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[sup.31]P magnetic resonance spectroscopy provides the possibility of obtaining bioenergetic data during skeletal muscle exercise and recovery. The time constant of phosphocreatine (PCr) recovery ([[tau].sub.PCr]) 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 [[tau].sub.PCr] 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 [[tau].sub.PCr]. We investigated the pH dependence of [[tau].sub.PCr] 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 [[tau].sub.PCr] differed among subjects, ranging from -33.0 to -75.3 s/pH unit. The slope of the relation between [[tau].sub.PCr] 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 [[tau].sub.PCr] have a higher proton efflux rate. Our study implies that simply correcting [[tau].sub.PCr] 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. [sup.31]P magnetic resonance spectroscopy; skeletal muscle; oxidative capacity; mitochondrial function; intracellular pH doi:10.1152/ajpcell.00023.2006

High-resolution magnetic resonance imaging (MRI) has evolved into one of the major non-invasive tools to study the healthy and diseased mouse heart. This study presents a Cartesian CINE MRI protocol based on a fast low-angle shot sequence with a navigator echo to generate cardiac triggering and respiratory gating signals retrospectively, making the use of ECG leads and respiratory motion sensors obsolete. MRI of the in vivo mouse heart using this sequence resulted in CINE images with no detectable cardiac and respiratory motion artefacts. The retrospective method allows for steady-state imaging of the mouse heart, which is essential for quantitative contrast-enhanced MRI studies. A comparison was made between prospective and retrospective methods in terms of the signal-to-noise ratio and the contrast-to-noise ratio between blood and myocardial wall, as well as global cardiac functional indices: end-diastolic volume, end-systolic volume, stroke volume and ejection fraction. The retrospective method resulted in almost constant left-ventricle wall signal intensity throughout the cardiac cycle, at the expense of a decrease in the signal-to-noise ratio and the contrast-to-noise ratio between blood and myocardial wall as compared with the prospective method. Prospective and retrospective sequences yielded comparable global cardiac functional indices. The largest mean relative difference found was 8% for the end-systolic volume. Copyright © 2006 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]