Sprint training increases human skeletal muscle Na⁺-K^-ATPase concentration and improves K⁺ regulation

This study investigated the effects of sprint training on muscle Na⁺- K⁺-adenosinetriphosphatase (ATPase) concentration, plasma [K⁺] regulation, muscle performance, and fatigue during severe intermittent exercise. Six untrained male subjects underwent intensive cycle-sprint training for 7 wk. Muscle biopsies were taken at rest from the vastus lateralis muscle before and after 7 wk of training and were assayed for Na⁺-K⁺-ATPase concentration using vanadate-facilitated [³H]ouabain binding to intact samples. Before and after the training period, subjects performed four maximal 30-s exercise bouts (EB) on a cycle ergometer, each separated by a 4-min recovery. Arterialized venous blood samples were drawn immediately before and after each sprint bout and were analyzed for plasma [K⁺]. The work output was significantly elevated (11%) across all four EBs after training. The muscle [³H]ouabain binding site concentration was significantly increased (16%) from 333 ± 19 to 387 ± 15 (SE) pmol/g wet wt after training but was unchanged in muscle obtained from three control subjects. Plasma [K⁺] rose by 1-2 mmol/l with each EB and declined rapidly by the end of each recovery period. The increases in plasma [K⁺] resulting from each EB were significantly lower (19%) after training. The ratios of rise in plasma [K⁺] relative to work output during each EB were also significantly lower (27%) after training. The increased muscle [³H]ouabain binding site concentration and the reduced ratio of rise in [K⁺] relative to work output with exercise are both consistent with improved plasma and skeletal muscle K⁺ regulation after sprint training.