Using the MIN6 B-cell line, we investigated the hypothesis that miniglucagon, the C-terminal (19 -29) fragment processed from glucagon and present in pancreatic A cells, modulates insulin release, and we analyzed its cellular mode of action. We show that, at concentrations ranging from 0.01 to 1000 pM, miniglucagon dosedependently (ID 50 ؍ 1 pM) inhibited by 80 -100% the insulin release triggered by glucose, glucagon, glucagonlike peptide-1-(7-36) amide (tGLP-1), or glibenclamide, but not that induced by carbachol. Miniglucagon had no significant effects on cellular cAMP levels. The increase in 45 Ca 2؉ uptake induced by depolarizing agents (glucose or extracellular K ؉ ), by glucagon, or by the Ca 2؉ channel agonist Bay K-8644 was blocked by miniglucagon at the doses active on insulin release. Electrophysiological experiments indicated that miniglucagon induces membrane hyperpolarization, probably by opening potassium channels, which terminated glucoseinduced electrical activity. Pretreatment with pertussis toxin abolished the effects of miniglucagon on insulin release. It is concluded that miniglucagon is a highly potent and efficient inhibitor of insulin release by closing, via hyperpolarization, voltage-dependent Ca 2؉ channels linked to a pathway involving a pertussis toxin-sensitive G protein.Like many other polypeptide hormones (1-3), glucagon is processed from a large precursor, the 160-amino acid proglucagon produced in the A-cells of the islets of Langerhans, in the L cells of the intestinal mucosa, and in specialized neurons of the central nervous system present mainly in the hypothalamus and in the medulla oblongata (4). Glucagon is known for its hyperglycemic activity through its action on liver via a seven-transmembrane domain receptor linked to adenylyl cyclase via a GTP-binding protein of the Gs sub-type (5). At the level of its target tissues such as the liver, glucagon is partially processed through a cleavage at the Arg 17 -Arg 18 basic doublet by a cell surface protease referred to as "miniglucagon-generating endopeptidase" (MGE) 1 (6) leading to the production of a C-terminal (19 -29) fragment called "miniglucagon" (7-9).Miniglucagon, which does not interfere with the adenylyl cyclase activity, inhibits at picomolar concentrations the hepatic plasma membrane calcium pump (10). On cultured cardiac myocytes, miniglucagon was shown to potentiate at nanomolar concentrations the positive inotropic effect of glucagon, whereas, when used alone at picomolar concentrations, it displayed a negative inotropic effect on myocyte contraction (11). These observations suggested a new role for glucagon as a prohormone and a biological role for miniglucagon as a daughter hormone that modulates the effects of the mother hormone (12). On the other hand, pancreas is the only known tissue in which miniglucagon is present in a stored form, at molar concentrations in the range of 2-5% of that of glucagon (13).In view of preliminary results suggesting that miniglucagon is able to inhibit glucose-and glucagon-induc...
