PHARMACOLOGICAL CHARACTERIZATION AND AUTORADIOGRAPHIC LOCALIZATION OF DOPAMINE-RECEPTORS IN THE PORTAL-VEIN

1 Dopamine (DA) and DA receptor agonists exert a variety of effects on the cardiovascular system through interaction with specific DA receptors, including decreases in blood pressure and heart rate. 2 The decrease in blood pressure is due primarily to arterial vasodilation. This phenomenon is due to the stimulation of both postjunctional (D-1-like or DA(1)) and prejunctional (D-2-like or DA(2)) receptors causing respectively relaxation of arterial smooth muscle and decrease of the sympathetic vasoconstriction tone. 3 In view of the lack of detailed information on the existence of DA receptors in venous tissue, we have analysed D-1-like and D-2-like receptors in the rat portal vein using radioligand binding techniques associated with light microscope autoradiography. 4 No D-1-like receptors were demonstrated in sections of the rat portal vein, whereas the D-2-like receptor ligand, [H-3]-spiroperidol, was bound to sections of the vein in a manner consistent with the labelling of D-2-like sites. Anatomically, D-2-like sites were located within the tunica adventitia, including the adventitia-media border, and in the endothelium. 5 These findings suggest the existence of D-2-like but not D-1-like receptor sites in the rat portal vein. D-2-like sites of the tunica adventitia are probably prejunctional and involved in the modulation of sympathetic outflow. The functional significance of endothelial D-2-like sites, if any, should be clarified in future studies.