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.     

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.     

Effects of cholecystokinin (CCK)-8, CCK-33, and gastric inhibitory polypeptide (GIP) on basal and meal-stimulated pancreatic hormone secretion in man.

Gastrointestinal hormones with insulinotropic effects, like cholecystokinin (CCK) and gastric inhibitory polypeptide (GIP) might tentatively be used in the treatment of non-insulin-dependent diabetes mellitus. We therefore examined the effects of intravenous injection of pharmacological dose levels of CCK-8 (100 and 300 pmol/kg), CCK-33 (100 pmol/kg), GIP (100 pmol/kg), and CCK-8 plus GIP (100 pmol/kg of each) on plasma levels of glucose, insulin, somatostatin, glucagon, and pancreatic polypeptide (PP) in healthy human volunteers. The peptides were given under basal conditions or in combination with a mixed meal. CCK-8, CCK-33, and GIP were all found to increase the basal plasma levels of insulin, somatostatin, and PP; the increases were observed already in samples taken at 2 min after the injection. In contrast, the plasma glucagon levels were unaffected by the peptides. CCK-8, CCK-33, and GIP (100 pmol/kg) all potentiated the meal-induced plasma responses of insulin and PP, whereas plasma levels of glucagon after the meal were not affected. Plasma somatostatin levels after the meal were increased by GIP but not affected by CCK-8 or CCK-33. CCK-8 and GIP together (100 pmol/kg for both) increased plasma levels of insulin, PP and somatostatin as much as each of the peptides given alone, both under basal conditions and after the meal intake. Plasma levels of glucagon were not affected by CCK-8 and GIP together. We conclude that in man, both CCK-8, CCK-33, and GIP moderately stimulate basal and meal related insulin release without any synergistic effects and that the peptides do not inhibit the secretion of glucagon.     

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.     

Effects of substance P and other peptides on the release of somatostatin, insulin, and glucagon in vitro

We studied the actions of substance P, bombesin, vasoactive intestinal peptide (VIP), and the octapeptide of cholecystokinin (CCK-8-S) on the release of somatostatin, insulin, and glucagon from the isolated perfused pancreatico-duodenal canine preparation. Substance P at concentrations ranging from 0.2-5.0 nM stimulated the secretion of somatostatin, insulin, and glucagon in a dose-dependent manner. However, the responses evoked by substance P were modified by the prevailing glucose level; higher somatostatin and insulin and lower glucagon responses were obtained at the high glucose concentration of 8.3 mM rather than at the low glucose concentration of 2.8 mM. At a glucose concentration of 5.5 mM, somatostatin release was above the prestimulation level in response to 1 nM substance P (89 +/- 15%; P less than 0.01), VIP (49 +/- 7%; P less than 0.01), or CCK-8-S (99 +/- 21%; P less than 0.01); bombesin was without effect (16 +/- 14; P = NS). Insulin release was enhanced by substance P (150 +/- 45%; P less than 0.05), bombesin (162 +/- 56%; P less than 0.05), VIP (44 +/- 5%; P less than 0.01), and CCK-8-S (190 +/- 17%; P less than 0.001). Furthermore, a significant release of glucagon was evoked by 1 nM substance P (501 +/- 158%; P less than 0.05), bombesin (30 +/- 10%; P less than 0.05), VIP (43 +/- 8%; P less than 0.01), or CCK-8-S (140 +/- 19%; P less than 0.001).     

Effects of cholecystokinin (CCK)-4, nonsulfated CCK-8, and sulfated CCK-8 on pancreatic somatostatin, insulin, and glucagon secretion in the dog: studies in vitro

The effect of cholecystokinin (CCK)-4, nonsulfated CCK-8 (CCK-8), and sulfated CCK-8 (CCK-8-S) on endocrine pancreas function was investigated in the isolated perfused dog pancreas in the presence of 5.5 mM glucose. CCK-4 and CCK-8 at concentrations of 1, 10, and 100 nM dose dependently stimulated pancreatic SRIF, insulin, and glucagon release. The insulinotropic and glucagonotropic potency of CCK-8 was significantly greater than that of CCK-4, whereas the effect on SRIF secretion was similar. Furthermore, CCK-8-S and CCK-8 at concentrations of 0.1, 1, and 10 nM caused a dose-dependent increase in pancreatic A, B, and D cell secretion. The CCK-8-S was a more potent insulinotropic agent than CCK-8. It is suggested that these principal molecular CCK forms qualify for a physiological modulatory role in the endocrine pancreas.     

