Time-courses of perfusion and phosphocreatine in rat leg during low-level exercise and recovery

PURPOSE: To develop a noninvasive protocol for measuring local perfusion and metabolic demand in muscle tissue with sufficient sensitivity and time resolution to monitor kinetics at the onset of low-level exercise and during recovery. MATERIALS AND METHODS: Capillary-level perfusion, the critical factor that determines oxygen and substrate delivery to active muscle, was measured by an arterial spin labeling (ASL) technique optimized for skeletal muscle. Phosphocreatine (PCr) kinetics, which signal the flux of oxidative phosphorylation, were measured by (31)P MR spectroscopy. Perfusion and PCr measurements were made in parallel studies before, during, and after three different intensities of low-level, stimulated exercise in rat hind limb. RESULTS: The data reveal close coupling between the perfusion response and PCr changes. The onset and recovery time constants for PCr changes were independent of contractile force over the range of forces studied. Perfusion time constants during both onset of exercise and recovery tended to increase with contractile force. CONCLUSION: These results demonstrate that the protocol implemented can be useful for probing the mechanisms that control skeletal muscle blood flow, the physiological limits to muscle performance, and the causes for the attenuated exercise-induced hyperemia observed in disease states.