. Relationship between intracellular PO2 recovery kinetics and fatigability in isolated single frog myocytes. J Appl Physiol 98: 2316 -2319, 2005. First published February 3, 2005 doi:10.1152/japplphysiol.00355.2004.-In single frog skeletal myocytes, a linear relationship exists between "fatigability" and oxidative capacity. The purpose of this investigation was to study the relationship between the intracellular PO2 (PIO 2 ) offset kinetics and fatigability in single Xenopus laevis myocytes to test the hypothesis that PIO 2 offset kinetics would be related linearly with myocyte fatigability and, by inference, oxidative capacity. Individual myocytes (n ϭ 30) isolated from lumbrical muscle were subjected to a 2-min bout of isometric peak tetanic contractions at either 0.25-or 0.33-Hz frequency while PIO 2 was measured continuously via phosphorescence quenching techniques. The mean response time (MRT; time to 63% of the overall response) for PIO 2 recovery from contracting values to resting baseline was calculated. After the initial square-wave constantfrequency contraction trial, each cell performed an incremental contraction protocol [i.e., frequency increase every 2 min from 0.167, 0.25, 0.33, 0.5, 1.0, and 2.0 Hz until peak tension fell below 50% of initial values (TTF)]. TTF values ranged from 3.39 to 10.04 min for the myocytes. The PIO 2 recovery MRT ranged from 26 to 146 s. A significant (P Ͻ 0.05), negative relationship (MRT ϭ Ϫ12.68TTF ϩ 168.3, r 2 ϭ 0.605) between TTF and PIO 2 recovery MRT existed. These data demonstrate a significant correlation between fatigability and oxidative phosphorylation recovery kinetics consistent with the notion that oxidative capacity determines, in part, the speed with which skeletal muscle can recover energetically to alterations in metabolic demand. muscle fiber; fatigue; metabolic control; oxidative capacity AN INVERSE, PROPORTIONAL RELATIONSHIP has been proposed to exist between the speed of phosphocreatine (PCr) resynthesis and muscle mitochondrial capacity (18,22). Previous investigations in both animals (23) and humans (e.g., Refs. 14, 20, 21) have confirmed this postulation. Specifically, Paganini and colleagues (23) demonstrated a linear relationship between PCr recovery times after electrically stimulated contractions and citrate synthase activity in rat skeletal muscle.O 2 uptake (V O 2 ) onset kinetics are considered dependent, at least in part, on maximal aerobic capacity. Indeed, aerobic exercise training that induces significant increases in muscle oxidative capacity also speeds V O 2 kinetics (4, 8, 10). However, this can occur before any discernible increase in maximal V O 2 and muscle oxidative capacity (24). To date, no rigorous investigation studying the relationship between maximal aerobic capacity and V O 2 off-kinetics exists. However, given that PCr and V O 2 kinetics appear to be reasonably well matched (1,19,26), it is possible that a strong relationship exists between maximal aerobic capacity and V O 2 off-kinetics.The purpose of the present inv...