Regulation of Islet Function by the Neuropeptide PACAP

Sammanfattning: It is known that parasympathetic, sympathetic and sensory nerves innervate the islets and that these nerves harbor both classical neurotransmitters and neuropeptides. However, the neural influence on islet hormone release is complex and not yet fully understood A recently discovered pancreatic neuropeptide is pituitary adenylate cyclase-activating polypeptide (PACAP). The aim of this thesis was to establish the role of PACAP in the regulation of islet function by using a variety of cell biological techniques in isolated islets and insulin producing clonal cells, in combination with in vivo studies in mice and humans. PACAP was localized to pancreatic nerves within the islets, and two of the three established PACAP receptors were expressed in islet cells. PACAP stimulated insulin secretion from isolated mouse and rat islets, as well as in the insulin producing clonal cell lines HIT-T15 and RINm5F. This action was accompanied by formation of cAMP and increases in cytoplasmic Ca2+ and Na+ and dependent on the extracellular concentrations of glucose, Ca2+ and Na+. Furthermore, addition of specific PACAP antisera to isolated rat islets inhibited glucose-induced insulin secretion. In anesthetized mice, injection of glucose together with PACAP27 increased plasma insulin levels, without affecting plasma glucose levels compared to controls injected with glucose alone. Plasma glucagon and plasma adrenaline levels were increased by injection of PACAP. By analyzing the data according to the minimal model technique, PACAP reduced insulin sensitivity without altering glucose effectiveness. Injection of a specific PACAP receptor antagonist in anesthetized mice prior to presentation of a gastric glucose gavage reduced plasma insulin levels compared to controls given saline. Finally, infusion of PACAP27 in humans increased circulating insulin and glucagon levels, without altering glucose elimination rate. These results shows that PACAP is exclusively a neuropeptide in the islets, that the insulinotropic action of PACAP is mediated by activation of adenylate cyclase in combination with cytoplasmic changes in Ca2+ and Na+, and that PACAP contributes to glucose- and meal-induced insulin secretion. This suggests that PACAP is a neuropeptide in the endocrine pancreas of physiological importance for the regulation of islet function.