THE EFFECT OF SODIUM PUMP BLOCKADE AND DENERVATION ON THE STEADY-STATE SODIUM PERMEABILITY OF MOUSE SKELETAL MUSCLE FIBRES

¹ Department of Physiology, University of Dundee, Dundee DD1 4HN

The Na⁺ pump inhibitor ouabain produced a 25 mV depolarization of mouse extensor digitorum longus muscle fibres in 2 h. Approximately 40% of this depolarization could be prevented by the inclusion of tetrodotoxin (TTX) in the incubation fluid, and the membrane potential of ouabain-depolarized fibres increased when external Na⁺ ion concentration was reduced. TTX and Na⁺ reduction were without effect on fibres incubated for 2 h in the control saline or on fibres depolarized by raising extemal K⁺ ion concentration. Ouabain induced a fall in input resistance in fibres of reduced chloride conductance. Part of the depolarization induced in the majority of fibres by K⁺-free solutions was also TTX sensitive. These results suggest that the steady-state Na⁺ permeability of muscle fibres is increased after Na⁺ pump blockade. The ouabain-induced depolarization of skeletal muscle therefore cannot be taken as evidence for a significant contribution of an electrogenic Na⁺ pump to the resting membrane potential. TTX also reverses much of the depolarization which occurs after denervation of skeletal muscle. Denervation and ouabain both increase the steady-state Na⁺ permeability of the sarcolemma, possibly by an action on the same population of Na⁺ channels.

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