Intracellular acidification and volume increases explain R2 decreases in exercising muscle

Exercise-induced decreases in the H-1 transverse relaxation rate (R-2) of muscle have been well documented, but the mechanism remains unclear. In this study, the hypothesis was tested that R-2 decreases could be explained by pH decreases and apparent intracellular volume (V-i') increases. P-31 and H-1 spectroscopy, biexponential R-2 analysis, and imaging were performed prior to and following fatiguing exercise in iodoacetate-treated (IAA, to inhibit glycolysis), NaCN-treated (to inhibit oxidative phosphorylation), and untreated frog gastrocnemii. In all exercised muscles, the apparent intracellular R-2 (R-2') and pH decreased, while intracellular osmolytes and V-i' increased. These effects were larger in NaCN-treated and untreated muscles than in IAA-treated muscles. Multiple regression analysis showed that pH and V-i' changes explain 70% of the R-2i' variance. Separate experiments in unexercised muscles demonstrated causal relationships between pH and R-2i' and between V-i' and R-2i'. These data indicate that the R-2 change of exercise is primarily an intracellular phenomenon caused by the accumulation of the end-products of anaerobic metabolism. In the NaCN-treated and untreated muscles, the R-2i' change increased as field strength increased, suggesting a role for pH-modulated chemical exchange. Magn Reson Med 47:14-23, 2002. (C) 2002 Wiley-Liss, Inc.