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J Appl Physiol 102: 1456-1461, 2007. First published January 4, 2007; doi:10.1152/japplphysiol.00422.2006
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Intracellular PO₂ kinetics at different contraction frequencies in Xenopus single skeletal muscle fibers

Department of Medicine, University of California, San Diego, La Jolla, California

Submitted 10 April 2006 ; accepted in final form 12 December 2006

Increasing contraction frequency in single skeletal muscle fibers has been shown to increase the magnitude of the fall in intracellular PO₂ (PI_O2), reflecting a greater metabolic rate. To test whether PI_O2 kinetics are altered by contraction frequency through this increase in metabolic stress, PI_O2 was measured in Xenopus single fibers (n = 11) during and after contraction bouts at three different frequencies. PI_O2 was measured via phosphorescence quenching at 0.16-, 0.25-, and 0.5-Hz tetanic stimulation. The kinetics of the change in PI_O2 from resting baseline to end-contraction values and end contraction to rest were described as a mean response time (MRT) representing the time to 63% of the change in PI_O2. As predicted, the fall in PI_O2 from baseline following contractions was progressively greater at 0.5 and 0.25 Hz than at 0.16 Hz (32.8 ± 2.1 and 29.3 ± 2.0 Torr vs. 23.6 ± 2.2 Torr, respectively) since metabolic demand was greater. The MRT for the decrease in PI_O2 was progressively faster at the higher frequencies (0.5 Hz: 45.3 ± 4.5 s; 0.25 Hz: 63.3 ± 4.1 s; 0.16 Hz: 78.0 ± 4.1 s), suggesting faster accumulation of stimulators of oxidative phosphorylation. The MRT for PI_O2 off-kinetics (0.5 Hz: 84.0 ± 11.7 s; 0.25 Hz: 79.1 ± 8.4 s; 0.16 Hz: 81.1 ± 8.3 s) was not different between trials. These data demonstrate in single fibers that the rate of the fall in PI_O2 is dependent on contraction frequency, whereas the rate of recovery following contractions is independent of either the magnitude of the fall in PI_O2 from baseline or the contraction frequency. This suggests that stimulation frequency plays an integral role in setting the initial metabolic response to work in isolated muscle fibers, possibly due to temporal recovery between contractions, but it does not determine recovery kinetics.

oxidative metabolism; metabolic control; oxygen utilization