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Department of Medicine, University of California, San Diego, California 92093-0623
To investigate the differential contribution of oxidative and substrate-level phosphorylation to force production during repetitive, maximal tetanic contractions, single skeletal muscle fiber performance was examined under conditions of high-oxygen availability and anoxia. Tetanic force development (P) was measured in isolated, single type-1 muscle fibers (fast twitch; n = 6) dissected from Xenopus lumbrical muscle while being stimulated at increasing frequencies (0.25, 0.33, and 0.5 Hz), with each frequency lasting 2 min. Two separate work bouts were conducted, with the perfusate PO2 being either 0 or 159 mmHg. No significant (P < 0.05) difference was found in the initial peak tensions (P0) between the high (334 ± 57 kPa) and the low (325 ± 41 kPa) PO2 treatment. No significant difference in P was observed between the treatments during the first 50 s. However, a significant difference in force production was observed between the high (P/P0 = 0.96 ± 0.02) and the low PO2 condition (P/P0 = 0.92 ± 0.02) by 60 s of work. After 60 s, steady-state force production was maintained during the high compared with the low PO2 condition until stimulation frequency was increased, at which point developed tension during the high PO2 condition began to decline. Time to fatigue (P/P0 = 0.3) was reached significantly sooner during the low (250 ± 16 s) than the high PO2 condition (367 ± 28 s). These results demonstrate that during the first 50 s of 0.25-Hz contractions, substrate-level phosphorylation has the capacity to maintain force and ATP hydrolysis when oxidative phosphorylation is absent. This period was followed by an oxygen-dependent phase in which force generation was maintained during the high PO2 condition (but not during the low PO2 condition) until the onset of a final fatiguing phase, at which a calculated maximal rate of oxidative phosphorylation was reached.
oxygen; glycolysis; aerobic metabolism; anaerobic metabolism; oxygen uptake; phosphocreatine; oxidative phosphorylation
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