Effects of islet hormones on nerve-mediated and acetylcholine-evoked secretory responses in the isolated pancreas of normal and diabetic rats.

This study employs the pancreas of normal and diabetic rats to investigate the relationship between the endocrine and exocrine pancreas in the control of exocrine secretion employing enzyme and immunohistochemical and physiological techniques. Acetylcholine esterase (ACh-E) positive nerves were distributed in the interacinar regions of the pancreas lying close to the exocrine cells. There was no difference between the cholinergic innervation of the pancreas in normal and diabetic rat. Insulin (INS) immunopositive cells were observed in the peripheral and central portions of the Islet of Langerhans in the pancreas of normal rat. In the diabetic animals the number of INS-positive cells were decreased. In contrast, glucagon (GLU) and somatostatin (SOM)-immunopositive cells were identified mainly in the peripheral parts of the Islets of Langerhans and their numbers increased markedly in the diabetic pancreas. Insulin alone had no significant effect on amylase secretion in the normal pancreas whereas GLU and SOM evoked small increases in amylase out compared to basal. In contrast, the islet hormones have no detectable secretory effect on the diabetic pancreas compared to control. Both electrical field stimulation (EFS) of intrinsic secretomotor nerves and exogenous application of acetylcholine (ACh) resulted in marked increases in amylase secretion. In pancreatic acini and acinar cells ACh evoked dose-dependent increases in amylase release. In normal pancreatic segments a combination of either INS or GLU with EFS or ACh resulted in marked potentiation of amylase output. In contrast, SOM inhibited the EFS-evoked amylase output but enhanced the secretory response to ACh. In pancreatic acini and acinar cells from normal rat and in pancreatic segments from diabetic rats, the islet hormones had no potentiating effect on the ACh-evoked secretory response. Similarly, in the diabetic rat the islet hormone had no effect on EFS-evoked amylase output. In fura-2 loaded pancreatic acinar cells ACh-induced a marked increase in intracellular free calcium concentration [Ca2+]i compared to basal. Either INS or GLU, but not SOM, elicited a small increase in [Ca2+]i. Combining either INS or GLU with ACh resulted in a potentiation of [Ca2+]i compared with ACh alone. In contrast, SOM had no significant effect on the ACh-induced [Ca2+]i compared to the response obtained with ACh alone. In pancreatic acinar cells of diabetic rat ACh-elicited similar magnitude of [Ca2+]i compared to acinar cells of normal rat. However, when the islet hormones were combined with ACh there was no enhancement of [Ca2+]i compared to ACh alone. The results indicate that the potentiation of either EFS or ACh-evoked secretory responses by the islet hormones seem to occur only in pancreatic segments which have intact viable Islets of Langerhans and not in either acini and acinar cells or from the pancreas of diabetic rat. Moreover, it is apparent that cellular Ca2+ is involved with the interaction of ACh with either INS or GLU.