Isolation and characterisation of GLP-1 7-36 amide from rat intestine. Elevated levels in diabetic rats

Gut-Proglucagon-Derived Peptides Are Essential for Regulating Glucose Homeostasis in Mice

The importance of pancreatic versus intestinal-derived GLP-1 for glucose homeostasis is controversial. We detected active GLP-1 in the mouse and human pancreas, albeit at extremely low levels relative to glucagon. Accordingly, to elucidate the metabolic importance of intestinal proglucagon-derived peptides (PGDPs), we generated mice with reduction of Gcg expression within the distal (Gcg^DistalGut-/-) or entire (Gcg^Gut-/-) gut. Substantial reduction of gut Gcg expression markedly reduced circulating levels of GLP-1, and impaired glucose homeostasis, associated with increased levels of GIP, and accelerated gastric emptying. Gcg^DistalGut-/- mice similarly exhibited lower circulating GLP-1 and impaired oral glucose tolerance. Nevertheless, plasma levels of insulin remained normal following glucose administration in the absence of gut-derived GLP-1. Collectively, our findings identify the essential importance of gut-derived PGDPs for maintaining levels of circulating GLP-1, control of gastric emptying, and glucose homeostasis.

GLP-1: a mediator of the beneficial metabolic effects of bariatric surgery?

There has been increasing interest in the role that gut hormones may play in contributing to the physiological changes produced by certain bariatric procedures, such as Roux-en-Y gastric bypass and sleeve gastrectomy. Here, we review the evidence implicating one such gut hormone, glucagon-like peptide-1, as a mediator of the metabolic benefits of these two procedures.

The short-chain fatty acid acetate reduces appetite via a central homeostatic mechanism

Increased intake of dietary carbohydrate that is fermented in the colon by the microbiota has been reported to decrease body weight, although the mechanism remains unclear. Here we use in vivo(11)C-acetate and PET-CT scanning to show that colonic acetate crosses the blood-brain barrier and is taken up by the brain. Intraperitoneal acetate results in appetite suppression and hypothalamic neuronal activation patterning. We also show that acetate administration is associated with activation of acetyl-CoA carboxylase and changes in the expression profiles of regulatory neuropeptides that favour appetite suppression. Furthermore, we demonstrate through (13)C high-resolution magic-angle-spinning that (13)C acetate from fermentation of (13)C-labelled carbohydrate in the colon increases hypothalamic (13)C acetate above baseline levels. Hypothalamic (13)C acetate regionally increases the (13)C labelling of the glutamate-glutamine and GABA neuroglial cycles, with hypothalamic (13)C lactate reaching higher levels than the 'remaining brain'. These observations suggest that acetate has a direct role in central appetite regulation.

Regulation of glucose homeostasis by GLP-1

Glucagon-like peptide-1(7-36)amide (GLP-1) is a secreted peptide that acts as a key determinant of blood glucose homeostasis by virtue of its abilities to slow gastric emptying, to enhance pancreatic insulin secretion, and to suppress pancreatic glucagon secretion. GLP-1 is secreted from L cells of the gastrointestinal mucosa in response to a meal, and the blood glucose-lowering action of GLP-1 is terminated due to its enzymatic degradation by dipeptidyl-peptidase-IV (DPP-IV). Released GLP-1 activates enteric and autonomic reflexes while also circulating as an incretin hormone to control endocrine pancreas function. The GLP-1 receptor (GLP-1R) is a G protein-coupled receptor that is activated directly or indirectly by blood glucose-lowering agents currently in use for the treatment of type 2 diabetes mellitus (T2DM). These therapeutic agents include GLP-1R agonists (exenatide, liraglutide, lixisenatide, albiglutide, dulaglutide, and langlenatide) and DPP-IV inhibitors (sitagliptin, vildagliptin, saxagliptin, linagliptin, and alogliptin). Investigational agents for use in the treatment of T2DM include GPR119 and GPR40 receptor agonists that stimulate the release of GLP-1 from L cells. Summarized here is the role of GLP-1 to control blood glucose homeostasis, with special emphasis on the advantages and limitations of GLP-1-based therapeutics.

