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.     

Long-term exposure of mouse pancreatic islets to oleate or palmitate results in reduced glucose-induced somatostatin and oversecretion of glucagon

AIMS/HYPOTHESIS: Long-term exposure to NEFAs leads to inhibition of glucose-induced insulin secretion. We tested whether the release of somatostatin and glucagon, the two other major islet hormones, is also affected. METHODS: Mouse pancreatic islets were cultured for 72 h at 4.5 or 15 mmol/l glucose with or without 0.5 mmol/l oleate or palmitate. The release of glucagon and somatostatin during subsequent 1 h incubations at 1 or 20 mmol/l glucose as well as the islet content of the two hormones were determined. Lipid-induced changes in islet cell ultrastructure were assessed by electron microscopy. RESULTS: Culture at 15 mmol/l glucose increased islet glucagon content by approximately 50% relative to that observed following culture at 4.5 mmol/l glucose. Inclusion of oleate or palmitate reduced islet glucagon content by 25% (at 4.5 mmol/l glucose) to 50% (at 15 mmol/l glucose). Long-term exposure to the NEFA increased glucagon secretion at 1 mmol/l glucose by 50% (when islets had been cultured at 15 mmol/l glucose) to 100% (with 4.5 mmol/l glucose in the culture medium) and abolished the inhibitory effect of 20 mmol/l glucose on glucagon secretion. Somatostatin content was unaffected by glucose and lipids, but glucose-induced somatostatin secretion was reduced by approximately 50% following long-term exposure to either of the NEFA, regardless of whether the culture medium contained 4.5 or 15 mmol/l glucose. Ultrastructural evidence of lipid deposition was seen in <10% of non-beta cells but in >80% of the beta cells. CONCLUSIONS/INTERPRETATION: Long-term exposure to high glucose and/or NEFA affects the release of somatostatin and glucagon. The effects on glucagon secretion are very pronounced and in type 2 diabetes in vivo may aggravate the hyperglycaemic effects due to lack of insulin.     

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 inhibits glucagon secretion by a direct effect on mouse pancreatic alpha cells

Aims/hypothesis The mechanisms by which glucose regulates glucagon release are poorly understood. The present study aimed to clarify the direct effects of glucose on the glucagon-releasing alpha cells and those effects mediated by paracrine islet factors. Materials and methods Glucagon, insulin and somatostatin release were measured from incubated mouse pancreatic islets and the cytoplasmic Ca²⁺ concentration ([Ca²⁺]_i) recorded in isolated mouse alpha cells. Results Glucose inhibited glucagon release with maximal effect at 7 mmol/l. Since this concentration corresponded to threshold stimulation of insulin secretion, it is unlikely that inhibition of glucagon secretion is mediated by beta cell factors. Although somatostatin secretion data seemed consistent with a role of this hormone in glucose-inhibited glucagon release, a somatostatin receptor type 2 antagonist stimulated glucagon release without diminishing the inhibitory effect of glucose. In islets exposed to tolbutamide plus 8 mmol/l K⁺, glucose inhibited glucagon secretion without stimulating the release of insulin and somatostatin, indicating a direct inhibitory effect on the alpha cells that was independent of ATP-sensitive K⁺ channels. Glucose lowered [Ca²⁺]_i of individual alpha cells independently of somatostatin and beta cell factors (insulin, Zn²⁺ and γ-aminobutyric acid). Glucose suppression of glucagon release was prevented by inhibitors of the sarco(endo)plasmic reticulum Ca²⁺-ATPase, which abolished the [Ca²⁺]_i-lowering effect of glucose on isolated alpha cells. Conclusions/interpretation Beta cell factors or somatostatin do not seem to mediate glucose inhibition of glucagon secretion. We instead propose that glucose has a direct inhibitory effect on mouse alpha cells by suppressing a depolarising Ca²⁺ store-operated current.     

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.     

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.     

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.     

Characterization of growth hormone-releasing hormone stimulation of the endocrine pancreas: studies with alpha- and beta-adrenergic and cholinergic antagonists

The 40 aminoacids residue of pancreatic growth hormone-releasing hormone stimulates the secretion of insulin, glucagon, an somatostatin from the pancreas. To determine whether this stimulation of islet hormone secretion is mediated via adrenergic or cholinergic receptor sites, we studied the effects of 30 nmol/l of the growth hormone-releasing hormone on the release of insulin, glucagon, and somatostatin in the presence of either alpha-adrenergic (phentolamine), beta-adrenergic (propranolol) or cholinergic (atropine) blocking agents. The responses to the growth hormone-releasing hormone were not significantly modified by adrenergic or cholinergic blockers. The findings rule out an interaction with adrenergic and cholinergic receptors on islet cells. It is at present unknown whether the growth hormone-releasing hormone stimulates islet hormone secretion via an interaction with specific growth hormone-releasing hormone receptors or vasoactive intestinal peptide receptors.     

