Selective impairment of pancreatic A cell suppression by glucose during acute alloxan-induced insulinopenia: in vitro study on isolated perfused rat pancreas

This study was performed in order to investigate the role of insulin in the modulation of pancreatic A cell response to glucose. The isolated perfused rat pancreas model was used: intraislet insulinopenia was induced in vitro by 0.56 mM alloxan infusion over 15 min. Alloxan caused a transitory insulin release but did not affect glucagon secretion. Exposure to alloxan completely abolished insulin response to 20 mM arginine, 1.6 mM glucose, and 11.1 mM glucose. Glucagon response to 20 mM arginine and 1.6 mM glucose was unchanged by alloxan pretreatment compared to control pancreata not treated with alloxan. However, the suppression of glucagon release by 11.1 mM glucose was abolished in the alloxan experiments. Twenty milliunits per ml of insulin infused during 11.1 mM glucose infusion restored glycemic suppression of glucagon release, but it produced only a slight inhibitory effect on A cell function in the presence of 3.9 mM glucose. Our study indicates that glucose is the physiological suppressor of the pancreatic A cell and that, in this regard, insulin exerts only a permissive effect.     

Pancreatic immunoreactive somatostatin release.

The location of the somatostatin-containing D-cells of the pancreatic islets between the A- and B-cells suggests that their function might be to inhibit insulin and/or glucagon secretion by these neighboring cells. To determine if insulin and/or glucagon, in concentrations that might be present in the extracellular space surrounding the D-cells, stimulate immunoreactive somatostatin (IRS) release, we perfused 10 microng of glucagon or 10 milliunits of insulin per ml in 11 isolated dog pancreases, for 40 min in seven experiments and for 100 min in four experiments. In eight of the nine experiments in which glucagon was perfused, a prompt and significant rise in mean IRS release, ranging from 71 to 128% above the control level, was observed. In the eight experiments in which insulin was perfused. IRS did not increase during the first 40 min; in the two 100-min insulin experiments, it did rise during the final 50 min, however. To determine the effect of an A- and B-cell secretogogue on IRS release, we perfused 20 mM arginine for 60 min in six experiments. In all, IRS rose within 3 min and reached a level 71-465% above the control, remaining significantly elevated throughout the perfusion, while glucagon and insulin rose to peak levels at 2 min and then declined somewhat despite continuing arginine perfusion. The results indicate that perfusion of the normal dog pancreas with high doses of glucagon or arginine is accompanied by a prompt increase in IRS release and are compatible with a local feedback circuit involving A- and D-cells. Insulin appears not to augment IRS release, at least not promptly, but IRS stimulated by local endogenous glucagon could inhibit the B-cell response to locally secreted glucagon and thereby influence the composition of the insulin/glucagon secretion mixture.     

Pancreastatin and islet hormone release.

The effect of pancreastatin on the release of insulin, glucagon, and somatostatin was studied in the isolated perfused rat pancreas. After an initial equilibration period (-20 to 0 min) with a basal glucose concentration (3.3 mM), the pancreata were perfused with either 16.7 mM glucose (0-40 min) or with 20 mM arginine (0-20 min). Pancreastatin was introduced 10 min prior to and throughout the administration of the high glucose and arginine and continued during their perfusion. As expected, the glucose and the arginine augmented insulin and somatostatin release. Pancreastatin (1 and 10 nM) markedly suppressed the first phase of insulin release with both insulinogogues used, while the early somatostatin secretion was not significantly decreased. However, the peak incremental somatostatin response to arginine was reduced by 50% (P less than 0.05). Conversely, the peptide (10 nM) tended to augment arginine-induced glucagon release. Pancreastatin (100 nM) also suppressed glucose-stimulated insulin release from isolated rat islets. These pancreastatin-mediated alterations in islet hormone release are reminiscent of those known to characterize non-insulin-dependent diabetes. Therefore, the significance of pancreastatin in islet physiology and pathophysiology deserves special consideration.     

