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The membrane-accessible diacylglycerol 1-oleoyl-2-acetyl-sn-glycerol (OAG, 5-500 µM) caused a dose-related activation of protein kinase C in rat islet homogenates. In islet cell membranes exposed to [γ-323P]ATP, OAG (100 µM) stimulated the net production of labeled phosphatidate and inhibited that of labeled phosphatidylinositol 4-phosphate. In intact islets exposed to 5.6 mM D-glucose, OAG (100 µM) decreased the outflow of 56Rb, increased that of 45Ca and caused a rapid stimulation of insulin release. The secretory response to OAG was dose-related in the 50-500 µM range, being most marked, in relative terms, at a glucose concentration close to the threshold value for stimulation of insulin release by this hexose. It was decreased but not abolished in the absence of CaCl2 and presence of EGTA. At variances with tumor-promoting phorbol esters, OAG failed to potentiate insulin release stimulated by a hypoglycaemic sulphonylurea. Although these findings support the view that activation of protein kinase C by diacylglycerol represents an efficient modality for stimulation of insulin release, they suggest that the effect of OAG upon islet function may not be solely attributable to such an activation. [ABSTRACT FROM AUTHOR]