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 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 O(2) consumption = 3.09 +/- 0.25 l/min). In normoxia, performed at 48 +/- 2.2% peak O(2) consumption, maximal Ca(2+)-dependent ATPase activity was reduced by approximately 25% at 30 min of exercise compared with rest (168 +/- 10 vs. 126 +/- 8 micromol.g protein(-1) x min(-1)), with no further reductions observed at 90 min (129 +/- 6 micromol x g protein(-1) x min(-1)). No changes were observed in the Hill coefficient or in the Ca(2+) concentration at half-maximal activity. The reduction in maximal Ca(2+)-dependent ATPase activity at 30 min of exercise was accompanied by oxalate-dependent reductions (P < 0.05) in Ca(2+) uptake by approximately 20% (370 +/- 22 vs. 298 +/- 25 micromol x g protein(-1) x min(-1)). Ca(2+) release, induced by 4-chloro-m-cresol and assessed into fast and slow phases, was decreased (P < 0.05) by approximately 16 and approximately 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 Ca(2+) cycling with prolonged moderate-intensity exercise in human muscle during normoxia are not modified when the exercise is performed in hypoxia.

Comparative effects of a low-carbohydrate diet and exercise plus a low-carbohydrate diet on muscle sarcoplasmic reticulum responses in males

“…It should be noted that in previous papers from our group (13,44,50), estimates of Ca 2ϩ uptake and Ca 2ϩ release were 100 times too high. This occurred because of errors in the software used to calculate [Ca 2ϩ ]f in conjunction with a dilution error.…”

“…Numerous studies have found this to be the case (3,5,13,31,50). Moreover, the reduction in Ca 2ϩ uptake appears to be mediated by reductions in Ca 2ϩ -ATPase activity (3,5,13,50), which appears to occur secondary to modifications in the region of the adenine nucleotide of the enzyme (34,35). These measurements, which are performed in vitro under supposedly optimal conditions by using exogenously generated ATP (47), support the hypothesis that the loss of catalytic activity occurs not as a result of deficiencies in ATP availability but, rather, as a result of structural modifications to the enzyme.…”

“…Differentiating the linear-fit curves allows determination of Ca 2ϩ release rates. Phase 1 Ca 2ϩ release is characterized by a rapid early release (ϳ0 -2 s), whereas phase 2 Ca 2ϩ release is characterized as the slower, more delayed release (ϳ5-15 s) (13,49).…”

Metabolic, enzymatic, and transporter responses in human muscle during three consecutive days of exercise and recovery

“…With the exception of the tissue samples, all other procedures and measurements were performed during the 3-day exercise protocol. We have previously reported that providing exercise patterns and dietary habits are not substantially altered, a wide range of muscle cellular properties remain stable over a similar time frame (15,69).…”

Abnormal sarcoplasmic reticulum Ca2+-sequestering properties in skeletal muscle in chronic obstructive pulmonary disease

“…For the Ca 2ϩ -ATPase, the reaction mixture (pH 7.0) contained 200 mM KCl, 20 mM HEPES, 15 mM MgCl2, 1 mM EGTA, 10 mM NaN3, 5 mM ATP, 0.3 mM NADH, 10 mM phosphoenolpyruvate (PEP), 18 U/ml LDH, 18 U/ml pyruvate kinase (PK), and 25 l homogenate. Enzyme activity was measured using a spectrophotometric method (45), as modified by our laboratory (9). All samples were measured in duplicate at 37°C.…”