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.     

Pancreastatin inhibits insulin secretion as induced by glucagon, vasoactive intestinal peptide, gastric inhibitory peptide, and 8-cholecystokinin in the perfused rat pancreas

Pancreastatin is a 49-amino acid straight chain molecule isolated from porcine pancreatic extracts. In the perfused rat pancreas, this peptide has been shown to inhibit unstimulated insulin release and the insulin responses to glucose, arginine, and tolbutamide. To further explore the influence of pancreastatin on islet cell secretion, the effect of synthetic porcine pancreastatin (a 2-micrograms priming dose, followed by constant infusion at a concentration of 15.7 nmol/L) was studied on the insulin, glucagon, and somatostatin responses to 1 nmol/L vasoactive intestinal peptide (VIP), 1 nmol/L gastric inhibitory peptide (GIP), and 1 nmol/L 26 to 33 octapeptide form of cholecystokinin (8-CCK). The effect of pancreastatin on the insulin and somatostatin secretion elicited by glucagon (20 nmol/L) was also examined. Pancreastatin infusion consistently reduced the insulin responses to VIP, GIP, and 8-CCK without modifying glucagon or somatostatin release. It also inhibited the insulin release but not the somatostatin output induced by glucagon. These observations broaden the spectrum of pancreastatin as an inhibitor of insulin release. The finding that pancreastatin does not alter glucagon or somatostatin secretion supports the concept that it influences the B cell directly, and not through an A cell or D cell paracrine effect.     

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.     

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.     

Multiple effects of leucine on glucagon, insulin, and somatostatin secretion from the perfused rat pancreas

The effects of increasing concentrations of leucine (0.2, 2.0, and 15.0 mmol/liter) on glucagon secretion from the perfused rat pancreas were examined at various glucose levels (0, 3.3, or 8.3 mmol/liter) and in the absence or presence of either arginine (5.0 mmol/liter) or glutamine (10.0 mmol/liter). At a low glucose concentration (3.3 mmol/liter), leucine caused a dose-related biphasic increase in glucagon output in the absence of arginine, but only a transient increase in the presence of the latter amino acid. These positive responses were markedly reduced and, on occasion, abolished at a high glucose concentration (8.3 mmol/liter). Moreover, at a low glucose concentration (3.3 mmol/liter) and in the presence of arginine, the highest concentration of leucine (15.0 mmol/liter) provoked a sustained and reversible inhibition of glucagon release. Likewise, leucine (15.0 mmol/liter) reversibly inhibited glucagon secretion evoked by glutamine in the absence of glucose. Thus, leucine exerted a dual effect on the secretion of glucagon, the inhibitory effect of leucine prevailing at a high concentration of the branched chain amino acid and when glucagon secretion was already stimulated by arginine or glutamine. At a physiological concentration (0.2 mmol/liter), however, leucine was a positive stimulus for glucagon release, especially in the absence of another amino acid. Concomitantly, leucine was always a positive stimulus for both insulin and somatostatin secretion. The intimate mechanisms involved in the dual effect of leucine on glucagon secretion remain to be elucidated.     

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.     

Glucose dependency of arginine vasopressin-induced insulin and glucagon release from the perfused rat pancreas

