title: Inactivation of human muscle Na+-K+-ATPase in vitro during prolonged exercise is increased with hypoxia.
abstract: 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.
authors.affiliation: Department of Kinesiology, University of Waterloo, Waterloo, ON, Canada N2L 3G1.
authors.forename: S D
authors.initials: SD
authors.lastname: Sandiford
authors.forename: H J
authors.initials: HJ
authors.lastname: Green
authors.forename: T A
authors.initials: TA
authors.lastname: Duhamel
authors.forename: J G
authors.initials: JG
authors.lastname: Perco
authors.forename: J D
authors.initials: JD
authors.lastname: Schertzer
authors.forename: J
authors.initials: J
authors.lastname: Ouyang
mesh_terms: D000328:Adult; D001642:Bicycling; D004558:Electric Stimulation; D004576:Electromyography; D004789:Enzyme Activation; D015444:Exercise; D005260:Female; D006801:Humans; D000860:Hypoxia / Q000201:enzymology*; D007537:Isometric Contraction; D008297:Male; D009119:Muscle Contraction; D018763:Muscle Fatigue; D018482:Muscle, Skeletal / Q000201:enzymology* / Q000503:physiopathology; D009469:Neuromuscular Junction / Q000503:physiopathology; D010101:Oxygen Consumption; D010807:Physical Endurance; D016037:Single-Blind Method; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*
chemical_list: D000254:Sodium-Potassium-Exchanging ATPase