Fatty Acid Signaling in the Hypothalamus and the Neural Control of Ins
http://www.100md.com
《糖尿病学杂志》
1 Laboratoire de Physiopathologie de la Nutrition, Universite Paris 7, National Center of Scientific Research, Paris, France
2 Department of Neurology and Neurosciences, New Jersey Medical School, Newark, New Jersey
3 Department of Pharmacology and Physiology, New Jersey Medical School, Newark, New Jersey
4 Neurology Service, VA Medical Center, East Orange, New Jersey
ARC, arcuate nucleus; FA, fatty acid; FLI, cfos-like immunoreactive; GIIS, glucose-induced insulin secretion; OA, oleic acid; OAE, OA-excited; OAI, OA-inhibited
ABSTRACT
It is now clearly demonstrated that fatty acids (FAs) may modulate neural control of energy homeostasis and specifically affect both insulin secretion and action. Indeed, pancreatic -cells receive rich neural innervation and FAs induce important changes in autonomic nervous activity. We previously reported that chronic infusion of lipids decreased sympathetic nervous system activity and led to exaggerated glucose-induced insulin secretion (GIIS), as would be expected from the known inhibitory effect of sympathetic splanchnic nerve activity on insulin secretion. Intracarotid infusion of lipids that do not change plasma FA concentrations also lead to increased GIIS. This effect of FAs on GIIS was prevented by inhibition of -oxidation. It is noteworthy that a single intracarotid injection of oleic acid also induced a transient increase in plasma insulin without any change in plasma glucose, suggesting that FAs per se can regulate neural control of insulin secretion. Finally, using whole cell current clamp recordings in hypothalamic slices and calcium imaging in dissociated hypothalamic neurons, we identified a hypothalamic subpopulation of neurons either excited (13%) or inhibited (6%) by FAs. Thus, FAs per se or their metabolites modulate neuronal activity, as a means of directly monitoring ongoing fuel availability by central nervous system nutrient-sensing neurons involved in the regulation of insulin secretion.
FATTY ACIDS AND NERVOUS CONTROL OF ENERGY HOMEOSTASIS
It is now well established that fatty acids (FAs) act on the central nervous system as important physiological regulators of glucose metabolism and overall energy homeostasis (1,2). For example, central administration of the long-chain FA oleate inhibits food intake and glucose production in rats (3,4). Such effects are partly related to a decreased expression of hypothalamic neuropeptide Y and of hepatic glucose-6-phosphatase after short-term overfeeding (4). This suggests that the slight increase in plasma FA concentrations in the postprandial state (5) might be detected by the central nervous system as a satiety signal. Beside these physiological effects, we previously showed that central triglyceride infusions into the brain through the carotid artery in rats over 24 h initiated hepatic insulin resistance independently of any change in plasma FA concentration (6). In this model, glucose-induced insulin secretion (GIIS) was markedly increased in lipid-infused rats compared with controls (6). Those toxic effects of lipids were prevented when -oxidation was inhibited (6). Finally, central inhibition of lipid oxidation is sufficient to restore the hypothalamic levels of long-chain fatty acyl-CoA and to normalize food intake and glucose homeostasis in overfed rats (7). Such studies suggest an important link between the central actions of FA on both food intake and glucose metabolism under both physiological but also pathological conditions.
FAs AND NERVOUS CONTROL OF INSULIN SECRETION
FAs directly act to regulate insulin exocytosis from the pancreatic -cell (8). In vitro, long-term exposure (48 h and more) to FAs results in a decreased insulin response to glucose in the pancreatic -cells (rev. in 9). However, when studied in vivo in healthy subjects, the precise effects of FAs on pancreatic -cells remain controversial. On the basis of experiments with lipid infusion, the insulin secretory response to glucose was reported to be increased (10), unchanged (11), or decreased (12). Different mechanisms might explain these discrepancies. The insulinotropic potency of FAs is influenced by chain length and degree of saturation (13). Moreover, the duration of lipid infusion also seems crucial, since opposite effects on insulin secretions are obtained by short-term (6 h) versus long-term (24 h) triglyceride infusions (12). It is also critical to carefully control the way in which glucose is administered (intravenously versus intraperitoneally) as a challenge of pancreatic -cell function (10–12). Discrepancies between in vivo and in vitro studies concerning the effect of a long-term increase in FA concentrations on pancreatic -cell function suggest the involvement of extrapancreatic factors. Among these factors, those of neural origin may be crucial. On the one hand, the endocrine pancreas is richly innervated by both parasympathetic and sympathetic inputs. Parasympathetic activation stimulates, whereas sympathetic activation inhibits, insulin secretion (14,15). On the other hand, several experiments clearly showed that lipids may alter autonomic nervous system activity in humans (16,17). More precisely, clinical and epidemiological studies showed that reduced autonomic activity, especially sympathetic, might be involved in the onset of obesity (18,19). Particularly in Pima Indians, a low sympathetic nervous system activity is also described as a predictor of weight gain (20). There is an important link between the development of obesity in rodents fed on diets of moderate to high FA content, pancreatic sympathetic function, and the development of abnormal insulin sensitivity. When outbred rats are fed such diets chronically, 50% become obese. Those rats do display reduced pancreatic sympathetic activity that precedes the development of obesity and that persists in association with hyperinsulinemia after obesity develops (21,22). These data suggest a critical link between impaired pancreatic sympathetic function, dietary FA intake, and the genetic propensity to develop diet-induced obesity.