Effects of neuromedin B, gastrin-releasing peptide-10 and their fragment peptides on secretion of gastrointestinal and pancreatic hormones in dogs

The effects of neuromedin B (NMB), gastrin-releasing peptide (GRP)-10 and their C-terminal fragment peptides on the pancreatic and gastrointestinal hormone release were studied in dogs. Intravenous bolus injections of NMB and GRP-10 (4.5 nmol/kg) into conscious dogs elicited a sharp and statistically significant rise in plasma gastrin and insulin levels, but only GRP-10 brought on a significant rise in the plasma glucagon and enteroglucagon levels. The degree of stimulation of gastrin and insulin secretion by NMB and GRP-10 was dose-dependent. With a dose of 4.5 nmol/kg, the minimum size of C-terminal fragment peptides of NMB and GRP-10 to stimulate gastrin secretion was NMB and GRP-10, respectively. Both NMB and GRP-10 (0.1-100 nmol/l) stimulated insulin release from the isolated canine pancreas. The glucagon release was stimulated by 10 and 100 nmol/l GRP-10 and was not stimulated by the same doses of NMB. The somatostatin release was not influenced by either peptide. It is concluded that 1) NMB and GRP-10 can stimulate gastrin and pancreatic hormone secretion, and the latter effect may be mainly due to a direct action on the islet cells; 2) the stimulatory effect of GRP-10 is stronger than that of NMB. The difference in the minimal active fragment between NMB and GRP-10 suggests that the amino acid of position 3 - NMB (Leu) and GRP-10 (His) - may play an important role in their biological activity.     

Effect of GIP on the secretion of insulin and somatostatin and the accumulation of cyclic AMP in vitro in the rat

The effects of gastric inhibitory polypeptide (GIP) on insulin secretion as well as on the intra-islet accumulation of [3H]cyclic AMP were investigated in isolated pancreatic islets of the rat. In the presence of 6.7 mmol/l of glucose, 3.0 and 30 nmol/l of GIP induced both insulin and [3H]cyclic AMP responses, while lower and higher concentrations of the peptide were ineffective. A coupling of the two parameters was also found with regard to interaction between glucose and GIP. Thus while 30 nmol/l of GIP was stimulatory together with 6.7, 16.7 or 33.3 mmol/l of glucose, the peptide stimulated neither insulin release, nor the accumulation of [3H]cyclic AMP in the presence of a low concentration of glucose (3.3 mmol/l). The concomittant release of insulin and somatostatin was studied in the perfused pancreas in order to assess a possible influence by somatostatin on the dose-response pattern for GIP-induced insulin release. In this preparation 1.0 to 10 nmol/l of GIP stimulated insulin and somatostatin secretion; however while these concentrations were equipotent on insulin release, 10 nmol/l of GIP stimulated somatostatin release more than 1 nmol/l, indicating differences in dose-response curves for the GIP-induced stimulation of the two hormones. It is concluded that 1) modulation of GIP-induced insulin release is coupled to changes in cyclc AMP response in the islet, 2) GIP-induced somatostatin secretion may influence the concomittant insulin response.     

Inhibition of dipeptidyl peptidase-4 augments insulin secretion in response to exogenously administered glucagon-like peptide-1, glucose-dependent insulinotropic polypeptide, pituitary adenylate cyclase-activating polypeptide, and gastrin-releasing peptide in mice

Inhibition of dipeptidyl peptidase-4 (DPP-4) is currently being explored as a new approach to the treatment of type 2 diabetes. This concept has emerged from the powerful and rapid action of the enzyme to inactivate glucagon-like peptide-1 (GLP-1). However, other bioactive peptides with potential influence of islet function are also substrates of DPP-4. Whether this inactivation may add to the beneficial effects of DPP-4 inhibition is not known. In this study, we explored whether DPP-4 inhibition by valine-pyrrolidide (val-pyr; 100 micromol/kg administered through gastric gavage at t = -30 min) affects the insulin and glucose responses to iv glucose (1 g/kg) together with GLP-1 (10 nmol/kg), glucose-dependent insulinotropic polypeptide (GIP; 10 nmol/kg), pituitary adenylate cyclase-activating polypeptide 38 (PACAP38; 1.3 nmol/kg), or gastrin-releasing peptide (GRP; 20 nmol/kg) given at t = 0 in anesthetized C57BL/6J mice. It was found that the acute (1-5 min) insulin response to GLP-1 was augmented by val-pyr by 80% (4.2 +/- 0.4 vs. 7.6 +/- 0.8 nmol/liter), that to GIP by 40% (2.7 +/- 0.3 vs. 3.8 +/- 0.4 nmol/liter), that to PACAP38 by 75% (4.6 +/- 0.5 vs. 8.1 +/- 0.6 nmol/liter), and that to GRP by 25% (1.8 +/- 0.2 vs. 2.3 +/- 0.3 nmol/liter; all P < 0.05 or less). This was associated with enhanced glucose elimination rate after GLP-1 [glucose elimination constant (K(G)) 2.1 +/- 0.2 vs. 3.1 +/- 0.3%/min] and PACAP38 (2.1 +/- 0.3 vs. 3.2 +/- 0.3%/min; both P < 0.01), but not after GIP or GRP. The augmented insulin response to GRP by val-pyr was prevented by the GLP-1 receptor antagonist, exendin(3) (9-39), raising the possibility that GRP effects may occur secondary to stimulation of GLP-1 secretion. We conclude that DPP-4 inhibition augments the insulin response not only to GLP-1 but also to GIP, PACAP38, and GRP.     

