Preserved incretin activity of glucagon-like peptide 1 [7-36 amide] but not of synthetic human gastric inhibitory polypeptide in patients with type-2 diabetes mellitus.

In type-2 diabetes, the overall incretin effect is reduced. The present investigation was designed to compare insulinotropic actions of exogenous incretin hormones (gastric inhibitory peptide [GIP] and glucagon-like peptide 1 [GLP-1] [7-36 amide]) in nine type-2 diabetic patients (fasting plasma glucose 7.8 mmol/liter; hemoglobin A1c 6.3 +/- 0.6%) and in nine age- and weight-matched normal subjects. Synthetic human GIP (0.8 and 2.4 pmol/kg.min over 1 h each), GLP-1 [7-36 amide] (0.4 and 1.2 pmol/kg.min over 1 h each), and placebo were administered under hyperglycemic clamp conditions (8.75 mmol/liter) in separate experiments. Plasma GIP and GLP-1 [7-36 amide] concentrations (radioimmunoassay) were comparable to those after oral glucose with the low, and clearly supraphysiological with the high infusion rates. Both GIP and GLP-1 [7-36 amide] dose-dependently augmented insulin secretion (insulin, C-peptide) in both groups (P < 0.05). With GIP, the maximum effect in type-2 diabetic patients was significantly lower (by 54%; P < 0.05) than in normal subjects. With GLP-1 [7-36 amide] type-2 diabetic patients reached 71% of the increments in C-peptide of normal subjects (difference not significant). Glucagon was lowered during hyperglycemic clamps in normal subjects, but not in type-2 diabetic patients, and further by GLP-1 [7-36 amide] in both groups (P < 0.05), but not by GIP. In conclusion, in mild type-2 diabetes, GLP-1 [7-36 amide], in contrast to GIP, retains much of its insulinotropic activity. It also lowers glucagon concentrations.

Preceding hyperinsulinemia prevents demonstration of insulin effect on fat-induced gastric inhibitory polypeptide (GIP).

The effect of insulin on fat-induced gastric inhibitory polypeptide (GIP) release has been studied in seven healthy volunteers during euglycemic blood glucose clamping. In the first protocol, insulin (0.1 U/kg/h) was infused 2 h before ingestion of 100 g fat and continued for 2 h thereafter. In protocol II, saline was infused 2 h before the fat load and the insulin infusion started at the time of fat ingestion. During both insulin infusion studies, glucose levels were clamped at the fasting level by means of the Biostator and plasma levels of insulin, C-peptide, and GIP were estimated by radioimmunoassay. The response of GIP to oral fat was inhibited by 63% if insulin infusion was started at the time of fat ingestion, whereas no inhibition was seen if a 2-h hyperinsulinemic period preceded the fat load. The plasma insulin levels were comparable at the end of each experiment, ranging from 110 to 130 microU/ml. Plasma C-peptide levels decreased during the insulin infusion and increased after fat ingestion. These findings were not the result of inhibition of gastric emptying by insulin because they could be confirmed in four volunteers with intraduodenal infusion of fat. The present data show that insulin does inhibit fat-induced GIP secretion in normal man, but preceding hyperinsulinemic glucose clamping masks this insulin effect, probably by decreasing the sensitivity of the GIP cells to insulin.

Inhibition of gastric inhibitory polypeptide (GIP) release by insulin and glucose in juvenile diabetes.

The effect of glucose and insulin on fat- and glucose-induced gastric inhibitory polypeptide (GIP) release has been studied in insulin-dependent juvenile-type diabetics. Blood glucose and serum immunoreactive GIP (IR-GIP) were measured after an oral load of 100 g glucose or 100 g fat was given and during an infusion of one of the following: saline, glucose, glucose plus insulin, or insulin. The infusion of insulin alone (in the presence of elevated glucose levels) or together with glucose significantly suppressed the IR-GIP rise after fat ingestion, but it did not alter the GIP response to oral glucose. Intravenous infusion of glucose had a slight but significant inhibitory effect on fat-stimulated increase of IR-GIP, which cannot be related to endogenous insulin release in these insulin-deficient diabetics. It is suggested that an insulin-mediated increase of glucose utilization in the GIP cell interferes only with increased GIP secretion stimulated by the utilization of fatty acids but not of glucose. This could explain the existence of a negative feedback control between insulin and GIP secretion for fat but not for glucose-induced GIP release.