To further study this potential relationship, we developed an in vivo model based on a 48-h intravenous infusion of triglyceride emulsion in rats (23). Briefly, a catheter was implanted under ketamine anesthesia (125 mg/kg, intraperitoneally; Imalgene, Merieux, Lyon, France) in the right atrium of a 250-g male Wistar rat via the jugular vein. A technique previously described (24) for a 48-h infusion in unrestrained rats was used for triglyceride or saline infusion. The infusion period started on day 2 after surgery. Rats were randomly divided into two groups. In the first group, rats were infused with a mixture of a 20% triglyceride emulsion (2,000 kcal/l Intralipid KabiVitrum and 20 units/ml heparin; Intralipid KabiVitrum, Stockholm, Sweden) at a rate of 20 μl/min. Control rats were infused with saline and heparin. At the end of infusion, we measured in vivo GIIS in response to a single intravenous injection of glucose after 5 h of food deprivation. In parallel, sympathetic firing-rate activities were recorded at the level of the superior cervical ganglion (Fig. 1). Such infusions approximately double plasma FA concentration in association with decreased sympathetic nerve activity (Fig. 1) and a concomitant increase in GIIS compared with control rats. These findings are in accord with the known inhibitory effect of sympathetic activity on insulin secretion. To demonstrate the potential role of the sympathetic activity in the control of insulin secretion after lipid infusion, GIIS was also studied in the presence of oxymetazoline, an 2A-adrenoceptor (the main -cell inhibitory adrenoceptor) agonist, to mimic an activated sympathetic nervous system. As expected, oxymetazoline produced a dose-related blunting of GIIS in lipid-infused rats, whereas controls were not sensitive to low concentrations of oxymetazoline. At a dose of 0.3 pmol/kg, GIIS became similar in both groups, suggesting that decreased sympathetic nervous system activity was partly responsible for pancreatic -cell hyperresponsiveness to glucose (23). Interestingly, there was also a reduced Bmax and Kd for 2-adrenoceptor binding in the pancreatic -cell of lipid-infused rats (25). Finally, neither -adrenoceptor agonists nor antagonists affected GIIS in either lipid-infused or control rats. This result suggests that the insulin stimulating -adrenergic pathway was not involved in the -cell hyperresponsiveness to glucose in our model. This is in keeping with the 100-fold lower number of -adrenoceptors compared with -adrenoceptors on the -cell surface (26,27) and is in agreement with the fact that -adrenergic control of insulin secretion predominates over the -adrenergic one in vivo (28). We have also demonstrated that these findings in rats are relevant to humans, where 48-h intravenous lipid infusion in healthy subjects also leads to glucose-induced insulin hypersecretion in association with a decrease in plasma norepinephrine concentration and urinary excretion, as an indirect measurement of sympathetic nervous system activity (29).
To assess whether these effects of FAs may be due to a direct action in the central nervous system, 250-g male Wistar rats were infused with lipids toward the brain through the carotid artery for 24 h at a rate of 2 μl/min (6). This approach allowed us to raise brain FA levels without bypassing the blood-brain barrier. Additionally, to assess the role of -oxidation in the effects of central FAs, rats were infused intracerebroventricularly with etomoxir, an inhibitor of carnitine palmitoyl transferase 1, before and during intracarotid infusion of FAs. Importantly, this route of administration produced increased GIIS without any change in plasma FA concentrations. This FA effect on GIIS was abolished by co-infusion of etomoxir, suggesting that -oxidation is required for the FA effects on insulin secretion (6). This is similar to the demonstration that -oxidation in the hypothalamus also appears to be an important regulator of food intake and hepatic glucose output (30). Because FAs induced hepatic insulin resistance in intracarotid lipid-infused rats, the increased GIIS could partly be related to changes in insulin sensitivity. To specifically study the effect of FAs on nervous control of insulin secretion, we measured both plasma insulin and glucose in response to an acute intracarotid injection of oleic acid (OA), which did not affect circulating lipid levels. The effect of OA was compared with saline, glucose, and both OA/glucose injection. As depicted in Fig. 2, an acute OA overload induced a transient significant increased insulinemia at time 1 and 3 min after injection, but did not affect glycemia. Glucose induced a greater increase in plasma insulin. Both OA and glucose injected together had an additive effect, thus leading to a higher insulinemia. It may be a bit surprising that blood glucose is not at all altered, although there is a rise in insulin secretion. It could suggest that insulin resistance is induced, in agreement with decreased hepatic insulin sensitivity observed in 24-h intracarotid interleukin-infused rats (6). The additive effect of both nutrients may partially lead to further dysregulation of nervous control of insulin secretion and finally contribute to the etiology of type 2 diabetes in predisposed subjects.
FA-SENSITIVE NEURONS
Whereas the in vivo effects of FA on insulin and glucose metabolism are interesting, it is imperative to demonstrate the cellular and molecular mechanisms by which FA can act to alter neural activity. Thirty years ago, Oomura et al. (31) showed that FAs modulated central nervous system activity in vivo. Experiments were performed on the rat lateral hypothalamic neurons. Indeed, the hypothalamus contains large populations of nutrient-sensitive neurons (either glucose or FA sensitive). Thus, it is now assumed that, like glucose, FAs per se or their metabolites could modulate neuronal activity as a means of directly monitoring ongoing fuel availability to allow central nervous system nutrient-sensing neurons to regulate energy homeostasis (2,32). We previously showed that intracarotid lipid infusion decreased the number of cfos-like immunoreactive (FLI) neurons (a marker of neuronal activity) in four of the five hypothalamic nuclei studied: arcuate nucleus (ARC), dorsomedial hypothalamus, ventromedial hypothalamus, and paraventricular nucleus. In contrast, the number of FLI neurons in the lateral hypothalamus was increased in lipid-infused rats (6). The decreased number of FLI neurons in ventromedial hypothalamus indicated a lower activity in this nucleus, which could likely promote a decreased sympathetic tone, since both ventromedial hypothalamus and ARC are known to activate the sympathetic nervous system via polysynaptic efferent pathways (28). On the other hand, stimulation of the lateral hypothalamus, which contributes to the control of parasympathetic activity, promotes insulin secretion in the presence of a simultaneous rise in glucose (33). Thus, a decrease in number of FLI neurons in the ventromedial hypothalamus and an increase in number of these FLI neurons in lateral hypothalamus could reflect a concomitant decrease in sympathetic activities and an increase in parasympathetic activities, which inhibit and stimulate insulin secretion, respectively.
