title: 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.
mesh_terms: D000818:Animals; D004553:Electric Conductivity; D066298:In Vitro Techniques; D007700:Kinetics; D008564:Membrane Potentials; D008954:Models, Biological; D009132:Muscles / Q000502:physiology*; D011188:Potassium / Q000502:physiology; D011896:Rana temporaria; D012964:Sodium / Q000502:physiology*
chemical_list: D012964:Sodium; D011188:Potassium