Cholecystokinin-33 potentiates and vasoactive intestinal polypeptide inhibits gastric inhibitory polypeptide--induced insulin secretion in the perfused rat pancreas

Gastric inhibitory polypeptide (GIP), cholecystokinin (CCK), and vasoactive intestinal polypeptide (VIP) stimulate insulin secretion. In this study we investigated whether CCK-33 and VIP could influence the insulinogenic effect of simultaneously administered GIP and 6.7 mmol/l glucose in the perfused rat pancreas. We found that at 0.1 nmol/l, GIP markedly potentiated glucose-induced insulin release whereas CCK-33 and VIP had a weak stimulatory effect and only during the late phase. At this low dose level, CCK-33 potentiated but VIP inhibited the late phase of insulin release stimulated by glucose and GIP. At 1.0 nmol/l, GIP, CCK-33, and VIP markedly potentiated both phases of glucose-induced insulin secretion. At this dose level CCK-33 and GIP exerted additive stimulatory effects on the late phase of insulin release triggered by glucose. In contrast, 1.0 nmol/l VIP inhibited insulin secretion augmented by glucose and GIP. In summary 1) GIP, CCK-33 and VIP all potentiate glucose-induced insulin secretion from the perfused rat pancreas, and 2) CCK-33 potentiates and VIP inhibits GIP-induced insulin secretion. We suggest that interactions of this kind are of importance for the precise regulation of insulin secretion.     

Studies on the mechanisms of somatostatin action on insulin release. IV. effect of somatostatin on cyclic AMP levels and phosphodiesterase activity in isolated rat pancreatic islets.

The effects of somatostatin on insulin release and cyclic AMP metabolism were studied in collagenase-isolated islets of Langerhans from the rat. Ceoncentrations from 500 to 2000 ng/ml significantly inhibited glucose stimulated insulin release, while 100 and 200 ng/ml were ineffective. Somatostatin (2000 ng/ml) inhibited insulin release and [3H]-cyclic AMP accumulation induced by 16.7 mM glucose after 10 and 30 min of incubation. In dose-response studies, the inhibition by somatostatin of the effect of glucose on [3H]cyclic AMP and insulin release could be overcome by a high concentration of the hexose (44.9 mM), suggesting competitive inhibition. In the absence of glucose, somatostatin inhibited [3H]cyclic AMP accumulation induced by the phosphodiesterase inhibitor, IBMX, while no inhibition was seen, again in the absence of hexose, when the [3H]cyclic AMP levels had been raised by the adenyl cyclase stimulator, cholera toxin. Somatostatin did not affect phosphodiesterase activity when added to islet homogenates, but preincubation of the islets with the peptide before homogenization decreased the activity by about 30%. It is suggested that somatostatin-induced inhibition of insulin release is, at least partially, mediated by cyclic AMP, probably through an action on islet adenyl cyclase.     

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 gastric inhibitory polypeptide, vasoactive intestinal polypeptide and peptide histidine isoleucine on the secretion of hormones by isolated mouse pancreatic islets

The release of insulin, glucagon, somatostatin and pancreatic polypeptide (PP) by isolated mouse pancreatic islets was determined during 30-min incubations at 5.6 and 16.7 mmol glucose/l in the absence and presence of gastric inhibitory polypeptide (GIP), vasoactive intestinal polypeptide (VIP) and peptide histidine isoleucine (PHI) at concentrations of 1-1000 nmol/l. Insulin release was enhanced (greater than 50%) by GIP (100-1000 nmol/l) and VIP (1 mumol/l) at 5.6 mmol glucose/l, but not at 16.7 mmol glucose/l. Glucagon release was increased by GIP (100-1000 nmol/l), and by VIP and PHI (1-1000 nmol/l) at both glucose concentrations in a dose-related manner (maximum increases greater than tenfold). Somatostatin release was similarly increased by GIP (10-1000 nmol/l) at both glucose concentrations. Only the highest concentration (1 mumol/l) of PHI tested increased somatostatin release (twofold) at 5.6 mmol glucose/l, whereas PHI and VIP (1-1000 nmol/l) reduced (greater than 37%) somatostatin release at 16.7 mmol glucose/l. PP release was increased (49-58%) by 100-1000 nmol GIP/l, but was not significantly altered by VIP, and was reduced (39-56%) by PHI. The results indicate that GIP, VIP and PHI each stimulate glucagon release in a dose-related manner, but they exert discretely different effects on other islet hormones depending upon the dose and the prevailing glucose concentration.     

