Effects of Catecholamines and Atropine on Cardiovascular Response to Exercise in the Dog

The manner in which the administration of epinephrine, norepinephrine, isoproterenol, and atrophic modifies the cardiovascular response to exercise has been studied in 12 dogs.In control observations, exercise increased the cardiac output by 116 per cent, the heart rate by 102 per cent, and the stroke volume by 8 per cent. When epinephrine was infused during identical exercise, the cardiac output was similar; however, the stroke volume was increased by 50 per cent. During infusion of norepinephrine, the cardiac output was reduced by 11 per cent compared with that during control exercise, and the stroke volume was increased by 25 per cent. With isoproterenol, the cardiac output exceeded that during control exercise, but the stroke volume was unchanged.Prior to administration of atropine, the cardiac output at rest averaged 4.2 L. per minute. After injection of atropine, the cardiac output averaged 3.7 L. per minute, the heart rate increased from 121 to 249 beats per minute, and the stroke volume decreased from 35 to 15 ml. Before atropinization, the cardiac output, heart rate, and stroke volume during exercise were increased by 95, .90, and 2 per cent over the resting values; after atropinization, the corresponding increases were 110, 7, and 100 per cent.Thus, while cardiac output during exercise normally is regulated mainly by changes in the heart rate, the stroke volume may make a major contribution under other circumstances.