Priming effect of glucagon-like peptide-1 (7-36) amide, glucose-dependent insulinotropic polypeptide and cholecystokinin-8 at the isolated perfused rat pancreas.

The priming effect of glucagon-like peptide-1 (7-36) amide (GLP-1 (7-36) amide), glucose-dependent insulin-releasing polypeptide (GIP) and cholecystokinin-8 (CCK-8) on glucose-induced insulin secretion from rat pancreas was investigated. The isolated pancreas was perfused in vitro with Krebs-Ringer bicarbonate buffer containing 2.8 mmol/l glucose. After 10 min this medium was supplemented with GLP-1 (7-36) amide, GIP or CCK-8 (10, 100, 1000 pmol/l) for 10 min. After an additional 10 min period with 2.8 mmol/l glucose alone, insulin secretion was stimulated with buffer containing 10 mmol/l glucose for 44 min. In control experiments the typical biphasic insulin response to 10 mmol/l glucose occurred. Pretreatment of the pancreas with GIP augmented insulin secretion: 10 pmol/l GIP enhanced only the first phase of the secretory response to 10 mmol/l glucose; 100 and 1000 pmol/l GIP stimulated both phases of hormone secretion. After exposure to CCK-8, enhanced insulin release during the first (at 10 and 1000 pmol/l CCK-8) and the second phase (at 1000 pmol/l) was observed. Priming with 100 pmol/l GLP-1 (7-36) amide significantly amplified the first and 1000 pmol/l GLP-1 (7-36) amide both secretion periods, 10 pmol/l GLP-1 (7-36) amide had no significant effect. All three peptide hormones influenced the first, quickly arising secretory response more than the second phase. Priming with forskolin (30 mM) enhanced the secretory response to 10 mM glucose plus 0.5 nM GLP-1 (7-36) amide 4-fold. With a glucose-responsive B-cell line (HIT cells), we investigated the hypothesis that the priming effect of GLP-1 (7-36) amide is mediated by the adenylate cyclase system. Priming with either IBMX (0.1 mM) or forskolin (2.5 microM) enhanced the insulin release after a consecutive glucose stimulation (5 mM). This effect was pronounced when GLP-1 (7-36) amide (100 pM) was added during glucose stimulation. Priming capacities of intestinal peptide hormones may be involved in the regulation of postprandial insulin release. The incretin action of these hormones can probably, at least in part, be explained by these effects. The priming effect of GLP-1 (7-36) amide is most likely mediated by the adenylate cyclase system.

Reduction of the incretin effect in rats by the glucagon-like peptide 1 receptor antagonist exendin (9-39) amide.

Glucagon-like peptide 1 (7-37)/(7-36) amide (GLP-1) is derived from the intestinal proglucagon processing. It is considered an important insulin-releasing gut hormone. This study uses exendin (9-39) amide as a GLP-1 receptor antagonist to evaluate the contribution of GLP-1 to the incretin effect. Anesthetized rats were challenged by an intraduodenal glucose infusion to evaluate maximally occurring GLP-1 and gastric inhibitory polypeptide (GIP) plasma levels. Maximal immunoreactive (IR) GLP-1 plasma levels amounted to 10 pmol/l (IR-GIP 11 pmol/l). Exendin (9-39) amide abolished the insulin-stimulatory effect of 60 pmol of GLP-1 or of the GLP-1 agonist exendin-4 (0.5 nmol) injected as bolus, respectively. An intravenous bolus injection of 5.94 nmol of exendin (9-39) amide 3 min before enteral glucose infusion grossly reduced the total insulin secretory response (by 60%) and significantly increased circulating blood glucose levels (P < 0.05). In contrast, the GLP-1 antagonist left the insulin response after an intravenous glucose or glucose plus GIP (60 pmol) load unaltered. Our data support the concept that GLP-1 is an important incretin factor. Exendin (9-39) amide is a useful GLP-1 antagonist for in vivo studies.

Insulinotropic glucagonlike peptide-I(7-37)/(7-36)amide A new incretin hormone.

