Selective contribution of two types of carotid sinus baroreceptors to the control of blood pressure.

This study was performed to determine if selective elimination of afferent input from two different types of previously described baroreceptors altered the ability of the dog to regulate blood pressure (BP), examining specifically if there was differential loss of baroreceptor control of tonic levels of baseline pressure versus dynamic changes in pressure. In the first series of experiments in this study, anodal block of the carotid sinus nerve was used to selectively block afferent input in a sequence from large-diameter A-fiber carotid baroreceptors (mostly type I) to smaller A-fiber and nonmyelinated C-fiber baroreceptors (mostly type II). In the second series of experiments, anesthetic block of the carotid sinus nerve with bupivacaine was used to selectively 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 baroreceptors. The effects of selective elimination of each baroreceptor type were determined by monitoring baseline BP during constant carotid sinus pressure (CSP) perfusion of a vascularly isolated carotid sinus (tonic control) and obtaining baroreflex sensitivity (slope) during ramp pressure stimulations of the carotid sinus (dynamic control) under various blocking conditions. Low levels of anodal block significantly attenuated baroreflex sensitivity (-0.84 +/- 0.11 versus -0.63 +/- 0.10 mm Hg BP/mm Hg CSP) at levels of block that had no effect on tonic baseline BP (158.41 +/- 9.5 versus 160.7 +/- 9.5 mm Hg BP). In contrast, low levels of bupivacaine block produced significant increases in tonic BP (158.8 +/- 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 Hg BP/mm Hg CSP). Thus, blocking large A-fiber baroreceptors resulted in significant decreases in baroreflex sensitivity without changes in baseline levels of BP, indicating primarily an attenuation in dynamic baroreflex regulation. Blocking of smaller A-fiber and unmyelinated C-fiber baroreceptors resulted in smaller decreases in baroreflex sensitivity and significant elevations in baseline BP, indicating a loss of tonic control of pressure. These results suggest that the two types of baroreceptors contribute differently to the regulation of blood pressure.