Aging augments interstitial K⁺ concentrations in active muscle of rats

Skeletal muscle performance declines with advancing age, and the underlying mechanism is not completely understood. A large body of convincing evidence has demonstrated a crucial role for interstitial K⁺ concentration ([K⁺]_o) in modulating contractile function of skeletal muscle. The present study tested the hypothesis that during muscle contraction there is a greater accumulation of [K⁺]_o in aged compared with adult skeletal muscle. Twitch muscle contraction was induced by electrical stimulation of the sciatic nerves of 8- and 32-mo-old Fischer 344 X Brown Norway rats. Levels of [K⁺]_o were measured continuously by a microdialysis technique with the probes inserted into the gastrocnemius muscle. Stimulation at 1, 3, and 5 Hz elevated muscle [K⁺]_o by 52, 64, and 88% in adult rats, and by 78, 98, and 104% in aged rats, respectively, and the increase was significantly higher in aged than in adult rats. Recovery for [K⁺]_o, as measured by the time for [K⁺]_o to recover by 20 and 50% from peak response after stimulation, was slower in aged rats. Ouabain (5 mM), a specific inhibitor of the Na⁺-K⁺ pump, was added in the perfusate to inhibit the reuptake of K⁺ into the cells to assess the role of the pump in the overall K⁺ balance. Ouabain elevated muscle [K⁺]_o at rest, and the effect was significantly attenuated in aged animals. The present data demonstrated an augmented [K⁺]_o in aged skeletal muscle compared with adult skeletal muscle, and the data suggested that an alteration in the function of the Na⁺-K⁺ pump may contribute, in part, to the deficiency in K⁺ balance in skeletal muscle of aged rats.