Angiotensin II modulates both slow and rapid baroreflex responses of 3 barosensitive bulbospinal neurons in the rabbit rostral ventrolateral medulla

31 32 This study investigated the effects of angiotensin II (Ang II) on slow and rapid 33 baroreflex responses of barosensitive bulbospinal neurons in the rostral ventrolateral medulla 34 (RVLM) in urethane-anesthetized rabbits to determine whether the sympathetic baroreflex 35 modulation induced by application of Ang II into the RVLM can be explained by the total 36 action of Ang II on individual RVLM neurons. In response to pharmacologically induced 37 slow ramp changes in mean arterial pressure (MAP), individual RVLM neurons exhibited a 38 unit activity–MAP relationship that was fitted by a straight line with upper and lower plateaus. 39 Iontophoretically applied Ang II raised the upper plateau without changing the slope, and 40 thereby increased the working range of the baroreflex response. An asymmetric sigmoid 41 curve that was determined by averaging individual unit activity–MAP relationship lines 42 became more symmetric with Ang II application. The characteristics of the average curves, 43 both before and during Ang II application, were consistent with the renal sympathetic nerve 44 activity–MAP relationship curves obtained under the same experimental conditions. Ang II 45 also affected rapid baroreflex responses of RVLM neurons that were induced by cardiac beats, 46 as application of Ang II predominantly raised the average unit activities in the downstroke 47 phase of arterial pulse waves. The present study provides a possible explanation for the Ang 48 II-induced sympathetic baroreflex modulation based on the action of Ang II on barosensitive 49 bulbospinal RVLM neurons. Our results also suggest that Ang II changes both static and 50 dynamic characteristics of baroreflex responses of RVLM neurons. 51 52