Effects of phorbol ester on spontaneous transmitter release at frog neuromuscular junction

Spontaneous transmitter release was studied at frog neuromuscular junctions exposed to the tumor promotor 12-O-tetradecanoylphorbol-13-acetate (TPA), a specific activator of protein kinase C (PrkC). TPA at concentrations between 10(-7) and 10(-6) M induced a dose-dependent increase in miniature end-plate potential frequency. This frequency increase was enhanced by raising [Ca]o, diminished by lowering the temperature of the bath and virtually abolished in Ca2+ -free Ringer. The TPA effect was only poorly reversible after washing. TPA was ineffective in increasing spontaneous release of transmitter at phosphatidylcholine-pretreated neuromuscular junctions. It is suggested that PrkC might play a role in neuromuscular transmission processes.