Two distinct mechanisms underlie the stimulation of neurotransmitter release by phorbol esters in clonal rat pheochromocytoma PC12 cells.

Phorbol ester treatment induces the phosphorylation of SNAP-25 at Ser(187) and the potentiation of Ca(2+)-induced dopamine (DA) and acetylcholine (Ach) release from PC12 cells. In order to evaluate the functional consequences of phosphorylation, quantitative analysis was carried out using an anti-phosphopeptide antibody that specifically recognizes SNAP-25 phosphorylated at Ser(187). DA and ACh release, assayed in low-K(+) as well as high-K(+) solution, increased by treating the cells with phorbol-12-myristate-13-acetate (PMA); however, the stimulation of high-K(+)-dependent release occurred at lower concentrations and with shorter exposures to PMA than that of the basal release in low-K(+)-solution. The PMA-induced phosphorylation of SNAP-25 did not correlate with the potentiation of high-K(+)-dependent neurotransmitter release. The potentiation of high-K(+)-dependent DA release by phorbol 12,13-diacetate (PDA), a water soluble phorbol ester, almost completely disappeared within 1 min after washing PDA in the presence of okadaic acid, conditions under which the phosphorylation of SNAP-25 persisted for at least 15 min. PMA-induced phosphorylation of SNAP-25 was inhibited by staurosporine, however, the potentiation of high-K(+)-dependent DA release was suppressed only partially. These results indicate that protein kinase activation does not account for a large fraction of the phorbol ester-induced potentiation of depolarization-dependent neurotransmitter release from PC12 cells.