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

Department of Kinesiology, University of Waterloo, Waterloo, Ontario, Canada

Submitted 3 January 2007 ; accepted in final form 30 April 2007

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 91% peak aerobic power (O_{2 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 O_{2 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 15% with exercise regardless of the number of repetitions performed. In addition, V_max at R9 and R16 was lower (P < 0.05) than at R1 and R2. Vanadate-facilitated [³H]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 ₂-isoform by R2 and a 29% increase in ₃ by R9. At R16, ₃ was lower (P < 0.05) than at R2 and R9. No changes were observed in ₁, ₁, or ₂. It is concluded that repeated sessions of heavy exercise, although resulting in increases in the ₂- and ₃-isoforms and decreases in ₃-isoform, also result in depression in maximal catalytic activity.

sodium-potassium pump; vastus lateralis; maximal activity; content; isoforms

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