Twitch potentiation of frog (Rana japonica) skeletal muscle by antipyrine.

1. The effects of antipyrine on single muscle fibres from the frog Rana japonica were examined. Antipyrine potentiated the twitch tension, but did not affect the tetanus response or induce any contracture by itself. 2. The duration of the action potential was prolonged in a dose-dependent manner. 3. Antipyrine did not affect the mechanical threshold, the uptake of Ca2+, or the Ca2+ induced release of Ca2+ from the sarcoplasmic reticulum. 4. Ca2+-channel blockers did not affect the twitch potentiation by antipyrine.