The purpose of this study was to investigate the glucose dependency of arginine vasopressin (AVP)-induced insulin, glucagon, and somatostatin release from the perfused rat pancreas. AVP (30 or 300 pmol/L) was tested in the presence of a glucose concentration of 0, 1.4, 5.5 (basal level), or 20 mmol/L. The rates of insulin release at 0 and 1.4 mmol/L glucose were approximately 70% to 80% and 60% to 70% less, respectively, than that at the baseline level. AVP (30 or 300 pmol/L) failed to change insulin release at 0 and 1.4 mmol/L glucose. At the basal glucose level, AVP (300 pmol/L) induced a biphasic insulin release, a peak followed by a sustained phase. In addition, the combination of glucose (20 mmol/L) and AVP (300 pmol/L) induced a higher insulin peak and sustained phase than 20 mmol/L glucose alone. The rates of glucagon release at 0 and 1.4 mmol/L glucose were about 3- and 2-fold more, respectively, than that at the baseline level. At 0 and 1.4 mmol/L glucose, both 30 and 300 pmol/L AVP caused a higher glucagon peak and sustained phase than 0 and 1.4 mmol/L glucose alone. At the basal glucose level, AVP (30 or 300 pmol/L) induced a biphasic glucagon release, a peak followed by a sustained phase. The rate of glucagon release at 20 mmol/L glucose was approximately 60% to 70% less than that at the baseline level. When AVP (300 pmol/L) was administered in 20 mmol/L glucose, it induced a transient glucagon peak, which was 2.4-fold of the baseline level. At all glucose concentrations tested, AVP (30 or 300 pmol/L) failed to change somatostatin release. These results suggested that (1) hypoglycemia directly increases glucagon and decreases insulin release; (2) AVP induces insulin and glucagon release by a direct action on beta and alpha cells, respectively; (3) AVP induces insulin and glucagon release in a glucose-dependent manner-the higher the glucose concentration, the greater the enhancement of AVP-induced insulin release, whereas the lower the glucose concentration, the higher the enhancement of AVP-induced glucagon release; and (4) alpha cells are more sensitive to AVP than beta cells in hormone release.     

Glucose regulates proinsulin and prosomatostatin but not proglucagon messenger ribonucleic acid levels in rat pancreatic islets

Insulin and glucagon are the major hormones involved in the control of fuel metabolism and particularly of glucose homeostasis; in turn, nutrients tightly regulate insulin and glucagon secretion from the islets of Langerhans. Nutrients have clearly been shown to affect insulin secretion, as well as insulin biosynthesis and proinsulin gene expression; by contrast, the effects of nutrients on proglucagon gene expression have not been studied. We have investigated the effect of glucose, arginine, and palmitate on glucagon release, glucagon cell content, and proglucagon messenger RNA (mRNA) levels from isolated rat islets in 24-h incubations. We report here that concentrations of glucose that clearly regulate insulin and somatostatin release as well as proinsulin and prosomatostatin mRNA levels, do not significantly affect glucagon release, glucagon cell content or proglucagon mRNA levels. In addition, though both 10 mM arginine and 1 mM palmitate strongly stimulated glucagon release, they did not affect proglucagon mRNA levels. We conclude that, in contrast to insulin and somatostatin, glucose does not affect glucagon release and proglucagon mRNA levels, and arginine and palmitate do not coordinately regulate glucagon release and proglucagon mRNA levels.     

Difference in calcium dependency of insulin,glucagon and somatostatin secretion in response to glibenclamide in perfused rat pancreas

Summary The extracellular calcium requirements for insulin, glucagon and somatostatin release induced by 1 g/ml of glibenclamide have been compared in the perfused, isolated rat pancreas. In the absence of glucose, the drug evoked insulin release equally well at physiological (2.6 mmol/l) and low (0.25 mmol/l) levels of total calcium. In contrast, glibenclamide evoked somatostatin release at 2.6 but not at 0.25 mmol/l of calcium. At 2.6 mmol/l of calcium, glibenclamide evoked bimodal effects (stimulation followed by inhibition) on glucagon secretion. At 0.25 mmol/l of calcium, basal secretory rates of glucagon were elevated and a small stimulatory effect of glibenclamide was seen. Addition of 0.5 mmol/l of EGTA to media with low calcium concentrations uniformly abolished the A, B and D cell secretory responses to glibenclamide. The possible modulation of calcium dependency by a non-stimulatory concentration of glucose was tested by its addition at 3.3 mmol/l to the perfusion media. Glucose enhanced glibenclamide-induced insulin secretion, both at 0.25 and 2.6 mmol/l of calcium. However, at 0.25 mmol/l of calcium, the enhancing effect of glucose was more pronounced than at 2.6 mmol/l. At 2.6 mmol/l of calcium, glucose diminished the somatostatin and abolished the glucagon response to glibenclamide. At 0.25 mmol/l of calcium, glucose did not influence somatostatin release while the presence of the sugar diminished basal and glibenclamide-induced glucagon secretion. The present data confirm the requirement of extracellular calcium for A, B and D cell secretion, demonstrating different calcium dependencies for the cell types and indicate that this dependency can, in part, be modulated by glucose.     

