Altered Contraction Frequency Yet Unchanged Duty Cycle Does Not Affect Single Myocyte Intracellular PO2 Kinetics

Abstract

0097 Single myocyte intracellular PO2 (PiO2) onset kinetics become more rapid as contraction frequency increases, whereas off-kinetics remain invariant. PURPOSE: This study examined whether cells subjected to electrical stimulation protocols consisting of altered contraction frequency but identical duty cycle would show differing kinetics for changes in PiO2, whether at the onset or cessation of contractions. METHODS: Single intact skeletal muscle cells (n = 11) were isolated from Xenopus lumbrical muscle, injected with an O2 sensitive porphyrin dye and stimulated electrically (200 ms trains, 70 Hz, 1 ms duration) to contract isometrically in a perfusion chamber with an initial PiO2 of ∼ 30 Torr. Cells were stimulated for 2 min at both a high frequency (HF) of 1 contraction every 2 s and a low frequency (LF) of 1 contraction every 4s in a randomized order with 15 min rest between trials. Contraction durations of 250 ms (HF) and 500 ms (LF) were utilized to maintain identical duty cycles. PiO2 was measured continuously throughout the protocol via phosphorescence quenching. The fall in PiO2 is proportional to the rise in VO2 in accordance with Fick's Law. RESULTS: Force production at the end of the stimulation protocol was significantly lower in HF compared to LF (56 ± 8.0 vs. 73 ± 7.0 % of initial). Initial resting PiO2 (HF: 33.7 ± 1.9 vs. LF: 31.4 ± 1.2 Torr), steady-state PiO2 following isometric contractions (HF: 8.8 ± 1.7 vs. LF: 11.1 ± 2.0 Torr) and PiO2 following recovery (HF: 30.3 ± 0.6 vs. LF: 29.3 ± 0.8 Torr) were all not significantly different (p>0.05) between trials. Mean response time (MRT), calculated as time to 63% of the difference between initial and final PiO2, was not different between treatments for either the on-response (HF: 48.7 ± 4.4 vs. LF: 52.5 ± 4.9 s) or off-response (HF: 107.9 ± 10.8 vs. LF: 116.9 ± 11.7 s). CONCLUSIONS: These data suggest that altered contraction frequency yet unchanged duty cycle does not affect either the speed or magnitude of the fall in PiO2 at contraction onset. Supported by NIH AR40155 and 1F32 AR48461. RAH and CAK are Parker B. Francis Fellows.