiol 276, H1137–H1143.
, 百拇医药
Baker KL, Taylor PM, Fowden AL & Bloomfield M (1999). Propofol-ketamine anaesthesia in late gestation ponies. J Vet Anaesthesia 26, 45–50.
Bell AW, Battaglia FC & Meschia G (1987). Relation between metabolic rate and body size in the ovine fetus. J Nutr 117, 1181–1186.
Boddy K, Dawes GS, Fisher R, Pinter S & Robinson JS (1974). Foetal respiratory movements electrocortical and cardiovascular responses to hypoxaemia and hypercapnia in sheep. J Physiol 243, 599–618.
, http://www.100md.com
Cohn HE, Sacks EJ, Heymann MA & Rudolph AM (1974). Cardiovascular responses to hypoxemia and acidemia in fetal lambs. Am J Obstet Gynecol 120, 817–824.
Dawes GS, Johnston BM & Walker DW (1980). Relationship of arterial pressure and heart rate in fetal new-born and adult sheep. J Physiol 309, 405–417.
Derks JB, Giussani DA, Jenkins SJ, Wentworth RA, Visser GHA, Padbury JF & Nathanielsz PW (1997). A comparative study of cardiovascular endocrine and behavioural effects of betamethasone and dexamethasone administration to fetal sheep. J Physiol 499, 217–226.
, 百拇医药
Fletcher AJW, McGarrigle HG, Edwards CMB, Fowden AL & Giussani DA (2002). Effects of low dose dexamethasone treatment on basal cardiovascular and endocrine function in fetal sheep during late gestation. J Physiol 545, 649–660.
Forhead AJ, Broughton Pipkin F & Fowden AL (2000b). Effect of cortisol on blood pressure and the renin-angiotensin system in fetal sheep during late gestation. J Physiol 526, 167–176.
Forhead AJ, Broughton Pipkin F, Taylor PM, Baker K, Balouzet V, Giussani DA & Fowden AL (2000a). Developmental changes in blood pressure and the renin-angiotensin system in Pony fetuses during the second half of gestation. J Reprod Fertil (Suppl.) 56, 693–703.
, 百拇医药
Fowden AL, Forhead AJ, White KL & Taylor PM (2000a). Equine uteroplacental metabolism at mid- and late gestation. Exp Physiol 85, 539–545.
Fowden AL, Li J & Forhead AJ (1998). Glucocorticoids and the preparation for life after birth: are there long-term consequences of the life insurance Proc Nutr Soc 57, 113–122.
Fowden AL & Silver M (1995). Comparative development of the pituitary-adrenal axis in the fetal foal and lamb. Reprod Domest Anim 30, 170–177.
, http://www.100md.com
Fowden AL, Taylor PM, White KL & Forhead AJ (2000b). Ontogenic and nutritionally induced changes in fetal metabolism in the horse. J Physiol 528, 209–219.
Gilbert M, Hay WW Jr, Johnson RL & Battaglia FC (1984). Some aspects of maternal metabolism throughout pregnancy in the conscious rabbit. Pediatr Res 18, 854–859.
Giussani DA, Spencer JAD & Hanson MA (1994). Fetal cardiovascular reflex responses to hypoxia. Fet Mat Med Rev 6, 17–37.
, http://www.100md.com
Giussani DA, Spencer JAD, Moore PJ, Bennet L & Hanson MA (1993). Afferent and efferent components of the cardiovascular reflex responses to acute hypoxia in term fetal sheep. J Physiol 461, 431–449.
Hay WW Jr (1995). Regulation of placental metabolism by glucose supply. Reprod Fertil Dev 7, 365–375.
Hedriana HL, Brace RA & Gilbert WM (1995). Changes in blood flow to the ovine chorion and amnion across gestation. J Soc Gynecol Invest 2, 727–734.
, http://www.100md.com
Jensen A & Berger R (1991). Fetal circulatory responses to oxygen lack. J Dev Physiol 16, 181–207.
Jorgensen CR, Wang K, Wang Y, Gobel FL, Nelson RR & Taylor H (1973). Effect of propranolol on myocardial oxygen consumption and its hemodynamic correlates during upright exercise. Circulation 48, 1173–1182.
Kitanaka T, Alonso JG, Gilbert RD, Siu BL, Clemons GK & Longo LD (1989). Fetal responses to long-term hypoxemia in sheep. Am J Physiol Regul Integr Comp Physiol 256, R1348–R1354.