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.     

The metabolism of cyclic AMP and glucose in isolated islets from Acomys cahirinus

Summary Glucose-induced cyclic (³H) AMP accumulation, insulin secretory responses and the metabolism of glucose were studied in pancreatic islets from Acomys cahirinus. 27.7 mmol/l of glucose stimulated neither islet cyclic (³H) AMP accumulation nor insulin release during the first 5 min of incubation. Stimulation by glucose of cyclic (³H) AMP was observed after 15 min of incubation and insulin release was markedly stimulated between 15 and 30 min. The utilization of glucose, measured as the production of (³H)₂O from (5-³H) glucose was stimulated by glucose after 10 min and proceeded at an apparently linear rate during a 20 min incubation period. In incubations of 5 min, glibenclamide, glucagon or chloromercuribenzene-p-sulphonic acid failed to stimulate islet cyclic (³H) AMP accumulation. 3-isobutyl-1-methylxanthine in a concentration of 1.0 mmol/l was the only agent tested that elevated rapidly (1 min) islet cyclic (³H) AMP. None of the agents tested elicited an insulin secretory response in 5 min incubations. It is concluded that 1) no gross defect is apparent in the utilization of glucose by Acomys islets, 2) the secretory derangement of the Acomys is associated with a delayed cyclic AMP response to glucose, 3) however a decreased level of cyclic AMP cannot be the sole explanation for the delayed insulin secretion in the Acomys.     

Comparison of GLP-1 (7-36amide) and GIP on release of somatostatin-like immunoreactivity and insulin from the isolated rat pancreas

The effect of equimolar doses of GIP and GLP-1 (7-36amide) on insulin and somatostatin secretion in the isolated perfused rat pancreas was compared. At a perfusate glucose concentration of 70 mg/dl GLP-1 (7-36amide) 10(-9) and 10(-8) M and GIP 10(-9) M elicited a significant stimulation of insulin while GIP 10(-8) M and lower doses of both peptides (10(-11) and 10(-10) M) were ineffective. At elevated perfusate glucose levels of 150 mg/dl both peptides stimulated insulin release at 10(-11), 10(-10), 10(-9) and 10(-8) M but not at 10(-12) M. The insulin response at the higher glucose level was significantly greater compared to the effect of the same doses at normoglycemic conditions. Somatostatin release was stimulated significantly by GLP-1 (7-36amide) at 10(-10) and 10(-9) M at perfusate glucose level 70 mg/dl. At a glucose concentration of 150 mg/dl this effect was abolished. GIP did not alter somatostatin release at a perfusate glucose concentration of 70 mg/dl while at 150 mg/dl only the highest dose of GIP (10(-8) M) stimulated somatostatin release significantly. In conclusion, the present data demonstrate that in vitro in the rat pancreas both peptides are equally effective secretagogues of insulin release at normal and moderately elevated perfusate glucose levels. In contrast, somatostatin secretion is stimulated by GLP-1 (7-36amide) at normoglycemic conditions while only a rather high and presumably pharmacological dose of GIP is a stimulus of somatostatin secretion at moderate hyperglycemia.(ABSTRACT TRUNCATED AT 250 WORDS)     

Synergistic impact of cholecystokinin and gastric inhibitory polypeptide on the regulation of insulin secretion

The modulation of insulin output from isolated perifused rat islets by the intestinal peptides cholecystokinin (CCK) and gastric inhibitory polypeptide (GIP) was assessed. In the presence of 7 mmol/L (126 mg/dL) glucose, but not 2.75 mmol/L (50 mg/dL) glucose, CCK (5 nmol/L) or GIP (50 ng/mL) alone evoke small insulin secretory responses. However, the combination of GIP (50 ng/mL) plus CCK (5 nmol/L) together with 7 mmol/L glucose results in a markedly amplified insulin secretory response. CCK (50 nmol/L) alone increases phosphoinositide (PI) hydrolysis in islets, an event reflected by an increase in 3H efflux from myo[2-3H]inositol prelabeled islets and parallel accumulations of labeled inositol phosphates. GIP (50 ng/mL) alone has no effect on PI hydrolysis. However, GIP reduces the quantitative impact of CCK on PI turnover, an effect attributable to the capacity of GIP to elevate islet cAMP levels. CCK has no significant effect on islet cAMP levels. The results support the concept that the synergistic action of these peptides on insulin output is mediated by their ability to generate separate beta cell second messenger molecules. Nutrient-regulated intestinal release of various peptides represents a remarkable control system to ensure the release of insulin from the beta cell in amounts commensurate with both the quantity and quality of nutrient intake.     

