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

• Published in: Journal of applied physiology (1985) 2004-05, Vol.96 (5), p.1767-1775
• Main Authors: Sandiford, S D, Green, HJ, Duhamel, T A, Perco, J G, Schertzer, J D, Ouyang, J
• Format: Article
• Language: English
• ISSN: 8750-7587
• DOI: 10.1152/japplphysiol.01273.2003

This study investigated the effects of prolonged exercise performed in normoxia (N) and hypoxia (H) on neuromuscular fatigue, membrane excitability, and Na super(+)-K super(+)-ATPase activity in working muscle. Ten untrained volunteers [peak oxygen consumption (V Vo sub(2) peak) = 42.1 plus or minus 2.8 (SE) ml times kg super(-1) times min super(-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 Vo sub(2) peak. During N, 3-O-methylfluorescein phosphatase activity (nmol times mg protein super(-1) times h super(-1)) in vastus lateralis, used as a measure of Na super(+)-K super(+)-ATPase activity, decreased (P < 0.05) by 21% at 30 min of exercise compared with rest (101 plus or minus 53 vs. 79.6 plus or minus 4.3) with no further reductions observed at 90 min (72.8 plus or minus 8.0). During H, similar reductions (P < 0.05) were observed during the first 30 min (90.8 plus or minus 5.3 vs. 79.0 plus or minus 6.3) followed by further reductions (P < 0.05) at 90 min (50.5 plus or minus 3.9). Exercise in N resulted in reductions (P < 0.05) in both quadriceps maximal voluntary contractile force (MVC; 633 plus or minus 50 vs. 477 plus or minus 67 N) and force at low frequencies of stimulation, namely 10 Hz (142 plus or minus 16 vs. 86.7 plus or minus 10 N) and 20 Hz (283 plus or minus 32 vs. 236 plus or minus 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 super(+)-K super(+)-ATPase activity compared with N, neuromuscular fatigue and membrane excitability are not differentially altered.