Loss of Baroreflex Bradycardia in Renal Hypertensive Rabbits

Abstract

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. p i them were tested intermittently during the 1 to 40 day period following unilateral riephrectomy 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 Rej.v setting of sino-aortic reflex activity under hypertensive conditions is dis-_ cussed. ADDITIONAL KEY WORDSheart rate reflex resetting of sino-aortic "reflexes baroreceptors and heart rate angiotensin and hypertension carotid sinus and aortic nerves conscious rabbits• Information concerning function of sinoaortic reflexes in man or in conscious laboratory animals with arterial hypertension is scanty, although it is important in the broader question of baroreflex resetting in hypertension. The term "reset" as applied to the sinoaortic baroreflex system has not been defined precisely in the literature, but is used here to mean that the effector response has returned to normal magnitude or rate under conditions that initially altered it. In this definition sino-aortic receptor activity, afferent or efferent nerve discharge can be substituted for effector response, the effector organs being, of course, the heart and blood vessels.

Alterations in the Baroreceptor Reflex in Conscious Dogs with Heart Failure

“…The higher baseline heart rate cannot explain our observation of a reduced sensitivity of the reflex; on the contrary, it has been noted that with higher initial heart rates the magnitude of reflex slowing is actually enhanced (16,20). The transient alterations in this reflex during exercise may be pertinent to our findings, since in both heart failure and during exercise heightened sympathetic tone is present (10,(25)(26)(27) and there is evidence that in states of high sympathetic tone there is a depression of parasympathetically mediated reflexes (28,29).…”

“…The observation in the present study that the animals with hypertrophy but without overt heart failure had less reduction of sensitivity than the group with overt heart failure suggests that the severity of this dysfunction of the baroreceptor reflex arc may be a function of the severity of the cardiac abnormality. Apparently, the sensitivity of this reflex is altered under other circumstances as well, since it has been shown to be accentuated during natural sleep (11) and depressed in hypertension in experimental animals (20) and man (21) and during anesthesia (13) and exercise (22,23) . 1 Baroreceptor Reflex in Heart Failure in man; resetting of the threshold without a change in sensitivity of the reflex has been produced by hypercapnia in normal subjects (24).…”

Carotid sinus baroreceptor reflex control of the circulation in medial sclerotic and renal hypertensive rabbits and its modification by the aortic baroreceptors.

“…Previous studies of the baroreceptor control of blood pressure in hypertensive animals have indicated that this control is either normal (McCubbin et al, 1956;Alexander and DeCuir, 1966;Ueda et al, 1966) or that blood pressure responses are exaggerated (Bouckaert et al, 1937). These views, however, neglect the factors that are fundamental to the reflex control of heart rate and vascular resistance.…”

“…These changes may be expected to produce a modification of arterial baroreceptors reflex control of the circulation in these conditions. Previous reports (Bouckaert et al, 1937;Goldblatt et al, 1940;Conway, 1955;McCubbin et al, 1956;Alexander and DeCuir, 1966) indicate that baroreceptor reflexes are active in the regulation of acute changes of blood pressure in hypertensive animals. Other evidence has indicated, however, that baroreflex control of the P-P or R-R interval induced by pressor drugs is reduced in hypertension (Bristow et al, 1969;Gribbin et al, 1971;Angell-James and George, 1980) and it is associated with increased pressor responses to infused vasoconstrictor substances (Brown and Maegraith, 1941;Doyle, 1968).…”

Altered control of hindlimb vascular resistance by vagal afferents in spontaneously hypertensive rats. Difference in the early and late stage of hypertension.

“…Thus, it may be possible that arrrial hypertension is present before resetting of the cardiopulmonary receptors develops. Second, there may be impaired arterial baroreflex in hyper-tension (Alexander and DeCuir, 1966;Brown et al, 1976), which may augment the effects of vagal afferents on control of sympathetic outflow. It is reported that the inhibitory influence of the cardiopulmonary receptors on sympathetic outflow is increased when the inhibitory input from the arterial baroreceptors is reduced (Koike et al, 1975;Mancia et al, 1976).…”