MOLECULAR MECHANISMS INVOLVED IN FA-SENSITIVE NEURONS
Molecular mechanisms involved in FA-sensitive neurons are still being debated. However, enzymes involved in their metabolism such as fatty acid synthase have been demonstrated to be present and active in hypothalamic neurons (34). Furthermore, carnitine palmitoyl transferase 1 is also expressed in some brain areas, including the hypothalamus, which supports our finding that -oxidation seems crucial to mediate some the effects of FA on glucose metabolism (6,30). Thus, while FA signaling in the hypothalamus is important for peripheral glucose and energy homeostasis, the mechanisms by which FAs alter neuronal activity are not clear. One possibility is via modulation of ion channels. A body of literature indicates that FAs regulate the conductance of a wide variety of ion channels, which include Cl–, GABAA (35), CIC-2 (36), potassium, K+-Ca2+ (37), ATP-sensitive K+ channel (37), and calcium channels (38). Additionally, FAs inhibit the Na+-K+ ATPase pump (39). These effects of FAs may be directly on ion channels or through the actions of metabolic intermediates. Because there is a reciprocal relationship between glucose and FA metabolism, flux through these metabolic pathways may interact to regulate neuronal activity in the hypothalamus (40). Using patch clamp recordings, we recently investigated the effects of OA on ARC neurons in hypothalamic slices from 14- to 21-day-old Sprague-Dawley (SD) rats (41). We also recorded discharge rates in ARC neurons in 8-week-old fed and fasted SD rats in vivo in response to a single injection of OA. We found that OA regulated three distinct subpopulations of ARC neurons in a glucose-dependent fashion. Patch clamp studies showed that in 2.5 mmol/l glucose, 12 of 94 (13%) ARC neurons were excited by 2 mmol/l OA (OA-excited [OAE] neurons, Fig. 3), whereas 6 of 94 (6%) were inhibited (OA-inhibited2.5 [OAI2.5] neurons). In contrast, in 0.1 mmol/l glucose, OA inhibited 6 of 20 (30%) ARC neurons (OAI0.1 neurons); none were excited. None of the OAI0.1 neurons responded to OA in 2.5 mmol/l glucose. Thus, OAI2.5 and OAI0.1 neurons are distinct. Similarly, in fed rats, 7 of 20 (35%) ARC neurons were OAE, whereas only 3 of 20 (15%) were OAI. Note that in these rats, there was an increase in insulin secretion in response to a single injection of OA (Fig. 2). In contrast, in fasted rats only, OAI neurons were observed (3 of 15 [20%]) in ARC neurons. Thus, neural control of insulin secretion by FAs probably involved more than one neuronal population and also several different molecular mechanisms. Indeed, we identified at least two types of channels relaying the effects of OA (41). Excitatory effects of OA were probably involved in the closure of Cl– channels assessed by whole-cell current clamp recording (41), whereas the inhibitory effects of OA were reversed by the ATP-sensitive K+ channel blocker tolbutamide (in 2.5 mmol/l glucose; V.H.R., R.W., unpublished data). Additionally, activation of AMP-activated kinase with 5-aminoimidazole-4-carboxamide-1--D-ribofuranoside (AICAR) reversed the excitatory effect of OA (Fig. 4). This suggests that AMP-activated kinase could be a key molecule-mediating effect of FAS in nutrient-sensing neurons.
We have further demonstrated the ability of FA to alter neuronal activity in dissociated neurons from the ventromedial hypothalamic nucleus of 3- to 4-week-old rats using the fura-2 Ca2+ imaging technique. These neurons either increase (glucose excited) or decrease (glucose inhibited) their intracellular Ca2+ flux when extracellular glucose levels are raised from 0.5 to 2.5 mmol/l (42). As mentioned above, in vivo OA infusion in ventromedial hypothalamic nucleus resulted in a decreased in FLI neuron number (6). In preliminary studies (Fig. 5), we found that OA induced a significant decrease in intracellular Ca2+ oscillations in both glucose-excited and glucose-inhibited neurons. These data independently reinforce the ability of FAs to alter activity in hypothalamic nutrient-sensitive neurons.
To summarize, neural control of insulin secretion by FAs is crucial to finely regulate pancreatic -cell function. As a consequence, central dysregulation of FA metabolism could be an early event leading to inappropriate GIIS and that could, at least in part, contribute to development of metabolic diseases such as type 2 diabetes in predisposed subjects.
FOOTNOTES
This article is based on a presentation at a symposium. The symposium and the publication of this article were made possible by an unrestricted educational grant from Servier.
The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
REFERENCES
Lam TK, Pocai A, Gutierrez-Juarez R, Obici S, Bryan J, Aguilar-Bryan L, Schwartz GJ, Rossetti L: Hypothalamic sensing of circulating fatty acids is required for glucose homeostasis. Nat Med 11:320–327, 2005
Seeley RJ, Woods SC: Monitoring of stored and available fuel by the CNS: implications for obesity. Nat Rev Neurosci 4:901–909, 2003
Obici S, Feng Z, Morgan K, Stein D, Karkanias G, Rossetti L: Central administration of oleic acid inhibits glucose production and food intake. Diabetes 51:271–275, 2002
Morgan K, Obici S, Rossetti L: Hypothalamic responses to long-chain fatty acids are nutritionally regulated. J Biol Chem 279:31139–31148, 2004
Meier JJ, Gethmann A, Gotze O, Gallwitz B, Holst JJ, Schmidt WE, Nauck MA: Glucagon-like peptide 1 abolishes the postprandial rise in triglyceride concentrations and lowers levels of non-esterified fatty acids in humans. Diabetologia 49:452–458, 2006
Cruciani-Guglielmacci C, Hervalet A, Douared L, Sanders NM, Levin BE, Ktorza A, Magnan C: Beta oxidation in the brain is required for the effects of non-esterified fatty acids on glucose-induced insulin secretion in rats. Diabetologia 47:2032–2038, 2004
Pocai A, Lam TK, Obici S, Gutierrez-Juarez R, Muse ED, Arduini A, Rossetti L: Restoration of hypothalamic lipid sensing normalizes energy and glucose homeostasis in overfed rats. J Clin Invest 116:1081–1091, 2006
Noushmehr H, D’Amico E, Farilla L, Hui H, Wawrowsky KA, Mlynarski W, Doria A, Abumrad NA, Perfetti R: Fatty acid translocase (FAT/CD36) is localized on insulin-containing granules in human pancreatic beta-cells and mediates fatty acid effects on insulin secretion. Diabetes 54:472–481, 2005
Robertson RP, Harmon J, Tran PO, Poitout V: Beta-cell glucose toxicity, lipotoxicity, and chronic oxidative stress in type 2 diabetes. Diabetes 53 (Suppl. 1):S119–S124, 2004
Boden G, Chen X, Rosner J, Barton M: Effects of a 48-h fat infusion on insulin secretion and glucose utilization. Diabetes 44:1239–1242, 1995
Carpentier A, Mittelman SD, Lamarche B, Bergman RN, Giacca A, Lewis GF: Acute enhancement of insulin secretion by FFA in humans is lost with prolonged FFA elevation. Am J Physiol 276:E1055–E1066, 1999
Paolisso G, Gambardella A, Amato L, Tortoriello R, D’Amore A, Varricchio M, D’Onofrio F: Opposite effects of short- and long-term fatty acid infusion on insulin secretion in healthy subjects. Diabetologia 38:1295–1299, 1995
Stein DT, Stevenson BE, Chester MW, Basit M, Daniels MB, Turley SD, McGarry JD: The insulinotropic potency of fatty acids is influenced profoundly by their chain length and degree of saturation. J Clin Invest 100:398–403, 1997
