Relationship between arterial baroreflex, cardiopulmonary vagal reflex andrenal natriuretic response to saline in conscious rabbits

1. We have previously shown that normotensive rabbits with a genetic impair ment in arterial baroreflex sensitivity showed a delayed sodium excretion a nd failed to increase their renal blood flow in response to a saline infusi on that did not after blood pressure. These renal haemodynamic and excretor y abnormalities were abolished by renal denervation. The present study dete rmined the sensitivity of the cardiopulmonary baroreceptors and the renal r esponse to a mild saline infusion in normotensive rabbits varying widely in their arterial baroreflex sensitivity. 2. Sensitivity of cardiopulmonary baroreceptors was assessed from the slope of the relationship of the change in both blood pressure and heart rate an d the dose of phenylbiguanide, a stimulator of vagal afferents. 3. The change in renal blood flow and lithium and sodium excretion was meas ured in response to saline, infused at a rate of 0.11 ml.min(-1).kg(-1) int o the ear vein. Urine was collected via a urethral catheter and renal blood flow was measured by para-aminohippurate clearance. 4. A significant correlation was found between the magnitude of the gain of the cardiac arterial baroreflex and the sensitivity of the cardiopulmonary baroreceptor response to phenylbiguanide. The latter was significantly cor related to renal blood flow and lithium clearance 60-90 min after the start of the saline infusion. 5. It was also found that in some normotensive rabbits there was a blunting of cardiovascular regulation as indicated by a reduced sensitivity of card iopulmonary and arterial baroreceptors. This may explain their abnormal hae modynamic and natriuretic response to salt.