It has been suggested that skeletal muscle O2 uptake (V̇o2) kinetics follow a first-order control model. Consistent with that, V̇o2 should show both 1) similar onset kinetics and 2) an on-off symmetry across submaximal work intensities regardless of the metabolic perturbation. To date, consensus on this issue has not been reached in whole body studies due to numerous confounding factors associated with O2 availability and fiber-type recruitment. To test whether single myocytes demonstrate similar intracellular Po2 (PiO2) on- and off-transient kinetics at varying work intensities, we studied Xenopus laevis single myocyte ( n = 8) PiO2 via phosphorescence quenching during two bouts of electrically induced isometric muscle contractions of 200 (low)- and 400 (high)-ms contraction duration (1 contraction every 4 s, 15 min between trials, order randomized). The fall in PiO2, which is inversely proportional to the net increase in V̇o2, was significantly greater ( P < 0.05) during the high (24.1 ± 3.2 Torr) vs. low (17.4 ± 1.6 Torr) contraction bout. However, the mean response time (MRT; time to 63% of the overall change) for the fall in PiO2 from resting baseline to end contractions was not different (high, 77.8 ± 11.5 vs. low, 76.1 ± 13.6 s; P > 0.05) between trials. The initial rate of change at contraction onset, defined as ΔPiO2/MRT, was significantly greater ( P < 0.05) in high compared with low. PiO2 off-transient MRT from the end of the contraction bout to initial baseline was unchanged (high, 83.3 ± 18.3 vs. low, 80.4 ± 21.6 s; P > 0.05) between high and low trials. These data revealed that PiO2 dynamics in frog isolated skeletal myocytes were invariant despite differing contraction durations and, by inference, metabolic demands. Thus these findings demonstrate that mitochondria can respond more rapidly at the initial onset of contractions when challenged with an augmented metabolic stimulus in accordance with an apparent first-order rate law.
The relationship between spatial patterns of calcium waves and localization of Inositol 1,4,5-triphosphate receptor in rat hepatocytes