Exercise‐induced changes in the Na + ,K + ‐ATPase Na+ affinity in rat skeletal muscle

Regulation of the Na + ,Kb‐ATPase in skeletal muscle is important for ion homeostasis and muscle function. We have used the inorganic phosphate release to quantify the Na + ‐induced Na + , K + ‐ATPase activity in purified membranes from oxidative and glycolytic rat muscle. In membranes purified from control rats the K m for Na + was lower in oxidative compared to glycolytic fibers (6.1 vs. 14.0 mM) (P<0.05). In membranes purified after 30 min of treadmill running the K m for Na + was increased (P<0.05) to 15.8 mM in oxidative fibers, whereas K m for Na + (11.6 mM) in glycolytic fibers was not different from control. Analysis of subunit (α, β and FXYD1) protein expression in the sample used revealed fiber type dependent differences in subunit content, a moderate activity‐dependent increase in α and βsubunits (translocation), and a two‐fold increase in the FXYD1(PLM) content. It is concluded that the affinity for Na + is higher in oxidative compared to glycolytic fibers, and that muscle activity reduces the Na + affinity in oxidative fibers. The underlying mechanisms may include translocation of subunits to the membrane fraction studied, phosphorylation of subunits, and increased association of the FXYD1 subunit with the αβ‐complex. The affinity differences may explain the fiber type dependent differences in ion changes associated with muscle activity.