Effect of the "calcium ionophore" A-23187 on transmitter release at the frog neuromuscular junction.

The ionophore A-23187 when added to the usual Ca²⁺-Ringer at the frog neuromuscular junction has almost no effect on the frequency of miniature end-plate potentials (min.e.p.p.s). The ionophore does increase the rate of Ca²⁺ efflux from frog muscle, so it is in effective concentrations in the Ringer. When added to Ringer containing Ni²⁺ instead of Ca²⁺, the ionophore increases the min.e.p.p. frequency. We suggest that the ionophore can carry divalent cations into the terminal, but there are mechanisms to keep the Ca²⁺ low.Apparently these mechanisms are unable to rapidly eject or sequester Ni²⁺.