Abnormal regulation by glucose of somatostatin secretion in the perfused pancreas of NIDDM rats

We have investigated the influence of non-insulin-dependent diabetes on the regulation of somatostatin secretion from the pancreatic D cell. These results were compared with the concomittantly measured secretory responses from A and B cells. Rats were rendered non-insulin-dependent diabetic by neonatal injection of streptozotocin (STZ). Secretion was studied in perfused pancreas at 6-10 weeks of age. At this age, STZ rats were mildly hyperglycemic, their nonfasting blood glucose being 9.0 +/- 0.8 vs. 5.6 +/- 0.2 mM in control rats. In perfused pancreas from the latter rats, high glucose, i.e., 16.7 mM, stimulated somatostatin secretion but completely failed to do so in STZ rats. Arginine (in the presence of low glucose, i.e., 3.3 mM) moderately stimulated somatostatin secretion in controls but fourfold more in STZ rats. Preperfusion with high glucose markedly potentiated subsequent arginine-induced somatostatin secretion in controls but failed to do so in STZ rats. Basal glucagon release was inhibited by ambient high glucose in control and STZ rats alike. Arginine-induced glucagon release was profoundly inhibited both by ambient and previous exposure to glucose in controls but only slightly and nonsignificantly in STZ rats. The insulin response to high glucose in controls was reduced by 90% in STZ. The insulin response to arginine (in the presence of low glucose) was 3.3-fold enhanced in STZ. Ambient and previous high glucose markedly enhanced arginine-induced insulin secretion in controls but only moderately so in STZ rats. We conclude that already mild hyperglycemia is associated with marked D-cell insensitivity to glucose that is qualitatively similar to A- and B-cell insensitivity.     

Acute effects of alloxan- and streptozotocin-induced insulin deficiency on somatostatin and glucagon secretion by the perfused isolated rat pancreatico-duodenal preparation

Summary The secretion of somatostatin and glucagon by the perfused rat pancreatico-duodenal preparation was examined in situ under control conditions and after the induction of acute insulin deficiency by alloxan or streptozotocin. A 10 min 0.625 mmol/l alloxan perfusion resulted in an immediate and transient increase in basal insulin and glucagon release and a slightly delayed and persistent increase in basal somatostatin secretion. The insulin responses to 16.7 mmol/l glucose, 1 mmol/l theophylline, and 19 mmol/l arginine alone or in combination were virtually eliminated by alloxan treatment, Somatostatin secretion in response to the stimuli was completely inhibited or markedly attenuated. The glucagon-suppressive effect of glucose was unaltered by alloxan and the stimulatory effect of arginine was enhanced. Addition of 1 g/ml porcine insulin to the perfusion medium did not modify the alterations in somatostatin and glucagon responses to arginine. Streptozotocin treatment 90 min prior to the onset of perfusion resulted in changes in somatostatin, glucagon, and insulin responses to glucose and arginine similar to those of alloxan. The present results are consistent with an effect of alloxan and streptozotocin on the D cell similar to that on the B cell, namely, interference with a glucose-mediated effect on hormone secretion.     

Inhibitory effect of ghrelin on insulin and pancreatic somatostatin secretion

OBJECTIVE: Ghrelin is a 28 amino acid residue peptide identified in both human and rat stomach and which acts as an endogenous ligand for the GH secretagogue receptor (GHS-R) and stimulates GH release. GHS-Rs are expressed in a number of tissues, including the pancreas, and ghrelin-like immunoreactivity is present in peripheral plasma, where its levels increase during fasting and decrease after food intake. The relationship between nutritional status and circulating ghrelin concentrations prompted us to investigate the effect of this peptide on pancreatic hormone secretion. METHODS: The study was performed in the isolated rat pancreas perfused in situ. Insulin, glucagon and somatostatin were measured by radioimmunoassay. RESULTS: Addition of 10 nM ghrelin to the perfusate significantly reduced the insulin response to the secretagogues glucose, arginine and carbachol, which act on the B-cell via different mechanisms, as well as the somatostatin response to arginine. Ghrelin was without effect on the glucagon output induced by this amino acid. At a lower concentration (2 nM) ghrelin was also found to inhibit glucose-induced insulin release. CONCLUSION: These findings support the proposal that the inhibitory effect of ghrelin on insulin release constitutes a tonic regulation of the B-cell, contributing to restrain its secretory activity in the state of food deprivation. On the other hand, the inhibition of pancreatic somatostatin release by ghrelin suggests a blocking effect of this hormone on the widely distributed D-cell population.     

Effects of rat pancreatic polypeptide on islet-cell secretion in the perfused rat pancreas.

