Human muscle sarcoplasmic reticulum function during submaximal exercise in normoxia and hypoxia

Abstract

In this study, the response of the sarcoplasmic reticulum (SR) to prolonged exercise, performed in normoxia (inspired O2fraction = 0.21) and hypoxia (inspired O2fraction = 0.14) was studied in homogenates prepared from the vastus lateralis muscle in 10 untrained men (peak O2consumption = 3.09 ± 0.25 l/min). In normoxia, performed at 48 ± 2.2% peak O2consumption, maximal Ca2+-dependent ATPase activity was reduced by ∼25% at 30 min of exercise compared with rest (168 ± 10 vs. 126 ± 8 μmol·g protein−1·min−1), with no further reductions observed at 90 min (129 ± 6 μmol·g protein−1·min−1). No changes were observed in the Hill coefficient or in the Ca2+concentration at half-maximal activity. The reduction in maximal Ca2+-dependent ATPase activity at 30 min of exercise was accompanied by oxalate-dependent reductions ( P < 0.05) in Ca2+uptake by ∼20% (370 ± 22 vs. 298 ± 25 μmol·g protein−1·min−1). Ca2+release, induced by 4-chloro- m-cresol and assessed into fast and slow phases, was decreased ( P < 0.05) by ∼16 and ∼32%, respectively, by 90 min of exercise. No differences were found between normoxia and hypoxia for any of the SR properties examined. It is concluded that the disturbances induced in SR Ca2+cycling with prolonged moderate-intensity exercise in human muscle during normoxia are not modified when the exercise is performed in hypoxia.