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.     

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.     

Influence of glucagon, gastric inhibitory polypeptide, pancreatic polypeptide and somatostatin on beta-adrenergically induced insulin secretion in the mouse

The effect of four polypeptides, glucagon, Gastric Inhibitory Polypeptide (GIP), Pancreatic Polypeptide (PP) and somatostatin on beta-adrenoceptor stimulated insulin secretion in vivo in the mouse was investigated. The beta-adrenoceptor stimulation was induced by isoprenaline (IPNA). It was found that at dose levels without influence on basal insulin secretion the polypeptides produced the following pattern of interaction with IPNA. Insulin secretion induced by IPNA was increased by glucagon and inhibited by somatostatin. GIP and PP did not change IPNA-induced insulin release. It is concluded from this and earlier published studies that glucagon, but not always GIP, serves as a positive modulator of basal and stimulated insulin secretion, and that somatostatin is a general inhibitor of insulin release. beta-Adrenoceptor-induced insulin secretion however, seems to be less sensitive to somatostatin than insulin release induced by glucose.     

Decreased cyclic AMP and Insulin responses to glucose in pancreatic islets of diabetic chinese hamsters

Summary The dose as well as the time kinetics of insulin and adenosine-3,5-monophosphate (cyclic AMP) responses to glucose were compared in pancreatic islets isolated from normal and diabetic Chinese hamsters. The insulin content in diabetic islets was about one-half that in normal islets. Insulin release in diabetic islets incubated for 10 min with glucose 60–1000 mg/l00 ml was from one-third to one-half that in normal islets. Glucose 1000 mg/l00 ml stimulated three-fold increases in insulin release without increasing the accumulation of [³H] cyclic AMP in either normal or diabetic islets prelabelled with [³H] adenine. However, in the presence of 1.0 mM of the phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (IBMX), glucose 150 mg/l00 ml elicited significant increases of insulin release (+ 134%) and [³H] cyclic AMP accumulation in islets (+ 44%) and incubation medium (+ 48%) of islets of normal but not diabetic hamsters. Also, in perifusion experiments with 0.1 mM IBMX, glucose 500 mg/l00 ml produced threefold greater increases in insulin release and two-fold greater increases in efflux of cyclic AMP in normal than diabetic islets. By contrast with the lesser effects of glucose in diabetic islets, 1.0 mM IBMX increased islet and medium cyclic AMP, as well as insulin release, similarly in normal and diabetic islets. It is suggested that the impairment of glucose induced insulin release in islets of the diabetic Chinese hamster may be due to a defective interaction of glucose with the adenylate cyclase-cyclic AMP system in the pancreatic B cell.This work was presented in part at a meeting of the European Association for the Study of Diabetes, Sept. 1975, Munich, Germanyon leave from the Department of Endocrinology, Karolinska Hospital, Stockholm     

Effects of alloxan on glucose-stimulated insulin secretion,glucose metabolism,and cyclic adenosine 3′, 5′-monophosphate levels in rat isolated islets of Langerhans

Summary Insulin secretion was stimulated and cyclic adenosine 3, 5-monophosphate (cAMP) levels were elevated in isolated rat islets by 27.5 mmol/l glucose. Alloxan caused a dose-dependent decrease in both variables with complete obliteration of insulin release at a concentration of 1.25 mmol/l. D-glucose, in the presence or absence of extracellular calcium, or 3-0-methyl-D-glucose (both at 27.5 mmol/l) protected completely against the effects of alloxan on both glucose-induced insulin release and cAMP levels. 3-0-Methylglucose did not stimulate insulin secretion or elevate cAMP and did not interfere with glucose-stimulated secretion or elevation of cAMP. When glucose-stimulated insulin release was abolished by alloxan, the metabolism of glucose, determined by the rate of³H₂O formation from [5-³H] glucose, was depressed by 20%. It is concluded that alloxan altered the adenylate cyclase system such that it could no longer be stimulated by glucose. Glucose-stimulated insulin secretion or elevation of cAMP did not appear essential for glucose to protect against alloxan. Protection by 3-0-methylglucose did not appear to be mediated through an alteration of cAMP metabolism. Alloxan did not inhibit glucose-induced insulin secretion by grossly altering glycolysis.     

