The islet-acinar axis of the pancreas: Is there a role for glucagon or a glucagon-like peptide?

Intravenous glucagon inhibits exocrine pancreatic secretion in vivo, but exogenous glucagon does not affect exocrine secretion in vitro. Recent work, however, suggested that endogenous glucagon may be involved in the regulation of exocrine secretion even in vitro. We therefore investigated the effects of exogenous and endogenous glucagon on exocrine secretion by the isolated perfused rat pancreas in the presence of 1.8 mM glucose. Exogenous glucagon did not affect CCK-stimulated amylase output. 20 mM arginine stimulated glucagon release, but did not affect basal enzyme secretion. CCK-stimulated amylase output, however, was significantly inhibited in the presence of arginine. This inhibitory effect of arginine on exocrine pancreatic secretion could be blocked by glucagon antibodies, but not by nonspecific gammaglobulins. Thus exogenous glucagon failed to affect exocrine pancreatic secretion in vitro, but endogenously released glucagon or a glucagon-like peptide inhibited amylase release in the isolated perfused pancreas. We conclude that glucagon or a glucagon-like peptide may be a mediator in the islet-acinar axis.     

Anomeric specificity of glucose-induced somatostatin secretion

In isolated perfused rat pancreases, the alpha-anomer of D-glucose is more potent than beta-D-glucose not solely in stimulating insulin release and suppressing glucagon output, but also in causing somatostatin secretion.     

Exocrine pancreatic enzymes in cycloheximide treated rats

Summary Cycloheximide, even in a dose of 0.25 mg/kg administered s.c. to rats stimulated by pancreozymin and secretin, inhibited lipase activity in pancreatic juice. Lipase activity in serum of control animals was inhibited by cycloheximide. The secretion of trypsin and chymotrypsin was also decreased.     

Effect of long acting somatostatin-analogue, SMS 201 995, on gut hormone secretion in normal subjects

SMS 201 995 is a new long acting analogue of somatostatin. We have investigated its effect on basal and meal stimulated secretion of gut hormones and have shown that after a single s.c. injection of 50 micrograms it lowers significantly the basal plasma levels of pancreatic polypeptide, secretin, motilin, pancreatic glucagon and insulin, it also effectively suppresses the postprandial release of pancreatic polypeptide, gastrin, secretin, gastric inhibitory peptide, pancreatic glucagon and insulin. Except for the usual brief discomfort of an injection, no symptoms or untoward effects were observed.     

Effect of long acting somatostatin-analogue,SMS 201995, on gut hormone secretion in normal subjects

Summary SMS 201 995 is a new long acting analogue of somatostatin. We have investigated its effect on basal and meal stimulated secretion of gut hormones and have shown that after a single s. c. injection of 50 g it lowers significantly the basal plasma levels of pancreatic polypeptide, secretin, motilin, pancreatic glucagon and insulin, it also effectively suppresses the postprandial release of pancreatic polypeptide, gastrin, secretin, gastric inhibitory peptide, pancreatic glucagon and insulin. Except for the usual brief discomfort of an injection, no symptoms or untoward effects were observed.     

Prostaglandin analogue protects pancreatic B-cells against cyclosporin A toxicity

Cyclosporin A toxicity on pancreatic B-cells and its prevention by rioprostil, a prostaglandin E1 analogue, were studied in the model of the isolated perfused pancreas of rats treated with both compounds for 8 days. At toxic doses of cyclosporin (10 and 20 mg/kg b.wt), the B-cells showed severe hydropic degeneration of the endoplasmatic reticulum and slight degranulation of the B-cells. Accordingly, the insulin secretion was markedly impaired. Administration of rioprostil ameliorated the insulin secretion significantly, but not the ultrastructural changes. At therapeutic levels of cyclosporin (5 mg/kg b.wt), the hydropic degeneration and the drop in insulin secretion were completely prevented by rioprostil. This observation might have therapeutic implications in the treatment of patients, in particular those undergoing pancreatic transplantation.     

