Intersubject differences in the effect of acidosis on phosphocreatine recovery kinetics in muscle after exercise are due to differences in proton efflux rates

31P magnetic resonance spectroscopy provides the possibility of obtaining bioenergetic data during skeletal muscle exercise and recovery. The time constant of phosphocreatine (PCr) recovery (tPCr) has been used as a measure of mitochondrial function. However, cytosolic pH has a strong influence on the kinetics of PCr recovery, and it has been suggested that tPCr should be normalized for end-exercise pH. A general correction can only be applied if there are no intersubject differences in the pH dependence of tPCr. We investigated the pH dependence of tPCr on a subject-by-subject basis. Furthermore, we determined the kinetics of proton efflux at the start of recovery. Intracellular acidosis slowed PCr recovery, and the pH dependence of tPCr differed among subjects, ranging from -33.0 to -75.3 s/pH unit. The slope of the relation between tPCr and end-exercise pH was positively correlated with both the proton efflux rate and the apparent proton efflux rate constant, indicating that subjects with a smaller pH dependence of tPCr have a higher proton efflux rate. Our study implies that simply correcting tPCr for end-exercise pH is not adequate, in particular when comparing patients and control subjects, because certain disorders are characterized by altered proton efflux from muscle fibers. Copyright © 2007 the American Physiological Society.