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The purpose of this study was to test the hypothesis that muscarinic cholinergic receptors are involved in the initial vasodilation in red muscle vascular beds of conscious rats performing slow locomotory exercise. Atropine sulfate (1 mg/kg, ia) was administered to one group of rats in which distribution of cardiac output was estimated with radiolabeled microspheres immediately before exercise while the animals were standing on the treadmill and at 30 s and 5 min of treadmill walking at 15 m/min. Blood flows within and among muscles in the atropine-treated animals were compared with flows in control rats that were given a sham injection of an equal volume of physiological saline. Heart rates were elevated above those of control animals in the atropinized rats during preexercise (+17%) and at 30 s of exercise (+15%). However, distributions and magnitudes of blood flows in nonmuscular tissues and within and among skeletal muscles were the same (P greater than 0.05) in atropinized and control rats during preexercise and at both exercise times, indicating that atropine had no effect on the distribution of cardiac output in the rats. It is concluded that muscarinic cholinergic receptors do not play a significant role in elevating muscle blood flow in conscious rats, either during the preexercise anticipatory phase or during slow locomotory exercise.
Armstrong et al. (Fri,) studied this question.