Effects of galanin on the release of insulin, glucagon and somatostatin from the isolated, perfused dog pancreas

Galanin is a 29 amino acid peptide which has been found in intrapancreatic nerves. The effects of galanin, adrenergic and cholinergic blockade as well as somatostatin on the hormone release from the isolated perfused dog pancreas were studied. It was found that galanin dose-dependently inhibited insulin (P less than 0.001) and somatostatin (P less than 0.001) but not glucagon secretion at normal glucose levels. The lowest galanin concentration that caused a significant suppression of insulin and somatostatin secretion was 10(-11) and 10(-10) mol/l, respectively. Similar effects were evident during stimulation with 2.5 mmol/l arginine. Galanin (10(-9) mol/l) caused a more pronounced inhibition of insulin and somatostatin secretion at high (10 mmol/l) and normal (5 mmol/l) than at low glucose (1.3 mmol/l). In contrast, suppression of the glucagon secretion was only seen at low glucose (1.3 mmol/l). Perfusion of 10(-6) mol/l of atropine, phentolamine and propranolol had no effect on the galanin-mediated (10(-10) mol/l) inhibition of insulin and somatostatin secretion. Galanin (10(-12)-10(-10) mol/l) and somatostatin (10(-12)-10(-10) mol/l) were equipotent in inhibiting insulin secretion whereas only somatostatin exerted a suppression of the glucagon secretion at normal glucose. Thus, galanin exerts a differential effect on islet hormone secretion and may participate in the hormonal control of insulin, glucagon and somatostatin secretion.     

Contribution of neural intrapancreatic non-cholinergic non-adrenergic mechanisms to glucose-induced insulin release in the isolated rat pancreas

Summary In the isolated rat pancreas the effect of intrapancreatic non-adrenergic non-cholinergic nerves was examined upon insulin, glucagon and somatostatin release during perturbations of perfusate glucose. Elevation of glucose from 1.6 to 8.3 mmol/l increased insulin and somatostatin secretion and inhibited glucagon release. The first phase of insulin secretion was significantly reduced by the neurotoxin tetrodotoxin to 55% of the controls (p<0.05). The somatostatin response was attenuated by tetrodotoxin while the change of glucagon remained unaffected. In contrast the combined adrenergic and cholinergic blockade with atropine, phentolamine and propranolol (10^–5mol/l) did not modify the insulin, glucagon and somatostatin response. When glucose was changed from 8.3 to 1.6 mmol/l, the reduction of insulin and somatostatin release was not modified by tetrodotoxin, but stimulation of glucagon was significantly attenuated by 60–70% (p<0.03), which was similar to the effect of combined adrenergic and cholinergic blockade. Subsequently, the effect of neural blockade was examined during more physiological perturbations of perfusate glucose levels. When glucose was changed from 3.9 to 7.2 mmol/l, tetrodotoxin also attenuated first phase insulin response by 40% while cholinergic and adrenergic blockade had no effect. The nitric oxide synthase inhibitor N^G-Nitro-l-arginine-methylester (l-NAME) did not alter the glucose-induced insulin response indicating that nitric oxide is not involved in this mechanism. It is concluded that neural non-adrenergic noncholinergic mechanisms contribute to the first, but not second phase of glucose-induced insulin release. Non-adrenergic non-cholinergic effects do not participate in regulation of glucagon and somatostatin secretion under the conditions employed. The non-adrenergic non-cholinergic effect is most likely of peptidergic nature and remains to be examined in greater detail.     