Insulin-like action of proinsulin on rat liver carbohydrate metabolism in vitro.

Short-term effects of human proinsulin on metabolic rates and its long-term action on enzyme induction were studied in primary cultures of rat hepatocytes and in the perfused rat liver, and compared with the effects of bovine insulin. In the perfused rat liver, proinsulin decreased the glucagon-dependent increase of glycogenolysis. The action of 0.5 nM glucagon was almost completely suppressed by 100 nM proinsulin. Proinsulin and insulin showed similar potency. In cultured rat hepatocytes, proinsulin stimulated glycolysis up to fivefold with a half-maximal effective dose of 30 nM. Proinsulin induced the key glycolytic enzymes glucokinase and pyruvate kinase by twofold and antagonized the glucagon-dependent induction of phosphoenolpyruvate carboxykinase with a half-maximal effective dose at 3 nM. For the effects in cultured hepatocytes, about 100-fold higher concentrations of proinsulin than of insulin were required.

Inhibition of glycogenolysis and glycogen phosphorylase by insulin and proinsulin in rat hepatocyte cultures.

The inhibitory action of insulin and proinsulin on basal and glucagon-activated glycogenolysis was studied in cultured rat hepatocytes containing [14C]glycogen. Insulin or proinsulin given as sole hormones in the presence of 5 mM glucose decreased basal release of [14C]glucose from [14C]glycogen to 20%. Half-maximal effective concentration of insulin was approximately 0.15 nM and of proinsulin was approximately 5 nM. Inhibition of [14C]lactate release from [14C]glycogen required slightly higher hormone concentrations with a similar difference in potency for insulin and proinsulin. The glucagon-stimulated release of [14C]glucose was completely blocked by insulin or proinsulin with half-maximal effective concentrations of approximately 0.2 and approximately 8 nM, respectively. In contrast, release of [14C]lactate in the presence of glucagon was increased slightly by insulin and proinsulin. Basal and glucagon-activated phosphorylase activity was inhibited by approximately 50% in a dose-dependent manner by both hormones, with differences in potency similar to those for the inhibition of glycogenolysis. These data point to a direct regulatory role of insulin in the control of hepatic glycogen breakdown even when acting as sole hormone. The results do not support the notion of a preferential inhibitory potency of proinsulin on hepatic glycogenolysis.

Glucagonostatic actions and reduction of fasting hyperglycemia by exogenous glucagon-like peptide I(7-36) amide in type I diabetic patients.

OBJECTIVE: Glucagon-like peptide I(7-36) amide (GLP-1) is a physiological incretin hormone that, in slightly supraphysiological doses, stimulates insulin secretion, lowers glucagon concentrations, and thereby normalizes elevated fasting plasma glucose concentrations in type II diabetic patients. It is not known whether GLP-1 has effects also in fasting type I diabetic patients. RESEARCH DESIGN AND METHODS: In 11 type I diabetic patients (HbA1c 9.1 +/- 2.1%; normal, 4.2-6.3%), fasting hyperglycemia was provoked by halving their usual evening NPH insulin dose. In random order on two occasions, 1.2 pmol . kg-1 . min-1 GLP-1 or placebo was infused intravenously in the morning (plasma glucose 13.7 +/- 0.9 mmol/l; plasma insulin 26 +/- 4 pmol/l). Glucose (glucose oxidase method), insulin, C-peptide, glucagon, GLP-1, cortisol, growth hormone (immunoassays), triglycerides, cholesterol, and nonesterified fatty acids (enzymatic tests) were measured. RESULTS: Glucagon was reduced from approximately 8 to 4 pmol/l, and plasma glucose was lowered from 13.4 +/- 1.0 to 10.0 +/- 1.2 mmol/l with GLP-1 administration (plasma concentrations approximately 100 pmol, P < 0.0001), but not with placebo (14.2 +/- 0.7 to 13.2 +/- 1.0). Transiently, C-peptide was stimulated from basal 0.09 +/- 0.02 to 0.19 +/- 0.06 nmol/l by GLP-1 (P < 0.0001), but not by placebo (0.07 +/- 0.02 to 0.07 +/- 0.02). There was no significant effect on nonesterified fatty acids (P = 0.34), triglycerides (P = 0.57), cholesterol (P = 0.64), cortisol (P = 0.40), or growth hormone (P = 0.53). CONCLUSIONS: Therefore, exogenous GLP-1 is able to lower fasting glycemia also in type I diabetic patients, mainly by reducing glucagon concentrations. However, this alone is not sufficient to normalize fasting plasma glucose concentrations, as was previously observed in type II diabetic patients, in whom insulin secretion (C-peptide response) was stimulated 20-fold.

