Fall in intracellular Po2at the onset of contractions inXenopussingle skeletal muscle fibers

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Abstract
It remains uncertain whether the delayed onset of mitochondrial respiration on initiation of muscle contractions is related to O2availability. The purpose of this research was to measure the kinetics of the fall in intracellular Po2at the onset of a contractile work period in rested and previously worked single skeletal muscle fibers. Intact single skeletal muscle fibers ( n = 11) from Xenopus laevis were dissected from the lumbrical muscle, injected with an O2-sensitive probe, mounted in a glass chamber, and perfused with Ringer solution (Po2= 32 ± 4 Torr and pH = 7.0) at 20°C. Intracellular Po2was measured in each fiber during a protocol consisting sequentially of 1-min rest; 3 min of tetanic contractions (1 contraction/2 s); 5-min rest; and, finally, a second 3-min contractile period identical to the first. Maximal force development and the fall in force (to 83 ± 2 vs. 86 ± 3% of maximal force development) in contractile periods 1 and 2, respectively, were not significantly different. The time delay (time before intracellular Po2began to decrease after the onset of contractions) was significantly greater ( P < 0.01) in the first contractile period (13 ± 3 s) compared with the second (5 ± 2 s), as was the time to reach 50% of the contractile steady-state intracellular Po2(28 ± 5 vs. 18 ± 4 s, respectively). In Xenopus single skeletal muscle fibers, 1) the lengthy response time for the fall in intracellular Po2at the onset of contractions suggests that intracellular factors other than O2availability determine the on-kinetics of oxidative phosphorylation and 2) a prior contractile period results in more rapid on-kinetics.