Loss of Baroreflex Bradycardia in Renal Hypertensive Rabbits

The degree of reflex bradycardia elicited by a sudden, brief rise in arterial pressure was used as an index of sino-aortic reflex activity in conscious rabbits with early renal hypertension. Change in heart rate was measured every 6 seconds after injected angiotensin which caused a rise in blood pressure of 15 to 60 mm Hg. Ninety-one tests for reflex bradycardia were made in 30 normal rabbits; 14 of them were tested intermittently during the 1 to 40 day period following unilateral nephrectomy and latex encapsulation of the opposite kidney. A total of 62 tests were made after arterial pressure had risen 10 to 80 mm Hg, average 35, above control values. Average decrease in heart rate was significantly less and frequency of negligible reflex bradycardia was much higher in hypertensives than controls. Individual rabbits showed no tendency for reflex bradycardia to return toward normal magnitude. However, the vasomotor component of the sino-aortic reflex mechanism in renal hypertensive rabbits buffered the pressor response to angiotensin in a normal manner. Resetting of sino-aortic reflex activity under hypertensive conditions is discussed.