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 (D1-like or DA1) and prejunctional (D2-like or DA2) 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 D1-like and D2-like receptors in the rat portal vein using radioligand binding techniques associated with light microscope autoradiography. 4. No D1-like receptors were demonstrated in sections of the rat portal vein, whereas the D2-like receptor ligand, [3H]-spiroperidol, was bound to sections of the vein in a manner consistent with the labelling of D2-like sites. Anatomically, D2-like sites were located within the tunica adventitia, including the adventitia-media border, and in the endothelium. 5. These findings suggest the existence of D2-like but not D1-like receptor sites in the rat portal vein. D2-like sites of the tunica adventitia are probably prejunctional and involved in the modulation of sympathetic outflow. The functional significance of endothelial D2-like sites, if any, should be clarified in future studies