Regulation of glucose homeostasis by GLP-1

Glucagon-like peptide-1(7-36)amide (GLP-1) is a secreted peptide that acts as a key determinant of blood glucose homeostasis by virtue of its abilities to slow gastric emptying, to enhance pancreatic insulin secretion, and to suppress pancreatic glucagon secretion. GLP-1 is secreted from L cells of the gastrointestinal mucosa in response to a meal, and the blood glucose-lowering action of GLP-1 is terminated due to its enzymatic degradation by dipeptidyl-peptidase-IV (DPP-IV). Released GLP-1 activates enteric and autonomic reflexes while also circulating as an incretin hormone to control endocrine pancreas function. The GLP-1 receptor (GLP-1R) is a G protein-coupled receptor that is activated directly or indirectly by blood glucose-lowering agents currently in use for the treatment of type 2 diabetes mellitus (T2DM). These therapeutic agents include GLP-1R agonists (exenatide, liraglutide, lixisenatide, albiglutide, dulaglutide, and langlenatide) and DPP-IV inhibitors (sitagliptin, vildagliptin, saxagliptin, linagliptin, and alogliptin). Investigational agents for use in the treatment of T2DM include GPR119 and GPR40 receptor agonists that stimulate the release of GLP-1 from L cells. Summarized here is the role of GLP-1 to control blood glucose homeostasis, with special emphasis on the advantages and limitations of GLP-1-based therapeutics.

Glucagon-like peptide-1 and-2 levels in children with diabetic ketoacidosis

OBJECTIVE

The aim of this study was to investigate whether insulin deficiency and increased catabolism may have a role in the regulation of plasma glucagon-like peptide (GLP)-1 and GLP-2 levels in children with diabetic ketoacidosis (DKA) and whether insulin treatment may affect the levels of these polypeptides.

METHODS

Plasma GLP-1 and -2 levels were measured in 24 patients with DKA aged 8 to 14 years before insulin infusion (time 0), when ketonemia and acidosis disappeared (time 1), and when weight gain started (time 2). Eighteen healthy children aged 8 to 14 years constituted the control group.

RESULTS

At time 0, mean plasma GLP-1 and GLP-2 levels were significantly elevated in the patients compared with the control group (p<0.05 and p<0.01, respectively). At time 1 when ketonemia and acidosis disappeared, GLP-1 and GLP-2 levels decreased significantly from the initial levels (p<0.05 and p<0.01, respectively). At this time, while GLP-1 level was not different from that of the controls, GLP-2 level was higher than that of the controls (p<0.05). GLP-1 and-2 levels did not show any significant differences between the patients and controls when weight gain started (time 2).

CONCLUSION

Our results show that DKA is associated with increased plasma GLP-1 and -2 concentrations. Effective fluid and insulin treatment resulted in a significant decrease in plasma GLP-1 and -2 levels. This may be due to the negative feedback effect of insulin on the production of these polypeptides.

Human duodenal enteroendocrine cells: source of both incretin peptides, GLP-1 and GIP

Among the products of enteroendocrine cells are the incretins glucagon-like peptide-1 (GLP-1, secreted by L cells) and glucose-dependent insulinotropic peptide (GIP, secreted by K cells). These are key modulators of insulin secretion, glucose homeostasis, and gastric emptying. Because of the rapid early rise of GLP-1 in plasma after oral glucose, we wished to definitively establish the absence or presence of L cells, as well as the relative distribution of the incretin cell types in human duodenum. We confirmed the presence of proglucagon and pro-GIP genes, their products, and glucosensory molecules by tissue immunohistochemistry and RT-PCR of laser-captured, single duodenal cells. We also assayed plasma glucose, incretin, and insulin levels in subjects with normal glucose tolerance and type 2 diabetes for 120 min after they ingested 75 g of glucose. Subjects with normal glucose tolerance (n=14) had as many L cells (15+/-1), expressed per 1,000 gut epithelial cells, as K cells (13+/-1), with some containing both hormones (L/K cells, 5+/-1). In type 2 diabetes, the number of L and L/K cells was increased (26+/-2; P<0.001 and 9+/-1; P < 0.001, respectively). Both L and K cells contained glucokinase and glucose transporter-1, -2, and -3. Newly diagnosed type 2 diabetic subjects had increased plasma GLP-1 levels between 20 and 80 min, concurrently with rising plasma insulin levels. Significant coexpression of the main incretin peptides occurs in human duodenum. L and K cells are present in equal numbers. New onset type 2 diabetes is associated with a shift to the L phenotype.

