Phosphorylating pathways and fatigue development in contracting Xenopus single skeletal muscle fibers

To investigate the differential contribution of oxidative and substrate-lev el phosphorylation to force production during repetitive, maximal tetanic c ontractions, single skeletal muscle fiber performance was examined under co nditions 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 incre asing frequencies (0.25, 0.33, and 0.5 Hz), with each frequency lasting 2 m in. Two separate work bouts were conducted, with the perfusate PO2 being ei ther 0 or 159 mmHg. No significant (P < 0.05) difference was found in the i nitial peak tensions (P-0) between the high (334 +/- 57 kPa) and the low (3 25 +/- 41 kPa) PO2 treatment. No significant difference in P was observed b etween the treatments during the first 50 s. However, a significant differe nce in force production was observed between the high (P/P-0 = 0.96 +/- 0.0 2) and the low PO2 condition (P/P-0 = 0.92 +/- 0.02) by 60 s of work. After 60 s, steady-state force production was maintained during the high compare d with the low PO2 condition until stimulation frequency was increased, at which point developed tension during the high PO2 condition began to declin e. Time to fatigue (P/P-0 = 0.3) was reached significantly sooner during th e low (250 +/- 16 s) than the high PO2 condition (367 +/- 28 s). These resu lts demonstrate that during the first 50 s of 0.25-Hz contractions, substra te-level phosphorylation has the capacity to maintain force and ATP hydroly sis when oxidative phosphorylation is absent. This period was followed by a n oxygen-dependent phase in which force generation was maintained during th e 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 oxidativ e phosphorylation was reached.