Inactivation of human muscle Na+-K+-ATPase in vitro during prolonged exercise is increased with hypoxia

This study investigated the effects of prolonged exercise performed in normoxia (N) and hypoxia (H) on neuromuscular fatigue, membrane excitability, and Na+-K+ -ATPase activity in working muscle. Ten untrained volunteers [peak oxygen consumption (Vo2peak) = 42.1 +/- 2.8 (SE) ml x kg(-1) x min(-1)] performed 90 min of cycling during N (inspired oxygen fraction = 0.21) and during H (inspired oxygen fraction = 0.14) at approximately 50% of normoxic Vo2peak. During N, 3-O-methylfluorescein phosphatase activity (nmol x mg protein(-1) x h(-1)) in vastus lateralis, used as a measure of Na+-K+-ATPase activity, decreased (P < 0.05) by 21% at 30 min of exercise compared with rest (101 +/- 53 vs. 79.6 +/- 4.3) with no further reductions observed at 90 min (72.8 +/- 8.0). During H, similar reductions (P < 0.05) were observed during the first 30 min (90.8 +/- 5.3 vs. 79.0 +/- 6.3) followed by further reductions (P < 0.05) at 90 min (50.5 +/- 3.9). Exercise in N resulted in reductions (P < 0.05) in both quadriceps maximal voluntary contractile force (MVC; 633 +/- 50 vs. 477 +/- 67 N) and force at low frequencies of stimulation, namely 10 Hz (142 +/- 16 vs. 86.7 +/- 10 N) and 20 Hz (283 +/- 32 vs. 236 +/- 31 N). No changes were observed in the amplitude, duration, and area of the muscle compound action potential (M wave). Exercise in H was without additional effect in altering MVC, low-frequency force, and M-wave properties. It is concluded that, although exercise in H resulted in a greater inactivation of Na+-K+-ATPase activity compared with N, neuromuscular fatigue and membrane excitability are not differentially altered.