Glucagon-like peptide 1 and gastric inhibitory polypeptide: potential applications in type 2 diabetes mellitus

Although the insulinotropic actions of gastric inhibitory polypeptide (GIP) and glucagon-like peptide 1 (GLP-1) have been known for almost 2 decades, the incretin hormones have not yet become available for clinical application. This can be explained by their unfavourable pharmacological properties. Both hormones are rapidly inactivated by the enzyme dipeptidyl peptidase IV (DPP IV), yielding biologically inactive fragments. There have been several attempts to make use of the antidiabetogenic potential of the incretin hormones. Various analogues of GLP-1 and GIP have been generated in order to achieve resistance to DPP IV degradation. The natural GLP-1 receptor agonist exendin-4, found in the saliva of the Gila monster, has a longer biological half-life after subcutaneous injection than GLP-1, and inhibition of DPP IV using, for example, pyrrolidine derivatives provides elevated concentrations of intact, biologically active GIP and GLP-1 endogenously released from the gut. A continuous intravenous infusion of native GLP-1 for a limited time may be suitable in certain clinical situations. Numerous clinical studies are currently underway to evaluate these approaches. Therefore, an antidiabetic treatment based on incretin hormones may become available within the next 5 years.

Evaluation of glycated glucagon-like peptide-1(7-36)amide in intestinal tissue of normal and diabetic animal models

Glucagon-like peptide-1(7-36)amide (tGLP-1) is an important insulin-releasing hormone of the enteroinsular axis which is secreted by endocrine L-cells of the small intestine following nutrient ingestion. The present study has evaluated tGLP-1 in the intestines of normal and diabetic animal models and estimated the proportion present in glycated form. Total immunoreactive tGLP-1 levels in the intestines of hyperglycaemic hydrocortisone-treated rats, streptozotocin-treated mice and ob/ob mice were similar to age-matched controls. Affinity chromatographic separation of glycated and non-glycated proteins in intestinal extracts followed by radioimmunoassay using a fully cross-reacting anti-serum demonstrated the presence of glycated tGLP-1 within the intestinal extracts of all control animals (approximately 19% of total tGLP-1 content). Chemically induced and spontaneous animal models of diabetes were found to possess significantly greater levels of glycated tGLP-1 than controls, corresponding to between 24--71% of the total content. These observations suggest that glycated tGLP-1 may be of physiological significance given that such N-terminal modification confers resistance to DPP IV inactivation and degradation, extending the very short half-life (<3 min) and bioactivity of the native peptide.

Elevated plasma levels of glucagon-like peptide-1 after oral glucose ingestion in patients with pancreatic diabetes

OBJECTIVE

The purpose of the present study was to evaluate plasma glucagon-like peptide-1 (GLP-1) responses after oral glucose ingestion in patients with chronic pancreatitis and to clarify how GLP-1 secretion relates to pancreatic diabetes.

METHODS

An oral glucose tolerance test (OGTT) was performed in 17 patients with chronic pancreatitis. Plasma glucose, immunoreactive insulin (IRI), C-peptide, glucagon, and GLP-1 levels at each time point during OGTT were measured. The diagnosis of chronic pancreatitis was made by the findings of endoscopic retrograde pancreatography (ERP): evident dilation of the main pancreatic duct with or without pancreatolithiasis.

