Effect of glucocorticoid treatment on the excitability of rat skeletal muscle

Dexamethasone treatment in the rat produced depolarization of extensor digitorum longus (EDL) muscle fibers but not soleus (SOL) fibers studies in vitro at 23° C. The depolarization of EDL fibers was most prominent after 1 day of treatment (treated-77.5±1.1 mV, control-87.2±0.8 mV; mean±S.E.), and was associated with elevation of the action potential threshold and reduction of the action potential overshoot. In vivo, or in vitro in chloride-free solution, the resting potential and action potential threshold and overshoot of EDL fibers from glucocorticoid-treated and control rats were similar. Sodium currents were studied with a patch voltage clamp. Glucocorticoid treatment did not alter the voltage dependence of sodium channel activation or inactivation currents at about -29 mV and half-maximal inward currented at about -50 mV. Sodium channels were half inactivated at about -71 mV. Glucocorticoid treatment did not alter the sarcolemmal resistance or capacitance. We conclude that glucocorticoid treatment does not produce muscle weakness or atrophy by altering the excitability of muscle fibers.