Activation and inactivation characteristics of the sodium permeability in muscle fibres from Rana temporaria

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Abstract
The steady-state and kinetic characteristics of the processes of activation and inactivation of the Na+ permeability, PNa, were measured in cut skeletal muscle fibers from R. temporaria under voltage-clamp conditions. The specific resistance, rss, in series with the surface sarcolemma, was estimated as 6 .OMEGA. cm2 by measuring the initial value of the membrane potential transient in response to current pulses under current-clamp conditions. To reduce the error in the potential across the sarcolemma introduced by rss, Na+ currents were recorded while using positive feedback compensation, in the presence of tetrodotoxin (2.4-5 nM). PNa (t) was fitted with m3h kinetics assuming a voltage-dependent delay, .delta.t, to the start of the activation process. The PNa-Vp curve exhibited saturation at potentials more positive than 30 mV. m.infin., calculated as (PNa, .infin./.hivin.PNa)1/3 as a function of Vp, was a sigmoid curve with a midpoint at -35 mV. The slope, dm.infin./dVp, at this point was 0.032 mV-1. By using a double-pulse protocol, a nonexponential time course for the development of fast inactivation at small depolarizations was observed. The time constant for activation, .tau.m, as a function of Vp, and .tau.h as a function of Vp, could be fitted with an approximately bell-shaped function, maximum of 430 .mu.s at -43 mV and 925 .mu.s at -78 mV, respectively, at 15.degree. C. The midpoint potential of the h.infin.-V1 curve occurred at -58 mV, and h.infin. approached 1 for V1 values more negative than -103 mV. By using a double-pulse procedure, the development of a slow inactivation of the Na+ current was demonstrated. Its time course could be described in terms of a single exponential function, time constant equal to 0.58 s. The recovery from slow inactivation could be described by a similar exponential for recovery times < 1 s.