Vagal C-fiber blockade abolishes sympathetic inhibition by atrial natriuretic factor

Administration of atrial natriuretic factor (ANF) to anesthetized rats decreases renal nerve activity (RNA), an effect prevented by vagotomy but not by atropine. We sought to determine whether afferent vagal C-fibers mediate the inhibition of sympathetic outflow. ANF (2.5 μg/kg iv) decreased mean arterial pressure (MAP) by 25 ± 2 mmHg, RNA by 11 ± 5%, and least splanchnic nerve activity (LSNA) by 10 ± 4% in anesthetized rats with arterial baroreceptors intact, and by 40 ± 3 mmHg, 28 ± 7%, and 23 ± 4%, respectively, in sinoaortic-denervated rats. Inhibition of RNA and LSNA by ANF was reduced slightly by cooling the vagi to 6 or 7°C, a temperature at which conduction in A-fibers was blocked and that in C-fibers attenuated; inhibition was abolished when C-fibers were blocked by cooling to 0°C. We conclude that the inhibition of RNA and LSNA by ANF was mediated by afferent vagal C-fibers. We also obtained evidence that the aortic nerves contribute to ANF-induced inhibition of RNA. Our results support the notion that ANF evokes a generalized decrease in sympathetic tone that contributes to the hypotension, cardiac inhibition, and natriuresis accompanying systemic administration of the peptide.