Effect of glucagon on digestive enzyme synthesis, transport and secretion in mouse pancreatic acinar cells

The islet-acinar axis of the pancreas: more than just insulin

Although the role of the islets in the regulation of acinar cell function seemed a mystery to investigators who observed their dispersion among pancreatic acini, over time an appreciation for this intricate and unique structural arrangement has developed. The last three decades have witnessed a steadily growing understanding of the interrelationship of the endocrine and the exocrine pancreas. The islet innervation and vascular anatomy have been more fully characterized and provide an appropriate background for our current understanding. The interrelationship between the endocrine and exocrine pancreas is mediated by islet-derived hormones such as insulin and somatostatin, other humoral factors including pancreastatin and ghrelin, and also neurotransmitters (nitric oxide, peptide YY, substance P, and galanin) released by the nerves innervating the pancreas. Although considerable progress has been achieved, further work is required to fully delineate the complex interplay of the numerous mechanisms involved. This review aims to provide a comprehensive update of the current literature available, bringing together data gleaned from studies addressing the actions of individual hormones, humoral factors, and neurotransmitters on the regulation of amylase secretion from the acinar cell. This comprehensive view of the islet-acinar axis of the pancreas while acknowledging the dominant role played by insulin and somatostatin on exocrine secretion sheds light on the influence of the various neuropeptides on amylase secretion.

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.

Dual actions of glucagon: direct stimulation and indirect inhibition of dog pancreatic secretion

The secretory actions of glucagon on the exocrine pancreas were examined using two kinds of canine preparations. In the isolated and blood-perfused dog pancreas with venous drainage, i.a. injection of glucagon did not inhibit secretin/cholecystokinin-octapeptide (CCK-8)-stimulated pancreatic secretion, but instead dose dependently enhanced both basal and stimulated pancreatic secretion. Glucagon-induced increase of pancreatic secretion was potentiated by 3-isobutyl-1-methylxanthine. In contrast, i.v. bolus injection of glucagon (3 and 10 nmol/kg) first augmented transiently then suppressed secretin/CCK-8-stimulated pancreatic secretion while simultaneously increasing circulating plasma somatostatin immunoreactivity from 14.2 to 214 fmol/ml in anesthetized intact dogs. The inhibition of secretin/CCK-8-stimulated pancreatic secretion and elevation of plasma somatostatin immunoreactivity induced by glucagon were comparable with those due to somatostatin-14. Thus, these results indicate that glucagon stimulates pancreatic secretion directly; the inhibitory action of glucagon is indirect and appears to be related to a rise in the circulating level of somatostatin immunoreactivity.

Effect of neurotensin on amylase secretion from rat pancreas in vivo and in vitro

The effects of neurotensin, a tridecapeptide localized mainly in the gut, on pancreatic secretion are controversial. It is not clear whether the effects are mediated by direct or indirect effects. The present study was done to determine the effect of threshold doses of neurotensin in vivo in the rat as well as in vitro to clarify whether the action is direct or indirect. For in vivo studies the pancreatic duct of rats was cannulated and threshold doses of neurotensin and CCK-8 alone or in combination were infused as an intravenous bolus and pancreatic juice collected at 10-min intervals for 1 hr. For in vitro studies dissociated lobules and acini were incubated for 1 hr with 10(-11) -10(-6) M neurotensin with or without 10(-9) M CCK-8 (maximally effective dose). Amylase secretion in the pancreatic juice or in the medium was determined. Neurotensin and CCK-8 at a dose of 50 pM/kg increased pancreatic protein secretion in vivo. Neurotensin in combination with CCK-8 did not potentiate protein secretion. In the pancreatic lobules and acini, neurotensin 10(-11) -10(-6) M alone or in combination with CCK-8 (10(-9) M) did not stimulate amylase secretion above the basal level or that stimulated with CCK-8. The combined data indicate that the effect of neurotensin is not mediated by a direct action on the pancreatic acinar cells.

