Inward rectification in frog skeletal muscle membrane during alternating current stimulation.

Abstract

1. Changes in membrane potential of frog skeletal muscle fibres (m. iliofibularis) due to alternating current (ac) up to 10 kHz and direct current (dc) are recorded with two intracellular microelectrodes. 2. Stimulation by ac with frequencies above 500 Hz induces a mean depolarization\(\bar Vm{\text{ }} \bullet {\text{ }}\bar Vm\) depends in a small range on the strength of the ac stimulus, and reaches a maximum of about 50 mV with stronger currents. When related to the absolute membrane potentialEm, this maximum\((\bar E_m )max = E_m + (\bar Vm)max = - 45 mV\) is hardly affected by alterations ofEm; below a membrane potential of −45 mV the depolarizing effect of ac stimulation is negligible. 3. The mean depolarization\(\bar V_m \) is explained mainly by rectification in the sodium channel; this interpretation is proved by the almost complete abolition of\(\bar V_m \) in sodium free solution or in Ringer's solution containing tetrodotoxin 10−7 g/ml. A slight residual amount of\(\bar V_m \) may be caused by anomalous rectification in the potassium system, as indicated by experiments in tetraethylammonium chloride 60 mM/l. 4. Ac and dc stimulation above threshold strength induce repetitive activity with trains of action potentials. The subsequent spikes are higher, the critical prepotentials are lower, and the time intervals are shorter with ac than with dc. 5. These facts agree, in principle, with findings on the nodal membrane; small differences may be attributed to anomalous rectification.