Depressed Na+-K+-ATPase activity in skeletal muscle at fatigue is correlated with increased Na+-K+-ATPase mRNA expression following intense exercise

We investigated whether depressed muscle Na⁺-K⁺-ATPase activity with exercise reflected a loss of Na⁺-K⁺-ATPase units, the time course of its recovery postexercise, and whether this depressed activity was related to increased Na⁺-K⁺-ATPase isoform gene expression. Fifteen subjects performed fatiguing, knee extensor exercise at ~40% maximal work output per contraction. A vastus lateralis muscle biopsy was taken at rest, fatigue, 3 h, and 24 h postexercise and analyzed for maximal Na⁺-K⁺-ATPase activity via 3-<i>O</i>-methylfluorescein phosphatase (3-<i>O</i>-MFPase) activity, Na⁺-K⁺-ATPase content via [3H]ouabain binding sites, and Na⁺-K⁺-ATPase α1-, α2-, α3-, ß1-, ß2- and ß3-isoform mRNA expression by real-time RT-PCR. Exercise [352 (SD 267) s] did not affect [³H]ouabain binding sites but decreased 3-<i>O</i>-MFPase activity by 10.7 (SD 8)% (<i>P</i> < 0.05), which had recovered by 3 h postexercise, without further change at 24 h. Exercise elevated α1-isoform mRNA by 1.5-fold at fatigue (<i>P </i>< 0.05). This increase was inversely correlated with the percent change in 3-<i>O</i>-MFPase activity from rest to fatigue (%Δ3-<i>O</i>-MFPaserest-fatigue) (<i>r </i>= –0.60, <i>P </i>< 0.05). The average postexercise (fatigue, 3 h, 24 h) {alpha}1-isoform mRNA was increased 1.4-fold (<i>P </i>< 0.05) and approached a significant inverse correlation with %Δ3-<i>O</i>-MFPaserest-fatigue (<i>r </i>= –0.56, <i>P </i>= 0.08). Exercise elevated α2-isoform mRNA at fatigue 2.5-fold (<i>P </i>< 0.05), which was inversely correlated with %Δ3-<i>O</i>-MFPaserest-fatigue (<i>r </i>= –0.60, <i>P </i>= 0.05). The average postexercise α2-isoform mRNA was increased 2.2-fold (<i>P </i>< 0.05) and was inversely correlated with the %Δ3-<i>O</i>-MFPaserest-fatigue (r = –0.68, P < 0.05). Nonsignificant correlations were found between %Δ3-<i>O</i>-MFPaserest-fatigue and other isoforms. Thus acute exercise transiently decreased Na^+-K⁺-ATPase activity, which was correlated with increased Na⁺-K⁺-ATPase gene expression. This suggests a possible signal-transduction role for depressed muscle Na⁺-K⁺-ATPase activity with exercise.<br>