REGULATION OF OXYGEN-CONSUMPTION IN FAST-TWITCH AND SLOW-TWITCH MUSCLE

Phosphorus nuclear magnetic resonance spectra and steady-state O2 consumption rates were obtained from ex vivo arterially perfused cat biceps brachii (fast twitch) and soleus (slow twitch) muscles during and after periods of isometric twitch stimulation at 30-degrees-C. In the biceps muscles, steady-state O2 consumption increased and phosphocreatine (PCr) concentration decreased progressively with stimulation. PCr recovery after these stimulation periods followed first-order kinetics with a half time of 10 min. The results in the biceps could be explained by a feedback control of cellular respiration by ADP concentration. In the soleus, steady-state O2 consumption also increased and PCr concentration decreased as stimulation rates increased. The half time for PCr recovery in the soleus was approximately 5 min, but, in contrast to the pattern in the biceps, the kinetics was not first order. There was an overshoot during the recovery period in the PCr content of soleus and a corresponding undershoot of P(i) compared with resting values. Mitochondrial regulation by ADP is not sufficient to account for respiratory control in slow-twitch soleus. The respiration rate in neither muscle was dependent on the P(i) content. Thus we conclude that the mechanism of control of cellular respiration is both quantitatively and qualitatively different in fast- and slow-twitch skeletal muscle.