Analysis of exercise‐induced Na⁺–K⁺ exchange in rat skeletal muscle in vivo

We aimed to quantify the Na+–K+ exchange occurring during exercise in rat skeletal muscle in vivo. Intracellular Na+ and K+ content, Na+ permeability (22Na+ influx), Na+–K+ pump activity (ouabain‐sensitive 86Rb+ uptake) and Na+–K+ pump α2 subunit content ([3H]ouabain binding) were measured. Six‐week‐old rats rested (control animals) or performed intermittent running for 10–60 min and were then killed or were killed at 15 or 90 min following 60 min exercise. In the soleus muscle, intracellular Na+ was 80% higher than in control rats after 60 min exercise, was still elevated (38%) after 15 min rest and returned to control levels after 90 min rest. Intracellular K+ showed corresponding decreases after 15–60 min exercise, returning to control levels 90 min postexercise. Exercise induced little change in Na+ and K+ in the extensor digitorum longus muscle (EDL). In soleus, the exercise‐induced rise in Na+ and reduction in K+ were augmented by pretreatment with ouabain or by reducing the content of muscular Na+–K+ pumps by prior K+ depletion of the animals. Fifteen minutes after 60 min exercise, ouabain‐sensitive 86Rb+ uptake in the soleus was increased by 30% but was unchanged in EDL, and there was no effect of exercise on [3H]ouabain binding measured in vitro or in vivo in either muscle. In conclusion, in the soleus, in vivo exercise induces a rise in intracellular Na+, which reflects the excitation‐induced increase in Na+ influx and leads to augmented Na+–K+ pump activity without apparent change in Na+–K+ pump capacity.