Pancreatic polypeptide (PP) secretory cells are abundant in the islets of Langerhans. Results concerning the effects of exogenous PP on islet-cell secretion are controversial. This might be due in part to species specificity, given that most reports refer to studies performed using PP of bovine, porcine, or human origin in a heterologous animal model. Thus, we have investigated the influence of synthetic rat PP (80 nmol/L) on unstimulated insulin, glucagon, and somatostatin release, and on the responses of these hormones to glucose (11 mmol/L) and to arginine (3.5 mmol/L) in a homologous animal model, the perfused rat pancreas. Infusion of rat PP (rPP) reduced unstimulated insulin release by 35% (P = .03), and the insulin responses to glucose by 65% (P = .029) and to arginine by 50% (P = .026), without modifying glucagon output. rPP did not affect somatostatin secretion, either in unstimulated conditions or in the presence of 11 mmol/L glucose. However, it induced a clear-cut increase in somatostatin release during 3.5 mmol/L arginine infusion. Our observation that rPP inhibited insulin secretion without affecting glucagon and somatostatin output points to a direct effect of PP on B-cell function. However, during aminogenic priming of the D cell, the inhibition of insulin output induced by rPP was accompanied by an increase in somatostatin release. Thus, in this circumstance, it might be considered that the blocking effect of PP on B-cell secretion could be, at least in part, mediated by a D-cell paracrine effect.     

Glucagon secretion in rats with non-insulin-dependent diabetes: an in vivo and in vitro study

Non-insulin-dependent diabetes ( NIDD ) was obtained in adult rats following a neonatal streptozotocin injection. Rats with NIDD exhibited a chronic low-insulin response to glucose in vivo, slightly elevated basal plasma glucose values (less than 2 g/l) and low pancreatic insulin stores (50% of the controls). Glucagon secretion was studied in this model, in vivo and in vitro using the isolated perfused pancreas technique. Normal basal plasma glucagon levels were observed in the fed state and were in accordance with normal basal glucagon release in vitro. The pancreatic glucagon stores were normal in the diabetics. In experiments with the perfused pancreas, the increased glucose concentration suppressed glucagon release as readily in the diabetics as in the controls. Moreover 5.5 mM glucose suppressed glucagon release stimulated by 19 mM arginine to the same extent in both groups. These data indicate that the suppression of A cell function by glucose is normal in rats with NIDD . Theophylline and isoproterenol also produced normal glucagon release in diabetics. By contrast, the glucagon secretion in response to arginine was lower in the diabetics. This was observed either in vivo (arginine infusion) or in vitro in the presence or the absence of glucose in the perfusate. But in the presence of theophylline the response to arginine was normalized in the diabetics. Impairment of A cell function of the diabetics is not limited to recognition of amino-acids, since acetylcholine evoked a lower glucagon response in the diabetics than in the controls. These defects are different from those described in their B cells.     

Effects of galanin on islet cell secretory responses to VIP, GIP, 8-CCK, and glucagon by the perfused rat pancreas

Exogenous galanin has been shown to suppress insulin secretion as elicited by a number of secretagogues such as glucose, arginine, tolbutamide, carbachol, and oral nutrients. To achieve further insight into the influence of galanin on the endocrine pancreas, we have investigated the effect of synthetic porcine galanin (a 200 ng bolus followed by constant infusion at a concentration of 16.8 ng/mL for 16 to 24 minutes) on unstimulated insulin, glucagon, and somatostatin release, as well as on the responses of these hormones to 1 nmol/L vasoactive intestinal peptide (VIP), 1 nmol/L gastric inhibitory peptide (GIP), 1 nmol/L 26 to 33 octapeptide form of cholecystokinin (8-CCK) or 10 nmol/L glucagon in the perfused rat pancreas. Galanin infusion reduced unstimulated insulin secretion by 60% without modifying glucagon and somatostatin output. Galanin also blocked insulin release elicited by VIP, GIP, and 8-CCK, it did not affect the glucagon responses to VIP and GIP, or the somatostatin responses to VIP, GIP, and 8-CCK. Finally, galanin inhibited the insulin output, but not the somatostatin release induced by glucagon. In conclusion, in the perfused rat pancreas, galanin appears to behave as a general inhibitor of insulin secretion. Since this neuropeptide does not modify glucagon or somatostatin release, a direct effect of galanin on the B-cell seems plausible.     