Gilon P, Henquin JC: Mechanisms and physiological significance of the cholinergic control of pancreatic beta-cell function. Endocr Rev 22:565–604, 2001
Ahren B: Autonomic regulation of islet hormone secretion-implications for health and disease. Diabetologia 43:393–410, 2000
Scherrer U, Owlya R, Lepori M: Body fat and sympathetic nerve activity. Cardiovasc Drugs Ther 10 (Suppl. 1):215–222, 1996
Peterson HR, Rothschild M, Weinberg CR, Fell RD, McLeish KR, Pfeifer MA: Body fat and the activity of the autonomic nervous system. N Engl J Med 318:1077–1083, 1988
Matsumoto T, Miyawaki T, Ue H, Kanda T, Zenji C, Moritani T: Autonomic responsiveness to acute cold exposure in obese and non-obese young women. Int J Obes Relat Metab Disord 23:793–800, 1999
Tataranni PA, Cizza G, Snitker S, Gucciardo F, Lotsikas A, Chrousos GP, Ravussin E: Hypothalamic-pituitary-adrenal axis and sympathetic nervous system activities in Pima Indians and Caucasians. Metabolism 48:395–399, 1999
Ravussin E, Gautier JF: Metabolic predictors of weight gain. Int J Obes Relat Metab Disord 23 (Suppl. 1):37–41, 1999
Young JB, Walgren MC: Differential effects of dietary fats on sympathetic nervous system activity in the rat. Metabolism 43:51–60, 1994
Levin BE, Triscari J, Sullivan AC: Relationship between sympathetic activity and diet-induced obesity in two rat strains. Am J Physiol 245:R364–R371, 1983
Magnan C, Collins S, Berthault MF, Kassis N, Vincent M, Gilbert M, Penicaud L, Ktorza A, Assimacopoulos-Jeannet F: Lipid infusion lowers sympathetic nervous activity and leads to increased beta-cell responsiveness to glucose. J Clin Invest 103:413–419, 1999
Laury MC, Penicaud L, Ktorza A, Benhaiem H, Bihoreau MT, Picon L: In vivo insulin secretion and action in hyperglycemic rat. Am J Physiol 257:E180–E184, 1989
Clement L, Kim-Sohn KA, Magnan C, Kassis N, Adnot P, Kergoat M, Assimacopoulos-Jeannet F, Penicaud L, Hsu F, Turk J, Ktorza A: Pancreatic beta-cell alpha2A adrenoceptor and phospholipid changes in hyperlipidemic rats. Lipids 37:501–506, 2002
Fyles JM, Cawthorne MA, Howell SL: The determination of alpha-adrenergic receptor concentration on rat pancreatic islet cells. Biosci Rep 7:17–22, 1987
Fyles JM, Cawthorne MA, Howell SL: The characteristics of beta-adrenergic binding sites on pancreatic islets of Langerhans. J Endocrinol 111:263–270, 1986
Ahren B, Taborsky GJ Jr, Porte D Jr: Neuropeptidergic versus cholinergic and adrenergic regulation of islet hormone secretion. Diabetologia 29:827–836, 1986
Magnan C, Cruciani C, Clement L, Adnot P, Vincent M, Kergoat M, Girard A, Elghozi JL, Velho G, Beressi N, Bresson JL, Ktorza A: Glucose-induced insulin hypersecretion in lipid-infused healthy subjects is associated with a decrease in plasma norepinephrine concentration and urinary excretion. J Clin Endocrinol Metab 86:4901–4907, 2001
Obici S, Feng Z, Arduini A, Conti R, Rossetti L: Inhibition of hypothalamic carnitine palmitoyltransferase-1 decreases food intake and glucose production. Nat Med 9:756–761, 2003
Oomura Y, Nakamura T, Sugimori M, Yamada Y: Effect of free fatty acid on the rat lateral hypothalamic neurons. Physiol Behav 14:483–486, 1975
Havel PJ: Peripheral signals conveying metabolic information to the brain: short-term and long-term regulation of food intake and energy homeostasis. Exp Biol Med (Maywood) 226:963–977, 2001
Berthoud HR, Bereiter DA, Jeanrenaud B: Role of the autonomic nervous system in the mediation of LHA electrical stimulation-induced effects on insulinemia and glycemia. J Auton Nerv Syst 2:183–198, 1980
Kim EK, Miller I, Landree LE, Borisy-Rudin FF, Brown P, Tihan T, Townsend CA, Witters LA, Moran TH, Kuhajda FP, Ronnett GV: Expression of FAS within hypothalamic neurons: a model for decreased food intake after C75 treatment. Am J Physiol Endocrinol Metab 283:E867–E879, 2002
Witt MR, Nielsen M: Characterization of the influence of unsaturated free fatty acids on brain GABA/benzodiazepine receptor binding in vitro. J Neurochem 62:1432–1439, 1994
Tewari KP, Malinowska DH, Sherry AM, Cuppoletti J: PKA and arachidonic acid activation of human recombinant ClC-2 chloride channels. Am J Physiol Cell Physiol 279:C40–C50, 2000
Zheng HF, Li XL, Jin ZY, Sun JB, Li ZL, Xu WX: Effects of unsaturated fatty acids on calcium-activated potassium current in gastric myocytes of guinea pigs. World J Gastroenterol 11:672–675, 2005
Honen BN, Saint DA, Laver DR: Suppression of calcium sparks in rat ventricular myocytes and direct inhibition of sheep cardiac RyR channels by EPA, DHA and oleic acid. J Membr Biol 196:95–103, 2003
Oishi K, Zheng B, Kuo JF: Inhibition of Na,K-ATPase and sodium pump by protein kinase C regulators sphingosine, lysophosphatidylcholine, and oleic acid. J Biol Chem 265:70–75, 1990
Randle PJ: Regulatory interactions between lipids and carbohydrates: the glucose fatty acid cycle after 35 years. Diabetes Metab Rev 14:263–283, 1998
Wang R, Cruciani-Guglielmacci C, Migrenne S, Magnan C, Cotero VE, Routh VH: Effects of oleic acid on distinct populations of neurons in the hypothalamic arcuate nucleus are dependent on extracellular glucose levels. J Neurophysiol 95:1491–1498, 2006
Kang L, Routh VH, Kuzhikandathil EV, Gaspers LD, Levin BE: Physiological and molecular characteristics of rat hypothalamic ventromedial nucleus glucosensing neurons. Diabetes 53:549–559, 2004(Stephanie Migrenne, Celine Cruciani-Gugl)
2 Department of Neurology and Neurosciences, New Jersey Medical School, Newark, New Jersey
3 Department of Pharmacology and Physiology, New Jersey Medical School, Newark, New Jersey
4 Neurology Service, VA Medical Center, East Orange, New Jersey
ARC, arcuate nucleus; FA, fatty acid; FLI, cfos-like immunoreactive; GIIS, glucose-induced insulin secretion; OA, oleic acid; OAE, OA-excited; OAI, OA-inhibited
ABSTRACT
It is now clearly demonstrated that fatty acids (FAs) may modulate neural control of energy homeostasis and specifically affect both insulin secretion and action. Indeed, pancreatic -cells receive rich neural innervation and FAs induce important changes in autonomic nervous activity. We previously reported that chronic infusion of lipids decreased sympathetic nervous system activity and led to exaggerated glucose-induced insulin secretion (GIIS), as would be expected from the known inhibitory effect of sympathetic splanchnic nerve activity on insulin secretion. Intracarotid infusion of lipids that do not change plasma FA concentrations also lead to increased GIIS. This effect of FAs on GIIS was prevented by inhibition of -oxidation. It is noteworthy that a single intracarotid injection of oleic acid also induced a transient increase in plasma insulin without any change in plasma glucose, suggesting that FAs per se can regulate neural control of insulin secretion. Finally, using whole cell current clamp recordings in hypothalamic slices and calcium imaging in dissociated hypothalamic neurons, we identified a hypothalamic subpopulation of neurons either excited (13%) or inhibited (6%) by FAs. Thus, FAs per se or their metabolites modulate neuronal activity, as a means of directly monitoring ongoing fuel availability by central nervous system nutrient-sensing neurons involved in the regulation of insulin secretion.