Influence of gastric inhibitory polypeptide antiserum on glucose-induced insulin secretion in rats.

The action of gastric inhibitory polypeptide (GIP) antiserum on glucose tolerance and insulin secretion after an intraduodenal glucose load (600 mg/kg) was examined in anesthetized rats. In control experiments the insulin secretion was nearly doubled when glucose was administered intraduodenally, as compared to an iv glucose load to simulate the blood glucose curve after the intraduodenal glucose administration. After injection of GIP antiserum, the glucose curve resulting from the intraduodenal glucose load was slightly elevated and the insulin response was significantly reduced. No free GIP could be measured in the plasma of antibody-treated rats. However, the GIP antiserum did not offset the incretin effect of the intraduodenal glucose load completely. In control experiments the same amount of GIP antibody completely blocked the insulinotropic effect of exogenous porcine GIP (0.6 microgram/kg . h). In nonanesthetized rats serial oral glucose tolerance tests were performed for 14 days after injection of the GIP antiserum. Despite the blockage of endogenous GIP, the glucose tolerance did not change significantly in the antibody-treated group of rats as compared to a control group. These data indicate that GIP is not the exclusive incretin and that additional gut factors with insulinotropic activity exist.

Pancreastatin: molecular and immunocytochemical characterization of a novel peptide in porcine and human tissues.

Pancreastatin, a novel 49-amino acid peptide isolated from porcine pancreas, shows over 70% sequence homology to the central part of bovine and human chromogranin-A. Using an N-terminal and C-terminal synthetic peptide, we developed two sensitive and specific RIAs for the detection of pancreastatin-like immunoreactivity (PLI) in porcine and human tissue extracts. PLI was present throughout the gastrointestinal tract and in most endocrine and neuronal tissues. Highest concentrations were measured in the pituitary, adrenal gland, and pancreas (1200-4000 pmol/g), similar to the distribution of chromogranin-A. PLI was also detected in human endocrine tumors, with large quantities in some carcinoids (up to 14 nmol/g). HPLC revealed that extracts from porcine pituitary and pancreas contained small pancreastatin-like peptides, whereas in adrenal medulla large chromogranin-A-like molecular forms predominated. Human endocrine tumors showed a different pattern, with intermediate forms distinct from chromogranin-A and pancreastatin. Biochemical analysis was confirmed by immunocytochemistry localizing PLI in pancreatic islets, adrenal medulla, pituitary, duodenum, and human endocrine tumors. Pancreastatin is present in a variety of gastrointestinal, endocrine, and neuronal tissues and may represent a novel peptide of unknown physiological function, derived from chromogranin-A by proteolytic cleavage.

Commercially available preparations of porcine glucose-dependent insulinotropic polypeptide (GIP) contain a biologically inactive GIP-fragment and cholecystokinin-33/-39.

Commercially available preparations of natural porcine glucose-dependent insulinotropic polypeptide (GIP) were subjected to reverse phase HPLC. The material was found to give rise to 4 peaks which were characterized by HPLC-retention time and N-terminal sequence analysis. They represented: intact porcine GIP(1-42), 58% (wt/wt); the GIP-fragment des-tyr-ala-GIP(3-42), 32% (wt/wt); cholecystokinin (CCK)-33 2% (wt/wt); CCK-39 2% (wt/wt). HPLC-pure GIP(1-42) stimulated insulin release in rat isolated pre-cultured pancreatic islets in the presence of 16.7 mM glucose up to 240% vs. control, whereas the fragment des-tyr-ala-GIP(3-42) did neither increase insulin release nor exhibit antagonistic activity to GIP(1-42) at 100 ng/ml. These results indicate that commercially available porcine GIP-preparations may contain the biologically inactive des-tyr-ala-GIP(3-42) in high amounts, and in addition may be contaminated by CCK-peptides. HPLC-characterization of these peptide preparations prior to any biological study is crucial.