Intestinal peptide hormones, released into circulation in response to a meal, are important augmentors of the postprandial insulin release stimulated by absorbed nutrients. In this "Incretin concept" glucagon-like peptide I(7-37) plays a major role because it is the most powerful glucose-dependent insulin secretagogue described so far. Glucagonlike peptide I(7-37) also stimulates proinsulin gene expression and proinsulin biosynthesis in insulinoma cells, and it may be involved in the regulation of the intracellular insulin pool of the B cell. Recent studies show elevated levels of glucagonlike peptide I(7-37) in patients with non-insulin-dependent diabetes mellitus. The physiology and pathophysiology of the enteroinsular axis is a promising field of basic and clinical research that has a relevance to diabetes mellitus.

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.

Insulinotropic hormone glucagon-like peptide-I(7-37) stimulation of proinsulin gene expression and proinsulin biosynthesis in insulinoma beta TC-1 cells.

Glucagon-like peptide-I(7-37) [GLP-I(7-37)] is an intestinal peptide hormone that is released in response to oral nutrients and that potently augments glucose-mediated insulin secretion. GLP-I(7-37) has potent insulin-releasing activities in vivo in response to oral nutrients, in situ in the isolated perfused pancreas, and in vitro in cultured pancreatic B-cells. As such GLP-I(7-37) is a potent hormonal mediator in the enteroinsular axis involved in the regulation of glucose homeostasis. We now show that in addition to stimulating the release of insulin, GLP-I(7-37) stimulates proinsulin gene expression at the levels of gene transcription and cellular levels of proinsulin messenger RNA as well as the translational biosynthesis of proinsulin. These findings of the positive anabolic actions of GLP-I(7-37) on the synthesis of insulin in B-cells support the notion that GLP-I(7-37) may be of therapeutic use in stimulating the production of insulin in patients with noninsulin-dependent diabetes mellitus and that overproduction of insulin with subsequent hypoglycemia will not occur in response to the administration of GLP-I(7-37). Furthermore, these positive actions of GLP-I(7-37) on insulin production obviate the possibility of B-cell exhaustion in response to such a potent secretagogue.

Galanin inhibits proinsulin gene expression stimulated by the insulinotropic hormone glucagon-like peptide-I(7-37) in mouse insulinoma beta TC-1 cells.

The neuropeptide hormone galanin, released by sympathetic stimulation of nerve terminals in the endocrine pancreas, inhibits insulin secretion via a receptor-linked pertussis toxin-sensitive (Gi) transmembrane signaling pathway. Glucagon-like peptide-I(7-37) [GLP-I(7-37)] is an intestinal hormone shown to have potent insulin-releasing activities in pancreatic B-cells and is believed to serve a physiological role in the augmentation of nutrient-induced insulin release. GLP-I(7-37) binds to specific Gs- and adenylate cyclase-coupled receptors on pancreatic B-cells and directly stimulates proinsulin gene transcription, thereby increasing cellular levels of proinsulin messenger RNA (mRNA) and proinsulin biosynthesis. This study examines the effects of galanin on GLP-I(7-37)-stimulated proinsulin gene expression in mouse beta TC1 cells. The degree of proinsulin gene transcription was assessed by measuring the activity of chloramphenicol acetyl transferase (CAT) expressed from a CAT reporter plasmid linked to the rat insulin-1 gene promoter transferred to beta TC1 cells and by measuring proinsulin mRNA levels by Northern blot analysis. Galanin inhibited both CAT activity and the rise in proinsulin mRNA levels stimulated by either GLP-I(7-37) or forskolin (0.1 microM). Notably, galanin was without effect on CAT activity induced by the cAMP analog, 8-bromo-cAMP, the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine, or higher concentrations of forskolin. The inhibitory effects of galanin on GLP-I(7-37) and forskolin-induced CAT activity were reversed by the addition of pertussis toxin, a toxin that inactivates inhibitory G-proteins (Gi). We conclude that galanin inhibits GLP-I(7-37)-stimulated proinsulin gene expression by inhibiting the activation of adenylate cyclase by GLP-I(7-37) and subsequently the production of cAMP in B-cells. Further, our data suggest that these actions of galanin are mediated by a pertussis toxin sensitive pathway involving one or more Gis that inhibit adenylate cyclase. Thus, in addition to its well known inhibitory effects on insulin secretion galanin can inhibit proinsulin gene expression stimulated by GLP-I(7-37) activation of the cAMP signaling pathway. These findings may be a unique demonstration of the inhibition of proinsulin gene expression by a substance (galanin) released endogenously within the pancreas.

