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1 Department of Kinesiology, University of Waterloo, Waterloo, Ontario, Canada
* To whom correspondence should be addressed. E-mail: green{at}healthy.uwaterloo.ca.
Acute regulation of the Na+-K+-ATPase activity in rat soleus muscle was investigated in response to 15 min and 90 min of electrically-induced contractile activity (500 ms trains at 30 Hz every 1.5 sec). Kinetic measurements of Na+-K+-ATPase activity, assessed using the 3-O-methylfluorescein K+-stimulated phosphatase assay (3-O-MFP), were performed on crude homogenates (HOM) and on tissue separated into 2 membrane fractions, the sarcolemmal/particulate (SLP) and endosomal (EN) in both stimulated (STIM) and contralateral control (CON) muscles. Maximal 3-O-MFP activity (Vmax, nmoles.mg protein-1.h-1) in STIM was elevated (P<0.05) in HOM by 40% and by 53 % at 15 min and 90 min, respectively. Increases (P<0.05) in Vmax were also observed in the SLP at 15 min (37%) and 90 min (40%) of stimulation while Vmax was unaltered in EN. No changes were observed in either Km or the Hill slope, 2 measures used to assess the affinity of the enzyme for K+. At 15 min of stimulation, 
2-subunit isoform distribution, as assessed by quantitative immunoblotting, increased (P<0.05) by 38% in SLP and decreased (P<0.05) by 42% in EN. The increase (P<0.05) in the 2- isoform in SLP but not EN persisted at 90 min of STIM. For the 1-subunit at 15 min, a 27% decrease (P<0.05) was observed in EN while the 13 % increase observed in SLP was not significant (P=0.08). At 90 min of STIM, 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. In HOM, only the 1-subunit was observed to change, and only at 90 min where a 15% increase (P<0.05) was observed. Immunoprecipation 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.
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