Differential effects of glucagon-like peptide-1 (7-36)amide versus cholecystokinin on arginine-induced islet hormone release in vivo and in vitro

We compared the effects of two incretin hormones, glucagon-like peptide-1 (7-36)amide (GLP-1) and cholecystokinin (CCK), on islet hormone secretion. GLP-1 strongly potentiated glucose-stimulated insulin secretion in the perfused rat pancreas and in vivo in mice (p < 0.001). In contrast, GLP-1 did not enhance arginine-induced insulin release under these experimental conditions. In the perfused rat pancreas, GLP-1 also potentiated glucose-stimulated somatostatin secretion but, again, had no effect on arginine-induced somatostatin release. However, GLP-1 promptly inhibited the arginine-induced glucagon release (p < 0.02). In contrast, CCK enhanced insulin release in response to arginine both in the perfused rat pancreas and in vivo in mice (p < 0.001). In conclusion, GLP-1, in contrast to CCK, failed to enhance arginine-induced insulin release both in vitro and in vivo. This suggests that a signal generated by nutrient metabolism is required for the potentiation of insulin secretion by GLP-1. Furthermore, GLP-1 directly inhibited arginine-induced glucagon release as no concurrent increase in insulin or somatostatin release was noted.     

Modulation by alloxan of glucagon and insulin secretion in the isolated perfused rat pancreas.

The acute in vitro effect of alloxan on glucagon and insulin secretion from the isolated perfused rat pancreas was examined. Alloxan alone produced transient insulin secretion. Pretreatment with alloxan attenuated both the stimulatory effect of glucose on insulin secretion and the inhibitory effect of glucose on glucagon secretion. Exposure to alloxan in varying doses either partially or completely inhibited insulin secretion induced by arginine in the presence or absence of glucose. On the contrary, pretreatment with alloxan produced complex effects on arginine-induced glucagon secretion. In the absence of glucose, the response of glucagon to arginine infusion was lower in the pancreas exposed to alloxan than in the control experiment. In the presence of glucose, however, an apparently augmented response of glucagon to arginine was observed after exposure to higher doses of alloxan, suggesting an impaired inhibitory effect of glucose on arginine-induced glucagon secretion. These effects of pretreatment with alloxan on glucagon secretion can not be explained by earlier or simultaneous insulin secretion. Therefore, we conclude that alloxan acts not only on beta-cells, but also directly on alpha-cells, although the latter are less sensitive to this agent.     

Somatostatin,glucagon, and insulin secretion from isolated perfused pancreas of new genetically obese hyperglycemic rats

Insulin, somatostatin, and glucagon release from the perfused pancreas was studied in the newly developed genetically obese hyperglycemic hyperinsulinemic (Wistar fatty) rat. Insulin and somatostatin levels rose significantly compared to those in lean littermate controls during arginine infusion. The glucagon increase, however, was significantly less when total amounts during arginine infusion were calculated. These results show that hypersecretion of insulin and somatostatin in vitro may suppress glucagon release in Wistar fatty rats.     

Modulation by IBMX, fasting and experimental diabetes of glibenclamide-induced islet hormone release from the perfused rat pancreas

The impact of increased c-AMP levels, short-term fasting as well as experimental diabetes on glibenclamide-induced secretion of somatostatin, insulin and glucagon was studied in the isolated perfused rat pancreas. Dose-response curves revealed that 1 microgram/ml of glibenclamide (in the presence of 3.3 mmol/l of glucose) induced maximal stimulation of insulin and near maximal stimulation of somatostatin release, but did not significantly affect glucagon release. A combination of glibenclamide and the phosphodiesterase inhibitor IBMX synergistically and equally increased both B- and D-cell secretion. Fasting the rats for 24 h significantly suppressed the insulin and glucagon responses to glibenclamide while the concomitant somatostatin response was slightly enhanced. Rats injected with alloxan 3 days prior to perfusion were rendered either moderately diabetic or severely ill with ketoacidosis. Their insulin responses were poor or absent, respectively. In the moderately diabetic rats glibenclamide-induced somatostatin release was blunted while it was abolished in the ketotic rats. The results indicate that glibenclamide-induced B- and D-cell secretion are both modulated by c-AMP, that short-term fasting differentially affects B- and D-cell secretion and that D-cell secretion is inhibited in alloxan diabetes of short duration. It is concluded that the balance of effects by glibenclamide on hormones of the endocrine pancreas may depend on the nutritional and metabolic environment.     

