Mechanisms of baroreceptor-induced changes in heart rate.

THAMES, MARC D., AND HERMES *4. KONTOS. Mechanisms of horeceptor-induced changes in heart rate. Am. J. Physiol. 2 18( I> : 25 l256. 1970.-The changes in heart rate in response to nitroglycerin-induced hypotension or phenylephrine-induced hypertension were studied in 8 dogs unanesthetized, anesthetized with chloralose and urethan, or with chloralose and morphine before and after beta-adrenergic receptor blockade with propranolol or parasympathetic efferent blockade with atropine. Propranolol reduced significantly but did not abolish the tachycardia in response to hypotension, and reduced significantly but did not abolish the bradycardia in response to hypertension. Atropine reduced the tachycardia in response to nitroglycerin and abolished the bradycardia in response to hypertension. In a second group of 8 dogs propranolol reduced significantly but did not abolish the increase in heart rate in response to bilateral carotid arterial occlusion. In a third group of 6 dogs atropine reduced but did not abolish the bradycardia in response to electrical stimulation of the carotid sinus nerves. These results show that baroreceptorinduced changes in heart rate are mediated by reciprocal alterations in both sympathetic and parasympathetic cardiac efferent activity.