Contrasting effects of moderate vagal stimulation on heart rate and carotid sinus baroreflex-mediated sympathetic arterial pressure regulation in rats

Aims: To examine whether moderate efferent vagal nerve stimulation (VNS) attenuates the carotid sinus baroreflex-mediated arterial pressure (AP) regulation via its antagonism to the sympathetic system. Main methods: Carotid sinus baroreceptor regions were isolated from the systemic circulation in eight anesthetized and vagotomized rats. A staircase-wise input was imposed on carotid sinus pressure (CSP) with or without efferent VNS (20 Hz, 2 ms, 1-4 V), while the responses in AP, heart rate (HR), and splanchnic sympathetic nerve activity (SNA) were measured. Key findings: A multiple linear regression analysis indicated that VNS decreased the minimum HR in the CSP-HR relationship by 58.2 ± 4.9 beats/min (P < 0.01) from its reference value of 387.0 ± 5.8 beats/min. Although VNS significantly decreased an intercept of the SNA-AP relationship, it did not affect parameters of the CSP-AP relationship or the CSP-SNA relationship significantly. The operating-point AP of the baroreflex was decreased by 2.8 ± 1.0 mm Hg (P < 0.01) during VNS, which was less than 3% of the reference value of 117.7 ± 1.2 mm Hg. Significance: VNS, at an intensity of decreasing HR by approximately 13%, does not acutely attenuate the baroreflex-mediated sympathetic AP regulation.