Evoked transmitter release at the frog neuromuscular junction in the presence of very low extracellular Ca2+.

We have examined the effect of ouabain, a Na,K-ATPase inhibitor known to secondarily increase intracellular Ca2+ levels, on evoked (phasic) transmitter release at the frog neuromuscular junction. As had been reported previously, we observed an increase in both spontaneous and evoked release with prolonged exposure to ouabain. We also found that following ouabain treatment, evoked release was maintained for a much longer period of time upon removal of extracellular Ca2+ than prior to ouabain exposure. These results indicate that after exposure to ouabain evoked transmitter release can occur in the absence of appreciable Ca2+ entry.