Functional receptors for the insulinotropic hormone glucagon-like peptide-I(7-37) on a somatostatin secreting cell line.

Glucagon-like peptide-I(7-37) [(GLP-I(7-37)] is an intestinal peptide hormone that has potent insulinotropic activities in vivo in response to oral nutrients, in the isolated perfused pancreas, and in vitro in cultured B cells. GLP-I(7-37) receptor binding and GLP-I(7-37)-induced cAMP generation and hormone secretion was studied using cell lines producing insulin/B cell (beta TC-1), glucagon/A cell (INR1G9) and somatostatin/D cell (RIN 1027-B2). [125I]GLP-I(7-37) bound specifically to both B and D cells but not to A cells. GLP-I(7-37) induced cAMP-formation in B and D cells with a maximum response at 10 nmol/l (B cells) or at 100 nmol/l (D cells). Insulin secretion from perifused B cells was stimulated by GLP-I(7-37) (maximum at 10 nmol/l) and 10 nmol/l GLP-I(7-37) released somatostatin from perifused D cells. GLP-I(7-37) did not influence cAMP or glucagon secretion from A cells. These data indicate that pancreatic B and D cells, but not the A cells are influenced directly by GLP-I(7-37) via binding to specific receptors. Our findings support a model of physiologic regulation of insulin secretion whereby GLP-I(7-37) released from the intestine in response to oral nutrients potently stimulates insulin secretion via an endocrine mechanism that in turn may be dampened by a feed-back suppression by the release of somatostatin. In addition, suppression of the secretion of glucagon, a hormone whose actions are counter-regulatory to those of insulin, may occur by paracrine mechanisms involving GLP-I(7-37)-mediated stimulation of both insulin and somatostatin secretion.

Cosecretion of amylin and insulin from isolated rat pancreas.

Amylin, a 37 amino acid C-terminal amidated peptide is an integral part of secretory granules of pancreatic beta-cells. Utilizing a specific radioimmunoassay system we demonstrate in the present study a cosecretion of amylin and insulin from the isolated rat pancreas. The secretion pattern of both peptides during glucose or glucose plus arginine stimulation is identical. The molar ratio of amylin amounts to 10% of that of insulin. The biological significance of amylin is still unknown, but a paracrine/endocrine role in glucose homeostasis is speculated.

Ras antagonizes cAMP stimulated glucagon gene transcription in pancreatic islet cell lines.

Ras, a GTP-binding protein, converts membrane tyrosine kinase signalling to changes in gene expression patterns. Utilising a rat glucagon promoter-CAT construct (p[-1.1]GLU-CAT) we demonstrate in transient transfection experiments that the oncogenic Ras inhibits cAMP-dependent activation of p[-1.1]GLU-CAT in both glucagonoma InR1-G9 and insulinoma beta-TC1 cells. Conversely, the expression of a dominant negative mutant of Ras enhances the cAMP-induced activation of p[-1.1]GLU-CAT transcription in these cells. Our data suggests a functional interference of Ras with the cAMP-dependent transcription of the glucagon gene.

Expression of the ras-related rab3a gene in insulinoma-derived cell lines.

