L-TYPE CA2+ CHANNEL IS INVOLVED IN THE REGULATION OF SPONTANEOUS TRANSMITTER RELEASE AT DEVELOPING NEUROMUSCULAR SYNAPSES

Involvement of an L-type Ca2+ channel in the regulation of spontaneous transmitter release was studied in Xenopus nerve-muscle cultures. The frequency of spontaneous synaptic currents, which reflects impulse-in dependent acetylcholine release from the nerve terminals, showed a mar ked increase in high-K+ medium or after treatment with a phorbol ester , 12-O-tetradecanoyl-phorbol 13-acetate, a drug that activates protein kinase C and depolarizes the presynaptic neuron. The potentiation eff ect of high K+ and 12-O-tetradecanoyl-phorbol 13-acetate requires Ca2 influx through the L-type Ca2+ channel in the plasma membrane, since it was significantly reduced by the presence of nifedipine, verapamil or diltiazem and enhanced by Bay K 8644, an L-type Ca2+ channel agonis t. It was shown recently that adenosine 5'-triphosphate markedly poten tiates the spontaneous acetylcholine release at these synapses through the binding of P-2-purinoceptors and the activation of protein kinase C. We found in the present study that potentiation effects of adenosi ne 5'-triphosphate are inhibited by L-type Ca2+ channel blockers, sugg esting that the L-type Ca2+ channel is responsible for the positive re gulation of spontaneous acetylcholine secretion by adenosine 5'-tripho sphate at the developing neuromuscular synapses. Our data suggest that modulation of the L-type Ca2+ channel in embryonic motor nerve termin als is important for the regulation of spontaneous transmitter release .