The chloride conductance of intermediate fibres from frog muscles

Sheets of muscle fibres dissected from surface portions of frog ileofibularis and semitendinosus muscles were soaked in solutions with elevated K and Cl concentrations. The KCl-loaded muscles were then bathed in low [Cl-] solutions, whereby the membrane potential became transiently inside positive. The repolarization of the twitch fibres from the tonus bundle ("intermediate fibres") was faster than that of the fibres adjacent to it ("fast fibres") when the preceding exposure to high KCl was brief (7-15 min), and it was slower than that of the fast fibres when KCl was applied for 4 hours. Measurements of the voltage displacement at constant current and of the current in a point voltage clamp showed that inwardly rectifying K channels were present in the membranes of both types of fibres. The ionic conductance ratio, gK/gCl, was 191/523 in fast fibres and 335/230 in "intermediate" fibres. The different repolarization rates may thus be explained by differences in the chloride conductance of the fast and intermediate fibre membranes. The smaller diameter of the latter fibres may be another factor.