Voltage‐dependent charge movement in frog slow muscle fibres.

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Abstract
Voltage-clamp experiments on frog slow and twitch fibers were carried out using the three-micro-electrode technique. K currents were blocked by tetraethylammonium. Contraction was blocked by 2 mM-tetracaine. After subtracting the linear capacitive and leakage currents, the .DELTA.V (test-control) traces from slow fibers show ''on'' and ''off'' charge movements similar to those observed in twitch fibers. The time integrals of the ''on'' and ''off'' transients, Qon and Qoff, in slow fibers are, as in twitch fibers, almost equal in magnitude but opposite in direction. The charge-voltage distribution is well fitted by a sigmoid curve given by .**GRAPHIC**. which has been successfully applied to twitch fibers. Data from 3 fibers gave .hivin.V = -25 mV, k = 13 mV, and Qmax = 7 nC/.mu.F. Intramembranous charge in slow fibers has the same steady-state voltage distribution as that in twitch fibers, but the quantity of maximum movable charge is only 1/4 to 1/3 as large. Charge movement in slow fibers does not inactivate completely when the fibers are held at -20 to 0 mV for durations as long as 30 min. Charge movement exists in slow fibers and may serve the same function in regulating contractile activation as that postulated for twitch fibers. The lack of complete inactivation may be consistent with the ability of slow fibers to maintain maximal tension during prolonged depolarizations.