Analysis of 'off' tails of intramembrane charge movements in skeletal muscle of Rana temporaria.

Components of non‐linear charge were isolated in transients obtained in response to hyperpolarizing ('off') voltage‐clamp steps in frog muscle fibres. 'Off' currents of the q gamma charge were isolated by comparing records obtained from long depolarizing steps with those resulting from short steps which intercepted the 'on' currents of the q gamma charge. The 'off' transients of the q gamma component so deduced were rapid decays lasting 10‐15 ms, in contrast with their prolonged time course in the preceding 'on' steps. 'Off' responses were rapid even at voltages when the 'on' current, obtained from imposed 10 mV steps made at a series of closely incremented conditioning voltages, was delayed and prolonged. Such slow transfers of charge could not be demonstrated in 'off' tails. Large depolarizing steps resulted in rapid q gamma transients in 'on' currents not distinguishable from the rest of the charge movement. Nevertheless, by separating the q gamma component through its inactivation by prolonged depolarization, it was possible to show that q gamma currents in the 'off' tails were still rapid decays. It is concluded that in contrast to the varied pattern shown by q gamma in 'on' transients, its 'off' responses are everywhere relatively fast decays. These features can be predicted by a simple two‐state model in which the forward and backward rate constants depend upon the amount of charge moved, as well as the membrane voltage.