This study was designed to search for the expression of the small-molecular-weight GTP-binding protein rab3a in endocrine pancreatic cell lines. Total RNA was isolated from five different cell lines (RINm5F, RIN 104836, beta-TC1, HIT-15, and INRI-G9) and from whole rat brain. The expression of rab3a was analyzed by Northern blots. Similar as in brain two transcripts of 1300 and 1800 bp were detected in RIN-cells at low stringency conditions with the predominant signal at 1300 bp. At high stringency the stronger signal was at 1800 bp. When a 300 bp PstI fragment derived from the coding region of rab3a was utilized as probe the 1800 bp signal was predominant under each condition. Only a faint band at 1800 bp occurred in preparations from beta TC1-cells and no signal at all was found in HIT-15 and INRI-G9-cells. In conclusion, rab3a is expressed in rat insulin-releasing insulinoma-derived RIN-cells with a specific 1800 bp transcription product.

Synergistic stimulatory effect of glucagon-like peptide-1 (7-36) amide and glucose-dependent insulin-releasing polypeptide on the endocrine rat pancreas.

The interaction of glucagon-like peptide-1 (7-36)amide (GLP-1) and glucose-dependent insulin-releasing polypeptide (GIP) on insulin release from the perfused rat pancreas was studied. The GLP-1 stimulated (0.5 nmol/l), glucose-induced (6.7 mmol/l) insulin secretory answer was enhanced by GIP (0.1, 1.0 and 10.0 nmol/l) to the arterial perfusate. This effect was maximal at 1 nmol/l GIP and smaller but still significant at 0.1 nmol/l GIP. The high GIP concentration of 10 nmol/l GIP did not further increase insulin secretion compared to the stimulation by 1 nmol/l GIP. Our data demonstrate an additive synergistic effect of GLP-1 and GIP on the glucose-induced insulin release. This supports the concept of an action "in concert' of gastrointestinal incretin hormones postprandially released on the endocrine pancreas to guarantee adequate insulin answers after meals.

Molecular cloning, functional expression, and signal transduction of the GIP-receptor cloned from a human insulinoma.

Glucose-dependent insulinotropic polypeptide (GIP) plays an important role in the regulation of postprandial insulin secretion and proinsulin gene expression of pancreatic beta-cells. This study demonstrates the molecular cloning of a cDNA for the GIP-receptor from a human insulinoma lambda gt11 cDNA library. The cloned cDNA encoded a seven transmembrane domain protein of 466 amino acids which showed high homology (41%) to the human glucagon-like peptide 1 (GLP-1) receptor. Homology to the GIP receptor from rat or hamster was 79% and 81%, respectively. When transfected stably into fibroblast CHL-cells a high affinity receptor was expressed which coupled to the adenylate cyclase with normal basal cAMP and increasing intracellular cAMP levels under stimulation with human GIP-1-42 (EC50 = 1.29 x 10(-13) M). The receptor accepted only human GIP 1-42 (Kd = 1.93 +/- 0.2 x 10(-8) M) and porcine truncated GIP 1-30 (Kd = 1.13 +/- 0.1 x 10(-8) M) as high affinity ligands. At 1 microM, exendin-4 and (9-39)amide weakly reduced GIP-binding (25%) whereas secretin, glucagon, glucagon-like peptide-1, vasoactive intestinal polypeptide, peptide histidine-isoleucine, and pituitary adenylyl cyclase activating peptide were without effect. In transfected CHL cells, GIP-1-42 did not increase intracellular calcium. Northern analysis revealed one transcript of human GIP receptor mRNA with an apparent size of 5.5 kb. The exact understanding of GIP receptor regulation and signal transduction will aid in the understanding of the incretin hormone's failure to exert its biological action at the pancreatic B-cell in type II diabetes mellitus.

Characterisation of the expression and post-translational processing of the preprotachykinin-I gene and the regulated release of tachykinins by the RINm5F cell-line.

Normal transcription and postranslational processing of the preprotachykinin (PPT)-I gene and regulated release of substance P and neurokinin A by the rat pancreatic endocrine cell-line, RINm5F, has been demonstrated, using radioimmunoassays (RIAs), reversed-phase (rp)HPLC and Northern blot analysis. This is the first stable cell-line found to express the PPT-I gene and provides an opportunity for investigating PPT-I gene expression and tachykinin biosynthesis. RIN5mF cells are a model for the pancreatic beta-cell, which is not known to exhibit PPT-I gene expression which may, therefore, be a feature of the transformed state of these cells. These data may imply that the tachykinins are important in pancreatic islet embryogenesis.