Additive insulinotropic effects of exogenous synthetic human gastric inhibitory polypeptide and glucagon-like peptide-1-(7-36) amide infused at near-physiological insulinotropic hormone and glucose concentrations.

Gastric inhibitory polypeptide (GIP) and glucagon-like peptide-1-(7-36) amide (GLP-1) are glucose-dependent insulinotropic gut hormones that may explain the greater insulin secretory response with oral compared to i.v. glucose (incretin effect). To study their individual and combined contributions, in eight healthy volunteers, on separate occasions, synthetic human GIP (1 pmol/kg.min) and/or GLP-1 (0.3 pmol/kg.min) or placebo were infused i.v. (-30 to 120 min), while at 0 min, a glucose infusion "isoglycemic" to the profile after an oral glucose load of 50 g/400 mL was started. After the administration of 50 g oral glucose, immunoreactive GIP rose several-fold to 337 +/- 43 pmol/L, while there was only a transient (10-30 min) and moderate increment in immunoreactive GLP-1 (from basal, 25-30, to 41 +/- 4 pmol/L). Isoglycemic i.v. glucose infusions led to smaller B-cell responses (estimated incretin effect, 41 +/- 5%). With single infusions of GIP or GLP-1 (circulating concentrations, 464 +/- 73 and 54 +/- 3 pmol/L, respectively), B-cell responses were significantly augmented compared to i.v. glucose alone and were no longer significantly different from those after oral glucose. The combination of GIP and GLP-1 led to B-cell responses that were significantly higher than those with either hormone alone (additive mode of cooperation). Plasma GIP concentrations were similar after endogenous secretion (oral glucose) and i.v. infusion, while exogenously administered GLP-1 led to plasma levels that were maintained at an elevated level for a longer period during exogenous infusion than after stimulation by oral glucose. When, in seven volunteers, a lower dose (0.15 pmol/kg.min) of GLP-1 was infused during isoglycemic glucose infusion experiments only for the duration of elevated plasma levels in the oral glucose challenges (0-30 min), a significant, but transient, increment in insulin and C-peptide concentrations was observed, which was equivalent to 26 +/- 10% of the estimated incretin effect. Therefore, in conclusion, circulating GIP seems to make a major contribution to the incretin effect after oral glucose, and GLP-1 appears to mediate a smaller proportion. GIP and GLP-1 can interact in an additive manner in normal man.

Insulinotropic properties of synthetic human gastric inhibitory polypeptide in man: interactions with glucose, phenylalanine, and cholecystokinin-8.

The quantitative contribution of glucose-dependent insulinotropic polypeptide [gastric inhibitory polypeptide (GIP)] to the incretin effect after oral glucose (augmentation of insulin secretion over the degree that is explained by the glycemic rise) is not known. Therefore, hyperglycemic clamp experiments (8 mmol/L, corresponding to postprandial glucose concentrations) were performed in healthy volunteers, and synthetic human GIP was infused for 60 min at a rate (approximately 1.3 pmol/kg.min) that results in plasma GIP concentrations similar to those occurring after oral glucose loads of 75 g. The MCR for exogenous GIP was approximately 6 mL/kg.min; the decay after ceasing infusion was exponential with a t1/2 of about 18 min, and the resulting volume of distribution was about 140 mL/kg. At euglycemic (basal) plasma glucose concentrations (5.0 mmol/L) similar values were found. Insulin secretion was stimulated by hyperglycemia alone, but was greatly (2.3-fold based on C-peptide) potentiated by GIP infusions (P less than or equal to 0.001 for integrated incremental values). When integrated incremental responses over 120 min of GIP, immunoreactive insulin, and immunoreactive C-peptide were compared after oral glucose and during GIP infusions, no significant differences were found. Peak glucose concentrations after oral glucose (7.6 +/- 0.6 mmol/L) were similar to mean plasma glucose values during clamp experiments (8.2 +/- 0.1 mmol/L; P = 0.124). However, mean glucose concentrations after oral glucose were lower (6.0 +/- 0.3 mmol/L; P = 0.0004). Additional infusion of sulfated cholecystokinin-8 (25 pmol/kg.h) or the amino acid phenylalanine (1.7 mumol/kg.min) did not further stimulate insulin secretion and had no influence on the pharmacokinetics of exogenous GIP. It is concluded that human synthetic GIP is insulinotropic in man and that this activity may well explain a substantial part of the incretin effect after oral glucose. There is no interaction with cholecystokinin or phenylalanine in concentrations found after mixed meals.

