Arterial baroreceptors have minimal physiological effects on adrenal medullary secretion

The influence of arterial baroreceptors on secretion of catecholamines from the adrenal medulla was evaluated by several methods. Conscious mongrel dogs with surgically denervated hearts were hemorrhaged until an estimated 16% of their blood volume had been removed. On a separate day they were anesthetized and their blood pressure was lowered with intravenous nitroglycerin. Neither of these maneuvers produced appreciable increases in heart rate in these dogs. In contrast, in a group of sham-operated control dogs, hemorrhage induced a mean increase in heart rate of 20 beats/min (P<0.05), and nitroglycerin-induced hypotension induced an increase of 50 beats/min (P<0.05). In a separate group of conscious dogs with aortic arch denervation but intact cardiac nerves, occlusion of the common carotid arteries for 5 min increased blood pressure and heart rate significantly but elicited only small, insignificant increases in plasma epinephrine and norepinephrine; the peak concentration of epinephrine achieved was considerably less than the amount necessary to cause appreciable effects on blood pressure and heart rate as determined in another experiment by infusing varying amounts of epinephrine into conscious, cardiac-denervated dogs. We conclude that the arterial baroreceptor reflex, within the range of activity likely to occur during most physiological and pathophysiological adjustments in the conscious dog, exerts only minimal effects on the secretion of catecholamines from the adrenal medulla.