Further investigation of the sites of vascular alpha 1 and alpha 2 adrenoceptors in the anaesthetised spontaneously hypertensive rat.

We have looked at the role played by alpha 1 and alpha 2 adrenoceptors in the control of blood pressure in the anaesthetised spontaneously hypertensive rat (SHR) by examining the blood-pressure-lowering effects of the alpha 1-adrenoceptor antagonist prazosin and the alpha 2-adrenoceptor antagonist rauwolscine in 5 groups of animals: unoperated, adrenal demedullated, shamoperated, sympathectomised with 6-hydroxydopamine, and vehicle treated. Prazosin (1 mg/kg) produced significant falls in diastolic blood pressure (DBP) in all groups of rat but the fall was significantly less in sympathectomised animals. In the presence of prazosin (1 mg/kg), rauwolscine (0.1-1 mg/kg) lowered DBP in all groups of rat except the adrenal demedullated animals. In the absence of prazosin, rauwolscine (0.1 mg/kg) produced a significant fall in DBP only in sympathectomised animals. These results demonstrate that alpha 2 adrenoceptors are involved in blood pressure control in the intact or sympathectomised SHR but not following adrenal demedullation, suggesting that circulating catecholamines rather than neurally released noradrenaline (NA) are responsible for these alpha 2-adrenoceptor-mediated pressor effects. In contrast, only alpha 1-adrenoceptor-mediated pressor responses are reduced by sympathectomy, suggesting that neurally released NA acts mainly on alpha 1-adrenoceptors.