Voltage‐clamp experiments on frog single skeletal muscle fibres: evidence for a tubular sodium current.

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Abstract
A late inward current following the initial Na current is sometimes observed on frog single skeletal muscle fibers studied in voltage-clamp conditions by the double sucrose-gap method. The late inward current was time and voltage dependent; it was sensitive to extracellular Na concentration and inhibited by tetrodotoxin. It was very probably carried by Na+. The late current reflected a regenerative increase of Na conductance in the tubular membrane. The main argument favoring this assumption was that the late current was never observed on detubulated fibers. Inactivation studies showed a marked difference between the evolution of the 2 inward currents when the membrane resting potential was increased by previous hyperpolarizations in a high range of values: the late current was decreased while the initial current remained constant. The decrease of the late current was suppressed by tetraethylammonium ions or in a K-free medium. On detubulated fibers, the Na current was never decreased by conditioning hyperpolarizations. A fast transient K current, located in the tubular membrane, may develop contemporaneously with the Na content. The decay of the Na inactivation was analyzed on semilogarithmic plot: on normal fibers, it ran in 2 phases while on detubulated fibers, one phase only was found.