Na+-K+-ATPase in rat skeletal muscle: muscle fiber-specific differences in exercise-induced changes in ion affinity and maximal activity

Juel C. Na+-K+-ATPase in rat skeletal muscle: muscle fiber-specific differences in exercise-induced changes in ion affinity and maximal activity. Am J Physiol Regul Integr Comp Physiol 296: R125-R132, 2009. First published November 5, 2008; doi:10.1152/ajpregu.90760.2008.- It is unclear whether muscle activity reduces or increases Na+-K+-ATPase maximal in vitro activity in rat skeletal muscle, and it is not known whether muscle activity changes the Na+-K+-ATPase ion affinity. The present study uses quantification of ATP hydrolysis to characterize muscle fiber type-specific changes in Na+-K+-ATPase activity in sarcolemmal membranes and in total membranes obtained from control rats and after 30 min of treadmill running. ATPase activity was measured at Na+ concentrations of 0-80 mM and K+ concentrations of 0-10 mM. K-m and V-max values were obtained from a Hill plot. K-m for Na+ was higher (lower affinity) in total membranes of glycolytic muscle (extensor digitorum longus and white vastus lateralis), when compared with oxidative muscle (red gastrocnemius and soleus). Treadmill running induced a significant decrease in K-m for Na+ in total membranes of glycolytic muscle, which abolished the fiber-type difference in Na+ affinity. K-m for K+ (in the presence of Na+) was not influenced by running. Running only increased the maximal in vitro activity (V-max) in total membranes from soleus, whereas V-max remained constant in the three other muscles tested. In conclusion, muscle activity induces fiber type-specific changes both in Na+ affinity and maximal in vitro activity of the Na+-K+-ATPase. The underlying mechanisms may involve translocation of subunits and increased association between PLM units and the alpha beta complex. The changes in Na+-K+-ATPase ion affinity are expected to influence muscle ion balance during muscle contraction.