Pancreatic polypeptide and peptide YY inhibit the denervated canine pancreas

Overview of exocrine pancreatic pathobiology

Exocrine pancreas is a source of several enzymes that are essential for the digestive process. The exocrine pancreatic secretion is tightly regulated by the neuroendocrine system. The endocrine pancreas is tightly integrated anatomically and physiologically with the exocrine pancreas and modulates its function. Compound-induced pancreatitis is not a common event in toxicology or drug development, but it becomes a significant liability when encountered. Understanding the species-specific differences in physiology is essential to understand the underlying pathobiology of pancreatic disease in animal models and its relevance to human disease. This review will mainly focus on understanding the morphology and physiology of the pancreas, unique islet-exocrine interactions, and pancreatitis.

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.

Insulin inhibits secretin-induced pancreatic bicarbonate output via cholinergic mechanisms

INTRODUCTION

Exogenous insulin inhibits secretin-stimulated pancreatic bicarbonate output via a dose-dependent mechanism; this effect is prevented by pancreatic denervation.

AIMS

To investigate possible cholinergic mediation, we examined the effect of bethanechol on secretin-stimulated pancreatic secretion during euglycemic, hyperinsulinemic clamp.

METHODOLOGY

In four dogs with chronic pancreatic fistulas, euglycemic, hyperinsulinemic clamp (1.25 mU/kg/min) was begun after a 30-minute basal period; computer-assisted glucose administration maintained euglycemia. Control studies were performed with volume-matched and rate-matched vehicle infusion. After 1 hour, secretin infusion was begun at a dosage of 16 ng/kg/h; the dose was doubled every 30 minutes. The studies were then repeated during background bethanechol infusion (90 microg/kg/h) begun 30 minutes after clamp initiation. Pancreatic juice was analyzed for bicarbonate and protein; serum samples were analyzed for glucose and insulin.

RESULTS

Exocrine outputs and serum glucose and insulin levels did not differ in the basal period. Insulin levels were significantly elevated during the euglycemic, hyperinsulinemic clamp (62 microU/mL versus 12 microU/mL; p < 0.01); glucose levels did not differ. As before, secretin-induced bicarbonate output was inhibited by euglycemic, hyperinsulinemic clamp. The inhibitory effect of insulin was reversed by bethanechol. Despite the euglycemic, hyperinsulinemic clamp, secretin-induced bicarbonate output was potentiated by bethanechol at higher secretin doses (p < 0.05).

CONCLUSION

These data confirm that cholinergic mechanisms mediate insulin's inhibition of secretin-induced pancreatic bicarbonate output.

Feedback regulation of pancreatic secretion by peptide YY

The present status of our understanding of the feedback regulation of pancreatic secretion by peptide YY (PYY) released from the distal intestine is reviewed. Exocrine pancreatic secretion is primarily controlled by the cephalic (the vagus nerve), gastric (acid and pepsin secretion, and nutrients delivered into the duodenum by gastric emptying), and intestinal (secretin and CCK) mechanisms. PYY acts on the multiple sites in the brain and gut, and inhibits pancreatic secretion by regulating these primary control mechanisms. The involvement of Y(1) and Y(2) receptors has been suggested in the regulation of pancreatic secretion. However, it remains to be studied which site of action or receptor subtype is physiologically most important for this regulation.

The effect of selective sympathetic denervation on pancreatic exocrine secretion

The purpose of this study was to investigate the influence of selective sympathetic denervation of the rat pancreas on exocrine secretion and to study whether the observed effects were due to pancreatic trophism. Sprague-Dawley rats were divided into two groups. One group underwent selective sympathetic denervation by skeletonizing the superior and inferior pancreaticoduodenal and splenic arteries. The other group underwent simple laparotomy and served as controls. One week after the operation a catheter was introduced into the bile-pancreatic duct and pancreatic juice was collected at 30-min intervals for 4 h. The output of bicarbonate, total protein, amylase, trypsin, chymotrypsin, lipase, colipase and carboxyesterlipase were determined. Following denervation secretion of pancreatic enzymes was significantly enhanced compared with sham-operated animals. We did not find any signs of pancreatic trophism 1 week after denervation.

Suppressive role of the islet-acinar axis in the perfused rat pancreas

BACKGROUND

The stimulating effects of insulin on the exocrine pancreas are well known. The effects of other islet hormones, however, are controversial. The aim of the present study was to determine whether the islet-acinar axis, as a whole, is stimulatory or inhibitory. Because we have shown that retrograde perfusion reverses the islet-acinar directed microcirculation, retrograde perfusion was expected to remove the overall effects of islet hormones from the acinar tissue.

METHODS

Rat pancreata were perfused (7 mmol/L glucose plus 3 mmol/L mixed amino acids) either anterogradely or retrogradely. Pancreatic juice flow, protein output, and amylase output were measured.

RESULTS

When perfusion was switched from anterograde to retrograde, juice flow increased threefold without changes in protein and amylase output. When cerulein (10(-10) mol/L) was infused, retrograde protein and amylase responses were larger than anterograde responses (each, n = 7; 2.71 +/- 0.23 vs. 1.71 +/- 0.11 mg/40 minutes; 173 +/- 17 vs. 98 +/- 8 U/40 minutes; mean +/- SE; both, P < 0.01). Somatostatin-14 and rat pancreatic polypeptide (each, 10(-9) mol/L) reduced the retrograde protein and amylase responses, but not juice flow, to the anterograde response levels. Conversely, these peptides did not affect exocrine function during anterograde perfusion.

CONCLUSIONS

A suppressive role of the islet-acinar axis via endogenous somatostatin and/or pancreatic polypeptide is suggested.

Effects of pancreatic polypeptide on insulin action in exocrine secretion of isolated rat pancreas

1. Effects of pancreatic polypeptide (PP) on insulin action in pancreatic exocrine secretion was investigated by using an isolated rat pancreas that was perfused with Krebs-Henseleit solution containing 2.5 mM glucose, 0.1% bovine serum albumin and 3% Dextran T-70 at a vascular flow rate of 1.2 ml min-1. 2. Cholecystokinin-8 (CCK-8) at a concentration of 14 pM stimulated basal flow rate and amylase output of the isolated pancreas. Twenty-five millimolar glucose not only increased the basal flow rate and amylase output but also potentiated the CCK-stimulated flow rate and amylase output. 3. Porcine insulin, administered intra-arterially at a concentration of 100 nM, also increased the basal flow rate and amylase output, and also potentiated the CCK-stimulated flow rate and amylase output. 4. Rat PP, given intra-arterially at a concentration of 10 pM, completely abolished the potentiation effects of both the 25 mM glucose and the exogenous insulin on the CCK-stimulated flow rate and amylase output. Rat PP also inhibited the flow rate and amylase output increased by either 25 mM glucose alone or exogenous insulin alone. However, rat PP did not change the flow rate and amylase output stimulated by CCK-8 alone. 5. These results indicate that insulin is an important stimulatory hormone of pancreatic exocrine secretion, and that PP exerts the inhibitory role in pancreatic exocrine secretion by modulating the insulin action.