Acid-induced gastric inhibitory polypeptide secretion in man.

This study has assessed the effect of oral or intraduodenal HCl, administered alone or in combination with glucose, on gastric inhibitory polypeptide (GIP) and insulin secretion. Eight young males were given the following three oral tests: 30 g glucose, 150 cc 0.1 N HCl and the glucose-acid combination. Another group of eight controls received intraduodenal infusions of 20 g glucose, 75 cc 0.1 N HCl and their combination. All tests were performed in a random fashion at weekly intervals. When given alone, HCl did not influence glucose or insulin levels. However, HCl did produce an increase in GIP. The GIP response to acid was less than that to glucose and was delayed. The peak insulin and GIP responses with the glucose and glucose/acid combinations were similar with both the oral and intraduodenal routes. There was, however, a potentiation of both the GIP and insulin responses when intraduodenal acid was given with glucose. This effect on GIP and insulin was not evident with the oral glucose/acid load. It is concluded that HCl by itself is capable of stimulating GIP secretion. Since there was only a potentiation of insulin and GIP secretion when large doses of HCl were given together with glucose via the intraduodenal route, the physiological relevance of acid-induced GIP secretion remains to be resolved.

Gastric emptying, glucose responses, and insulin secretion after a liquid test meal: effects of exogenous glucagon-like peptide-1 (GLP-1)-(7-36) amide in type 2 (noninsulin-dependent) diabetic patients.

The aim of the study was to investigate whether inhibition of gastric emptying of meals plays a role in the mechanism of the blood glucose-lowering action of glucagon-like peptide-1-(7-36) amide [GLP-1-(7-36) amide] in type 2 diabetes. Eight poorly controlled type 2 diabetic patients (age, 58 +/- 6 yr; body mass index, 30.0 +/- 5.2 kg/m2; hemoglobin A1c, 10.5 +/- 1.2%) were studied in the fasting state (plasma glucose, 11.1 +/- 1.1 mmol/L). A liquid meal of 400 mL containing 8% amino acids and 50 g sucrose (327 Kcal) was administered at time zero by a nasogastric tube. Gastric volume was determined by a dye dilution technique using phenol red. In randomized order, GLP-1-(7-36) amide (1.2 pmol/kg.min; Saxon Biochemicals) or placebo (0.9% NaCl with 1% human serum albumin) was infused between -30 and 240 min. In the control experiment, gastric emptying was completed within 120 min, and plasma glucose, insulin, C-peptide, GLP-1-(7-36) amide, and glucagon concentrations transiently increased. With exogenous GLP-1-(7-36) amide (plasma level, approximately 70 pmol/L), gastric volume remained constant over the period it was measured (120 min; P < 0.0001 vs. placebo), and plasma glucose fell to normal fasting values (5.4 +/- 0.7 mmol/L) within 3-4 h, whereas insulin was stimulated in most, but not all, patients, and glucagon remained at the basal level or was slightly suppressed. In conclusion, GLP-1-(7-36) amide inhibits gastric emptying in type 2 diabetic patients. Together with the stimulation of insulin and the inhibition of glucagon secretion, this effect probably contributes to the blood glucose-lowering action of GLP-1-(7-36) amide in type 2-diabetic patients when studied after meal ingestion. At the degree observed, inhibition of gastric emptying, however, must be overcome by tachyphylaxis, reduction in dose, or pharmacological interventions so as not to interfere with the therapeutic use of GLP-1-(7-36) amide in type 2 diabetic patients.

