The phorbol ester, 12-O-tetradecanoyl-phorbol-13-acetate, enhances the evoked quanta release of acetylcholine at the frog neuromuscular junction.

The possible role of protein kinase C in the regulation of quantal transmitter release was studied at the frog neuromuscular junction by using the phorbol ester 12-O-tetradecanoyl-phorbol-13-acetate (TPA), a compound known to mimic the effects of the physiological activator of the enzyme, endogenous diacylglycerol. The main effect of the phorbol ester was to increase the quantal content, m, of the endplate potential. The initial values of m were adjusted over a wide range by changing the Ca2+ concentration of the extracellular medium, and the TPA-induced fractional increase in m was significantly greater at junctions with a lower initial quantal content. On the other hand, the absolute increases in m induced by the phorbol ester were positively correlated with the square root of the initial quantal content. The possible physiological significance of this correlation is discussed in view of the well known relationship between extracellular Ca2+ concentration and the quantal content of the end plate potential.