EFFECT OF DISUSE ON MUSCLE ENERGY-METABOLISM STUDIED BY INVIVO 31-PHOSPHORUS MAGNETIC-RESONANCE SPECTROSCOPY

Chronic unloading induces disuse atrophy in rat hindlimb muscles. The purpose of this study was to examine the effect of hindlimb suspension on energy metabolism in hindlimb muscles during tetanic contraction using 31-phosphorus magnetic resonance spectroscopy (P-31-MRS). Muscular contraction was induced by electrical stimulation of the sciatic nerve at 40 Hz. Energy metabolism was evaluated by measuring the relative intensities of phosphocreatine (PCr), inorganic phosphate (Pi), beta-ATP and phosphomonoester (PME). The intracellular pH was also estimated from the chemical shift of Pi. Two weeks of hindlimb suspension induced a greater weight loss in the soleus compared with the gastrocnemius muscle. The maximum tension at 40 Hz was significantly lower (P<0.05) in the hindlimb suspended group than in the control group. However, fatigability did not differ significantly. The energy level at rest was similar in the two groups. During electrical stimulation, the Pi/(Pi+PCr) ratio was higher in the hindlimb suspended group compared with the control group significantly (P<0.01), but the intracellular pH did not differ. These results suggest that the metabolic capacity of hindlimb muscle decreases after a prolonged period of disuse.