RESULTS

The patients were divided into three groups according to the World Health Organization classification of diabetes based on plasma glucose levels after OGTT. The groups were: normal (three patients), impaired glucose tolerant (IGT) (six patients), and diabetic (DM) (eight patients). In the DM group, IRI and C-peptide response levels after oral glucose ingestion were significantly reduced as compared with those of the normal and IGT groups. No significant glucagon responses to oral glucose ingestion were found in the three groups. In contrast, plasma GLP-1 levels were significantly elevated after oral glucose ingestion in the DM groups as compared with normal and IGT groups.

CONCLUSIONS

The present study affords evidence that plasma GLP-1 levels become elevated with development of pancreatic diabetes, although the precise mechanism of this elevation remains undetermined.

Glucagon-like peptide-1 activates the adenylyl cyclase system in rockfish enterocytes and brain membranes

Glucagon-like peptide (GLP) exerts important physiological functions in fish liver, but extrahepatic sites of action and physiological roles have been largely ignored. We show here that GLP activates adenylyl cyclase in isolated brain and enterocyte membranes and increases cellular cyclic adenosine monophosphate (cAMP) levels in isolated enterocytes of rockfish (Sebastes caurinus). Following exposure to synthetic zebrafish GLP (zf-GLP) (1 nM-1 microM), a concentration-dependent increase in enterocyte cAMP is noted. The maximum increase in cAMP levels is observed at 1 microM zf-GLP, and represents a 30% increase above control values. Exendin-4, a GLP receptor agonist in mammals, elicits a similar concentration-dependent increase in enterocyte cAMP. In contrast, norepinephrine or prostaglandin E2 (at 1 microM) increased cAMP levels by 2 and 4-fold, respectively. Brain membrane adenylyl cyclase is activated 20-40% by zf-GLP, and to a smaller extent by zf-glucagon, while exendin-4 is as effective as zf-GLP at a dose of 100 nM. These results suggest potential physiological roles of GLP in brain and intestine in piscine systems analogous to GLP-1 functions in these tissues described for mammals.

Sympathetic pathways mediate GLP-1 actions in the gastrointestinal tract of the rat

The aim of this study was to establish the actions of GLP-1 (7-37) on gastrointestinal motility in rats. We prepared anaesthetized Sprague-Dawley rats with strain-gauges in the antrum, duodenum and the proximal jejunum and a catheter in the aorta close to the coeliac artery for close infusion of substances. Intraarterial GLP-1 infusions (3 x 10(-10) and 3 x 10(-9) moles/kg per 10 min) (n = 8) induced inhibition of spontaneous motor activity in the antrum, duodenum and proximal jejunum. Inhibition induced by GLP-1 was reversed by i.v. infusion of GLP-1 receptor antagonist, Exendin (9-39) (3 x 10(-8) moles/kg per 10 min) (n = 6). Neither the presence of L-NNA (10(-5) moles/kg) (n = 9) nor the VIP receptor antagonist [4-Cl-D-Phe6, Leu17]-VIP (3 x 10(-8) moles/kg per 10 min) (n = 8) modified responses to GLP-1. However, a combination of the adrenergic blockers phentolamine and propranolol (1 mg/kg each) (n = 8) completely blocked motor actions of GLP-1 in all the organs studied. Moreover, inhibition of gastrointestinal motor activity by GLP-1 was blocked by previous infusion of hexamethonium (10 mg/kg) (n = 4). This study demonstrates that GLP-1 inhibits gastrointestinal motor activity of the rat acting on specific GLP-1 receptors and via stimulation of adrenergic pathways.

