Effects of epinephrine on firing characteristics of two functionally different types of carotid baroreceptors.

Sympathetic stimulation and catecholamine exposure have been shown to sensitize the arterial baroreceptors, but the extent or importance of this effect is not known. We performed this study to investigate the effects of sympathetic feedback on the carotid sinus baroreceptors, specifically examining the effect of the stimulation on the two different functional types of baroreceptors characterized in an earlier study. The existence of two baroreceptor function-response curves has suggested that the roles of the two functionally different baroreceptors may not be the same. If true, the effects of epinephrine exposure on baroreceptor firing characteristics may contribute to differential roles played by each baroreceptor type in the control of blood pressure. Single-fiber baroreceptor activity from a vascularly isolated carotid sinus was recorded during slow increases in carotid sinus pressure before and during exposure to epinephrine (10(-8) to 10(-6) M). Baroreceptor firing characteristics were determined from function curves plotting carotid sinus pressure versus nerve activity, with curve-fitting analysis of the hyperbolic type I and sigmoidal type II baroreceptor curves used to obtain threshold (Pth) and saturation (Psat) pressures, threshold (Fth) and saturation (Fsat) firing rates, and sensitivity (slope) for each baroreceptor before and during epinephrine exposure. The possible mechanisms of observed changes were examined using our previously published baroreceptor computer model. Epinephrine exposure was found to significantly increase sensitivity, Fth, and Fsat of both types of baroreceptors, with a relatively greater effect on type I sensitivity and on type II Fth and Fsat. Epinephrine also was found to increase the level of spontaneous discharge for type II baroreceptors.(ABSTRACT TRUNCATED AT 250 WORDS)