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.     

Stimulation of (Pro-)insulin biosynthesis and release by gastric inhibitory polypeptide in isolated islets of rat pancreas.

(Pro-)Insulin biosynthesis ([3H]leucine incorporation) and insulin secretion were studied in collagenase-isolated rat islets incubated for 3 hours at 1 and 2 mg/ml glucose in the presence of gastric inhibitory polypeptide (GIP). GIP augmented [3H]leucine incorporation and release of insulin at both glucose concentrations. In a second series of experiments it was found that an amino acid mixture was without influence on the insulotrophic action of GIP. Combined stimulation of insulin release by GIP and glucagon did not result in higher insulin output than observed in the presence of each substance alone. Thus GIP, in constrast to many other gastrointestinal peptides, however similar to glucagon, enhances not only release but also biosynthesis of insulin. This insulinotrophic action can be observed already at a glucose concentration of 1 mg/ml. The results underline the outstanding role which GIP appears to play in the regulation of beta-cell function.     

Antidiabetogenic action of cholecystokinin-8 in type 2 diabetes

Cholecystokinin (CCK) is a gut hormone and a neuropeptide that has the capacity to stimulate insulin secretion. As insulin secretion is impaired in type 2 diabetes, we explored whether exogenous administration of this peptide exerts antidiabetogenic action. The C-terminal octapeptide of CCK (CCK-8) was therefore infused i.v. (24 pmol/kg x h) for 90 min in six healthy postmenopausal women and in six postmenopausal women with type 2 diabetes. At 15 min after start of infusion, a meal was served and ingested during 10 min. On a separate day, saline was infused instead of CCK-8. In both healthy subjects and subjects with type 2 diabetes, CCK-8 reduced the increase in circulating glucose after meal ingestion and potentiated the increase in circulating insulin. The ratio between the area under the curves for serum insulin and plasma glucose during the 15- to 75-min period after meal ingestion was increased by CCK-8 by 198 +/- 18% in healthy subjects (P = 0.002) and by 474 +/- 151% (P = 0.038) in subjects with type 2 diabetes. In contrast, the increase in the circulating levels of gastric inhibitory polypeptide (GIP), glucagon-like peptide-1 (GLP-1), or glucagon after meal ingestion was not significantly affected by CCK-8. The study therefore shows that CCK-8 exerts an antidiabetogenic action in both healthy subjects and type 2 diabetes through an insulinotropic action that most likely is exerted trough a direct islet effect. As at the same time, CCK-8 was infused without any adverse effects, the study suggests that CCK is a potential treatment for type 2 diabetes.     

Evidence that glucagon stimulates insulin secretion through its own receptor in rats

Summary Since glucagon-like peptide-1 (7–36) amide (7–37) (GLP-1) has been found to be a potent insulinotropic hormone, it has been postulated that glucagon stimulates insulin secretion from islet beta cells through the GLP-1 receptor. We therefore examined the effects of a GLP-1 receptor antagonist, exendin (9–39) amide, on glucagon- or GLP-1-stimulated insulin release from isolated perfused rat pancreas. When infusion of 100 nmol/l exendin (9–39) amide was started 5 min before that of 1 nmol/l glucagon, the stimulation of insulin release by glucagon was similar to that found in the control situation (preinfusion with vehicle alone). By contrast, when 0.3 nmol/l GLP-1 was used in the same experimental setting, exendin (9–39) amide clearly inhibited insulin release. These results indicate that glucagon stimulates insulin release mainly through glucagon receptors but not GLP-1 receptors on islet beta cells.Abbreviations GLP-1 Glucagon-like peptide-1 - BSA bovine serum albumin - IRI immunoreactive insulin     

GLP-1 and GLP-1(7-36) amide: influences on basal and stimulated insulin and glucagon secretion in the mouse