, http://www.100md.com
Liggins GC (1994). The role of cortisol in preparing the fetus for birth. Reprod Fertil Dev 6, 141–150.
Louey S, Cock ML, Stevenson KM & Harding R (2000). Placental insufficiency and fetal growth restriction lead to postnatal hypotension and altered postnatal growth in sheep. Pediatr Res 48, 808–814.
Macdonald AA, Colenbrander B & Wensing CJG (1983). The effects of gestational age and chronic fetal decapitation on arterial blood pressure in the fetus. Eur J Obstet Gynaecol Reprod Biol 16, 63–70.
, 百拇医药
Machida N, Yasuda J & Too K (1988). A morphometric study of foetal and newborn cardiac growth in the horse. Equine Vet J 20, 261–267.
Meschia G, Battaglia FC, Hay WW & Sparks JW (1980). Utilization of substrates by the ovine placenta in vivo. Fed Proc 39, 245–249.
Molina RD, Meschia G, Battaglia FC & Hay WW Jr (1991). Gestational maturation of placental glucose transfer capacity in sheep. Am J Physiol Regul Integr Comp Physiol 261, R697–R704.
, http://www.100md.com
Nelson RR, Gobel FL, Jorgensen CR, Wang K, Wang Y & Taylor HL (1974). Hemodynamic predictors of myocardial oxygen consumption during static and dynamic exercise. Circulation 50, 1179–1189.
Ostrowski NL, Young WS, Knepper MA & Lolait SJ (1993). Expression of vasopressin V1a and V2 receptor messenger ribonucleic acid in the liver and kidney of embryonic developing and adult rats. Endocrinology 133, 1849–1859.
Reeves JT, Daoud FS & Gentry M (1972). Growth of the fetal calf and its arterial pressure blood gases and hematologic data. J Appl Physiol 32, 240–244.
, http://www.100md.com
Rossdale PD, Silver M, Ellis L & Frauenfelder H (1982). Response of the adrenal cortex to tetracosactrin (ACTH 1–24) in the premature and full term foal. J Reprod Fertil (Suppl.) 32, 545–553.
Rudolph AM (1984). The fetal circulation and its response to stress. J Dev Physiol 6, 11–19.
Rudolph AM & Heymann MA (1970). Circulatory changes during growth in the fetal lamb. Circ Res 26, 289–299.
, http://www.100md.com
Silver M, Barnes RJ, Comline RS, Fowden AL & Mitchell MD (1979). Prostaglandins in maternal and fetal plasma and in allantoic fluid during the second half of gestation in the mare. J Reprod Fertil (Suppl.) 27, 531–539.
Silver M & Fowden AL (1984). Prepartum adrenocortical maturation in the fetal foal: responses to ACTH1–24. J Endocrinol 142, 417–425.
Silver M & Fowden AL (1995). Sympathoadrenal and other endocrine and metabolic responses to hypoglycaemia in the fetal foal during late gestation. Exp Physiol 80, 651–662.
, 百拇医药
Soothill PW, Nicolaides KH, Rodeck CH & Campbell S (1986). Effect of gestational age on fetal and intervillous blood gas and acid-base values in human pregnancy. Fetal Ther 1, 168–175.
Sutton MS, Theard MA, Bhatia SJ, Plappert T, Saltzman DH & Doubilet P (1990). Changes in placental blood flow in the normal human fetus with gestational age. Pediatr Res 28, 383–387.
Tangalakis K, Lumbers ER, Moritz KM, Towstoless MK & Wintour EM (1992). Effect of cortisol on blood pressure and vascular reactivity in the ovine fetus. Exp Physiol 77, 709–717.
, 百拇医药
Taylor PM, White KL, Fowden AL, Giussani DA, Bloomfield M & Sear JW (2001). Propofol anaesthesia for surgery in late gestation pony mares. Vet Anaesth Analg 28, 177–187.
Unno N, Wong CH, Jenkins SJ, Wentworth RA, Ding XY, Li C, Robertson SS, Smotherman WP & Nathanielsz PW (1999). Blood pressure and heart rate in the ovine fetus: ontogenic changes and effects of fetal adrenalectomy. Am J Physiol Heart Circ Physiol 276, H248–H256.
Wood CE, Cheung CY & Brace RA (1987). Fetal heart rate arterial pressure and blood Volume responses to cortisol infusion. Am J Physiol Regul Integr Comp Physiol 253, R904–R909., 百拇医药(Dino A Giussani, Alison J)