SELECTIVE CONTRIBUTION OF 2 TYPES OF CAROTID-SINUS BARORECEPTORS TO THE CONTROL OF BLOOD-PRESSURE

This study was performed to determine if selective elimination of affe rent input from two different types of previously described barorecept ors altered the ability of the dog to regulate blood pressure (BP), ex amining specifically if there was differential loss of baroreceptor co ntrol of tonic levels of baseline pressure versus dynamic changes in p ressure. In the first series of experiments in this study, anodal bloc k of the carotid sinus nerve was used to selectively block afferent in put in a sequence from large-diameter A-fiber carotid baroreceptors (m ostly type I) to smaller A-fiber and nonmyelinated C-fiber barorecepto rs (mostly type II). In the second series of experiments, anesthetic b lock of the carotid sinus nerve with bupivacaine was used to selective ly eliminate afferent input in reverse order from anodal block, first blocking input from baroreceptors with small afferent fibers and then additionally eliminating input from the larger-diameter A-fiber barore ceptors. The effects of selective elimination of each baroreceptor typ e were determined by monitoring baseline BP during constant carotid si nus pressure (CSP) perfusion of a vascularly isolated carotid sinus (t onic control) and obtaining baroreflex sensitivity (slope) during ramp pressure stimulations of the carotid sinus (dynamic control) under va rious blocking conditions. Low levels of anodal block significantly at tenuated baroreflex sensitivity (-0.84+/-0.11 versus -0.63+/-0.10 mm H g BP/mm Hg CSP) at levels of block that had no effect on tonic baselin e BP (158.41+/-9.5 versus 160.7+/-9.5 mm Hg BP). In contrast, low leve ls of bupivacaine block produced significant increases in tonic BP (15 8.8+/-6.4 versus 169.0+/-6.5 mm Hg BP), whereas there was no effect on dynamic baroreflex sensitivity (-0.85+/-0.08 versus -0.73+/-0.08 mm H g BP/mm Hg CSP). Thus, blocking large A-fiber baroreceptors resulted i n significant decreases in baroreflex sensitivity without changes in b aseline levels of BP, indicating primarily an attenuation in dynamic b aroreflex regulation. Blocking of smaller A-fiber and unmyelinated C-f iber baroreceptors resulted in smaller decreases in baroreflex sensiti vity and significant elevations in baseline BP, indicating a loss of t onic control of pressure. These results suggest that the two types of baroreceptors contribute differently to the regulation of blood pressu re.