Effects of trifluoperazine and promethazine on the release of transmitter quanta at the mouse neuromuscular junction.

The present experiments examined the effects of phenothiazine derivatives, such as trifluoperazine and promethazine, on the release of transmitter quanta in preparations of the mouse diaphragm. The frequency (F, s-1) of miniature end-plate potentials and the quantal content (m) of endplate potentials were measured intracellularly at the same endplate in a bathing solution that contained 0.5-0.8 mM Ca2+ ions and 5 mM Mg2+ ions. Trifluoperazine (4 microM) significantly reduced both F and m. The inhibitory effect on m, but not on F, was subject to competition by Ca2+ ions. Promethazine at 48 microM, but not at 16 microM, reduced the quantal release. It was apparent that the effect of trifluoperazine was competitively antagonized by Ca2+ ions at motor nerve terminals.