Role of Vagal Afferents In the Control of Renal Sympathetic Nerve Activity in the Rabbit

Abstract

The influence of vagal afferents on renal sympathetic nerve activity was studied in nine rabbits anesthetized with sodium pentobarbital and artificially ventilated with oxygen. Arterial Pco 2 and pH were kept normal. The aortic depressor and the carotid sinus nerves were cut. The mean impulse frequency was determined from multifiber preparations of the renal nerve. Blood volume was altered either by bleeding or by infusing dextran. With vagi intact, an increase in blood volume of approximately 1OSE caused the nerve activity to decrease 41 ± 4%; it returned toward the control level after withdrawal of the same volume. A similar decrease in volume increased the nerve activity 33 ± 7%; it decreased toward the control level after reinfusion. These changes were unaffected by cutting the vagi at the diaphragm but were abolished or markedly attenuated by cooling or cutting the vagi in the neck. Interruption of vagal afferents resulted in a 21 ± 2% increase in nerve activity, indicating that the afferents exerted a continuous inhibition on the sympathetic outflow to the kidney. These experiments demonstrated a role for the low-pressure intrathoracic receptors in the control of renal sympathetic nerve activity in response to changes in blood volume.KEY WORDS low-pressure receptors blood volume control cardiopulmonary receptors hemorrhage vagal cooling hypervolemia bilateral vagotomy• Evidence is accumulating that, in response to hemorrhage, receptors in the cardiopulmonary area are important in the control of the renal resistance vessels (1, 2). However, the magnitude of the renal response to the same hemorrhage can be strikingly different depending on the technique used to determine the reactions of the renal vessels (2, 3). Local factors in the kidney, such as autoregulation, may cause these differences and make it difficult to assess the role of renal sympathetic nerve activity in determining the magnitude of the response. In the present experiments, From the Mayo Clinic and Mayo Foundation, Rochester, Minnesota 55901.This investigation was supported in part by U. S. Public Health Service Grant HL-5883 from the National Heart and Lung Institute and by a grant from the Medical Research Council of Canada (Dr. Pelletier). Dr. Clement is a U. S. Public Health Service International Fellow on leave from the State University of Ghent, Belgium.Received July 17, 1972. Accepted for publication September 29, 1972. electroneurographic recordings from the renal nerve were made to establish more directly the relationship between the impulse frequency in the renal sympathetic nerves and the activation of the low-pressure mechanoreceptors during increases and decreases in blood volume.Kezdi and Geller (4) have shown an inverse relationship between the carotid sinus pressure and the frequency of renal sympathetic nerve discharge. The influence of the receptors subserved by the vagal afferents on renal sympathetic nerve activity in response to alterations in blood volume has not yet been investigated. MethodsPreparation.-Nin...

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