Role of Na⁺,K⁺‐pumps and transmembrane Na⁺,K⁺‐distribution in muscle function

Abstract

Na(+),K(+)-ATPase situated in the plasma membrane mediates active extrusion of Na(+) and intracellular accumulation of K(+). This transport system the Na(+),K(+)-pump is the major regulator of the transmembrane distribution of Na(+) and K(+), and is itself subject to regulation by a wide variety of factors in skeletal muscles. The excitation of skeletal muscles is elicited by a rapid influx of Na(+), followed by an equivalent efflux of K(+) across sarcolemmal and t-tubular membranes. Due to their size and sudden onset, these events constitute the major transport challenge for the Na(+),K(+)-pumps. Skeletal muscles contain the largest single pool of K(+) in the organism. During intense exercise, the Na(+),K(+)-pumps cannot readily reaccumulate K(+) into the muscle cells. Therefore, the working muscles undergo a net loss of K(+), causing up to a doubling of the K(+) concentration in the arterial blood plasma in less than 1 min and even larger increases in interstitial K(+). This may induce depolarization, loss of excitability and force, in particular in muscles, where the excitation-induced passive Na(+),K(+)-fluxes are large. During continuous stimulation of isolated rat muscles, there is a highly significant correlation between the rise in extracellular K(+) and the rate of force decline. Fortunately, excitation increases the Na(+),K(+)-pumping rate within seconds. Thus, maximum activation of up to 20-fold above the resting transport rate may be reached in 10 s, with utilization of all available Na(+),K(+)-pumps. In muscles, where excitability is reduced by pre-exposure to high [K(+)]o, acute activation of the Na(+),K(+)-pumps by hormones or intermittent electrical stimulation restores excitability and contractility. In working muscles, the Na(+),K(+)-pumps, due to rapid activation of their large transport capacity, play a dynamic regulatory role in the from second to second ongoing restoration and maintenance of excitability and force. Excitation is a self-limiting process that depends on the leak/pump ratio for Na(+) and K(+). Acute inhibition of the Na(+),K(+)-pumps with ouabain or downregulation of the Na(+),K(+)-pump capacity clearly reduces contractile endurance in isolated muscles. The Na(+),K(+)-pumps are a limiting factor for contractile force and endurance. This is in particular noted if their capacity is reduced because of inactivity or disease. For these reasons, tight regulation of the Na(+),K(+)-pumps is crucial for the maintenance of plasma K(+), membrane potential and excitability in skeletal muscle. This is achieved by: (1) acute activation of the Na(+),K(+)-pumps elicited by excitation, catecholamines, insulin, insulin-like growth factor I, calcitonins and amylin; and (2) long-term regulation of the content of Na(+),K(+)-pumps exerted by thyroid hormones, adrenal steroids, insulin, training, inactivity, fasting, K(+)-deficiency or K(+)-overload. In conclusion, the Na(+),K(+)-pump is a central target for regulation of Na(+),K(+)-distribution, important for the contractile performance of skeletal ...

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