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

1982; Wiley; Volume: 324; Issue: 1 Linguagem: Inglês

10.1113/jphysiol.1982.sp014114

1469-7793

Carol A. Collins, Emilio Rojas, Benjamín A. Suárez‐Isla,

Neurobiology and Insect Physiology Research

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 Ω cm 2 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 n m ). 3. P Na ( t ) was fitted with m 3 h kinetics assuming a voltage‐dependent delay, δt , 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 , ∞ / P̄ Na ) ⅓ as a function of V p , was a sigmoid curve with a mid point at ‐35 mV. The slope, d m ∞ /d V 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, τ m , as a function of V p , and τ 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 V 1 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.