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.

Glucagon-like peptide-1(7-37)/(7-36)amide is a new incretin.

Glucagon-like peptide-1 (GLP-1) is the main product of the intestinal processing of proglucagon. It is released from the intestinal K-cells into the circulation in response to the oral ingestion of food. At the pancreatic beta cell GLP-1 is a potent insulin secretagogue in the presence of elevated glucose levels, defining glucagon-like peptide-1 as a new incretin. Its action is mediated by specific receptors coupled to the adenylate cyclase system by a stimulatory G-protein. Finally, glucagon-like peptide-1 stimulates proinsulin gene expression and it is thus involved at several levels in the regulation of insulin synthesis and secretion.

Interaction of glucagon-like peptide-I (7-37) and somatostatin-14 on signal transduction and proinsulin gene expression in beta TC-1 cells.

The interactions of glucagon-like peptide-I(7-37)/(7-36)amide (GLP-I) and somatostatin-14 were characterized on the cyclic adenosine monophosphate (cAMP)-dependent signal transduction pathway and on proinsulin gene expression using mouse insulinoma beta TC-1 cells. GLP-I stimulated the activity of adenylate cyclase maximally at 1 mumol/L (151%). This effect was inhibited by 1 mumol/L somatostatin (119%). Forskolin also stimulated adenylate cyclase activity (10 mumol/L forskolin, 265%), and this action was inhibited by somatostatin (220%). Somatostatin alone left the basal adenylate cyclase activity unaltered. Somatostatin reduced the GLP-I-stimulated increase of intracellular cAMP levels (100 nmol/L GLP-I, 141%; 100 nmol/L GLP-I + 1 mumol/L somatostatin, 110%). GLP-I stimulated concentration-dependently the activity of protein kinase A (PKA), with a maximum at 10 nmol/L (181%). This action was inhibited by 100 nmol/L somatostatin (118%), but somatostatin did not influence the basal PKA activity. Furthermore, somatostatin reduced the GLP-I-induced stimulation of proinsulin gene expression (10 nmol/L GLP-I, 176%; 10 nmol/L GLP-I + 1 mumol/L somatostatin, 77%). Somatostatin itself inhibited concentration-dependently proinsulin gene expression (1 mumol/L somatostatin, 53%). These data demonstrate that GLP-I increases the activities of both adenylate cyclase and cAMP-dependent PKA, whereas somatostatin counteracts the stimulatory effect of GLP-I on adenylate cyclase activity, cAMP generation, PKA activity, and proinsulin gene expression. The interaction of both hormones occurs at the level of adenylate cyclase. Therefore, the interaction of both peptide hormones regulates downstream events, including gene expression.

Functional active receptors for insulin-like growth factors-I (IGF-I) and IGF-II on insulin-, glucagon-, and somatostatin-producing cells.

Insulin-like growth factors I and II (IGF-I and IGF-II) are expressed at high levels in the endocrine pancreas during development and tissue regeneration. However, their effects at the endocrine pancreas are poorly understood. We searched for receptors of IGF-I and IGF-II and possible biological effects on clonal insulin-secreting (HIT), glucagon-secreting (INR1G9), and somatostatin-secreting (RIN 1027 B2) cell lines. Our data showed that HIT cells and RIN 1027 B2 cells express specific type I and type 11 IGF receptors. INR1G9 cells possess type II IGF receptors and IGF-I binding sites with the same affinity for both IGF-I and IGF-II. In HIT cells, insulin secretion was not influenced by either peptide. Proinsulin gene transcription was stimulated by IGF-II but not by IGF-I. IGF-I potently inhibited proglucagon gene transcription and glucagon secretion in INR1G9 cells, whereas IGF-II only inhibited glucagon release. In RIN 1027 B2 cells, IGF-I but not IGF-II increased somatostatin output, whereas both stimulated somatostatin gene expression. These data demonstrate the presence of classic type I and type II IGF receptors on insulin-, glucagon-, and somatostatin-secreting cells. Both peptides may be important regulators of endocrine pancreatic function in terms of islet hormone release and gene expression. Therefore, both peptides may be involved in the regulation of intraislet cellular homeostasis.

