The Na+, K+‐pump and muscle contractility

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Abstract
In skeletal muscle, the excitation induced influx of Na+ and efflux of K+ may be sufficient to exceed the activity or even the capacity of the available Na+, K+‐pumps. This leads to a rise in intracellular Na+ and extracellular K+ Both events interfere with excitability and may present important limitations for the continuation of contractile activity. Furthermore, inhibition of the Na+, K+‐pump or reduction of the concentration of functional Na+, K+‐pumps decrease excitability and the maintenance of force during continued stimulation. Conversely, in muscles where contractile force is inhibited by exposure to high extracellular K+, acute stimulation of the Na+, K+‐pump with catecholamines, CGRP or insulin leads to a rapid recovery of force. The large passive fluxes of Na+ and K+ associated with excitation constitute the major drive on the activity of the Na+, K+‐pump, giving rise to up to 20‐fold stimulation of the transport rate. In keeping with this, training induces an upregulation of the total concentration of Na+, K+‐pumps in skeletal muscle. The activity and the capacity of the Na+, K+‐pump are important limiting factors determining the maintenance of excitability and contractile performance.