The rostral ventrolateral medulla mediates sympathetic baroreflex responses to intraventricular angiotensin II in rabbits

Abstract

The present study examined the role of the rostral ventrolateral medulla (RVLM) in mediating the pressor and renal sympathetic baroreflex effects of intraventricularly administered angiotensin II (Ang II) in urethane anaesthetised rabbits. Microinjection of Ang II over a wide range of medullary sites showed that pressor responses were observed only in the RVLM. Ang II was particularly potent in producing a transient pressor response at this site with a half maximal dose of 9 fmol. The administration of the Ang II antagonist Sar(1)-Ile(8)-Ang II (10 pmol) bilaterally into the RVLM inhibited the pressor response to local and fourth ventricular Ang II, but not the pressor response to RVLM applied glutamate. To determine the contribution of the RVLM to the renal sympathetic baroreflex effects of Ang II, blood pressure-renal sympathetic nerve activity (RSNA) curves were constructed with intravenous infusion of phenylephrine or nitroprusside before and after Ang II, vehicle or glutamate infusions into the RVLM. Ang II infusion of 4 pmol/min into the RVLM increased blood pressure by 8+/-3 mm Hg and shifted the renal sympathetic baroreflex curve to the right. The maximum RSNA evoked by lowering blood pressure increased by 36+/-6%, similar to the effect seen with fourth ventricular Ang II and RVLM glutamate. These studies suggest that the major medullary pressor site of action of Ang II when injected into the hindbrain cerebro-spinal fluid of anaesthetized rabbits is the RVLM where it facilitates baroreflex control of RSNA.