Na channels of frog skeletal muscle are studied under voltage clamp and their properties compared with those of frog myelinated nerve. A standard mathematical model is fitted to the sodium currents measured in nerve and in muscle to obtain a quantitative description of the gating kinetics. At 5 degrees C the kinetics in frog nerve and skeletal muscle are similar except that activation proceeds five times faster in nerve. Block of Na channels by saxitoxin is measured in nerve and in muscle. The apparent dissociation constants for the inhibitory complex are about 1 nM and not significantly different in nerve and muscle. Block of Na channels by external protons in muscle is found to have an apparent pKalpha of 5.33 and a voltage dependence corresponding to action of 27% of the membrane potential drop. Both values are like those for nerve. Shift of the peak sodium permeability-membrane potential curve with changes of external pH and Ca++ are found to be the same in nerve and muscle. It is concluded that Na channels of nerve and muscle are nearly the same.

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