FATTY ACIDS AND NERVOUS CONTROL OF ENERGY HOMEOSTASIS
It is now well established that fatty acids (FAs) act on the central nervous system as important physiological regulators of glucose metabolism and overall energy homeostasis (1,2). For example, central administration of the long-chain FA oleate inhibits food intake and glucose production in rats (3,4). Such effects are partly related to a decreased expression of hypothalamic neuropeptide Y and of hepatic glucose-6-phosphatase after short-term overfeeding (4). This suggests that the slight increase in plasma FA concentrations in the postprandial state (5) might be detected by the central nervous system as a satiety signal. Beside these physiological effects, we previously showed that central triglyceride infusions into the brain through the carotid artery in rats over 24 h initiated hepatic insulin resistance independently of any change in plasma FA concentration (6). In this model, glucose-induced insulin secretion (GIIS) was markedly increased in lipid-infused rats compared with controls (6). Those toxic effects of lipids were prevented when -oxidation was inhibited (6). Finally, central inhibition of lipid oxidation is sufficient to restore the hypothalamic levels of long-chain fatty acyl-CoA and to normalize food intake and glucose homeostasis in overfed rats (7). Such studies suggest an important link between the central actions of FA on both food intake and glucose metabolism under both physiological but also pathological conditions.
FAs AND NERVOUS CONTROL OF INSULIN SECRETION
FAs directly act to regulate insulin exocytosis from the pancreatic -cell (8). In vitro, long-term exposure (48 h and more) to FAs results in a decreased insulin response to glucose in the pancreatic -cells (rev. in 9). However, when studied in vivo in healthy subjects, the precise effects of FAs on pancreatic -cells remain controversial. On the basis of experiments with lipid infusion, the insulin secretory response to glucose was reported to be increased (10), unchanged (11), or decreased (12). Different mechanisms might explain these discrepancies. The insulinotropic potency of FAs is influenced by chain length and degree of saturation (13). Moreover, the duration of lipid infusion also seems crucial, since opposite effects on insulin secretions are obtained by short-term (6 h) versus long-term (24 h) triglyceride infusions (12). It is also critical to carefully control the way in which glucose is administered (intravenously versus intraperitoneally) as a challenge of pancreatic -cell function (10–12). Discrepancies between in vivo and in vitro studies concerning the effect of a long-term increase in FA concentrations on pancreatic -cell function suggest the involvement of extrapancreatic factors. Among these factors, those of neural origin may be crucial. On the one hand, the endocrine pancreas is richly innervated by both parasympathetic and sympathetic inputs. Parasympathetic activation stimulates, whereas sympathetic activation inhibits, insulin secretion (14,15). On the other hand, several experiments clearly showed that lipids may alter autonomic nervous system activity in humans (16,17). More precisely, clinical and epidemiological studies showed that reduced autonomic activity, especially sympathetic, might be involved in the onset of obesity (18,19). Particularly in Pima Indians, a low sympathetic nervous system activity is also described as a predictor of weight gain (20). There is an important link between the development of obesity in rodents fed on diets of moderate to high FA content, pancreatic sympathetic function, and the development of abnormal insulin sensitivity. When outbred rats are fed such diets chronically, 50% become obese. Those rats do display reduced pancreatic sympathetic activity that precedes the development of obesity and that persists in association with hyperinsulinemia after obesity develops (21,22). These data suggest a critical link between impaired pancreatic sympathetic function, dietary FA intake, and the genetic propensity to develop diet-induced obesity.
