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Department of Kinesiology, University of Waterloo, Waterloo, Ontario, Canada
Submitted 16 May 2005 ; accepted in final form 11 August 2005
Acute regulation of the Na+-K+-ATPase activity in rat soleus muscle was investigated in response to 15 and 90 min of electrically induced contractile activity (500-ms trains at 30 Hz every 1.5 s). Kinetic measurements of Na+-K+-ATPase activity, assessed by the 3-O-methylfluorescein K+-stimulated phosphatase assay (3-O-MFP), were performed on crude homogenates (Hom) and on tissue separated into two membrane fractions, the sarcolemmal/particulate (SLP) and endosomal (En), in both stimulated (Stim) and contralateral control (Con) muscles. Maximal 3-O-MFP activity (Vmax, nmol·mg protein–1·h–1) was elevated (P < 0.05) in Stim by 40% and by 53% in Hom and by 37 and 40% in SLP at 15 and 90 min, respectively. The 38% increase (P < 0.05) in the 
2-isoform subunit distribution in SLP at 15 min, as assessed by quantitative immunoblotting, persisted at 90 min, whereas for En a 42% decrease (P < 0.05) was observed only at 15 min. For the 1-subunit at 15 min, a 27% decrease (P < 0.05) was observed in En, whereas the 13% increase observed in SLP was not significant (P = 0.08). At 90 min, 1 was increased (P < 0.05) by 14% in SLP and by 29% in En. No changes were observed in 
1-subunit distribution in En and SLP regardless of time of stimulation. Immunoprecipitation with antiphosphotyrosine antibody and quantitative immunoblotting with 1- and 2-antibodies indicated increases (P < 0.05) in tyrosine phosphorylation of 51% in 2 at 15 min only. These results suggest that the increases in Vmax during contractile activity are mediated both by increased phosphorylation and by translocation of the enzyme to the plasma membrane.
skeletal muscle; Na+-K+ pump; exercise; translocation; intrinsic activity
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