Muscle Studies by P-31 MRS

P-31 Magnetic resonance spectroscopy (P-31 MRS) yields information about skeletal muscle metabolism in vivo which is hard to obtain in other ways. The ability of muscle to alter ATP turnover allows us to probe energy metabolism and cellular pH homeostasis dynamically by analyzing the kinetics of pH, [PCr], [Pi], and [ADP] during exercise and recovery. Quantitative interpretation depends on understanding the relevant physiology, and exploiting three principles: first, the creatine kinase (CK) equilibrium ensures that any imbalance between ATP supply (by anaerobic glycolysis and by oxidative metabolism) and ATP demand (by basal processes, plus that related to contraction) is balanced by net breakdown or resynthesis of PCr; second, any imbalance between H+ production (by anaerobic glycolysis, and by PCr synthesis) and H+ disposal (consumption by PCr breakdown, and H+ efflux) results in a pH change moderated by cytosolic buffers; and third, the CK system acts as comparator of ATP supply and demand with an important role in the feedback regulation of ATP production. Here, we review the application of these interpretative principles to exercise/recovery studies of various kinds, and some other uses of 31P MRS data in muscle.