Rat muscle blood flows as a function of time during prolonged slow treadmill exercise

The purpose of these experiments was to follow blood flows (BF) within and among rat hindlimb skeletal muscles as a function of time during prolonged low-speed treadmill locomotion. Male Sprague-Dawley rats were chronically instrumented with two Silastic catheters, one in the ascending aorta via the right carotid artery for microsphere infusion and one in the left renal artery for arterial reference blood sample withdrawal. BFs were measured, using the radio-labeled microsphere technique, within and among 23 major skeletal muscles of rats before exercise and during treadmill locomotion at 15 m/min at 0.5, 1, 5, 15, 30, 54, and 71 min of exercise. During preexercise, BF was highest to deeply situated slow-twitch muscles (210 ml . min-1 . 100 g-1 in vastus intermedius) in the antigravity extensor muscle groups. During the 1st min of exercise each of the hindlimb muscles displayed one of four general BF patterns. 1) Many muscles had an "overshoot" in BF during the first 30 s of exercise; 2) some muscles attained steady-state exercise levels in the first 30 s of exercise; 3) others showed a decrease below preexercise levels; and 4) some muscles showed no change from preexercise. Most muscles showed a gradual increase in BF from 5 min through 54 min of exercise. The elevations in BF over preexercise were primarily directed to fast-twitch oxidative muscle fibers in the antigravity extensor muscles, and BFs to extensor muscle groups were generally higher than those to flexor muscle groups. The data demonstrate that BFs within and among rat muscles are heterogeneous, both before exercise and during prolonged low-intensity treadmill walking to fatigue. Mechanisms regulating the distribution of flow to the muscles remain to be elucidated.