Prostaglandin analogue protects pancreatic B-cells against cyclosporin A toxicity

Summary Cyclosporin A toxicity on pancreatic B-cells and its prevention by rioprostil, a prostaglandin E₁ analogue, were studied in the model of the isolated perfused pancreas of rats treated with both compounds for 8 days. At toxic doses of cyclosporin (10 and 20 mg/kg b.wt), the B-cells showed severe hydropic degeneration of the endoplasmatic reticulum and slight degranulation of the B-cells. Accordingly, the insulin secretion was markedly impaired. Administration of rioprostil ameliorated the insulin secretion significantly, but not the ultrastructural changes. At therapeutic levels of cyclosporin (5 mg/kg b.wt), the hydropic degeneration and the drop in insulin secretion were completely prevented by rioprostil. This observation might have therapeutic implications in the treatment of patients, in particular those undergoing pancreatic transplantation.     

Lack of influence of vitamin D deficiency on insulin release from the isolated pancreatic islets of rats

Pancreatic islets were isolated from young (100 g) and adult (390 g), normal and vitamin D deficient male Sprague-Dawley rats. The release of insulin from leucine-stimulated or glucose-stimulated islet was not altered by vitamin D deficiency. The in vitro addition of either 25-hydroxy- or 1,25-dihydroxyvitamin-D had no effect on insulin release from either normal or vitamin D deficient islets. We conclude that the earlier report (Normal et al., Science 209 (1980) 823-825) on vitamin D deficiency depressing insulin secretion from the perfused pancreas must be related to the vitamin's effect on insulin synthesis and not the islet's release of insulin.     

Spontaneous phospholipase A activity of rat pancreatic homogenates]

Rat pancreas presents a spontaneous phospholipase A activity which appears before trypsin activation at optimal pH 6.5. The responsible enzyme is independent of pancreatic prophospholipase A, as can be seen through experiments done in the presence of trypsin inhibitors. On the other hand, this enzyme is distinct from excretory phospholipase which is more active and whose optimal pH is 8.8. Thermostability and insensibility of spontaneously active phospholipase A to DFP differentiate it from lipase, carboxyl-esterhydrolase and lysophospholipase, respectively.     

Factors influencing the intestinal phase of pancreatic exocrine secretion in the turkey

The present study was done to investigate the factors regulating the intestinal phase of exocrine pancreatic secretion in the turkey. The intestine of turkeys equipped with pancreatic fistulas was perfused with peptone solution, fat emulsion and hydrochloric acid (HCl), and pancreatic flow and protein output were measured. Neither peptone solution nor fat emulsion had any effects on pancreatic secretion. HCl enhanced the flow rate of pancreatic juice but not protein output. To clarify the neural mechanism of this phenomenon, the vagal postganglionic blocker atropine was continuously infused and pancreatic secretion in response to intestinal HCl was measured. Atropine completely suppressed both pancreatic flow and protein output. It is suggested that the avian intestinal phase of pancreatic secretion is mainly controlled by cholinergic action though HCl stimulation.     

Effect of phosphate omission on glucose-induced insulin release in vitro.

In the isolated perfused rat pancreas, omission of extracellular phosphate (H2PO-4) significantly reduces the insulin secretion in response to 16.7 mM glucose.     

Pancreastatin (33–49) enhances the priming effect of glucose in the rat pancreas

Short-term exposure to glusoe increases insulin secretion during subsequent stimulation. We investigated the effect of the new regulatory peptide pancreastatin on this priming effect of glucose in the perfused rat pancreas. Pancreastatin (33–49) at a concentration of 10^–8 M inhibited insulin release when stimulated by glucose at a concentration of 16.7 mM. However, after a second pulse of 16.7 mM glucose, pancreastatin potentiated the priming effect of glucose on insulin secretion. The modulation of insulin secretion by pancreastatin results in a potentiation of the priming effect of glucose in the rat pancreas, suggesting a role for pancreastatin in the adaptation of the B cell to glucose-stimulated insulin secretion.     

Bombesin promotes pancreatic growth in suckling rats

The pancreatic growth promoting effect of long term administration of bombesin was investigated in suckling rats. The authors showed that bombesin given in 10 micrograms/kg b.wt doses s.c. every 8 h for 10 days from the day of parturition stimulated pancreatic growth: it increased pancreatic weight, protein and DNA content, trypsin and amylase activity and trypsin/DNA ratio. Conclusion: Bombesin is an effective stimulator of pancreatic growth in suckling rats.     

