Muscle Na+-K+-ATPase response during 16 h of heavy intermittent cycle exercise

This study investigated the effects of a 16-h protocol of heavy intermittent exercise on the intrinsic activity and protein and isoform content of skeletal muscle Na+-K+-ATPase. The protocol consisted of 6 min of exercise performed once per hour at similar to 91% peak aerobic power (V-O2 peak) with tissue sampling from vastus lateralis before (B) and immediately after repetitions 1 (R1), 2 (R2), 9 (R9), and 16 (R16). Eleven untrained volunteers with a V-O2 peak of 44.3 +/- 2.3 ml (.) kg(-1) (.) min(-1) participated in the study. Maximal Na+-K+-ATPase activity (V-max, in nmol (.) mg protein(-1.)h(-1)) as measured by the 3-O-methylfluorescein K+-stimulated phosphatase assay was reduced (P < 0.05) by similar to 15% with exercise regardless of the number of repetitions performed. In addition, Vmax at R9 and R16 was lower (P < 0.05) than at R1 and R2. Vanadate-facilitated [H-3] ouabain determination of Na+-K+-ATPase content (maximum binding capacity, pmol/g wet wt), although unaltered by exercise, increased (P < 0.05) 8.3% by R9 with no further increase observed at R16. Assessment of relative changes in isoform abundance measured at B as determined by quantitative immunoblotting showed a 26% increase (P < 0.05) in the alpha(2)-isoform by R2 and a 29% increase in alpha(3) by R9. At R16, beta(3) was lower (P < 0.05) than at R2 and R9. No changes were observed in alpha(1), beta(1), or beta(2). It is concluded that repeated sessions of heavy exercise, although resulting in increases in the alpha(2)- and alpha(3)-isoforms and decreases in beta(3)-isoform, also result in depression in maximal catalytic activity.