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

Abstract

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 (V̇o2 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 V̇o2 peak of 44.3 ± 2.3 ml·kg−1·min−1 participated in the study. Maximal Na+-K+-ATPase activity ( Vmax, 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, Vmax 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 α2-isoform by R2 and a 29% increase in α3 by R9. At R16, β3 was lower ( P < 0.05) than at R2 and R9. No changes were observed in α1, β1, or β2. It is concluded that repeated sessions of heavy exercise, although resulting in increases in the α2- and α3-isoforms and decreases in β3-isoform, also result in depression in maximal catalytic activity.