Insulin,glucagon, and somatostatin release from the prediabetic Chinese hamster

Summary Diabetes mellitus in the adult Chinese hamster is characterized by subnormal pancreatic insulin release in vitro, decreased insulin content, and lack of obesity. The cause of the islet B-cell failure is not clear. We measured insulin, glucagon, and somatostatin release from in vitro perfused pancreases of young (mean age 10 and 20 weeks), genetically diabetic animals (subline AC, mean plasma glucose 8.0 and 16.6mmol/l, respectively). Compared to age- and sex-matched normal hamsters (subline M, mean plasma glucose 5.3 mmol/l), the younger diabetic animals had a significantly elevated mean plasma glucose level, but net in vitro pancreatic release of insulin, glucagon, and somatostatin was normal. Pancreatic content of insulin and glucagon was also not significantly different from normal. At age 20 weeks, when the plasma glucose of the diabetic animals was even more elevated, pancreatic content and release of insulin were significantly subnormal, whereas glucagon and somatostatin release were normal, and pancreatic content of glucagon was normal. In a similar group of young (mean age 10 weeks) diabetic animals, non-fasting plasma insulin levels were within the normal range, but the corresponding glucose levels were excessive in most of the animals (13 out of 19). In conclusion, 10-week-old diabetic hamsters show mild hyperglycaemia which cannot be accounted for directly by decreased pancreatic release in response to a glucose plus arginine stimulus in vitro. Decreased ability of the B cell to respond in vivo to hyperglycaemia or peripheral resistance to insulin may contribute to later B-cell failure in the older diabetic hamster.     

Insulin, glucagon, somatostatin, and thyrotropin-releasing hormone content and secretion by perifused fetal rat islets during culture

In the neonatal period of the rat, pancreatic thyrotropin-releasing hormone content decreases and the sensitivity of insulin secretion to glucose increases. In adult rat islets, TRH inhibits glucose-induced insulin release. The aim of this study was to investigate whether a high TRH content and release can be part of the explanation for the functional immaturity of neonatal islets. For that purpose, we have measured the tissue content and the secretion of immunoreactive insulin, glucagon, somatostatin and TRH in islets from 21.5-day-old rat fetuses cultured for up to one week. Insulin, glucagon and somatostatin content increased during one week of culture in the presence of 11.1 mmol/l glucose. The TRH content decreased during culture, but did not equal adult values. Insulin, glucagon and somatostatin responses to glucose were present after one week of culture. Glucose had no effect on TRH release in cultured fetal islets, but inhibited TRH release in adult islets. We conclude that glucose can stimulate insulin secretion without inhibiting TRH release, but that a decrease in islet TRH content and a sensitization of TRH secretion to glucose may be important in the full maturation of fetal pancreatic islets.     

Previous exposure to glucose enhances somatostatin secretion from the isolated perfused rat pancreas

Summary Previous exposure to glucose enhances insulin and depresses glucagon secretion by the pancreas. We have investigated whether secretion of somatostatin is also influenced by a glucose priming effect. In perfused rat pancreas from 36 h fasted rats a 5 min pulse of arginine (8 mmol/l) rapidly elicited a peak of somatostatin release. A similar somatostatin response was evoked by a second, identical, pulse of arginine after perfusion with basal glucose (3.9 mmol/l) for 45 min. On the other hand when 27.7 mmol/l D-glucose, was administered for 20 min between arginine pulses, there was significant stimulation of somatostatin secretion. When arginine was re-introduced 15 min after the cessation of the pulse of elevated glucose the magnitude of the arginine-induced peak (min 0–2 of stimulation) was increased from 16.2±4.1 to 33.1±4.7 pg/2 min, p<0.01, relative to the first stimulation with arginine. None of these effects of glucose could be reproduced by Dgalactose. The somatostatin response to arginine was higher in pancreata from fed than from 36 h fasted animals as was also basal release (22.8±5.0 vs 9.0±2.0 pg/min). In the fed state the response to the second pulse of arginine was however reduced by 50% after perfusion with basal glucose. This decrease in responsiveness was counteracted by perfusion with 27.7 mmol/l glucose for 20 min between the arginine pulses. It is concluded that previous exposure to an elevated concentration of glucose enhances D-cell responsiveness to arginine in the fasted as well as the fed state.     

