Intracellular Po2kinetics at different contraction frequencies inXenopussingle skeletal muscle fibers

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Abstract
Increasing contraction frequency in single skeletal muscle fibers has been shown to increase the magnitude of the fall in intracellular Po2(PiO2), reflecting a greater metabolic rate. To test whether PiO2kinetics are altered by contraction frequency through this increase in metabolic stress, PiO2was measured in Xenopus single fibers ( n = 11) during and after contraction bouts at three different frequencies. PiO2was measured via phosphorescence quenching at 0.16-, 0.25-, and 0.5-Hz tetanic stimulation. The kinetics of the change in PiO2from 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 PiO2. As predicted, the fall in PiO2from 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 PiO2was 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 PiO2off-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 PiO2is dependent on contraction frequency, whereas the rate of recovery following contractions is independent of either the magnitude of the fall in PiO2from 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.

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