Mastoparan, a tetradecapeptide purified from wasp venom, stimulates insulin and glucagon release by rat pancreatic islets in a dose-related manner. In perifusion experiments, mastoparan produces monophasic hormone release, which ceases within 10 min of removal of the peptide. After exposure of the isles to mastoparan, glucose-induced insulin release is clearly retained. In incubation experiments, mastoparan-induced insulin release is greatly blocked by pretreatment of the islets with pertussis toxin or neomycin (inhibitor of phosphoinositide turnover) or by lowering the ambient temperature to 17 C. Pretreatment of the islets with nifedipine (calcium channel blocker), H-7 (inhibitor of A- and C-kinase), somatostatin, or divalent cation-free medium does not affect the response to mastoparan. Pretreatment with parabromophenacylbromide (phospholipase-A2 inhibitor) does not block the response induced by a high concentration of (58 microM) mastoparan. The peptide does not stimulate insulin synthesis during 30 min of incubation. Mastoparan raises the cytosolic free Ca2+ concentration, measured by fura-2, in isolated islet cells at normal (1.9 mM) and very low (6.5 microM) extracellular Ca2+ concentrations. Intravenous administration of mastoparan in rats causes a significant elevation of both insulin and glucagon. Together with the previous data, we conclude that mastoparan stimulates islet hormone release through a temperature-dependent process mediated by pertussis toxin-sensitive GTP-binding protein(s). Activation of phospholipase-C and liberation of intracellular Ca2+ are likely to be coupled to exocytosis. Ca2+ influx through the Ca2+ channel and protein kinase-A and -C appear not to be involved in mastoparan's hormone-releasing action. Phospholipase-A2 may be involved in the hormone release induced by low, but not high, concentrations of the peptide.