Effects of galanin on hormone secretion from the in situ perfused rat pancreas and on glucose production in rat hepatocytes in vitro

Galanin is a novel peptide, widely distributed throughout the central and peripheral nervous system, including nerve endings surrounding the pancreatic islets. In dogs, galanin infusion has been reported to induce hyperglycemia along with a reduction of circulating insulin. In this work, we have studied the effect of galanin (a 200 ng bolus followed by constant infusion at a concentration of 16.8 ng/ml for 22-24 min) on insulin, glucagon, and somatostatin secretion in the perfused rat pancreas. In addition, we have investigated the effect of galanin (10 and 100 nM) on glycogenolysis and gluconeogenesis in isolated rat hepatocytes. In the rat pancreas, galanin infusion marked inhibited unstimulated insulin release as well as the insulin responses to glucose (11 mM), tolbutamide (100 mg/liter) and arginine (5 mM). Galanin failed to alter the glucagon and somatostatin responses to glucose, tolbutamide, and arginine. In isolated rat hepatocytes, galanin did not influence glycogenolysis or glucagon phosphorylase a activity. Gluconeogenesis and the hepatocyte concentration of fructose 2,6-bisphosphate were also unaffected by galanin. In conclusion: in the perfused rat pancreas, galanin inhibited insulin secretion without modifying glucagon and somatostatin output, thus pointing to a direct effect of galanin on the B cell; and in rat hepatocytes, galanin did not affect glycogenolysis or gluconeogenesis; hence, the reported hyperglycemia induced by exogenous galanin does not seem to be accounted for by a direct effect of this peptide on hepatic glucose production.     

Effects of a thiazide diuretic (hydroflumethiazide) and a loop diuretic (bumetanide) on the endocrine pancreas: studies in vitro

Treatment with thiazide diuretics causes an impairment of the glucose metabolism. To study whether this is due to a direct effect on the endocrine pancreas, the effects of the thiazide hydroflumethiazide on the release of glucagon, insulin, and somatostatin from the isolated perfused pancreas of normal and alloxan diabetic dogs were examined. Hydroflumethiazide at concentrations ranging from 1 to 50 micrograms/mL stimulated the normal secretion of glucagon (P less than 0.001), insulin (P less than 0.001), and somatostatin (P less than 0.001) in a dose-dependent manner. The normal hormone responses evoked by 50 micrograms/mL of the thiazide were, however, modified by the prevailing glucose level: higher insulin (P less than 0.05) and somatostatin (P less than 0.05) and lower glucagon (P less than 0.05) were obtained at the high glucose concentration of 11 mmol/L rather than at the low glucose concentration of 1.3 mmol/L. In alloxan diabetes, insulin secretion was almost extinct and did not respond to hydroflumethiazide, whereas glucagon was dose-dependently stimulated (P less than 0.001). In addition, we looked at the effect of the loop diuretic, bumetanide. The infusion of bumetanide at doses ranging from 0.5 to 3 micrograms/mL did not alter the release of glucagon, insulin, and somatostatin in the presence of 5.5 mmol/L glucose. The results suggest that hydroflumethiazide possesses the ability to directly stimulate A cell secretion in the normal and alloxan diabetic pancreas. Whether this effect is of clinical importance for the diminution in glucose tolerance observed during thiazide therapy remains, however, uncertain.     

Effect of neurotensin on insulin, glucagon, and somatostatin release from isolated pancreatic islets.

The effects of neurotensin on the release of insulin, glucagon, and somatostatin were investigated in isolated pancreatic islets prepared from 3- to 4-day-old rats and maintained in culture for 48 h before use. Islets were incubated for 20 and 60 min in the presence of 3 or 23 mM glucose with or without neurotensin. In 20-min incubations at 3 mM glucose, neurotensin (10-100 nM) increased the release of insulin, glucagon, and somatostatin by 60%, 90%, and 110%, respectively. These increases were not detected in 60-min incubations. Neurotensin (100 nM) inhibited the release of both insulin (by 60-90%) and somatostatin (by 100%) which was induced by 23 mM glucose in 60-min incubations; this inhibitory effect could be detected with neurotensin at a concentration of 1 nM. Neurotensin also significantly inhibited the elevations in glucagon, insulin, and somatostatin release induced by 20 mM arginine. It is concluded that neurotensin exerts a dual effect on the endocrine pancreas in vitro: 1) at low glucose concentration and over short term (20 min) incubations, the peptide stimulates insulin, glucagon, and somatostatin release; and 2) under stimulated conditions (high glucose or arginine), neurotensin inhibits insulin, glucagon, and somatostatin release.     

