Submitted on April 10, 2006
Accepted on December 12, 2006
INTRACELLULAR PO2 KINETICS AT DIFFERENT CONTRACTION FREQUENCIES IN XENOPUSSINGLE SKELETAL MUSCLE FIBERS
Richard A. Howlett1, Casey (Deceased) A Kindig2, and Michael C. Hogan3*
1 Dept. of Medicine, 0623A, University of California, San Diego, La Jolla, California, United States
2 Medicine, University of California-San Diego, La Jolla, California, United States
3 Department of Medicine, 0623, University of California-San Diego, La Jolla, California, United States
* To whom correspondence should be addressed. E-mail: mchogan{at}ucsd.edu.
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 PiO2 kinetics are altered by contraction frequency through this increase in metabolic stress, PiO2 was measured in Xenopus fibers (n=11) during and after contraction bouts at three different frequencies. PiO2 was measured via phosphorescence quenching at 0.16, 0.25 and 0.5Hz tetanic stimulation. The kinetics of the change in PiO2 from rest to end-contraction and back 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 PiO2 from baseline (
PiO2) following contractions was progressively greater at 0.5 and 0.25Hz than 0.16Hz (32.8±2.1 and 29.3±2.0 Torr vs. 23.6±2.2 Torr) as metabolic demand was greater. The MRT for the decrease in PiO2 was progressively faster at the higher frequencies (0.5Hz: 45.3±4.5s; 0.25Hz: 63.3±4.1s; 0.16Hz: 78.0±4.1s) suggesting faster accumulation of stimulators of oxidative phosphorylation. The MRT for PiO2 off-kinetics (0.5Hz: 84.0±11.7s; 0.25Hz: 79.1±8.4s; 0.16Hz: 81.1±8.3s) was not different between trials. These data demonstrate that the rate of the fall in PiO2 is dependent on contraction frequency while the rate of recovery following contractions is independent of either the magnitude of PiO2 or the contraction frequency. This suggests that stimulation frequency plays an integral role in setting the initial metabolic response to work in single fibers, possibly due to temporal recovery between contractions, but it does not determine recovery kinetics.