Presence of "pancreatic polypeptide" cells, and gastrin immunoreactivity in immuno-induced exocrine pancreas carcinoma]

1) In electively immuno-induced carcinomas of the exocrine pancrease in Mice, where A (glucagon) and B (insulin) endocrine cells persist, cells with a pancreatic polypeptide immunoreactivity are also detected, even in late evolution stages. These cells, like D cells, containing somatostatin, are localized only in the pancreatic remains surrounding the anaplasic carcinomatous tissue: islets, adenomatous parenchyma, and ductular epithelium. Ultrastructure of these cells shows their active elaboration of numerous chracteristic secretion granules. (2) Immunocytoenzymatic detection of gastrin is negative in the exocrine and endocrine pancreatic tissues. However one of the anti-gastrin sera used gives a positive reaction, in some carinomatous cells only. Does this immunoreactivity characterize a polypeptide specific to the pancreatic carcinomatous cell?     

The role of the autonomic nervous system in mediating pancreatic endocrine responses to arginine in the calf

The release of insulin which occurred in response to arginine, in the conscious calf, differed from that which occurs in response to glucose in that it was not significantly affected by either adrenergic or muscarinic blocking agents. Release of pancreatic glucagon was reduced by pretreatment with phentolamine.     

Secretion of insulin and of glucagon by the perfused pancreas of newborn rats: effect of vasoactive intestinal peptide Vip]

A method is described for perifusion of the splenic part of the pancreas from 48-64 hour-old rat. In different basal conditions, the secretion of insulin and glucagon is stable and reproducible for 90 mn. The addition of the vasoactive intestinal peptide (VIP) to these perifusion media, at a concentration as low as 2 ng/ml, determines a remarkable increase of insulin and of glucagon secretion. These results suggest the possibility of a VIP action in the physiology of endocrine pancreas.     

Endocrine cells producing regulatory peptides

Recent data on the immunolocalization of regulatory peptides and related propeptide sequences in endocrine cells and tumors of the gastrointestinal tract, pancreas, lung, thyroid, pituitary (ACTH and opioids), adrenals and paraganglia have been revised and discussed. Gastrin, xenopsin, cholecystokinin (CCK), somatostatin, motilin, secretin, GIP (gastric inhibitory polypeptide), neurotensin, glicentin/glucagon-37 and PYY (peptide tyrosine tyrosine) are the main products of gastrointestinal endocrine cells; glucagon, CRF (corticotropin releasing factor), somatostatin, PP (pancreatic polypeptide) and GRF (growth hormone releasing factor), in addition to insulin, are produced in pancreatic islet cells; bombesin-related peptides are the main markers of pulmonary endocrine cells; calcitonin and CGRP (calcitonin gene-related peptide) occur in thyroid and extrathyroid C cells; ACTH and endorphins in anterior and intermediate lobe pituitary cells, alpha-MSH and CLIP (corticotropin-like intermediate lobe peptide) in intermediate lobe cells; met- and leu-enkephalins and related peptides in adrenal medullary and paraganglionic cells as well as in some gut (enterochromaffin) cells; NPY (neuropeptide Y) in adrenaline-type adrenal medullary cells, etc.. Both tissue-appropriate and tissue-inappropriate regulatory peptides are produced by endocrine tumours, with inappropriate peptides mostly produced by malignant tumours.     

Ontogenic development of peptide hormones in the mammalian fetal pancreas

The ontogeny of insulin, glucagon, PP and somatostatin in the mammalian fetal pancreas has been examined in recent years largely by immunocytochemistry and in some instances by radioimmunoassay. Complete ontogenic data are available only for the rat, human, pig and sheep. Figure 3 compares the time of appearance of the endocrine cell-types within the fetal pancreas when the periods of gestation of the four species are converted to a uniform scale. The striking ontogenic difference in the rat probably reflects the immaturity of the rodent fetus at birth compared with the human, pig and sheep. In the fetal pancreas, differences in cell number of glucagon and PP cells in the dorsal and ventral lobes become apparent from an early gestational period. Factors responsible for the functional and structural maturation of the fetal pancreatic endocrine cells and the processes involved in pancreatic organogenesis are poorly understood. Studies in these areas would have clinical implications since it may be possible in the future to employ agents for selective replication of fetal beta-cells for transplantation in patients with Type I diabetes, bearing in mind that such cells must have the capacity to respond to normal stimuli and repressors when transplanted. The presence of the other islet cell-types may be obligatory for these appropriate responses. This would require a more complete knowledge of those factors which produce the normal selectivity of the four hormonal cell-types.     