To further study this potential relationship, we developed an in vivo model based on a 48-h intravenous infusion of triglyceride emulsion in rats (23). Briefly, a catheter was implanted under ketamine anesthesia (125 mg/kg, intraperitoneally; Imalgene, Merieux, Lyon, France) in the right atrium of a 250-g male Wistar rat via the jugular vein. A technique previously described (24) for a 48-h infusion in unrestrained rats was used for triglyceride or saline infusion. The infusion period started on day 2 after surgery. Rats were randomly divided into two groups. In the first group, rats were infused with a mixture of a 20% triglyceride emulsion (2,000 kcal/l Intralipid KabiVitrum and 20 units/ml heparin; Intralipid KabiVitrum, Stockholm, Sweden) at a rate of 20 μl/min. Control rats were infused with saline and heparin. At the end of infusion, we measured in vivo GIIS in response to a single intravenous injection of glucose after 5 h of food deprivation. In parallel, sympathetic firing-rate activities were recorded at the level of the superior cervical ganglion (Fig. 1). Such infusions approximately double plasma FA concentration in association with decreased sympathetic nerve activity (Fig. 1) and a concomitant increase in GIIS compared with control rats. These findings are in accord with the known inhibitory effect of sympathetic activity on insulin secretion. To demonstrate the potential role of the sympathetic activity in the control of insulin secretion after lipid infusion, GIIS was also studied in the presence of oxymetazoline, an 2A-adrenoceptor (the main -cell inhibitory adrenoceptor) agonist, to mimic an activated sympathetic nervous system. As expected, oxymetazoline produced a dose-related blunting of GIIS in lipid-infused rats, whereas controls were not sensitive to low concentrations of oxymetazoline. At a dose of 0.3 pmol/kg, GIIS became similar in both groups, suggesting that decreased sympathetic nervous system activity was partly responsible for pancreatic -cell hyperresponsiveness to glucose (23). Interestingly, there was also a reduced Bmax and Kd for 2-adrenoceptor binding in the pancreatic -cell of lipid-infused rats (25). Finally, neither -adrenoceptor agonists nor antagonists affected GIIS in either lipid-infused or control rats. This result suggests that the insulin stimulating -adrenergic pathway was not involved in the -cell hyperresponsiveness to glucose in our model. This is in keeping with the 100-fold lower number of -adrenoceptors compared with -adrenoceptors on the -cell surface (26,27) and is in agreement with the fact that -adrenergic control of insulin secretion predominates over the -adrenergic one in vivo (28). We have also demonstrated that these findings in rats are relevant to humans, where 48-h intravenous lipid infusion in healthy subjects also leads to glucose-induced insulin hypersecretion in association with a decrease in plasma norepinephrine concentration and urinary excretion, as an indirect measurement of sympathetic nervous system activity (29).
To assess whether these effects of FAs may be due to a direct action in the central nervous system, 250-g male Wistar rats were infused with lipids toward the brain through the carotid artery for 24 h at a rate of 2 μl/min (6). This approach allowed us to raise brain FA levels without bypassing the blood-brain barrier. Additionally, to assess the role of -oxidation in the effects of central FAs, rats were infused intracerebroventricularly with etomoxir, an inhibitor of carnitine palmitoyl transferase 1, before and during intracarotid infusion of FAs. Importantly, this route of administration produced increased GIIS without any change in plasma FA concentrations. This FA effect on GIIS was abolished by co-infusion of etomoxir, suggesting that -oxidation is required for the FA effects on insulin secretion (6). This is similar to the demonstration that -oxidation in the hypothalamus also appears to be an important regulator of food intake and hepatic glucose output (30). Because FAs induced hepatic insulin resistance in intracarotid lipid-infused rats, the increased GIIS could partly be related to changes in insulin sensitivity. To specifically study the effect of FAs on nervous control of insulin secretion, we measured both plasma insulin and glucose in response to an acute intracarotid injection of oleic acid (OA), which did not affect circulating lipid levels. The effect of OA was compared with saline, glucose, and both OA/glucose injection. As depicted in Fig. 2, an acute OA overload induced a transient significant increased insulinemia at time 1 and 3 min after injection, but did not affect glycemia. Glucose induced a greater increase in plasma insulin. Both OA and glucose injected together had an additive effect, thus leading to a higher insulinemia. It may be a bit surprising that blood glucose is not at all altered, although there is a rise in insulin secretion. It could suggest that insulin resistance is induced, in agreement with decreased hepatic insulin sensitivity observed in 24-h intracarotid interleukin-infused rats (6). The additive effect of both nutrients may partially lead to further dysregulation of nervous control of insulin secretion and finally contribute to the etiology of type 2 diabetes in predisposed subjects.
FA-SENSITIVE NEURONS
Whereas the in vivo effects of FA on insulin and glucose metabolism are interesting, it is imperative to demonstrate the cellular and molecular mechanisms by which FA can act to alter neural activity. Thirty years ago, Oomura et al. (31) showed that FAs modulated central nervous system activity in vivo. Experiments were performed on the rat lateral hypothalamic neurons. Indeed, the hypothalamus contains large populations of nutrient-sensitive neurons (either glucose or FA sensitive). Thus, it is now assumed that, like glucose, FAs per se or their metabolites could modulate neuronal activity as a means of directly monitoring ongoing fuel availability to allow central nervous system nutrient-sensing neurons to regulate energy homeostasis (2,32). We previously showed that intracarotid lipid infusion decreased the number of cfos-like immunoreactive (FLI) neurons (a marker of neuronal activity) in four of the five hypothalamic nuclei studied: arcuate nucleus (ARC), dorsomedial hypothalamus, ventromedial hypothalamus, and paraventricular nucleus. In contrast, the number of FLI neurons in the lateral hypothalamus was increased in lipid-infused rats (6). The decreased number of FLI neurons in ventromedial hypothalamus indicated a lower activity in this nucleus, which could likely promote a decreased sympathetic tone, since both ventromedial hypothalamus and ARC are known to activate the sympathetic nervous system via polysynaptic efferent pathways (28). On the other hand, stimulation of the lateral hypothalamus, which contributes to the control of parasympathetic activity, promotes insulin secretion in the presence of a simultaneous rise in glucose (33). Thus, a decrease in number of FLI neurons in the ventromedial hypothalamus and an increase in number of these FLI neurons in lateral hypothalamus could reflect a concomitant decrease in sympathetic activities and an increase in parasympathetic activities, which inhibit and stimulate insulin secretion, respectively.
