|Budget Amount *help
¥3,100,000 (Direct Cost : ¥3,100,000)
Fiscal Year 1998 : ¥900,000 (Direct Cost : ¥900,000)
Fiscal Year 1997 : ¥2,200,000 (Direct Cost : ¥2,200,000)
We examined whether pituitary adenylate cyclase-activating polypeptide (PACAP) serves as an intra-islet regulator of glucose-induced insulin secretion. High glucose-stimulated insulin release from isolated rat islets was attenuated by an antiserum specific for PACAP, but not by nonimmune sera. The islet incubation medium with high glucose possessed a capacity, which was neutralized by the PACAP antiserum, to increase cytosotic Ca^<2+> concentration ([Ca^<2+>]_i) in beta-cells. PACAP antiserum also neutralized the [Ca^<2+>]_i-increasing action of synthetic PACAP, but not VIP and glucagon. In islets and a beta-cell line, MIN6, expression of PACAP mRNA was detected by reverse-transcription polymerase chain reaction (RT-PCR) and biosynthesis of PACAP was detected by metabolic labeling and immunoblotting. Immunoreactivity for PACAP-selective receptor (PAC1-R) was observed in islets. [Ca^<2+>]_i measurements combined with immunocytochemistry with insulin antiserum demonstrated that glucose-u
nresponsive beta-cells are recruited by PACAP into [Ca^<2+>]_i responses. These results reveal that PACAP is a novel islet substance, which is synthesized and released by islet cells and then) in an autocrine/paracrine manner, potentiates and arouses beta-cell responses to glucose, thereby amplifying glucose-induced insulin secretion in islets.
Next, we explored a possible extra-pancreatic action of PACAP.PAC1-R was expressed in the rat fat tissue and an adipocyte cell line, 3T3-Li. PACAP significantly enhanced the insulin-induced 2-deoxyglucose uptake by 333-Li adipocytes. Both insulin receptor beta-subunit and insulin receptor substrate-1 (IRS-1) were tyrosine-phosphorylated by insulin, on which PACAP had no additional effect. In contrast, the insulin-stimulated phosphatidylinositol (PI) 3-kinase activity was further increased by PACAP.The basal 2-deoxyglucose uptake and PI 3-kinase activity were not altered by PACAP.These results indicate that PACAP promotes insulin-induced glucose uptake by increasing PI 3-kinase activity, and reveal that PACAP not only potentiates glucose-induced insulin secretion in islets but also potentiates insulin action in adipocytes. Less