Twitch activation in Ca2+ -free solutions in the myotomes of the lancelet (Branchiostoma lanceolatum).

The question of whether a Ca2+ influx is necessary to activate twitches in the very thin myotome cells of the lancelet was reinvestigated. Though twitches were blocked in EGTA containing Ca2+- free bathing solutions, they reappeared under certain conditions: a) when the Mg2+ level was lowered, b) when small amounts of caffeine were added, and c) when Cl- in the bathing solution was partly replaced by SCN-. In Ca2+ -containing solutions the changes a) to c) increased the twitch height. The results suggest that a Ca2+ release from the intracellular stores is involved in twitch activation and that external Ca2+ modifies the coupling between excitation and Ca2+ release rather than initiating contractile activation.