MOLECULAR MECHANISMS INVOLVED IN FA-SENSITIVE NEURONS
Molecular mechanisms involved in FA-sensitive neurons are still being debated. However, enzymes involved in their metabolism such as fatty acid synthase have been demonstrated to be present and active in hypothalamic neurons (34). Furthermore, carnitine palmitoyl transferase 1 is also expressed in some brain areas, including the hypothalamus, which supports our finding that -oxidation seems crucial to mediate some the effects of FA on glucose metabolism (6,30). Thus, while FA signaling in the hypothalamus is important for peripheral glucose and energy homeostasis, the mechanisms by which FAs alter neuronal activity are not clear. One possibility is via modulation of ion channels. A body of literature indicates that FAs regulate the conductance of a wide variety of ion channels, which include Cl–, GABAA (35), CIC-2 (36), potassium, K+-Ca2+ (37), ATP-sensitive K+ channel (37), and calcium channels (38). Additionally, FAs inhibit the Na+-K+ ATPase pump (39). These effects of FAs may be directly on ion channels or through the actions of metabolic intermediates. Because there is a reciprocal relationship between glucose and FA metabolism, flux through these metabolic pathways may interact to regulate neuronal activity in the hypothalamus (40). Using patch clamp recordings, we recently investigated the effects of OA on ARC neurons in hypothalamic slices from 14- to 21-day-old Sprague-Dawley (SD) rats (41). We also recorded discharge rates in ARC neurons in 8-week-old fed and fasted SD rats in vivo in response to a single injection of OA. We found that OA regulated three distinct subpopulations of ARC neurons in a glucose-dependent fashion. Patch clamp studies showed that in 2.5 mmol/l glucose, 12 of 94 (13%) ARC neurons were excited by 2 mmol/l OA (OA-excited [OAE] neurons, Fig. 3), whereas 6 of 94 (6%) were inhibited (OA-inhibited2.5 [OAI2.5] neurons). In contrast, in 0.1 mmol/l glucose, OA inhibited 6 of 20 (30%) ARC neurons (OAI0.1 neurons); none were excited. None of the OAI0.1 neurons responded to OA in 2.5 mmol/l glucose. Thus, OAI2.5 and OAI0.1 neurons are distinct. Similarly, in fed rats, 7 of 20 (35%) ARC neurons were OAE, whereas only 3 of 20 (15%) were OAI. Note that in these rats, there was an increase in insulin secretion in response to a single injection of OA (Fig. 2). In contrast, in fasted rats only, OAI neurons were observed (3 of 15 [20%]) in ARC neurons. Thus, neural control of insulin secretion by FAs probably involved more than one neuronal population and also several different molecular mechanisms. Indeed, we identified at least two types of channels relaying the effects of OA (41). Excitatory effects of OA were probably involved in the closure of Cl– channels assessed by whole-cell current clamp recording (41), whereas the inhibitory effects of OA were reversed by the ATP-sensitive K+ channel blocker tolbutamide (in 2.5 mmol/l glucose; V.H.R., R.W., unpublished data). Additionally, activation of AMP-activated kinase with 5-aminoimidazole-4-carboxamide-1--D-ribofuranoside (AICAR) reversed the excitatory effect of OA (Fig. 4). This suggests that AMP-activated kinase could be a key molecule-mediating effect of FAS in nutrient-sensing neurons.
We have further demonstrated the ability of FA to alter neuronal activity in dissociated neurons from the ventromedial hypothalamic nucleus of 3- to 4-week-old rats using the fura-2 Ca2+ imaging technique. These neurons either increase (glucose excited) or decrease (glucose inhibited) their intracellular Ca2+ flux when extracellular glucose levels are raised from 0.5 to 2.5 mmol/l (42). As mentioned above, in vivo OA infusion in ventromedial hypothalamic nucleus resulted in a decreased in FLI neuron number (6). In preliminary studies (Fig. 5), we found that OA induced a significant decrease in intracellular Ca2+ oscillations in both glucose-excited and glucose-inhibited neurons. These data independently reinforce the ability of FAs to alter activity in hypothalamic nutrient-sensitive neurons.
To summarize, neural control of insulin secretion by FAs is crucial to finely regulate pancreatic -cell function. As a consequence, central dysregulation of FA metabolism could be an early event leading to inappropriate GIIS and that could, at least in part, contribute to development of metabolic diseases such as type 2 diabetes in predisposed subjects.
FOOTNOTES
This article is based on a presentation at a symposium. The symposium and the publication of this article were made possible by an unrestricted educational grant from Servier.
The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
REFERENCES
Lam TK, Pocai A, Gutierrez-Juarez R, Obici S, Bryan J, Aguilar-Bryan L, Schwartz GJ, Rossetti L: Hypothalamic sensing of circulating fatty acids is required for glucose homeostasis. Nat Med 11:320–327, 2005
Seeley RJ, Woods SC: Monitoring of stored and available fuel by the CNS: implications for obesity. Nat Rev Neurosci 4:901–909, 2003
Obici S, Feng Z, Morgan K, Stein D, Karkanias G, Rossetti L: Central administration of oleic acid inhibits glucose production and food intake. Diabetes 51:271–275, 2002
Morgan K, Obici S, Rossetti L: Hypothalamic responses to long-chain fatty acids are nutritionally regulated. J Biol Chem 279:31139–31148, 2004
Meier JJ, Gethmann A, Gotze O, Gallwitz B, Holst JJ, Schmidt WE, Nauck MA: Glucagon-like peptide 1 abolishes the postprandial rise in triglyceride concentrations and lowers levels of non-esterified fatty acids in humans. Diabetologia 49:452–458, 2006
Cruciani-Guglielmacci C, Hervalet A, Douared L, Sanders NM, Levin BE, Ktorza A, Magnan C: Beta oxidation in the brain is required for the effects of non-esterified fatty acids on glucose-induced insulin secretion in rats. Diabetologia 47:2032–2038, 2004
Pocai A, Lam TK, Obici S, Gutierrez-Juarez R, Muse ED, Arduini A, Rossetti L: Restoration of hypothalamic lipid sensing normalizes energy and glucose homeostasis in overfed rats. J Clin Invest 116:1081–1091, 2006
Noushmehr H, D’Amico E, Farilla L, Hui H, Wawrowsky KA, Mlynarski W, Doria A, Abumrad NA, Perfetti R: Fatty acid translocase (FAT/CD36) is localized on insulin-containing granules in human pancreatic beta-cells and mediates fatty acid effects on insulin secretion. Diabetes 54:472–481, 2005
