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

In this study, the response of the sarcoplasmic reticulum (SR) to prolonged exercise, performed in normoxia (inspired O-2 fraction = 0.21) and hypoxia (inspired O-2 fraction = 0.14) was studied in homogenates prepared from the vastus lateralis muscle in 10 untrained men (peak 02 consumption = 3.09 +/- 0.25 l/min). In normoxia, performed at 48 +/- 2.2% peak O-2 consumption, maximal Ca2+-dependent ATPase activity was reduced by similar to25% at 30 min of exercise compared with rest (168 +/- 10 vs. 126 +/- 8 mumol(.)g protein(-1.)min(-1)), with no further reductions observed at 90 min (129 +/- 6 mumol(.)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 similar to20% (370 +/- 22 vs. 298 +/- 25 mumol(.)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 similar to16 and similar to32%, 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.