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 glucagon or somatostatin on desensitized insulin secretion

In this study we have examined the role of glucagon and somatostatin in regulating glucose-induced desensitization of insulin secretion from rat islets. Measured in batch incubations with medium routinely used to induce three phases of insulin secretion, secreted glucagon levels fell off over 24 h to 20% of peak secretion levels. Although more responsive to various secretagogues, somatostatin secretion also declined to the same degree. Thus, the A- and D-cells desensitize to chronic stimulation as does the B cell. In other experiments, added glucagon (10(-6) M) enhanced glucose (11 X 10(-3) M)-stimulated insulin secretion 34% in the first 3 h; however, islets became insensitive to continuous glucagon by 4 h. The exogenous glucagon did not prevent or delay glucose-induced desensitization of insulin secretion. When glucagon was administered as acute 1-h tests over continuous glucose administration, the degree of B-cell response did not differ in the 1st, 3rd, or 6th hours and appeared to increase in the 21st hour. When islets were perifused continuously with glucose (22 X 10(-3) M) plus 3 X 10(-7) M somatostatin, glucose-induced insulin secretion was suppressed 50% in the first 3 h, but this inhibitory effect disappeared after 6 h. Desensitization was slightly delayed, but not prevented. When somatostatin was administered as acute 1-h tests over continuous glucose perifusion, the B-cell response was relatively constant in the 3rd, 6th, and 21st hours. Results show that 1) islet release of glucagon and somatostatin desensitizes during constant stimulation; and 2) islet release of insulin desensitizes to chronic potentiation or inhibition, respectively, by these hormones. Furthermore, 3) changing B-cell sensitivity to either glucagon or somatostatin cannot account for observed desensitization of insulin secretion with chronic glucose exposure.     

Insulin and glucagon secretory responses to arginine, glucagon, and theophylline during perifusion of human fetal islet-like cell clusters

The secretory responsiveness of human fetal pancreatic endocrine cells was studied by perifusion of cultured islet-like cell clusters (ICC). ICC were obtained from 7 fetuses at 13-15 weeks gestation and 21 fetuses at 17-22 weeks gestation. The ICC were challenged with glucose (20 mmol/L), arginine (10 mmol/L), glucagon (1.4 mumol/L), and theophylline (10 mmol/L) combined with zero, low (2 mmol/L), or high (20 mmol/L) glucose. At 13-15 weeks, glucose and arginine enhanced insulin release in some experiments, whereas glucagon and theophylline were always potent stimuli (mean response, 4-fold regardless of the glucose concentration). At 17-22 weeks, both glucose alone (20 mmol/L) and arginine (10 mmol/L, with 2 mmol/L glucose) induced a small (1.4- to 1.5-fold) increase in insulin release. When arginine was combined with 20 mmol/L glucose, the response was potentiated to become a 2.3-fold increase. In contrast, glucagon was equally effective in 2 and 20 mmol/L glucose (2.9- and 2.6-fold responses, respectively) and produced a half-maximal response even in the absence of glucose. In this age range the most potent stimulus for insulin release was clearly theophylline. The effect of theophylline was also remarkably independent of the glucose concentration of the perifusate (5.6-, 8.1-, and 8.6-fold responses at 0, 2, and 20 mmol/L glucose, respectively). Glucagon release from the ICC of the 17- to 22-week-old fetuses was low (mean basal glucagon release, 2.9; insulin, 24.8 fmol/100 ICC/min). Glucagon release was not affected by 20 mmol/L glucose, but was stimulated by arginine and theophylline. These findings suggest that in the fetal pancreas, in contrast to the adult organ, insulin release results from elevation of intracellular cAMP concentrations (by glucagon or theophylline) relatively independent of the exogenous glucose concentration. Therefore, glucagon may have an important role in regulating insulin release during the early development of human fetal B-cells.     

