Transmitter release during repetitive stimulation: selective changes produced by Sr²⁺ and Ba²⁺

The addition of Sr²⁺ or Ba²⁺ to the solution bathing the frog neuromuscular junction leads to an increased release of transmitter by each nerve impulse during and following repetitive stimulation. The mechanisms by which Sr²⁺ and Ba²⁺ increase release are not the same. Each ion appears to act selectively on a different process involved in transmitter release.