Incretin hormone expression in the gut of diabetic mice and rats.

To elucidate the question of whether production of the insulinotropic gut hormones glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) is altered by a diabetic metabolic state, their intestinal expression pattern was evaluated. Two rodent models for diabetes mellitus were used, non-obese diabetic (NOD) mice as a model for insulin-dependent diabetes and Zucker diabetic fatty (ZDF) rats for non-insulin-dependent diabetes mellitus (NIDDM). Expression of both incretin hormones followed typical patterns, which were similar in both animals and unaltered by the diabetic state. The GIP gene was greatly expressed in the duodenum, jejunum, and ileum, with a continuous decrease from the upper to lower intestines. This pattern was observed in both NOD mice and ZDF rats regardless of the diabetic state. This expression data was corroborated by radioimmunoassay (RIA) analysis of the gene product GIP. Expression of the proglucagon gene encoding GLP-1 had an opposite appearance. The highest expression was seen in the large bowel and the ileum. RIA analysis of the gene product GLP-1 mirrored these data. Although the distribution pattern was similar in both animal models, in contrast to diabetic NOD mice, a regulated expression was found in diabetic ZDF rats. Compared with lean nondiabetic controls, fatty hyperglycemic animals showed an increased expression of the proglucagon gene in the colon and a concomitant reduction in the small intestine. This was mirrored by the GLP-1 content of the colon and ileum. Overall, basal GLP-1 plasma levels were increased in ZDF rats (17.0 +/- 2.8 pmol) compared with lean Zucker rats (12.4 +/- 1.8 pmol). In conclusion, incretin hormone expression (GIP and GLP-1) follows specific patterns throughout the gut and is unaltered by the diabetic state. In ZDF rats, regulation of proglucagon expression occurs mainly in the large intestine.

Characterization of GIP(1-30) and GIP(1-42) as stimulators of proinsulin gene transcription.

Originally characterized in terms of its gastric acid inhibitory properties, GIP (gastric inhibitory polypeptide) expressed in the upper small intestine, was subsequently shown to exert strong glucose-dependent insulin-releasing properties. This action is generally attributed to GIP(1-42) and, so far, no evidence for the contribution of other relevant GIP forms exists. In this study, we compared the effects of GIP(1-42) and C-terminally truncated GIP(1-30) on cAMP production and proinsulin gene transcription at clonal insulin-secreting cell lines (RIN 1046-38, beta TC-3). Both peptides were equally potent stimulators of cAMP generation in both cell lines. Insulin release from RIN 1046-38 cells stimulated by both GIP forms was identical. In both B-cell lines GIP(1-42) and GIP(1-30) stimulated proinsulin gene expression equipotently. GIP not only enhances insulin secretion but also insulin gene expression and, therefore, it is a true insulinotropic hormone.

Effects of VIP and related peptides on airway mucus secretion from isolated rat trachea.

Vasoactive intestinal polypeptide (VIP) is known as an important regulator of airway function. It has been suggested that VIP is involved in the pathogenesis of asthma due to its relaxant effects on smooth muscles. The present study was designed to characterize the effects of the peptides of the VIP family on airway mucus secretion. The peptides VIP, PHI, PACAP-27, PACAP-38, GLP-I, exendin-4, helodermin, helospectin I and helospectin II were investigated using isolated rat trachea. Data show that PACAP-27 is the most potent stimulator of airway mucus secretion (225% stimulation). The rank order of potency was PACAP-27 > VIP > helospectin II > PHI > exendin-4 = helodermin = helospectin I = PACAP-38. The addition of the protease inhibitor thiorphan enhanced the effects of PHI and helodermin, but not of the other peptides. These data show that the peptides of the VIP family stimulate airway mucus secretion differently.