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Mechanisms underlying increases in rat soleus Na⁺-K⁺-ATPase activity by induced contractions

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 (V_max, 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 V_max 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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