Incretin effects of increasing glucose loads in man calculated from venous insulin and C-peptide responses.

Integrated insulin secretion rates calculated from peripheral venous C-peptide measurements by two-compartment kinetic analysis were measured in six young normal subjects after increasing oral glucose loads of 25, 50, and 100 g and respective isoglycemic glucose infusions. The differences in B-cell secretory responses between oral and iv glucose challenges were attributed to factors other than glycemia itself (incretin effect). Both insulin and C-peptide concentrations as well as calculated integrated insulin secretion rates increased with increasing oral glucose loads. Due to the similarity in the glucose profiles after all oral loads, almost identical amounts of iv glucose (approximately 20 g) were infused in all "isoglycemic" infusion experiments, with resulting similar hormone profiles and insulin secretion rates. The percent contribution of incretin factors to total immunoreactive insulin responses after 25, 50, and 100 g glucose (85.6%, 74.9%, and 93.0%; response to oral load, 100%) was significantly higher than their contribution to integrated C-peptide responses (27.6-62.9%) or calculated integrated insulin secretion rates (19.2-61.0%). These findings indicate that the degree of incretin stimulation of insulin secretion depends on the amount of glucose ingested. A discrepancy between the estimates of the incretin effect derived from peripheral venous insulin responses, on the one hand, and C-peptide responses or calculated insulin secretion rates, on the other hand, exists. Inasmuch as peripheral insulin values reflect both insulin secretion and hepatic insulin removal, this discrepancy suggests that elimination kinetics of insulin differ between oral and iv glucose administration. This difference can be related to a significantly reduced fractional hepatic insulin extraction after oral (46.9-54.6%) compared to iv (63.4-76.5%) glucose administration when calculated by a three-compartment kinetic model. This reduction in fractional hepatic insulin extraction could be caused by gastrointestinal factors (hormones or nerves) stimulated in the course of glucose ingestion.

Valosin: isolation and characterization of a novel peptide from porcine intestine.

The isolation and primary structure of a novel gastrointestinal peptide, designated valosin, is described. The peptide was purified from porcine upper gut extracts using an HPLC and N-terminal sequence screening strategy which depends on chromatographic and structural characteristics as isolation criterion. The amino acid sequence of this peptide consists of 25 amino acid residues:

Isolation and characterization of proSS1-32, a peptide derived from the N-terminal region of porcine preprosomatostatin.

A peptide derived from the N-terminal region of porcine prosomatostatin, proSS1-32, has been purified to homogeneity from extracts of porcine upper intestine. Amino acid analysis revealed that the peptide consists of 32 residues. The complete primary structure was determined as: A P S D P R L R Q F L Q K S L A A A A G K Q E L A K Y F L A E L. This sequence obviously comprises residues 1-32 of porcine prosomatostatin since it is identical to the corresponding sequence in human preprosomatostatin. The postulated cleavage site in porcine prosomatostatin is a Leu-Leu bond between residues 32 and 33, thus confirming previous studies of the processing of the somatostatin precursor in the rat and transgenic mouse.

Glucagon-like peptide-1(7-36)amide: characterization of the domain responsible for binding to its receptor on rat insulinoma RINm5F cells.

Glucagon-like peptide-1(7-36)amide (GLP-1(7-36)amide) is a potent stimulator of insulin secretion. Receptors for this hormone have been found on different insulinoma-derived cell lines, e.g. the RINm5F cell line which is derived from a radiation-induced rat insulinoma. To characterize the part of the GLP-1(7-36)amide molecule that is responsible for binding to its receptor on RINm5F cells, binding studies with synthetic C-terminal (GLP-1(21-36)amide) and synthetic N-terminal (GLP-1(7-25] GLP-1 fragments were carried out. GLP-1(21-36)amide showed dose-dependent binding to the GLP-1(7-36)amide receptor but was approximately 1500 times less potent in inhibiting binding of 125I-labelled GLP-1(7-36)amide than the intact hormone. GLP-1(7-25) at concentrations up to 10 mumol/l did not inhibit binding of label. Neither fragment changed intracellular cyclic AMP concentrations, in contrast to GLP-1(7-36)amide which increased intracellular cyclic AMP. GLP-1(21-36)amide, however, acted as a weak partial antagonist of GLP-1(7-36)amide with respect to GLP-1(7-36)amide-dependent stimulation of cyclic AMP production.

