Influence of protein kinase C-stimulation by a phorbol ester on neurotransmitter release at frog end-plates.

1. The effect of the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) on the stimulation-evoked neurotransmitter release has been investigated by measuring the quantal content (m) of end-plate potentials at frog neuromuscular junctions (Rana temporaria, M. sartorius). 2. After addition of TPA (0.1 up to 1 mumol/l) to the Ringer solution the m-values increased in a concentration-dependent manner up to more than 3 times the control values. 3. Inhibition of the activity of the protein kinase C through the inhibitor 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H-7) blocked this effect of TPA. 4. The TPA effect was much more conspicuous when the m-value was reduced by raising the extracellular Mg2+ concentration. Between the control m-values and the n-fold increase in the m-value enhanced by TPA a hyperbolic relation was observed. 5. It is concluded that protein kinase C stimulation affects predominantly the spontaneous release of neurotransmitter at the frog neuromuscular junction and only very poorly the stimulation-evoked one.