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Fast charge movements in skeletal muscle fibres from Rana temporaria

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

10.1113/jphysiol.1982.sp014115

1469-7793

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

Ion channel regulation and function

1. Fast charge movements were measured in cut skeletal muscle fibres from Rana temporaria . 2. The initial time course of the current in response to a sudden displacement of the membrane potential from ‐110 to ‐60 mV was analysed in terms of an electrical equivalent circuit modified from Falk & Fatt (1964). 3. The specific resistance in series with the sarcolemma was estimated as 7·4 Ω cm 2 . The total capacity (surface sarcolemma plus tubular membrane) was estimated as 3·43 μF/cm 2 . 4. The asymmetry currents settling within 1 ms during depolarizing pulses of increasing size (on‐response), from a holding potential around ‐120 mV, could be described in terms of a single exponential. The asymmetry currents after the pulses (off‐response) exhibited at least two components. 5. The integral of the on‐response, Q on , as a function of V p , could be fitted using a function of the Boltzmann type. At the mid‐point of the distribution curve, equal to ‐38 mV, the slope was 0·012 mV −1 . A saturating value of 28 pC was reached at 40 mV. 6. The off‐response to pulses not exceeding 3 ms exhibited two components. The first one had an exponential time course. The charge Q off associated with this fast component was always equal to Q on . 7. τ on (the relaxation time constant), as a function of membrane potential was asymmetrical, exhibiting a maximum value of 233 μs at about ‐38 mV. 8. For V p values smaller than ‐20 mV the Q on — V p and τ on — V p curves could be analysed using the two‐state transition model. From this analysis the average transition potential V ′ was estimated as ‐38 mV and the effective valence of the mobile charges as 1·36. 9. Double‐pulse protocols (duration of pre‐pulses referred to as T in the range 0‐3 s) showed that Q on and τ on decreased as T increased. Single transient analysis shows that the changes are confined to the transient for depolarizing pulses. 10. This immobilization of the charges is reversible and follows a similar time course to the slow inactivation of the Na + conductance described in the preceding paper. 11. A differential effect of the depolarizing pre‐pulse on the ionic and asymmetry currents is seen in the decrease of τ on with increasing T while τ m remains constant.