Pre- and post-synaptic actions of McN-A-343 in the neuromuscular junction: myographic and electrophysiologic studies in vitro.

McN-A-343 (McN) concentrations up to 1.0 micrograms/ml increased the amplitude of the indirectly elicited muscular contractions of the isolated rat diaphragm. The effect was accompanied by an increase in both frequency and amplitude of miniature endplate potentials. The latter effects were abolished by tetrodotoxin, thus suggesting that McN increases the permeability of presynaptic membrane to sodium ions. Larger doses of McN block the neurotransmission probably acting at post-synaptic level.