The O2cost of the tension-time integral in isolated single myocytes during fatigue

Abstract

One proposed explanation for the V̇o2slow component is that lower-threshold motor units may fatigue and develop little or no tension but continue to use O2, thereby resulting in a dissociation of cellular respiration from force generation. The present study used intact isolated single myocytes with differing fatigue resistance profiles to investigate the relationship between fatigue, tension development, and aerobic metabolism. Single Xenopus skeletal muscle myofibers were allocated to a fast-fatiguing (FF) or a slow-fatiguing (SF) group, based on the contraction frequency required to elicit a fall in tension to 60% of peak. Phosphorescence quenching of a porphyrin compound was used to determine Δ intracellular Po2(PiO2; a proxy for V̇o2), and developed isometric tension was monitored to allow calculation of the time-integrated tension (TxT). Although peak ΔPiO2was not different between groups ( P = 0.36), peak tension was lower ( P < 0.05) in SF vs. FF (1.97 ± 0. 17 V vs. 2. 73 ± 0.30 V, respectively) and time to 60% of peak tension was significantly longer in SF vs. FF (242 ± 10 s vs. 203 ± 10 s, respectively). Before fatigue, both ΔPiO2and TxT rose proportionally with contraction frequency in SF and FF, resulting in ΔPiO2/TxT being identical between groups. At fatigue, TxT fell dramatically in both groups, but ΔPiO2decreased proportionately only in the FF group, resulting in an increase in ΔPiO2/TxT in the SF group relative to the prefatigue condition. These data show that more fatigue-resistant fibers maintain aerobic metabolism as they fatigue, resulting in an increased O2cost of contractions that could contribute to the V̇o2slow component seen in whole body exercise.