The phorbol ester 4 beta-phorbol 12-myristate 13-acetate (PMA), at concentrations of 0.1 microM and above, stimulated secretion of glucagon and of insulin from isolated rat islets of Langerhans incubated in the presence of 5.5 mM-glucose. Stimulation of secretion of both hormones by 1 microM-PMA persisted in the absence of external Ca2+, and could be abolished by incubating the islets at 4 degrees C. These findings suggest a role of protein kinase C in the alpha-cell (and beta-cell) secretory mechanism.

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