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EXERCISE AND RESPIRATORY PHYSIOLOGY

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

Department of Biology, University of Copenhagen, Copenhagen, Denmark

Submitted 11 September 2008 ; accepted in final form 2 November 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 β complex. The changes in Na⁺-K⁺-ATPase ion affinity are expected to influence muscle ion balance during muscle contraction.

adaptational modifications; Na⁺-K⁺-pump; ion sensitivity

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