Original Research Time-Courses of Perfusion and Phosphocreatine in Rat Leg During Low-Level Exercise and Recovery Kenneth I. Marro, PhD, 1 * Jennifer L. Olive, PhD, 2 Outi M. Hyyti, MS, 1 and Martin J. Kushmerick, MD, PhD 1,3 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 recov- ery. Materials and Methods: Capillary-level perfusion, the crit- ical factor that determines oxygen and substrate delivery to active muscle, was measured by an arterial spin labeling (ASL) technique optimized for skeletal muscle. Phosphocre- atine (PCr) kinetics, which signal the flux of oxidative phos- phorylation, were measured by 31 P MR spectroscopy. Per- fusion 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 perfu- sion response and PCr changes. The onset and recovery time constants for PCr changes were independent of con- tractile 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 atten- uated exercise-induced hyperemia observed in disease states. Key Words: capillary perfusion; blood flow; metabolic de- mand; stimulated exercise; skeletal muscle; arterial spin labeling J. Magn. Reson. Imaging 2007;25:1021–1027. © 2007 Wiley-Liss, Inc. IN THIS WORK we describe a noninvasive protocol for acquiring parallel perfusion measurements and 31 P magnetic resonance spectroscopy (MRS) measure- ments of high-energy phosphorous metabolites in skel- etal muscle with sufficient time resolution and sensi- tivity to monitor kinetics at the onset of low-level exercise and during recovery. Our research group has previously developed a quantitative, magnetic reso- nance (MR)-based perfusion measurement technique that is particularly well suited for skeletal muscle (1–3). This technique, flow-driven arterial water stimulation with elimination of tissue signal (FAWSETS), is a form of arterial spin labeling (ASL). In this work we have combined FAWSETS perfusion measurements with standard 31 P-MRS measurements of metabolic activity. Our primary goal was to achieve adequate time resolu- tion to determine the time-course of perfusion and phosphocreatine (PCr) changes during entire cycles of exercise and recovery. Our secondary goal was to probe the limits of sensitivity to perfusion for the FAWSETS technique at low intensity exercise. In normal subjects, skeletal muscle blood flow in- creases rapidly during exercise and achieves a steady state that is dependent on metabolic demand (4,5). Ex- ercise-related increases in muscle perfusion are atten- uated in a number of diseases, including insulin resis- tance and diabetes (6 – 8), congestive heart failure (9 –11), compartment syndrome (12,13), peripheral vas- cular disease (PVD) (14,15), and systemic sclerosis (scleroderma) (16). It is likely that the exercise intoler- ance, muscle fatigue, and atrophy related to these dis- eases arise from insufficient delivery of oxygen and sub- strates to meet metabolic demands during exercise. A noninvasive protocol for probing the relationship be- tween muscle perfusion and metabolic demand within the same volume of tissue could provide new insights into the mechanisms involved in regulation of blood flow as well as the reasons for regulatory failure in disease states. Most information on muscle blood flow is derived from noninvasive measurements made at the level of larger arteries or from invasive measurements made at the capillary level (5,17). ASL techniques represent a substantial advancement in this field because they can provide noninvasive quantification of capillary-level perfusion (18 –21). The widest application of ASL tech- 1 Department of Radiology, University of Washington, Seattle, Washing- ton, USA. 2 Department of Health and Sport Sciences, University of Louisville, Louisville, Kentucky, USA. 3 Department of Physiology and Biophysics, University of Washington, Seattle, Washington, USA. Contract grant sponsor: University of Washington Royalty Research Fund; Contract grant sponsor: National Institutes of Health; Contract grant number: HL64946, AR41928. *Address reprint requests to: K.M., PhD, Department of Radiology, Box 357115, University of Washington, Seattle, WA 98195-7115. E-mail: marro@u.washington.edu Received June 9, 2006; Accepted December 11, 2006. DOI 10.1002/jmri.20903 Published online in Wiley InterScience (www.interscience.wiley. com). JOURNAL OF MAGNETIC RESONANCE IMAGING 25:1021–1027 (2007) © 2007 Wiley-Liss, Inc. 1021