Insulin response of cultured islets from diabetic and nondiabetic BB rats.

This study examines the insulin response of pancreatic islets isolated from diabetic BB rats (BBD), nondiabetic BB rats (BBN), and Wistar rats to in vitro stimulation. After a 48-hour culture period, insulin release in response to glucose (17.8 mmol/L) either alone, with glucose-dependent insulinotropic polypeptide (GIP) +/- somatostatin (SS), or with Arg +/- SS was measured. A static incubation system was used. Insulin secretion from islets cultured in 4.4 mmol/L glucose (basal) did not differ between BBN and BBD rats (0.50% +/- 0.08%, 0.67% +/- 0.25% of total islet cell content [TCC], respectively). High glucose concentrations (17.8 mmol/L) stimulated a modest increase in insulin release from BBD and BBN islets (1.8% +/- 0.48% and 2.1% +/- 0.19% TCC, respectively). The addition of GIP (1 nmol/L) enhanced glucose-stimulated insulin secretion from BBN rat islets (2.9% +/- 0.42% TCC), but had no effect on BBD islets (2.04% +/- 0.57% TCC). Somatostatin (1 mumol/L) completely reversed the glucose- and/or GIP-stimulated insulin secretion from both BBN and BBD rat islets to basal levels (0.42% +/- 0.043%, 0.42% +/- 0.09% TCC, respectively). Arg (1 mmol/L) enhanced glucose-stimulated insulin secretion in both groups, although the greatest response was elicited from BBD rat islets (8.4-fold v 3.2-fold). Experiments comparing BB rats with Wistar rats demonstrated significant differences in the glucose-stimulated (17.8 mmol/L) insulin response of the islets. Islets taken from BBN and BBD were less responsive to glucose than those from Wistar rats. However, islets from BBD rats were hyperresponsive to Arg when compared with islets from Wistar rats.(ABSTRACT TRUNCATED AT 250 WORDS)     

Mechanisms underlying the insulinostatic effect of peptide YY in mouse pancreatic islets

Summary Peptide YY is an insulinostatic peptide which is released into the circulation from the intestinal mucosa upon food intake. Peptide YY is also co-stored with glucagon in the secretory granules of the pancreatic alpha cells. We examined the mechanisms underlying the insulinostatic effect of peptide YY in isolated mouse pancreatic islets. We found that peptide YY (0.1 nmol/l-1 mol/l) inhibited glucose (11.1 mmol/l)-stimulated insulin secretion from incubated isolated islets, with a maximal inhibition of approximately 70% observed at a dose of 1 nmol/ 1 (p<0.001). Also in perifused islets the peptide (1 nmol/l) inhibited insulin secretion in response to 11.1 mmol/l glucose (p<0.001). Furthermore, peptide YY inhibited glucose-stimulated cyclic AMP formation (by 67%, p<0.05), and insulin secretion stimulated by dibutyryl cyclic AMP (p<0.01). In contrast, the peptide was without effect both on the cytoplasmic Ca²⁺ concentration in dispersed mouse islet-cell suspensions as measured by the FURA 2-AM technique, and on insulin release in isolated islets, when stimulated by the protein kinase C-activator 12-O-tetradecanoyl phorbol 13-acetate. Finally, in pre-labelled perifused islets, peptide YY caused a small and transient increase in the ⁸⁶Rb⁺ efflux (p<0.001), but only in the absence of extracellular Ca²⁺. We conclude that peptide YY inhibits glucose-stimulated insulin secretion from isolated mouse islets by inhibiting two different steps in the cyclic AMP cascade, that is, both the accumulation and the action of the cyclic nucleotide. In contrast, the data suggest that protein kinase C, K⁺ channels, the cytoplasmic Ca²⁺ concentration or other processes directly regulating the exocytosis are not involved in the signal transduction underlying peptide YY-induced inhibition of insulin secretion.Abbreviations PYY Peptide YY - TPA 12-O-tetradecanoylphorbol 13-acetate     

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.     

Correlation between Ca(2+)-ATPase activity of rat islet cells and insulin secretion.

