Kinetic properties of the sodium current in striated muscle fibres on the basis of the Hodgkin-Huxley theory.

1. The kinetic properties of the sodium current in skeletal muscle fibre, obtained from records in a double sucrose-gap apparatus, have been analysed in terms of the Hodgkin-Huxley formulation by means of a curve fitting computer program.2. The net sodium currents were obtained by subtracting records in Ringer and Ringer tetrodotoxin; the currents are inward for depolarizations V from +25 to +128 mV (V(Na)). The I(Na)-V characteristic shows a small inward rectification for V > + 128 mV.3. The characteristics of both sodium conductance and permeability are described.4. The activation and inactivation parameters, m and h, are calculated and compared with those experimentally established in a previous work (Ildefonse & Rougier, 1972) and with the results of Adrian, Chandler & Hodgkin (1970).5. The currents calculated with these parameter values fit rather well with the experimental ones, the permeability approach being slightly better.6. The main difference between the experimental and calculated curves is the slower inactivation of the calculated current at V = +55 mV; this difference is envisaged as a possible consequence of some activity in the tubular system of the muscle fibre.