1. We examined whether pituitary adenylate cyclase-activating polypeptide with 38 or 27 residues (PACAP_38 or PACAP_27) serves as an intra-islet regulator of glucose-induced insulin secretion in rats. PACAP antiserum specific for PACAP_38 and PACAP_27 was used to neutralize the effect of endogenous PACAP in islets. PACAP release from islets was bioassayed using the response of cytosolic Ca¥ concentration ([Ca¥]é) in single â-cells, monitored by dual-wavelength fura_2 microfluorometry. Expression of PACAP mRNA was studied by reverse transcription-polymerase chain reaction (RT_PCR), while expression of PACAP was studied by metabolic labelling and immunoblotting. Localization of PACAP receptors was studied immunohistochemically. 2. High glucose-stimulated insulin release from isolated islets was attenuated by PACAP antiserum but not by non-immune sera. 3. The islet incubation medium with high glucose (Med) possessed a capacity, which was neutralized by PACAP antiserum, to increase [Ca¥]é in â_cells. PACAP antiserum also neutralized the [Ca¥]é-increasing action of synthetic PACAP_38 and PACAP_27, but not that of vasoactive intestinal polypeptide (VIP) and glucagon. 4. Both Med and synthetic PACAP increased [Ca¥]é in â_cells only in the presence of stimulatory, but not basal, glucose concentrations. In contrast, ATP, a substance that is known to be released from â_cells, increased [Ca¥]é in â_cells at both basal and stimulatory glucose concentrations. 5. Expression of PACAP mRNA and biosynthesis of PACAP_38 were detected in islets and a â_cell line, MIN6. 6. Immunoreactivity for PACAP-selective type-I receptor was observed in islets. 7.[Ca¥]é measurements combined with immunocytochemistry with insulin antiserum revealed a substantial population of glucose-unresponsive â_cells, many of which were recruited by PACAP_38 into [Ca¥]é responses. 8. These results indicate that PACAP_38 is a novel islet substance that is synthesized and released by islet cells and then, in an autocrine andÏor paracrine manner, potentiates and arouses â-cell responses to glucose, thereby amplifying glucose-induced insulin secretion in islets.

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