Fast charge movements in skeletal muscle fibres from Rana temporaria

Abstract

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 Omega cm(2). The total capacity (surface sarcolemma plus tubular membrane) was estimated as 3.43 muF/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. tau(on) (the relaxation time constant), as a function of membrane potential was asymmetrical, exhibiting a maximum value of 233 mus at about -38 mV.8. For V(p) values smaller than -20 mV the Q(on)-V(p) and tau(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 tau(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 tau(on) with increasing T while tau(m) remains constant.