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Carol A. Collins^*, E. Rojas and B. A. Suarez-Isla

Department of Biophysics, School of Biological Sciences, University of East Anglia, Norwich NR4 7TJ

1. The steady-state and kinetic characteristics of the processes of activation and inactivation of the Na⁺ permeability, P_Na, were measured in cut skeletal muscle fibres from Rana temporaria under voltage-clamp conditions.

2. The specific resistance, r_ss, in series with the surface sarcolemma, was estimated as 6 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 r_ss, Na⁺ currents were recorded using positive feed-back compensation, in the presence of tetrodotoxin (2·4-5 nM).

3. P_Na(t) was fitted with m³h kinetics assuming a voltage-dependent delay, t, to the start of the activation process.

4. The P_Na—V_p curve exhibited saturation at potentials more positive than 30 mV. m, calculated as (P_Na, /_Na) as a function of V_p, was a sigmoid curve with a mid point at -35 mV. The slope, dm/dV_p, at this point was 0·032 mV^-1.

5. Using a double-pulse protocol a non-exponential time course for the development of fast inactivation at small depolarizations was observed.

6. The time constant for activation, _m, as a function of V_p, and _h as a function of V_p, could be fitted with an approximately bell-shaped function, maximum of 430 µs at -43 mV and 925 µs at -78 mV respectively, at 15 °C.

7. The mid-point potential of the h—V_l curve occurred at -58 mV, and h approached 1 for V₁ values more negative than -103 mV.

8. 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 smaller than 1 s.

Present address: Laboratory of Neurosciences, Gerontology Research Center, Baltimore City Hospitals, Baltimore, MD 21224, U.S.A.