Dynamic insulin secretion from perifused rat pancreatic islets

Insulin secretion from isolated pancreatic islets of 8- to 12-day-old rats was investigated in a dynamic in vitro (perifusion) system. The aims of the study were (i) to describe a carefully controlled in vitro method to study the mechanism of insulin secretion and to analyse the effects and dynamic interactions of bioactive compounds on isolated rat pancreatic islets, (ii) to validate the method by comparing fundamental data on the functions of the islets obtained with this method to those collected with other techniques; and (iii) to find novel features of the control of insulin secretion. The method was carefully designed to maintain the functional capacity of the explanted cells. A functional standardization system was elaborated consisting of (i) analysis of the changes in the basal hormone secretion of the cells; (ii) evaluating responses to a standard, specific stimuli (50 mM glucose for 3 min); (iii) determining the alteration of the momentary size of the hormone pool with responses to KCl; and (iv) direct determination of the total intracellular hormone content from the extract of the column. The technique provides accurate quantitative data on the dynamic responses to biologically active compounds that act directly on the pancreatic islets. The islets maintained their full responsiveness for up to 7 days, and responses as close as in 1-min intervals could be distinguished. A linear dose-response relationship was found on the glucose-induced insulin release in case of 3-min stimulation with 4 and 500 mM of glucose (lin-log graph). Utilizing this method, we showed that no desensitization to glucose-induced insulin release can be observed if the responsiveness of the cells is properly maintained and the parameters of the stimulation are carefully designed. Exposure of the explanted islets to 10 μM acetylcholine or 30 mM arginine (Arg) induced a transitory elevation of insulin release similar in shape to that experienced after glucose stimulation. Norepinephrine (NE), dopamine (DA) and somatostatin (SS) did not induce any detectable alteration on the basal insulin secretion of the islets. However, 100 nM SS given together with 50 mM glucose, 30 mM Arg or 10 μM acetylcholine significantly reduced the insulin-releasing effect of these substances (by 75.5, 71.5 and 72.5%, respectively). At the same time, SS did not alter the insulin response of the islets to 100 mM elevation of K⁺ concentration. SS also inhibited glucose-induced insulin release in a dose-dependent way (ED₅₀ = 22 nM). A similar dose-dependent inhibitory effect on glucose-induced insulin release was found with NE (ED₅₀ = 89 nM) and DA (ED₅₀ = 2.2 μM). γ-Aminobutyric acid (GABA) did not influence insulin release under similar circumstances. Received 16 January 1998; received after revision 6 May 1998; accepted 8 May 1998     

Loss of huntingtin-associated protein 1 impairs insulin secretion from pancreatic β-cells

Hap1 was originally identified as a neuronal protein that interacts with huntingtin, the Huntington’s disease (HD) protein. Later studies revealed that Hap1 participates in intracellular trafficking in neuronal cells and that this trafficking function can be adversely affected by mutant huntingtin. Hap1 is also present in pancreatic β-cells and other endocrine cells; however, the role of Hap1 in these endocrine cells remains unknown. Using the Cre-loxP system, we generated conditional Hap1 knockout mice to selectively deplete the expression of Hap1 in mouse pancreatic β-cells. Mutant mice with Hap1 deficiency in pancreatic β-cells had impaired glucose tolerance and decreased insulin release in response to intraperitoneally injected glucose. Using cultured pancreatic β-cell lines and isolated mouse pancreatic islets, we confirmed that decreasing Hap1 could reduce glucose-mediated insulin release. Electron microscopy suggested that there was a reduced number of insulin-containing vesicles docked at the plasma membrane of pancreatic islets in Hap1 mutant mice following intraperitoneal glucose injection. Glucose treatment decreased the phosphorylation of Hap1A in cultured β-cells and in mouse pancreatic tissues. Moreover, this glucose treatment increased Hap1’s association with kinesin light chain and dynactin p150, both of which are involved in microtubule-dependent trafficking. These studies suggest that Hap1 is important for insulin release from β-cells via dephosphorylation that can regulate its intracellular trafficking function.