Effects of K+ and arginine on insulin, glucagon, and somatostatin release from the in vitro perfused rat pancreas

Rat pancreases were perfused in vitro for 5-min periods with K+ alone (8, 10, and 12 mM) or in the presence of arginine (10 mM). Alone, K+ caused bursts of insulin, glucagon, and somatostatin (SRIF) release; with arginine, it caused a burst of insulin and sustained SRIF release, but caused sustained suppression of glucagon. This suppression correlated better with SRIF than insulin release. Therefore, if a paracrine effect is responsible for the inhibition of glucagon secretion under these circumstances, SRIF is a more likely candidate than insulin.     

Effect of glucose/sulfonylurea interaction on release of insulin, glucagon, and somatostatin from isolated perfused rat pancreas.

The effect of a sulfonylurea, glibenclamide, on the release of insulin, glucagon, and somatostatin was studied in the isolated perfused rat pancreas. At glucose concentrations of 1.1 mM or less, the drug stimulated somatostatin release, whereas glucagon release, after 2-3 min of increase, was markedly inhibited. Insulin release was moderately stimulated, and maximal release occurred relatively late. A moderate glucose load (6.7 mM) inhibited glibenclamide-induced release of somatostatin, whereas the two in combination exerted an additive action on insulin release. Greater glucose loads, which by themselves would stimulate somatostatin release, only marginally suppressed glibenclamide-induced somatostatin release. The insulinogenic effect of these glucose levels was not modified by glibenclamide. Glibenclamide may thus stimulate both the alpha and beta as well as delta cells of the pancreas, depending on glucose concentration. We suggest a paracrine (local) interaction of somatostatin with the alpha and beta cells, which has an important role in the kinetics of insulin and glucagon release induced by sulfonylureas.     

Characterization of the inhibitory effect of somatostatin upon insulin and glucagon release in the isolated perfused canine pancreas: evidence for interaction with calcium

Somatostatin is a potent inhibitor of insulin and glucagon release from the isolated perfused canine pancreas. The present investigation was undertaken to characterize the pancreatic effects of somatostatin by studying its ability to influence insulin and glucagon release from the same perfusion preparation in response to various well-known stimuli and modulators. Somatostatin inhibited insulin and glucagon release in all test situations chosen but one. Thus, somatostatin inhibits pancreatic hormone secretion irrespective of whether it is modulated by (1) a primary initiator of insulin release--glucose (1.3 or 8.3 mM), leucine (4.1 mM), tolbutamide (2.6 mM); (2) a potentiator of insulin release, i.e., a substance that requires the presence of glucose--arginine (1 mM); (3) substances known to increase the level of cyclic AMP (cAMP) in the islets--glucagon (2 ng/ml), cAMP (1 mM), theophylline (1 mM); (4) an autonomic agent--epinephrine (2 ng/ml), acetylcholine (10 microM); or (5) alpha and beta adrenergic antagonists--phentolamine (1 microM), propranolol (1 microM). In contrast, high Ca++ concentrations (4.8 and 8.2 mM) abolished the inhibitory action of somatostatin on both insulin and glucagon release. These findings lend support to the hypothesis that somatostatin acts at a stage of secretory processes, possibly related to Ca++ inactivation, late in the chain of events leading to hormone release.     

The interaction of Vasoactive Intestinal Polypeptide (VIP), glucose and arginine on the secretion of insulin,glucagon and somatostatin in the perfused rat pancreas

Summary Vasoactive Intestinal Polypeptide (VIP) increased the release of insulin, glucagon and somatostatin from the perfused rat pancreas. The amount of these hormones released was dependent upon the prevailing glucose concentration. VIP stimulated glucagon release in the absence of glucose, while insulin and somatostatin release were increased by VIP only in the presence of glucose concentrations of 4.4 mmol/l and above. Glucagon secretion stimulated by arginine in the presence of 4.4 mmol/l glucose was potentiated by VIP. In contrast, VIP did not induce any further increase in the secretion of insulin and somatostatin over that stimulated by arginine. At higher concentrations of glucose (6.7, 16.7, and 33.3 mmol/l) VIP continued to stimulate insulin and somatostatin release, this effect being synergistic on early-phase insulin release. The effects of VIP on islet cells thus depend on the levels of modulating nutrients.