Exercise-induced regulation of phospholemman (FXYD1) in rat skeletal muscle: implications for Na+/K+-ATPase activity.

Background: Na+/K+-ATPase activity is upregulated during muscle exercise to maintain ionic homeostasis. One mechanism may involve movement of α-subunits to the outer membrane (translocation). Aim: We investigated the existence of exercise-induced translocation and phosphorylation of phospholemman (PLM, FXYD1) protein in rat skeletal muscle and exercise-induced changes in Vmax and Km for Na+ of the Na+/K+-ATPase. Methods: Two membrane fractionation methods and immunoprecipitation were used. Results: Both fractionation methods revealed a 200–350% increase in PLM in the sarcolemma after 30 min of treadmill running, while the phosphorylation of Ser-68 of PLM appeared to be unchanged. Exercise did not change Vmax or Km for Na+ of the Na+/K+-ATPase in muscle homogenate, but induced a 67% increase in Vmax in the sarcolemmal giant vesicle preparation; Km for Na+ remained constant. The main part of the increase in Vmax is related to a 36–53% increase in the level of α-subunits; the remainder may be related to increased PLM content. Similar results were obtained with another membrane purification method. In resting muscle, 29% and 32% of α1- and α2-subunits, respectively, were co-immunoprecipitated by PLM antibodies. In muscle homogenate prepared after exercise, immunoprecipitation of α1-subunits was increased to 227%, whereas the fraction of precipitated α2 remained constant. Conclusion: Exercise translocates PLM to the muscle outer membrane and increases its association with mainly the α1-subunit, which may contribute to the increased Vmax of the Na+/K+-ATPase. [ABSTRACT FROM AUTHOR]