Binding specificity and signal transduction of receptors for glucagon-like peptide-1(7-36)amide and gastric inhibitory polypeptide on RINm5F insulinoma cells.

Glucagon-like peptide-1(7-36)amide (GLP-1(7-36) amide) and gastric inhibitory polypeptide (GIP), peptides of the glucagon family, stimulate insulin secretion in vitro and in vivo. They possess high N-terminal sequence homology. Binding studies with 125I-labelled GIP and 125I-labelled GLP-1(7-36)amide were performed in RINm5F insulinoma cells to investigate receptor specificity and to compare both receptors directly. Both binding sites were highly ligand-specific: GIP did not bind to the GLP-1(7-36)amide receptor and vice versa. Both peptides increased intracellular cyclic AMP levels; GLP-1(7-36)amide was 100-fold more potent in stimulating cyclic AMP production when compared with GIP. At ranges of 1-10 nmol GLP-1(7-36)amide/l and 0.1-10 nmol GIP/l, corresponding to submaximal binding concentrations, the hormones showed an additive effect on cyclic AMP production. The N-terminal portion of GIP was important for binding, as GIP(1-30) showed almost full binding and biological activity. GIP(17-42) bound in a concentration-dependent manner with approximately 500-fold lower potency than GIP. At concentrations of up to 10 mumol GIP(17-42)/l no stimulation of cyclic AMP was observed.

Effect of somatostatin analogue (SMS 201-995, Sandostatin) on pancreatic secretion in humans.

The effect of the long-acting somatostatin analogue SMS 201-995 on exocrine pancreatic function and hormone release was investigated in a double-blind, placebo-controlled study in healthy subjects. SMS 201-995 was administered subcutaneously at a dose of 25 micrograms twice daily, and all tests were performed 30 minutes after the morning injection. Pancreatic enzyme secretion, gall bladder contraction, and cholecystokinin response after a Lundh meal were completely inhibited by SMS, while pancreatic enzyme secretion elicited by intravenous injection of secretin and pancreozymin was suppressed by 80 percent. The inhibitory effect of SMS on endogenous cholecystokinin release was fully operative on the sixth day of injection treatment, whereas the inhibitory effect on exogenous cholecystokinin injection significantly decreased after SMS administration for seven days, indicating desensitization of the end organ by somatostatin. The release of pancreatic polypeptide by a solid test meal is abolished by administration of 25 micrograms of SMS, the release of gastric inhibitory polypeptide is nearly completely suppressed, the response of insulin and C-peptide are significantly lowered, and the gastrin response is only slightly reduced.

Impairment of stimulated insulin release from the isolated perfused rat pancreas by cyclosporine pretreatment.

Cyclosporine was fed to male Wistar rats in a dose of 5, 10, or 50 mg/kg b.wt. for 7 days, and the effect on insulin secretion from the isolated perfused pancreas was investigated. Dose-dependently plasma insulin and pancreatic insulin content decreased while whole-blood CsA levels increased. An increase in blood glucose was only observed after feeding 50 mg/kg b.wt. CsA resulting in whole-blood CsA levels of 7735 ng/ml. Glucose (20 mM)-stimulated total insulin secretion (ng/50 min) was not affected during feeding 5 mg/kg b.wt. CsA, but was significantly reduced after feeding 10 or 50 mg/kg b.wt. CsA. The biphasic insulin secretion was reduced after 5 mg/kg b.wt. during the initial peak (0-10 min) but not during the second peak (10-50 min), whereas after 10 or 50 mg/kg b.wt. CsA both peaks were markedly reduced. The arginine (20 mM) and the arginine (20 mM)-plus-glucose (20 mM) stimulated insulin secretion was less affected after feeding 10 mg/kg b.wt. CsA than after stimulation with glucose (20 mM) alone. The addition of CsA to the perfusate did not influence glucose-stimulated insulin release from normal rat pancreas. Our results demonstrate a toxic effect of CsA on the pancreatic beta cell that is dose dependent and possibly influences both insulin secretion and biosynthesis.