Glucagon-like peptide-1 stimulates luteinizing hormone-releasing hormone secretion in a rodent hypothalamic neuronal cell line

To examine the influence of the putative satiety factor (GLP-1) on the hypothalamo-pituitary-gonadal axis, we used GT1-7 cells as a model of neuronal luteinizing hormone- releasing hormone (LHRH) release. GLP-1 caused a concentration-dependent increase in LHRH release from GT1-7 cells. Specific, saturable GLP-1 binding sites were demonstrated on these cells. The binding of [125I]GLP-1 was time-dependent and consistent with a single binding site (Kd = 0.07+/-0.016 nM; binding capacity = 160+/-11 fmol/mg protein). The specific GLP-1 receptor agonists, exendin-3 and exendin-4, also showed high affinity (Ki = 0.3+/-0.05 and 0.32+/-0.06 nM, respectively) as did the antagonist exendin-(9-39) (Ki = 0.98+/-0.24 nM). At concentrations that increased LHRH release, GLP-1 (0.5-10 nM) also caused an increase in intracellular cAMP in GT1-7 cells (10 nM GLP-1: 7.66+/-0.4 vs. control: 0.23+/-0.02 nmol/mg protein; P < 0.001). Intracerebroventricular injection of GLP-1 at a single concentration (10 microg) produced a prompt increase in the plasma luteinizing hormone concentration in male rats (GLP-1: 1.09+/-0.11 vs. saline: 0.69+/-0.06 ng/ml; P < 0.005). GLP-1 levels in the hypothalami of 48-h-fasted male rats showed a decrease, indicating a possible association of the satiety factor with the low luteinizing hormone levels in animals with a negative energy balance.

Intestinal growth is associated with elevated levels of glucagon-like peptide 2 in diabetic rats

Glucagon-like peptide 2 (GLP-2) has recently been identified as a novel intestinal growth factor. Because experimental diabetes is associated with bowel growth, we examined the relationship between GLP-2 and intestinal growth in rats made diabetic by streptozotocin (STZ) injection and treated with or without insulin for 3 wk. Ileal concentrations of the intestinal proglucagon-derived peptides, i.e., glicentin + oxyntomodulin, and GLPs 1 and 2, were increased by 57 +/- 20% above those of controls in untreated STZ diabetes (P < 0.05-0.001). Similar increases in plasma concentrations of glicentin + oxyntomodulin (77 +/- 15% above controls, P < 0.01) and GLP-2 (91 +/- 32% above controls, P < 0.05) were seen in untreated STZ diabetes. Both wet and dry small intestinal weight increased by 74 +/- 20% above controls (P < 0.01) in STZ diabetes, and macromolecular analysis indicated parallel increases in both protein (P < 0.001) and lipid (P < 0.05) content. Villus height (P < 0.001) and crypt depth (P < 0.01) were also increased in untreated diabetic rat intestine. Insulin therapy prevented the changes in plasma GLP-2 and intestinal mass seen in untreated STZ diabetes. Thus STZ diabetes is associated with both increased production of GLP-2 and enhanced bowel weight, thereby suggesting a role for GLP-2 in diabetes-associated bowel growth.

Identification of amidated forms of GLP-1 in rat tissues using a highly sensitive radioimmunoassay

The development of a sensitive radioimmunoassay (RIA) for C-terminally amidated forms of glucagon-like peptide-1 (GLP-1) is described. Rabbits immunized with GLP-1(7-36)amide conjugated to bovine serum albumin with glutaraldehyde produced antisera containing high-affinity antibodies directed against an epitope that included the free amidated C-terminus of the peptide. These antisera could be used in a sensitive RIA (detection limit 0.1 fmol/tube) that measured GLP-1(7-36)amide and GLP-1(1-36)amide equally. Total concentrations of amidated GLP-1 immunoreactivity in extracts of rat hypothalamus, pancreas and intestine were determined by RIA, and resolved into GLP-1(7-36)amide, GLP-1(1-36)amide and unidentified cross-reacting substances by HPLC. Whereas only GLP-1(7-36)amide could be identified in the hypothalamus, in amounts that represented 55-94% of total glucagon-like immunoreactivity (GLI), the pancreas produced chiefly GLP-1(1-36)amide, representing 0.8-3.4% of total GLI, and only trace or undetectable amounts of GLP-1(7-36)amide (0-0.36% of total GLI). This argues against any role of intrapancreatic GLP-1(7-36)amide in the secretion of insulin. In the terminal ileum total amidated GLP-1 immunoreactivity represented 27-73% of total GLI, and in five of six specimens only GLP-1(7-36)amide could be identified on HPLC, in amounts representing 13-17% of total GLI. Only one specimen of terminal ileum contained HPLC-identified GLP-1(1-36)amide (13% of total GLI) in addition to GLP-1(7-36)amide (31% of total GLI). Acid-ethanol extraction of peptide-free rat plasma with added GLP-1(7-36)amide gave recoveries of 91+/-SEM 2% in the range 20-200 pmol/l. Basal plasma amidated GLP-1 in six unanaesthetized rats was 4.1+/-1.1 pmol/l and rose to a maximum of 15.4+/-3.0 pmol/l 10 min after intragastric glucose 1 g/kg, illustrating the modest level of plasma responses of amidated forms of GLP-1.

