Fall in intracellular PO2 at the onset of contractions in Xenopus single skeletal muscle fibers

It remains uncertain whether the delayed onset of mitochondrial respiration on initiation of muscle contractions is related to O-2 availability. The p urpose of this research was to measure the kinetics of the fall in intracel lular PO2 at the onset of a contractile work period in rested and previousl y worked single skeletal muscle fibers. Intact single skeletal muscle fiber s (n = 11) from Xenopus laevis were dissected from the lumbrical muscle, in jected with an O-2-sensitive probe, mounted in a glass chamber, and perfuse d with Ringer solution (PO2 = 32 +/- 4 Torr and pH = 7.0) at 20 degreesC. I ntracellular PO2 was measured in each fiber during a protocol consisting se quentially 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, re spectively, were not significantly different. The time delay (time before i ntracellular PO2 began to decrease after the onset of contractions) was sig nificantly 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 c ontractile steady-state intracellular PO2 (28 +/- 5 vs. 18 +/- 4 s, respect ively). In Xenopus single skeletal muscle fibers, 1) the lengthy response t ime for the fall in intracellular PO2 at the onset of contractions suggests that intracellular factors other than O-2 availability determine the on-ki netics of oxidative phosphorylation and 2) a prior contractile period resul ts in more rapid on-kinetics.