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 ((V) over dot O-2 peak) = 42.1 +/- 2.8 (SE) ml . kg(-1) . min(-1)] performed 90 min of cycling during N ( inspired oxygen fraction = 0.21) and during H ( inspired oxygen fraction = 0.14) at similar to 50% of normoxic (V) over dot O-2 peak. During N, 3-O-methylfluorescein phosphatase activity (nmol . mg protein(-1) . 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.