Pituitary adenylate cyclase-activating polypeptide (PACAP) is an islet substance serving as an intra-islet amplifier of glucose-induced insulin secretion in rats.
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 Ca2+ concentration ([Ca2+]i) in single beta-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 [Ca2+]i in beta-cells. PACAP antiserum also neutralized the [Ca2+]i-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 [Ca2+]i in beta-cells only in the presence of stimulatory, but not basal, glucose concentrations. In contrast, ATP, a substance that is known to be released from beta-cells, increased [Ca2+]i in beta-cells at both and stimulatory glucose concentrations. 5. Expression of PACAP mRNA and biosynthesis of PACAP-38 were detected in islets and a beta-cell line, MIN6. 6. Immunoreactivity for PACAP-selective type-I receptor was observed in islets. 7. [Ca2+]i measurements combined with immunocytochemistry with insulin antiserum revealed a substantial population of glucose-unresponsive beta-cells, many of which were recruited by PACAP-38 into [Ca2+]i 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 and/or paracrine manner, potentiates and arouses beta-cell responses to glucose, thereby amplifying glucose-induced insulin secretion in islets.
Yada
T
T
Department of Physiology, Kagoshima University School of Medicine, Japan. yada@med2.kufm.kagoshima-u.ac.jp
Sakurada
M
M
Ishihara
H
H
Nakata
M
M
Shioda
S
S
Yaekura
K
K
Hamakawa
N
N
Yanagida
K
K
Kikuchi
M
M
Oka
Y
Y
D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism; D002478:Cells, Cultured; D003600:Cytosol / Q000378:metabolism; D005934:Glucagon / Q000096:biosynthesis; D005947:Glucose / Q000494:pharmacology*; D007106:Immune Sera / Q000494:pharmacology; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D007515:Islets of Langerhans / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D009479:Neuropeptides / Q000096:biosynthesis / Q000494:pharmacology / Q000502:physiology*; D051219:Pituitary Adenylate Cyclase-Activating Polypeptide; D016133:Polymerase Chain Reaction; D012333:RNA, Messenger / Q000096:biosynthesis; D051381:Rats; D017208:Rats, Wistar; D014158:Transcription, Genetic; D014660:Vasoactive Intestinal Peptide / Q000096:biosynthesis
C495403:Adcyap1 protein, rat; D007106:Immune Sera; D007328:Insulin; D009479:Neuropeptides; D051219:Pituitary Adenylate Cyclase-Activating Polypeptide; D012333:RNA, Messenger; D014660:Vasoactive Intestinal Peptide; D000255:Adenosine Triphosphate; D005934:Glucagon; D005947:Glucose; D002118:Calcium