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.
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