Islet secretion of immunoreactive thyrotropin-releasing hormone and the 'paracrine-like' effects of its exogenous administration

In order to know more about the secretory pattern of islet TRH in response to glucose and its possible physiological relevance, the release of this hormone as well as that of insulin, glucagon, and somatostatin was radioimmunologically measured. Whereas the secretion of immunoreactive insulin and somatostatin by incubated rat islets is known to be dose-dependently stimulated by glucose, that of glucagon and TRH was inhibited by glucose. Similarly, palmitate dose-dependently inhibited islet glucagon and TRH release. Exogenous TRH exerted strong and dose-dependent effects on islet secretion of the other hormones at the same concentration range at which its hypophysiotropic effects are produced (10(-10) to 10(-8) mol/l). It inhibited the insulin response to glucose and blocked that of glucagon, whereas it enhanced glucose-induced stimulation of somatostatin. These results are suggestive of a possible paracrine inhibitory role of islet TRH, either directly exerted on the secretion of insulin and glucagon or partially mediated through the stimulation of somatostatin release.     

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.     

Glucose, beta adrenergic effects, and pancreatic endocrine function in the isolated perfused duck pancreas

Duck isolated perfused pancreas was used to assess glucose, adrenergic mediated effects and pancreatic function interrelationships. A moderate physiological 50% increase in glucose level, corresponding closely to the difference observed between 24-h-fasted and fed animals, induced a significant decrease of pancreatic glucagon not due to a rise in somatostatin secretion. The great responsiveness of the A cell was still found after glucagon stimulation by catecholamines or beta adrenergic agonism. Insulin was irresponsive to the glucose load we used, suggesting that glucose-induced glucagon suppression was also insulin independent. As far as the D cell was concerned, glucose had no effect on pancreatic somatostatin output; however, an interesting finding was that beta adrenergic agonism has a permissive effect on D cell responsiveness to the nutriment.     

Selective effect of some somatostatin analogs on glucagon as opposed to insulin release in rats in vivo

Cyclic somatostatin, at a dose of 700 but not 70 ng/kg/min, inhibited arginine-induced insulin and glucagon release as well as glucose stimulated insulin release in rats in vivo. Three somatostatin (S-S) analogs (D-Cys14-S-S, D-Trp8-D-Cys14-S-S and Ala2-D-Trp8-D-Cys14-S-S), at a dose of 70 ng/kg/min, suppressed arginine-induced glucagon but not insulin release. At the same dose, the first two of these analogs had no effect on glucose-induced insulin release, while the third one. Ala2-D-Trp8-D-Cys14-somatostatin, enhanced insulin release induced by glucose. A fourth analog, D-Trp8-somatostatin, was more potent than somatostatin with regard to arginine stimulated insulin and glucagon release, and equipotent with somatostatin with respect to glucose stimulated insulin release. These studies show, firstly, that the inhibitory effect of somatostatin analogs on arginine induced insulin release may be different from that when glucose is used as a stimulant and, secondly, that Ala2-D-Trp8-D-Cys14-somatostatin inhibits arginine-induced glucagon release while enhancing insulin release on glucose stimulation.     

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,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.     

Effect of gastrin-releasing peptide on the secretion of mouse islet hormones in vitro

Collagenase-isolated mouse islets were incubated with gastrin-releasing peptide (GRP). At 5.6 mmol glucose/l. 10 nmol GRP/l increased the release of insulin (by 50%) and glucagon (by twofold), decreased the release of pancreatic polypeptide (by 35%), but did not significantly affect the release of somatostatin. At 16.7 mmol glucose/l, 10 nmol GRP/l increased glucagon release (by fivefold) and decreased pancreatic polypeptide release (by 46%), without significantly altering insulin and somatostatin release. GRP (200 nmol/l) did not affect insulin release by perifused mouse islets at 2.8 mmol glucose/l, but increased both first and second phase insulin release after a square wave increase in the glucose concentration to 11.1 mmol/l. At 5.6 mmol glucose/l, GRP (100 pmol/1-100 nmol/l) increased (by 50-70%) insulin release by the RINm5F clonal cell line. GRP did not affect glucose oxidation or the cyclic adenosine monophosphate content of RINm5F cells. However, the intracellular free Ca2+ concentration of RINm5F cells was rapidly and transiently increased by GRP (maximum increase of 64% about 10 s after exposure to 1 mumol GRP/l). The rise of intracellular free Ca2+ was approximately halved in the absence of extracellular Ca2+. The results suggest that GRP may contribute to the normal regulation of the endocrine pancreas. The insulin-releasing effect of GRP is mediated via increased cytosolic free Ca2+, derived both from an increased net influx of extracellular Ca2+ and from mobilization of intracellular Ca2+ stores.     