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.

Purification and sequence of rat oxyntomodulin

Structural information about rat enteroglucagon, intestinal peptides containing the pancreatic glucagon sequence, has been based previously on cDNA, immunologic, and chromatographic data. Our interests in testing the physiological actions of synthetic enteroglucagon peptides in rats required that we identify precisely the forms present in vivo. From knowledge of the proglucagon gene sequence, we synthesized an enteroglucagon C-terminal octapeptide common to both proposed enteroglucagon forms, glicentin and oxyntomodulin, but sharing no sequence overlap with glucagon. We then developed a radioimmunoassay using antibodies raised against the octapeptide that was specific for enteroglucagon peptides without cross-reacting with glucagon. Rat intestine was extracted, and one presumptive enteroglucagon form was purified by following the enteroglucagon C-terminal octapeptide-like immunoreactivity through several HPLC purification steps. Structural characterization of the material by amino acid composition, microsequence, and mass spectral analyses identified the peptide as rat oxyntomodulin. The 37-residue peptide consists of pancreatic glucagon plus the C-terminal extension, Lys-Arg-Asn-Arg-Asn-Asn-Ile-Ala. This now permits synthesis of an unambiguous duplicate of endogenous rat oxyntomodulin for physiological studies.

Glucagon-like peptide-1, a new hormone of the entero-insular axis

The post-translational processing of proglucagon in the small intestine gives rise to glucagon-like peptide-1 (PG 78-107 amide) which has profound effects on the endocrine pancreas, and in many species also on the stomach. Glucagon-like peptide-1 (PG 78-107 amide) is secreted in man in response to physiological stimuli e.g. a mixed meal. Glucagon-like peptide-1, in concentrations corresponding to those observed in response to meals, strongly stimulates insulin secretion, in all mammals studied, even more potently than the gastric inhibitory peptide. Thus, glucagon-like peptide-1 fulfills the classic criteria for being a hormone and is likely to be a new incretin. The glucagon inhibitory effect of glucagon-like peptide-1 (PG 78-107 amide) probably further potentiates the effect of glucagon-like peptide-1 on glucose metabolism and distinguished this peptide from other intestinal peptides which have been proposed as incretins. Glucagon-like peptide-1 also inhibits gastric acid secretion and gastric emptying in man. The latter delays nutrient entry to the intestine and thereby diminishes meal-induced glucose excursions. Elevated plasma concentrations of immunoreactive glucagon-like peptide-1 have been reported in Type 2 (noninsulin-dependent) diabetic patients, however, the consequences of the elevation are not yet known. However, elevated levels of glucagon-like peptide-1 in patients with increased gastric emptying rate (post-gastrectomy syndromes) may be responsible for the exaggerated insulin secretion seen in these patients.

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.

cDNA cloning, sequence analysis and tissue distribution of a precursor for vasoactive intestinal contractor (VIC)

A full-length cDNA encoding preprovasoactive intestinal contractor (PPVIC) has been cloned. From the deduced 160 amino acid PPVIC, the mature VIC is predicted to be produced via a 37 residue intermediate, big VIC. The PPVIC also contains a VIC-like peptide of 16 amino acids structurally related to to the amino-terminal residues of VIC and flanked by pairs of dibasic amino acids, putative processing sites. RNA blot hybridization with PPVIC cDNA confirmed the PPVIC gene to be expressed in the small and large intestinal tract in a tissue specific manner.