We studied the cellular distribution of glucagon-like peptide-1 (GLP-1) in the pancreas and gut and the effects of GLP-1 and its truncated form, GLP-1(7-36) amide, on basal and stimulated insulin and glucagon secretion in the mouse. Immunofluorescence staining showed that GLP-1 immunoreactivity occurred within peripheral islet cells and in cells located mainly distally in the small intestine and in the entire large intestine. Double-immunostaining revealed that the GLP-1-immunoreactive cells were identical to the glucagon/glicentin cells. Experiments in vivo revealed that basal insulin secretion was stimulated by GLP-1(7-36) amide at the dose levels of 8 and 32 nmol/kg, and by GLP-1 at 32 nmol/kg. Furthermore, GLP-1(7-36) amide showed additive stimulatory influence with glucose (2.8 mmol/kg), the cholinergic agonist carbachol (0.16 mumol/kg), and the C-terminal octapeptide of cholecystokinin (CCK-8, 5.3 nmol/kg), when injected at 8 or 32 nmol/kg. In contrast, stimulated insulin secretion was unaffected by GLP-1. Moreover, the glucagon secretory responses to carbachol and CCK-8 were inhibited by GLP-1(7-36) amide but were unaffected by the entire GLP-1. We conclude that GLP-1(7-36) has the potential for being a modulator of islet hormone secretion.     

Glucose-dependent insulinotropic hormone potentiates the hypoglycemic effect of glibenclamide in healthy volunteers: evidence for an effect on insulin extraction

Glucose-dependent insulinotropic hormone (GIP) is an intestinal hormone considered to be an important mediator of the incretin effect, i.e. the augmented insulin release observed in response to orally, compared with iv, administered glucose, despite isoglycemic glucose profiles. Stimulation of beta-cell secretion of insulin by GIP is seen both in vitro and in vivo at permissive extracellular glucose concentrations (> 6 mmol/L). It has also been claimed that part of the incretin effect is due to decreased insulin extraction. We now show that an infusion of GIP in healthy volunteers in whom blood glucose levels were maintained at 5 mmol/L, increased glibenclamide-stimulated levels of plasma insulin without significantly changing the C peptide profile. The increased plasma insulin levels necessitated extra glucose infusion to maintain euglycemia, demonstrating the biological significance of the elevated insulin levels. Infusion of GIP alone caused neither glucose changes nor elevation of C peptide or insulin levels. Hence, our results show that at a blood glucose concentration of 5 mmol/L, GIP augments the increase in plasma insulin levels stimulated by glibenclamide, possibly acting through a mechanism involving decreased insulin extraction in the liver or peripheral tissues, thus increasing insulin availability.     

Intestinal nutrient influence on the enteroinsular axis.

The gastrointestinal contribution to carbohydrate metabolism includes carbohydrate absorption and the release of gastrointestinal hormones that interact with the endocrine pancreas. To learn the contributions to the enteroinsular axis from different levels of the gastrointestinal tract and different nutrients in chyme, we determined serum concentrations of glucose, gastric inhibitory peptide (GIP), insulin, and glucagon postprandially in six normal subjects who underwent diversion of chyme just proximal to an occlusive balloon at the ligament of Treitz and jejunal infusion of saline or chyme carbohydrate, protein, and lipid, separately or in combination. Postprandial elevations of serum glucose, GIP, and insulin and decrease of serum glucagon were elicited predominantly from the bowel and its contents distal to the ligament of Treitz. In this segment, each chyme nutrient (but especially carbohydrate) significantly stimulated factors affecting carbohydrate metabolism. Protein and lipid were able to block carbohydrate-induced glucagon inhibition. The gastroduodenal segment, although containing several proposed insulinotropic hormones (gastrin, secretin, and cholecystokinin), had no effect on serum glucose of glucagon and stimulated only small insulin and GIP responses.     

A comparison of the insulinotropic and the insulin-inhibitory actions of gut peptides on newborn and adult rat islet cells

The objective of this study was to examine the release of insulin from cultured islet cells, taken from the pancreas of newborn and adult rats, in response to gastric inhibitory polypeptide (GIP), cholecystokinin-8 (CCK-8), calcitonin gene-related peptide (CGRP), and pancreastatin. GIP (10(-9)-10(-7) M) potentiated glucose-stimulated release of insulin in a dose-dependent fashion from both newborn and adult islet cells. CCK-8 (greater than 10(-8) M) also increased glucose-stimulated release of insulin from newborn islet cells, however its effect was not significant and not as strong as that observed with adult islet cells. Culture of newborn islet cells for 3 weeks with media containing high concentrations of glucose (16.7 mM) enhanced insulin release in response to CCK-8. CGRP did not affect the release of insulin from newborn islet cells, whereas at 10(-10) M, it reduced the release of insulin from adult islet cells by 66 +/- 4%. Pancreastatin (10(-9)-10(-8) M) did not affect the release of insulin from newborn islet cells when cells were incubated with 4.2 mM glucose, whereas it stimulated the release of insulin from adult islet cells in a dose-dependent fashion. When incubated with 16.7 mM glucose, pancreastatin inhibited the release of insulin from both newborn and adult islet cells. These results indicate that newborn islet cells experience developmental changes which render them responsive to enteric peptides.     

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.