Glucagon-like peptide-1, but not glucose-dependent insulinotropic peptide,inhibits glucagon secretion via somatostatin (receptor subtype 2) in the perfused rat pancreas

Aims/hypothesis  The glucose-lowering effect of glucagon-like peptide-1 (GLP-1) is based not only upon its potent insulinotropic actions but also on its ability to restrain glucagon secretion. Surprisingly, the closely related glucose-dependent insulinotropic peptide (GIP) stimulates glucagon release. We examined whether the islet hormone somatostatin, which strongly inhibits glucagon secretion, is involved in this divergent behaviour. Methods  At 1.5 mmol/l glucose and therefore minimal insulin secretion, the glucagon, insulin and somatostatin responses to 20 mmol/l glucose, GLP-1, GIP and somatostatin were studied in the presence of a high-affinity monoclonal somatostatin antibody and of a highly specific somatostatin receptor subtype 2 (SSTR2) antagonist (PRL-2903) in the isolated perfused rat pancreas. Results  In control experiments, GLP-1 at 1 and 10 nmol/l reduced glucagon secretion significantly to 59.0 ± 6.3% (p < 0.004; n = 5; SSTR2 series; each vs pre-infusion level) and to 48.0 ± 2.6% (p < 0.001; n = 6; somatostatin antibody series) respectively. During somatostatin antibody administration, GLP-1 still inhibited glucagon secretion significantly, but the effect was less pronounced than in control experiments (p < 0.018). Co-infusion of the SSTR2 antagonist completely abolished the GLP-1-induced suppression of glucagon secretion. In contrast, neither the GIP-induced stimulation of glucagon release nor its inhibition by 20 mmol/l glucose was altered by somatostatin antibody or SSTR2 antagonist administration. Conclusions/interpretation  We conclude that GLP-1 is capable of inhibiting glucagon secretion even in the absence of secretory products from the beta cell. It is highly likely that this is mediated via somatostatin interacting with SSTR2 on rat alpha cells. In contrast, GIP and glucose seem to influence the alpha cell independently of somatostatin secretion.     

Diminished inhibitory effect of noradrenaline on insulin release from mouse islets transplanted to kidney

Insulin release is inhibited by adrenergic alpha-2 agonism in normal beta-cells. To test whether the inhibitory response to noradrenaline is modified by transplantation, we studied insulin release from freshly isolated islets and from syngeneic islets transplanted under the kidney capsule of non-diabetic C57BL/6 mice. When perifused in vitro, fresh islets, as well as grafts harvested 1 or 3 weeks after transplantation, reacted to 2.5 mol/l noradrenaline with a complete inhibition of insulin release induced by 16.7 mmol/ld-glucose. In contrast, islet grafts harvested after 6, 12, or 21 weeks exhibited a conspicuous insulin secretory response to 16.7 mmol/l glucose in the presence of 2.5 mol/l noradrenaline. Also a concentration of 0.25 mol/l, noradrenaline inhibited the glucose-induced insulin release from fresh islets but not from 6-week-old islet grafts. It is concluded that transplantation under the kidney capsule induces a decreased inhibitory responsiveness to noradrenaline in islet grafts.     

Regulation of glucagon release: Effects of insulin on the pancreatic A2-cell of the guinea pig

Summary The effect of exogenous insulin on glucagon release by guinea pig A₂-cells of isolated islets from normal and streptozotocin treated animals has been studied to test the hypothesis that insulin directly affects glucagon secretion. Glucose utilization and ATP concentration were also measured. In addition, the effects of exogenous somatostatin on glucagon release and glucose utilization of these cells have been investigated. In the A₂-cell rich islets from streptozotocin treated guinea pigs glucagon release was 76.9±7.8 pg/g islet dry weight/h in 1.7 mmol/l glucose, not being significantly inhibited when the glucose concentration was increased up to 16.7 mmol/l. Glucagon release was, however, strongly suppressed by 30 mU/ml insulin independent of the glucose concentration, while release from the normal islets was unaffected by exogenous insulin. Glucose utilization increased four-fold in the A₂cell rich islets when the glucose level was raised from 1.7 to 16.7 mmol/l, but was nevertheless at all times less than 70 per cent of that of the normal islets. Addition of exogenous insulin caused a further significant stimulation (40–100 per cent) in the A₂-cell rich islets, but not in the normal islets. The ATP concentration of the A₂-cell rich islets increased in parallel with the glucose concentration. Addition of insulin effected the highest ATP levels, about 15 mmol/kg islet dry weight, irrespective of the glucose concentration. Somatostatin (2.5 g/ml) strongly inhibited glucagon release, but failed to affect glucose utilization. The present observations support the view that the A₂-cell is sensitive to insulin, and suggest that the suppressive effect of insulin on glucagon release is mediated via an increased intracellular ATP concentration generated by stimulated glucose metabolism.This work was supported by grants from the Swedish Medical Research Council (B79-12X-001-15B and B79-12X-0229712C), the Swedish Diabetes Association, the Medical Faculty of the University of Uppsala and the Swedish Society for Medical Research.     

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.