Journal
JOURNAL OF APPLIED PHYSIOLOGY
Volume 90, Issue 5, Pages 1871-1876Publisher
AMER PHYSIOLOGICAL SOC
DOI: 10.1152/jappl.2001.90.5.1871
Keywords
oxygen uptake; exercise; oxidative phosphorylation; mitochondria; phosphorescence; cellular respiration
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Funding
- NIAMS NIH HHS [AR-40155] Funding Source: Medline
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It remains uncertain whether the delayed onset of mitochondrial respiration on initiation of muscle contractions is related to O-2 availability. The purpose of this research was to measure the kinetics of the fall in intracellular PO2 at 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 O-2-sensitive probe, mounted in a glass chamber, and perfused with Ringer solution (PO2 = 32 +/- 4 Torr and pH = 7.0) at 20 degreesC. Intracellular PO2 was 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 PO2 began 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 PO2 at the onset of contractions suggests that intracellular factors other than O-2 availability determine the on-kinetics of oxidative phosphorylation and 2) a prior contractile period results in more rapid on-kinetics.
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