Downregulation of Na+-K+-ATPase pumps in skeletal muscle with training in normobaric hypoxia

To investigate the effects of training in normoxia vs. training in normobar ic hypoxia (fraction of inspired O-2 = 20.9 vs. 13.5%, respectively) on the regulation of Na+-K+-ATPase pump concentration in skeletal muscle (vastus lateralis), 9 untrained men, ranging in age from 19 to 25 yr, underwent 8 w k of cycle training. The training consisted of both prolonged and intermitt ent single leg exercise for both normoxia (N) and hypoxia (H) during a sing le session (a similar work output for each leg) and was performed 3 times/w k. Na+-K+-ATPase concentration was 326 +/- 17 (SE) pmol/g wet wt before tra ining (Control), increased by 14% with N (371 +/- 18 pmol/g wet wt; P < 0.0 5), and decreased by 14% with H (282 +/- 20 pmol/g wet wt; P < 0.05). The m aximal activity of citrate synthase, selected as a measure of mitochondrial potential, showed greater increases (P < 0.05) with H (1.22 +/- 0.10 mmol. h(-1).g wet wt(-1); 70%; P < 0.05) than with N (0.99 +/- 0.10 mmol.h(-1).g wet wt(-1) 51%; P < 0.05) compared with pretraining (0.658 +/- 0.09 mmol.h( -1).g wet wt(-1)). These results demonstrate that normobaric hypoxia induce d during exercise training represents a potent stimulus for the upregulatio n in mitochondrial potential while at the same time promoting a downregulat ion in Na+-K+-ATPase pump expression. In contrast, normoxic training stimul ates increases in both mitochondrial potential and Na+-K+-ATPase concentrat ion.