Robertson RP, Harmon J, Tran PO, Poitout V: Beta-cell glucose toxicity, lipotoxicity, and chronic oxidative stress in type 2 diabetes. Diabetes 53 (Suppl. 1):S119–S124, 2004
Boden G, Chen X, Rosner J, Barton M: Effects of a 48-h fat infusion on insulin secretion and glucose utilization. Diabetes 44:1239–1242, 1995
Carpentier A, Mittelman SD, Lamarche B, Bergman RN, Giacca A, Lewis GF: Acute enhancement of insulin secretion by FFA in humans is lost with prolonged FFA elevation. Am J Physiol 276:E1055–E1066, 1999
Paolisso G, Gambardella A, Amato L, Tortoriello R, D’Amore A, Varricchio M, D’Onofrio F: Opposite effects of short- and long-term fatty acid infusion on insulin secretion in healthy subjects. Diabetologia 38:1295–1299, 1995
Stein DT, Stevenson BE, Chester MW, Basit M, Daniels MB, Turley SD, McGarry JD: The insulinotropic potency of fatty acids is influenced profoundly by their chain length and degree of saturation. J Clin Invest 100:398–403, 1997
Gilon P, Henquin JC: Mechanisms and physiological significance of the cholinergic control of pancreatic beta-cell function. Endocr Rev 22:565–604, 2001
Ahren B: Autonomic regulation of islet hormone secretion-implications for health and disease. Diabetologia 43:393–410, 2000
Scherrer U, Owlya R, Lepori M: Body fat and sympathetic nerve activity. Cardiovasc Drugs Ther 10 (Suppl. 1):215–222, 1996
Peterson HR, Rothschild M, Weinberg CR, Fell RD, McLeish KR, Pfeifer MA: Body fat and the activity of the autonomic nervous system. N Engl J Med 318:1077–1083, 1988
Matsumoto T, Miyawaki T, Ue H, Kanda T, Zenji C, Moritani T: Autonomic responsiveness to acute cold exposure in obese and non-obese young women. Int J Obes Relat Metab Disord 23:793–800, 1999
Tataranni PA, Cizza G, Snitker S, Gucciardo F, Lotsikas A, Chrousos GP, Ravussin E: Hypothalamic-pituitary-adrenal axis and sympathetic nervous system activities in Pima Indians and Caucasians. Metabolism 48:395–399, 1999
Ravussin E, Gautier JF: Metabolic predictors of weight gain. Int J Obes Relat Metab Disord 23 (Suppl. 1):37–41, 1999
Young JB, Walgren MC: Differential effects of dietary fats on sympathetic nervous system activity in the rat. Metabolism 43:51–60, 1994
Levin BE, Triscari J, Sullivan AC: Relationship between sympathetic activity and diet-induced obesity in two rat strains. Am J Physiol 245:R364–R371, 1983
Magnan C, Collins S, Berthault MF, Kassis N, Vincent M, Gilbert M, Penicaud L, Ktorza A, Assimacopoulos-Jeannet F: Lipid infusion lowers sympathetic nervous activity and leads to increased beta-cell responsiveness to glucose. J Clin Invest 103:413–419, 1999
Laury MC, Penicaud L, Ktorza A, Benhaiem H, Bihoreau MT, Picon L: In vivo insulin secretion and action in hyperglycemic rat. Am J Physiol 257:E180–E184, 1989
Clement L, Kim-Sohn KA, Magnan C, Kassis N, Adnot P, Kergoat M, Assimacopoulos-Jeannet F, Penicaud L, Hsu F, Turk J, Ktorza A: Pancreatic beta-cell alpha2A adrenoceptor and phospholipid changes in hyperlipidemic rats. Lipids 37:501–506, 2002
Fyles JM, Cawthorne MA, Howell SL: The determination of alpha-adrenergic receptor concentration on rat pancreatic islet cells. Biosci Rep 7:17–22, 1987
Fyles JM, Cawthorne MA, Howell SL: The characteristics of beta-adrenergic binding sites on pancreatic islets of Langerhans. J Endocrinol 111:263–270, 1986
Ahren B, Taborsky GJ Jr, Porte D Jr: Neuropeptidergic versus cholinergic and adrenergic regulation of islet hormone secretion. Diabetologia 29:827–836, 1986
Magnan C, Cruciani C, Clement L, Adnot P, Vincent M, Kergoat M, Girard A, Elghozi JL, Velho G, Beressi N, Bresson JL, Ktorza A: Glucose-induced insulin hypersecretion in lipid-infused healthy subjects is associated with a decrease in plasma norepinephrine concentration and urinary excretion. J Clin Endocrinol Metab 86:4901–4907, 2001
Obici S, Feng Z, Arduini A, Conti R, Rossetti L: Inhibition of hypothalamic carnitine palmitoyltransferase-1 decreases food intake and glucose production. Nat Med 9:756–761, 2003
Oomura Y, Nakamura T, Sugimori M, Yamada Y: Effect of free fatty acid on the rat lateral hypothalamic neurons. Physiol Behav 14:483–486, 1975
Havel PJ: Peripheral signals conveying metabolic information to the brain: short-term and long-term regulation of food intake and energy homeostasis. Exp Biol Med (Maywood) 226:963–977, 2001
Berthoud HR, Bereiter DA, Jeanrenaud B: Role of the autonomic nervous system in the mediation of LHA electrical stimulation-induced effects on insulinemia and glycemia. J Auton Nerv Syst 2:183–198, 1980
Kim EK, Miller I, Landree LE, Borisy-Rudin FF, Brown P, Tihan T, Townsend CA, Witters LA, Moran TH, Kuhajda FP, Ronnett GV: Expression of FAS within hypothalamic neurons: a model for decreased food intake after C75 treatment. Am J Physiol Endocrinol Metab 283:E867–E879, 2002
Witt MR, Nielsen M: Characterization of the influence of unsaturated free fatty acids on brain GABA/benzodiazepine receptor binding in vitro. J Neurochem 62:1432–1439, 1994
Tewari KP, Malinowska DH, Sherry AM, Cuppoletti J: PKA and arachidonic acid activation of human recombinant ClC-2 chloride channels. Am J Physiol Cell Physiol 279:C40–C50, 2000
Zheng HF, Li XL, Jin ZY, Sun JB, Li ZL, Xu WX: Effects of unsaturated fatty acids on calcium-activated potassium current in gastric myocytes of guinea pigs. World J Gastroenterol 11:672–675, 2005
Honen BN, Saint DA, Laver DR: Suppression of calcium sparks in rat ventricular myocytes and direct inhibition of sheep cardiac RyR channels by EPA, DHA and oleic acid. J Membr Biol 196:95–103, 2003
Oishi K, Zheng B, Kuo JF: Inhibition of Na,K-ATPase and sodium pump by protein kinase C regulators sphingosine, lysophosphatidylcholine, and oleic acid. J Biol Chem 265:70–75, 1990
Randle PJ: Regulatory interactions between lipids and carbohydrates: the glucose fatty acid cycle after 35 years. Diabetes Metab Rev 14:263–283, 1998
Wang R, Cruciani-Guglielmacci C, Migrenne S, Magnan C, Cotero VE, Routh VH: Effects of oleic acid on distinct populations of neurons in the hypothalamic arcuate nucleus are dependent on extracellular glucose levels. J Neurophysiol 95:1491–1498, 2006
Kang L, Routh VH, Kuzhikandathil EV, Gaspers LD, Levin BE: Physiological and molecular characteristics of rat hypothalamic ventromedial nucleus glucosensing neurons. Diabetes 53:549–559, 2004(Stephanie Migrenne, Celine Cruciani-Gugl)