POSTSYNAPTIC EFFECTS OF THE PHORBOL ESTER TPA ON FROG END-PLATES

The effects of the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA), a specific activator of protein kinase C (PKc), were examined on the frog neuromuscular junction. The depolarization elicited by iontophoretically applied acetylcholine (ACh) was reversibly decreased by 20-60% when muscle fibres were exposed to 1-5 x 10-7 M TPA. Liposome-delivered phosphatidylcholine (100 μg/ml) prevented this effect. A similar decrease in ACh-sensitivity was produced by diacylglycerol (diolein), a physiological activator of PKc, but in this case the decrease was only partially reversible. In TPA-Ringer, (1) the peak size of miniature end-plate potentials exhibited a small decrease; (2) miniature end-plate currents were reduced in size and their decay time constant became longer and relatively independent of membrane potential. The possibility that these TPA-induced actions are mediated by activation of PKc is discussed.