The O2 cost of the tension-time integral in isolated single myocytes during fatigue

Hepple RT, Howlett RA, Kindig CA, Stary CM, Hogan MC. The O-2 cost of the tension-time integral in isolated single myocytes during fatigue. Am J Physiol Regul Integr Comp Physiol 298: R983-R988, 2010. First published February 3, 2010; doi: 10.1152/ajpregu.00715.2009.-One proposed explanation for the (V) over dotO(2) slow component is that lower-threshold motor units may fatigue and develop little or no tension but continue to use O-2, 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 Delta intracellular PO2 (PIO2; a proxy for (V) over dotO(2)), and developed isometric tension was monitored to allow calculation of the time-integrated tension (TxT). Although peak Delta PIO2 was 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 Delta PIO2 and TxT rose proportionally with contraction frequency in SF and FF, resulting in Delta PIO2/TxT being identical between groups. At fatigue, TxT fell dramatically in both groups, but Delta PIO2 decreased proportionately only in the FF group, resulting in an increase in Delta PIO2/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 O-2 cost of contractions that could contribute to the (V) over dotO(2) slow component seen in whole body exercise.