Adrenoreceptor effects on rat muscle blood flow during treadmill exercise.

The purpose of this study was to examine the effects of the adrenergic receptors on the distribution of blood flow within and among skeletal muscles in rats. Blood flow was measured with the radiolabeled microsphere technique before exercise and during treadmill exercise at 15 or 60 m/min. Alpha- (phentolamine) or beta- (propranolol) adrenergic blocking drugs were administered, and then blood flow was measured and results compared with those from saline-treated rats. Before exercise, alpha-blockade caused increases in total muscle blood flow and in all fast-twitch muscles, whereas muscles composed of greater than 20% slow-twitch fibers showed no effect. During exercise at 15 m/min, the normal increase in total muscle blood flow was attenuated by alpha-blockade. Compared with controls, blood flow was less in the high-oxidative (fast and slow) muscle fiber areas of extensor muscles, whereas blood flow to white areas of extensor muscles was increased. beta-Blockade tended to decrease muscle blood flow before exercise and during exercise at 15 m/min with no apparent relationship between the effects of blockade on blood flow and muscle fiber type. These effects of beta-blockade were not apparent during exercise at 60 m/min. We conclude that before exercise alpha-receptor effects are limited to fast muscle, whereas beta-receptor influences are independent of fiber type, beta-receptors contribute to the initial hyperemia of exercise at 15 m/min, and beta-receptor influence is inversely related to metabolic rate.