Autoradiographic localization of dopamine D2-like receptors in the rabbit pulmonary vascular tree

In the present study, the pharmacological characteristics and the anatomical localization of dopamine D2-like receptor sites in the extraparenchymal and in the intraparenchymal portion of the rabbit pulmonary artery were investigated using combined radioligand binding and light microscope autoradiography with [3H]-spiroperidol (spiperone) as a ligand. The ligand was bound to sections of the pulmonary artery in a manner consistent with the labelling of dopamine D2-like receptors with an equilibrium dissociation constant (Kd) of about 2.4 +/- 0.07 nmol/l and a maximum density of binding sites of 65 +/- 4.5 fmol/mg tissue. In contrast, binding experiments made with sections of rabbit lung did not allow the evaluation of specific binding. Light microscope autoradiography showed the development of specific silver grains within the tunica adventitia of extraparenchymal branches of rabbit pulmonary artery and of large and, to a lesser extent, of medium-sized intraparenchymal branches of the pulmonary artery. No silver grains were found within small branches of the pulmonary artery or of the pulmonary vein. Development of adventitial silver grains was inhibited by compounds active at dopamine receptors. The greater sensitivity to displacement by domperidone, haloperidol, (-)-sulpiride and bromocriptine than to displacement by N-propyl-norapomorphine, quinpirole or clozapine suggests that the [3H]-spiroperidol binding sites observed in extraparenchymal, large and medium-sized branches of the pulmonary artery belong, probably, to the dopamine D2 receptor subtype. The possible pre-junctional localization of these sites is discussed.