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Picrotoxin potentiates contraction while inhibiting Ca current but increasing birefringence signal in frog skeletal muscle fibers.

In single frog skeletal muscle fibers, picrotoxin (5 mM) potentiated the voltage-dependent component of contractions in response to 2-s depolarizing pulses while greatly inhibiting the simultaneously recorded tubular Ca current in a normal-Ca, Na- and CI-deficient solution, provided the contractions were generated at long time intervals (2 min). In normal Ringer's solution, picrotoxin reversibly increased the amplitude of the early large birefringence signal and the amplitude and duration of the simultaneously recorded twitch tension, suggesting that the drug may increase, directly or indirectly, the release of Ca from the SR.