Subregions of rostral ventral medulla control arterial pressure and regional hemodynamics

The cardiovascular effects of inactivating rostral ventromedial medulla (RVMM) under conditions of normal (2.5 ml) and reduced (1.5 ml) tidal volume were studied in urethan-anesthetized rats. Bilateral microinjection of lidocaine (200 nl, 4%) reduced mean arterial pressure (MAP), renal, mesenteric, and particularly hindquarter vascular resistance. These effects were not significantly altered by reducing tidal volume. Electrical stimulation of RVMM increased MAP and regional vascular resistances, again with the hindquarter change most prominent. The integrated cardiovascular response to stimulating rostral ventrolateral medulla (RVLM) appears to require integrity of RVMM; however, the converse is not true. Overall, these studies indicate that 1) the potential for RVMM to maintain neurogenic control of arterial pressure is as great as RVLM; 2) RVMM is capable of playing a proportionally greater role in the controlling hindquarter vascular resistance; 3) the integrity of RVMM appears to be important for responses elicited from RVLM; and 4) unlike RVLM, neural control of arterial pressure is sustained by RVMM under conditions of reduced tidal volume. We conclude that RVLM and RVMM are functionally and anatomically distinct subregions of rostral ventral medulla with equivalent capacity to maintain vasomotor tone.