PII S0196-9781(97)00202-7 Interactions of Islet Hormones with Acetylcholine in the Isolated Rat Pancreas LUBNA M. O. JUMA,* JAIPAUL SINGH,* 1 DAVID J. PALLOT,† GINES M. SALIDO‡ AND ERNEST ADEGHATE† *Department of Applied Biology, University of Central Lancashire, Preston PRI 2HE, England; †Department of Human Anatomy, Faculty of Medicine & Health Sciences, United Arab Emirates University, Al Ain, United Arab Emirates; ‡Department of Physiology, University of Extremadura, 10071-Caceres, Spain Received 9 May 1996; Accepted 25 June 1997 JUMA, L. M. O., J. SINGH, D. J. PALLOT, G. M. SALIDO AND E. ADEGHAE. Interactions of islet hormones with acetylcholine in the isolated rat pancreas. PEPTIDES 18(9) 1415–1422, 1997.—This study investigates the effects of the islet hormones, insulin (INS), glucagon (GLU) and somatostatin (SOM) on acetylcholine (ACh)-evoked amylase secretion and calcium (Ca 2+ ) mobilization in the isolated rat pancreas. Stimulation of pancreatic segments and acini with either INS, GLU or SOM resulted in small increases of amylase output compared to much large increases in enzyme output with ACh. Combinations of the peptide hormones with ACh resulted in enhanced secretory responses compared to the effects obtained with either ACh or each of the islet hormone alone. Genistein, the tyrosine kinase inhibitor, evoked a decrease in amylase output from pancreatic segments. It had no effect on the ACh-evoked secretory response but it markedly inhibited the potentiation of the islet hormones with ACh. In pancreatic acinar cells either INS, GLU or SOM elicited moderate increases in amylase output compared to much larger responses with ACh. Furthermore, the islet hormones failed to potentiate the secretory effect of ACh in pancreatic acini. In fura-2 AM loaded acinar cells both INS and GLU evoked small increases in intracellular free calcium concentration [Ca 2+ ] i compared to a much larger elevation with ACh. Both INS and GLU enhanced the ACh-evoked [Ca 2+ ] i . Genistein elicited a decrease in [Ca 2+ ] i both in the absence and presence of both INS and GLU. It also decreased the rise in [Ca 2+ ] i resulting from the combined presence of ACh with both INS and GLU. SOM had no significant effect on the ACh-induced [Ca 2+ ] i . When genistein was combined with ACh and SOM there was a decrease in [Ca 2+ ] i compared to the response obtained with SOM and ACh alone. The results indicate that both tyrosine kinase and cellular Ca 2+ seem to be the intracellular mediators associated with the enhanced secretory responses obtained with a combination of the islet hormones with ACh. Finally, our results using immunohistochemical techniques confirm the presence of INS-, GLU- SOM- and ACh- immunoreactive cells in the endocrine and neural elements of the rat pancreas. © 1997 Elsevier Science Inc. Amylase secretion Pancreas Calcium Somatostatin Insulin Glucagon ACh Rat IN THE exocrine pancreas there are three main stimulus-secretion coupling mechanisms by which secretagogues can elicit enzyme secretion (6,8,22,32,39,43,45). One pathway involves the changes in intracellular adenosine 3,5 cyclic monophosphate (cAMP) lev- els following stimulation of pancreatic acinar cells with either vasoactive intestinal polypeptide (VIP), secretin or noradrenaline (NA) (22,30,31). The cAMP in turn stimulates protein kinase A (PKA) to mediate enzyme secretion. The other two pathways are associated with the metabolism of membrane bound phosphatidyl inositol 4,5 bisphosphate (PIP 2 ) by secretagogues such as acetyl- choline (ACh) and cholecystokinin (CCK) resulting in the eleva- tion of inositol 1,4,5 triphosphate (IP 3 ), inositol 1,2,4,5 tetraki- phosphate (IP 4 ) and diacylglycerol (DG) (5,6,28,34). IP 3 and IP 4 mobilize cellular Ca 2+ while DG activates protein kinase C (PKC) which in turn mediate enzyme secretion. Hitherto, unknown intra- cellular events in the stimulus-secretion coupling mechanism in- cluding the mediators such as cAMP, Ca 2+ and PKC can either potentiate or attenuate the secretory actions of one another (4,12,15,22,38,42,43,47). The precise biochemical mechanism(s) for these interactions between the intracellular mediators is still unresolved. However, it is the belief that they may be associated with zymogen granular phosphorylation (7,46). In comparison to the classical secretagogues including CCK, ACh, NA, VIP and secretin, very little is known about the physiological roles of the islet hormones INS, GLU and SOM in the control of exocrine pancreatic secretion. Some previous studies have demonstrated that INS alone has little or no effect on enzyme secretion but it can markedly potentiate the effects of ACh and CCK-8 (23,36,37,42,46). The potentiatory action of INS on ACh-evoked enzyme secretion is believed to be associated with cellular Ca 2+ mobilization and cAMP metabolism (41,42) as well as with gran- ular regulatory protein phosphorylation (7,46). However, the role of the other islet hormones such as GLU and SOM on enzyme secretion and their interactions with cholinergic agonist, ACh are not yet understood. Thus, the study was designed to investigate the in vitro effects of these islet hormones on exocrine enzyme secre- 1 To whom requests for reprints should be addressed. Peptides, Vol. 18, No. 9, pp. 1415–1422, 1997 Copyright © 1997 Elsevier Science Inc. Printed in the USA. All rights reserved 0196-9781/97 $17.00 + .00 1415