Proglucagon-derived peptides in the neuroendocrine system

Using several novel in vitro culture systems, we have examined the tissue-specific regulation of the proglucagon-derived peptides, at the levels of proglucagon gene expression and pGdp synthesis and secretion. Our studies indicate that proglucagon gene expression in intenstine, hypothalamus and pancreas is under the regulatory control of protein kinase A- but not a protein kinase C-dependent pathway. PKA and PKC stimulate secretion of the intestinal pGdp's, whereas only PKA stimulates secretion of the hypothalamic peptides. Pancreatic glucagon secretion in response to PKA is subject to further modulation by prevailing glucose concentrations. This diversity in intracellular regulation of the pGdp's may account for some of the tissue-specific differences in synthesis and secretion of the pGdp's that we have observed in diabetes and during development.

Nucleotide sequence determination of chicken glucagon precursor cDNA. Chicken preproglucagon does not contain glucagon-like peptide II

cDNA clones coding for glucagon were isolated from a chicken pancreas cDNA library, and the nucleotide and amino acid sequences were determined. The amino acid sequence of chicken glucagon was HSQGTFTSDYSKYLDSRRAQDFVQWLMST, which was contained in the 151-amino acid long precursor, being preceded by a signal sequence and an amino-terminal peptide (NH2-peptide) and followed by an intervening peptide and a glucagon-like peptide I (GLP-I). Chicken preproglucagon, however, lacked GLP-II and intervening peptide II which have been shown to be contained in mammalian glucagon precursors.

Alterations of plasma immunoreactive glucagon-like peptide-1 behavior in non-insulin-dependent diabetics

The basal level of plasma immunoreactive glucagon-like peptide-1 (IR GLP-1) was significantly elevated in non-insulin-dependent diabetics (NIDD), and this elevation of IR GLP-1 was mainly due to an increase in the large component of IR GLP-1, corresponding to the pancreatic form. During the oral glucose-tolerance test (OGTT), the total plasma IR GLP-1 decreased in normal subjects but increased significantly in diabetic patients. Chromatographic analysis showed that IR GLP-1 consisted of several different molecular forms. OGTT caused a decrease in the pancreatic form but increased the intestinal form in normal subject, resulting into a net decrease in total plasma IR GLP-1. Whereas in NIDD the increase in the intestinal form was more prominent and the suppression of the pancreatic form was practically abolished to result in a net increase of total plasma IR GLP-1. This observation is consistent with the fact that in normal subjects the total change in IR GLP-1 was significantly correlated with both the total change of gut glucagon as well as that of pancreatic glucagon, but in diabetics the total change of GLP-1 only correlated to that of gut glucagon. The impaired suppression of pancreatic GLP-1 and enhanced release of intestinal GLP-1 could have some physiological importance in NIDD.

Characterization of glucagon-like peptide-1-(7-36)amide in the hypothalamus

The distribution of glucagon-like peptide-1-(7-36)amide like immunoreactivity (GLP-1-(7-36)NH2 IR) in rat brain was determined. Highest concentrations were found in the hypothalamus. A combination of gel chromatography and anion exchange chromatography showed that the majority of hypothalamic immunoreactivity exactly corresponded in position to synthetic GLP-1-(7-36)NH2. Chromatographic analyses of rat ileum demonstrated a similar pattern, whereas in rat pancreas mainly a large proglucagon fragment and GLP-1 were indicated. Determination of the subcellular distribution by differential centrifugation of hypothalamic tissue revealed that most of the GLP-1-(7-36)NH2 IR was present in the synaptosome fraction. GLP-1-(7-36)NH2 was released from hypothalamic tissue slices in a calcium-dependent fashion by potassium-induced depolarization. Thus GLP-1-(7-36)NH2 appears to fulfil two criteria for a neurotransmitter. No change was found in its hypothalamic content in streptozocin-induced diabetic rats compared to normal controls but a decrease was seen in hyperinsulinemic hyperglycemic KKAy mice compared to KK mice.