Phorbol esters enhance exocytosis from chromaffin cells by two mechanisms.

Treatment with phorbol esters such as 12-O-tetradecanoylphorbol acetate (TPA) rapidly enhances [3H]norepinephrine secretion from digitonin-permeabilized adrenal chromaffin cells. When TPA treatment was prolonged for several hours, a second distinct enhancing effect was observed. This later enhancement was most prominent at intracellular Ca2+ concentrations of 3-30 microM, and did not require the continued presence of membrane-bound protein kinase C for its expression. The effect could be elicited by as little as 30-min exposure to TPA, followed by several hours in TPA-free medium. This effect of TPA was blocked by actinomycin D and cycloheximide, indicating a requirement for RNA and protein synthesis. Similar effects were seen when intact cells that had been pretreated with TPA were stimulated to secrete by depolarizing concentrations of K+. Thus, protein kinase C enhances secretion by two mechanisms. One is rapid and probably reflects the effects of immediate protein phosphorylation. The other occurs over several hours and requires gene transcription and protein synthesis.