Sulfonylurea-induced inhibition of glucagon secretion from the perfused rat pancreas: evidence for a direct,non-paracrine effect

Summary The effects of sulfonylurea on glucagon secretion were characterized in the perfused rat pancreas using glibenclamide (1 g/ml) or tolazamide (10 g/ml) in the presence of 3.3 mmol/1 glucose. Glucagon release, which was unaffected by glibenclamide at 2.75 mmol/1 calcium, was suppressed at 1.19 and 0.64 mmol/l but transiently stimulated at 0.25 mmol/l extracellular calcium. The insulinogenic effect of glibenclamide at 0.64 and 0.25 mmol/1 calcium was enhanced by 35% and 89%, respectively, compared to the response at 2.75 mmol/1 calcium. The stimulatory effect of the compound on somatostatin secretion, however, was lost at the lower calcium levels. The effects of tolazamide at 2.75 and 0.64 mmol/1 calcium mimicked those of glibenclamide, thus indicating that our results with the latter compound may be representative for all sulfonylureas. In pancreata from insulin-deficient alloxan diabetic rats, glibenclamide completely lost its inhibitory effect on glucagon release at 0.64 mmol/1 calcium. Inhibition was not restored by adding insulin (25 U/1) to the perfusate. However, when diabetic rats had been treated with insulin for 6–7 days, glibenclamide suppressed glucagon release at low calcium levels in the absence of stimulated insulin and somatostatin release. It is concluded that, at low calcium concentrations, sulfonylureas suppress glucagon secretion by a direct action on the A cell and not through paracrine interactions by insulin and somatostatin. Prolonged insulin deficiency impairs the sulfonylurea action on glucagon secretion.     

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.     

Characterisation of the abnormal pancreatic D and A cell function in streptozotocin diabetic dogs: Studies with D-glyceraldehyde,dihydroxyacetone, D-mannoheptulose,D-glucose,and L-arginine

Summary Pancreatic D and A cell function is deranged in streptozotocin diabetes. To investigate this, the effect of D-glyceraldehyde, dihydroxyacetone, D-mannoheptulose and glucose variations during arginine stimulation on the release of somatostatin and glucagon from the isolated pancreas of normal and streptozotocin diabetic dogs was studied. Concentrations of the trioses, D-glyceraldehyde (1.25 and 2.5 mmol/l) and dihydroxyacetone (11 mmol/l), which normally stimulate D cells, did not influence the release of somatostatin in the diabetic dog. However, the higher concentration of D-glyceraldehyde (5 mmol/l) suppressed D cell secretion in the diabetic animals at 0 and 8.3 mmol/l glucose. A cell secretion was significantly suppressed at the higher glucose level in response to both 2.5 and 5 mmol/l of the triose. This inhibition may be explained by a non-specific effect induced by the high dose of this triose. The addition of 5 mmol/l mannoheptulose, which normally reduces glucose-induced somatostatin secretion and stimulates glucagon release, did not affect hormone secretion. In both the diabetic and the normal animals, arginine (5 mmol/l) stimulated somatostatin and glucagon secretion. Although arginine was able to stimulate D and A cell secretion in the diabetic dogs, it was however unable to restore the response to changes in glucose concentration between 1.4 and 8.3 mmol/l to normal. These results demonstrate that the abnormal pancreatic D and A cell function in streptozotocin diabetes is characterised by an impaired response to glucose and certain glucose metabolites and probably results from a specific defect in glucose recognition.