Existence of a sodium current in the tubular membrane of frog twitch muscle fibre; its possible role in the activation of contraction.

1. The membrane current of frog twitch muscle fibre has been recorded together with contraction in the test gap of a double sucrose gap apparatus. 2. In Ringer, the inward current sometimes shows 2 phases with the same threshold and the same reversal potential: a rapid one (early inward current) and a slower one (late inward current). The mechanical threshold is near the inward current threshold. The amplitude of the contraction increases progressively with depolarizations, without modification of its time to peak. Experiments with variation of the pulse duration and with conditioning depolarization show that a part of the contraction seems to be correlated with the late inward current. 3. Experiments in a sodium free solution, with low TTX concentration, and on glycerol treated fibres show that the late inward current corresponds to a tubular sodium current. 4. A method is described to separate the two phases of inward current. The smooth development of the current-voltage relation of the late inward current, its diminution without modification in time to peak under the action of TTX, and the exponential decay of its tail current all suggest that the tubular membrane potential is sufficiently well controlled. 5. In the experiments where the tubular membrane potential seems to be controlled, a part of the contraction depends on the tubular sodium current, perhaps involving a mechanism of sodium induced calcium release.