Amylase secretion from isolated pure acinar cells

Isolation of pure acinar cells of the rat pancreas was achieved employing counterflow sedimentation filtration technique (CSFT). The preparation of purified acinar cells contained an occasional red blood cell (RBC, 200:1) with total absence of endocrine and duct cells. A significant stimulation of amylase secretion from isolated pure acinar cells was produced by octapeptide of cholecystokinin (CCK8) and insulin produced potentiation of the effect of CCK8. Synthetic glucagon inhibited basal and CCK8 stimulated amylase secretion. Non-synthetic purified glucagon stimulated amylase secretion and potentiated the effect of CCK8. Vasoactive intestinal polypeptide (VIP) did not stimulate amylase secretion but potentiated the effect of CCK8. No leakage of lactic dehydrogenase (LDH) was detected from the cells in any of the secretion studies. Thus a highly purified preparation of isolated pure acinar cells of rat pancreas could be obtained with excellent morphologic and functional integrity.

Effect of chronic ethanol feeding on pancreatic enzyme secretion in rats in vitro

The effect of long-term ethanol intake on pancreatic digestive enzyme secretion was determined in Sprague-Dawley rats. Weight-matched triplets were fed Lieber-DeCarli diet containing 5% w/v concentration of ethanol, isocaloric amounts of Lieber-DeCarli diet, or rat chow ad libitum for 6, 12, and 18 months. Basal and bethanechol-stimulated secretion of amylase, lipase, trypsinogen, chymotrypsinogen, and pancreatic secretory trypsin inhibitor (PSTI) was determined. In the ethanol-fed group, basal secretion of trypsinogen and chymotrypsinogen was increased at 6, 12, and 18 months. In addition, basal secretion of amylase and lipase was increased and that of PSTI was decreased at 12 and 18 months. Secretion of PSTI was stimulated by bethanechol (10(-4)M), whereas the secretion of digestive enzymes was not stimulated in the ethanol-fed versus two control groups. At 12 months the dose-response curve of amylase and lipase secretion was shifted upwards in the ethanol-fed group with increase in ED50. These data are suggestive of membrane perturbations leading to increased basal secretion and a subsensitivity of the cholinergic receptors in the ethanol-fed group. Increased basal secretion of proteases in the presence of diminished trypsin inhibitor indicates that premature activation of proenzymes could occur resulting in pancreatitis.

Role of cyclic adenosine monophosphate in amylase release from dissociated rat pancreatic acini

1. The effect of octapeptide of cholecystokinin-pancreozymin (CCK(8)), bethanechol, cholera toxin, glucagon and vasoactive intestinal polypeptide (VIP) on amylase secretion and lactic dehydrogenase (LDH) release from isolated rat pancreatic acini was studied.2. In isolated rat pancreatic acini, in the absence of theophylline in the medium, amylase secretion was increased by 65-78% with 10(-7) and 10(-6) M-cholera toxin. In the presence of theophylline, amylase secretion was increased by 43-56% with 10(-7) and 10(-6) M-cholera toxin following a 90 min incubation. No effect was observed in the presence of theophylline at 30 and 60 min. The effect of cholera toxin was potentiated by CCK(8) at 60 and 90 min.3. In the absence of theophylline in the medium, amylase secretion was increased by 81-118% with 10(-5) and 10(-4) M-glucagon and 86% with 10(-6) M-VIP at 60 min. In the presence of theophylline in the medium, amylase secretion was increased by 53-246% with 10(-9) to 10(-6) M-glucagon and 111-158% with 10(-7) and 10(-6) M-VIP respectively. The effect of glucagon and VIP was potentiated by CCK(8).4. Potentiation of the rate of amylase release due to glucagon (10(-5) M) and VIP (10(-6) M) occurred during the first 15 min of incubation.5. Release of LDH was not increased by any of these agents.6. It is concluded that cyclic AMP rise (due to cholera toxin, glucagon and VIP effect) increased amylase secretion from rat pancreatic acinar cells. This effect is less marked than in the guinea-pig pancreas and is potentiated by agents mobilizing cellular Ca(2+) (CCK(8) and bethanechol).7. These data indicate species-specific variation in the action of cyclic AMP in the pancreas.

Contacts between endocrine and exocrine cells in the pancreas

Close contacts between exocrine and endocrine cells were observed in human and rat pancreas. The presence of junctional specializations, including desmosomes, tight and gap junctions, as well as interdigitations between endocrine and exocrine cells, implies that these cells are structurally and functionally associated.