Using medium with a low ionic strength, a low concentration of Ca2+ and Mg2+ and devoid of K+, we have measured Ca(2+)-ATPase activity in the homogenates of rat islets preincubated for 3 min with several hormones in the presence of 3.3 mmol glucose/l. Insulin secretion was also measured in islets incubated for 5 min under identical experimental conditions. Islets preincubated with glucose (3.3 mmol/l) and glucagon (1.4 mumol/l) plus theophylline (10 mmol/l), ACTH (0.11 nmol/l), bovine GH (0.46 mumol/l), prolactin (0.2 mumol/l) or tri-iodothyronine (1.0 nmol/l) have significantly lower Ca(2+)-ATPase activity than those preincubated with only 3.3 mmol glucose/l. All these hormones increased the release of insulin significantly. Dexamethasone (0.1 mumol/l) and somatostatin (1.2 mumol/l) enhanced the Ca(2+)-ATPase activity while adrenaline (10 mumol/l) did not produce any significant effect on the activity of the enzyme. These hormones decreased the release of insulin significantly. These results demonstrated that islet Ca(2+)-ATPase activity was modulated by the hormones tested. Their inhibitory or enhancing effect seemed to be related to their effect on insulin secretion; i.e. those which stimulated the secretion of insulin inhibited the activity of the enzyme and vice versa. Hence, their effect on insulin secretion may be due, in part, to their effect on enzyme activity and consequently on the concentration of cytosolic Ca2+. These results reinforce the assumption that Ca(2+)-ATPase activity participates in the physiological regulation of insulin secretion, being one of the cellular targets for several agents which affect this process.     

Characterisation of somatostatin release from the pancreas

Summary The effect of calcium on somatostatin secretion was investigated in the isolated, perfused canine pancreas preparation and compared with those of acetylcholine, glucose, isoproterenol and arginine. Calcium (5 mmol/l) stimulated somatostatin release in a typical biphasic response pattern being about 5 times as potent as acetylcholine (1 mol/l), arginine (5 mmol/l), and isoproterenol (2 ng/ml) while the release of insulin and glucagon in response to calcium and the other secretagogues were of the same magnitude. Somatostatin release increased progressively when perfusate calcium was increased step-wise from 0 through 1.25 and 2.5 to 5.0 mmol/l. Calcium stimulated the secretion of somatostatin in the absence of glucose. The stimulatory effect of calcium was, however, modulated by the glucose concentration being about twice as large at 200 mg/100 ml as at 25 mg/100 ml glucose in the perfusion medium.     

Homologous desensitization of the insulinotropic glucagon-like peptide-I (7-37) receptor on insulinoma (HIT-T15) cells.

Glucagon-like peptide-I(7-37) [GLP-I(7-37)] is an intestinal peptide with potent insulinotropic activities on pancreatic beta-cells in vivo and in vitro. In earlier studies elevated concentrations GLP-I(7-37) inhibited insulin release and cAMP generation in beta-cells. We now show that the GLP-I(7-37) receptor in the glucose-responsive B-cell line HIT-T15 undergoes rapid and reversible homologous desensitization in response to supraphysiological concentrations of GLP-I(7-37). GLP-I(7-37) stimulated insulin release and cAMP generation in a glucose-dependent biphasic manner with a maximum stimulation at 10 nmol/liter. The first-phase insulin secretory response was reduced by 41% at doses of GLP-I(7-37) of 100 nmol/liter and higher. Preperifusion of B-cells with 100 nmol/liter GLP-I(7-37) for 5 or 10 min reduced a subsequent insulin secretory response to 10 nmol/liter GLP-I(7-37) after hormone washout and recovery periods of 10 min (52% and 55% reduction) or 30 min (33% reduction or full recovery). Preperifusion of HIT-T15 cells with 100 nmol/liter glucagon (10 min) or 100 nmol/liter gastric inhibitory peptide (GIP) (10 min) had no effect on the insulin secretory response to 10 nmol/liter GLP-(7-37). Prior exposure of cells to 100 nmol/liter GLP-(7-37) (10 min) did not alter the GIP-induced (10 nmol/liter) insulin release, but 100 nmol/liter GIP (10 min) reduced the insulin secretion during stimulation with 10 nmol/liter GIP by 56%. These data indicate that: 1) the GLP-I(7-37) receptor is subject to rapid and reversible homologous desensitization and, 2) the GLP-I(7-37) receptor on beta-cells is distinct from that of GIP. The recent finding of elevated GLP-I(7-36)amide levels in subjects with noninsulin-dependent diabetes suggest the possibility that a homologous desensitization of the GLP-I(7-37) receptor might contribute to the impaired insulin secretion in this disorder.