AUTORADIOGRAPHIC LOCALIZATION OF DOPAMINE D-2-LIKE RECEPTORS IN THE RABBIT PULMONARY VASCULAR TREE

In the present study, the pharmacological characteristics and the anat omical localization of dopamine D-2-like receptor sites in the extrapa renchymal and in the intraparenchymal portion of the rabbit pulmonary artery were investigated using combined radioligand binding and light microscope autoradiography with [H-3]-spiroperidol (spiperone) as a li gand. The ligand was bound to sections of the pulmonary artery in a ma nner consistent with the labelling of dopamine D-2-like receptors with an equilibrium dissociation constant (K-d) 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 n ot allow the evaluation of specific binding. Light microscope autoradi ography showed the development of specific silver grains within the tu nica adventitia of extraparenchymal branches of rabbit pulmonary arter y and of large and, to a lesser extent, of medium-sized intraparenchym al branches of the pulmonary artery. No silver grains were found withi n small branches of the pulmonary artery or of the pulmonary vein. Dev elopment of adventitial silver grains was inhibited by compounds activ e at dopamine receptors. The greater sensitivity to displacement by do mperidone, haloperidol, (-)-sulpiride and bromocriptine than to displa cement by N-propyl-norapomorphine, quinpirole or clozapine suggests th at the [H-3]-spiroperidol binding sites observed in extraparenchymal, large and medium-sized branches of the pulmonary artery belong, probab ly, to the dopamine D-2 receptor subtype. The possible pre-junctional localization of these sites is discussed.