β-Adrenoceptors in vascular capacitance responses to unloading of carotid baroreceptors in anesthetized dogs.

The role of β- and α-adrenoceptors in the total vascular capacitance responses to changing pressure in vascularly isolated carotid sinuses of anesthetized and atropinized dogs was investigated. A change in vascular capacitance was determined by measuring the shift of blood in and out of a reservoir that was connected to the aorta and maintained at a constant pressure. Changes in carotid sinus pressure from 135 to 57 mmHg and back to 137 mmHg resulted in a rapid vascular capacitance response of ∼30 ml in the absence of adrenoceptor antagonists. Administration of a β2-adrenoceptor antagonist (ICI-118551) caused a significant enhancement of the capacitance responses to similar decreases and increases in carotid sinus pressure (∼130%). Administration of a β1-adrenoceptor antagonist (CGP-20712A) did not cause any further enhancement of the responses. However, an α-blocker (phentolamine) reduced the responses by 75%. The results suggest that in the presence of a β2-adrenoceptor antagonist vascular capacitance responses to loading and unloading of baroreceptors are greatly enhanced and that patients suffering from orthostatic syncope may benefit from this kind of drug.