ALTERATION OF DOPAMINE RECEPTOR BINDING CHARACTERISTICS IN RAT LUNGS: RESPONSE TO RECEPTOR ACTIVATION WITH FENOLDOPAM ♦ 334

We have previously demonstrated that the DA₁ agonist, fenoldopam(fen) produces vasodilation in the preconstricted pulmonary vascular bed of the isolated perfused lung of the rat. We concluded that activation of vascular dopamine receptors were responsible for the results. However, we were unable to confirm these functional aspects of DA₁ receptor activity with anatomical correlation with radio-ligand binding studies in the rat lung. Our initial experiments demonstrated low (0-3%) specific binding of a radiolabeled dopamine receptor antagonist, ³H-SCH 23390, on the adult rat lung.

To assess whether the receptor binding characteristics could change in response to functional receptor activation, we combined isolated perfused lung experiments with radio-ligand binding studies. Adult Sprague-Dawley rats (n=8) were anesthetized and instrumented for isolated perfused lung studies. They were randomly assigned to either a control group (baseline conditions for 20 minutes) or fen treatment group (vasoconstriction with PGF₂α followed by injection of fen, 0.1μg/g body weight). At the end of the experiment (20 minutes for control and the point of maximum vasodilation for fen treated group) the lungs were immediately frozen in liquid N₂ and sectioned for radiolabeled binding studies. The sections were incubated in a medium consisting of buffer, protease inhibitors and ³H-SCH 23390. At the end of the incubation, the tissue sections were wiped from the slides with filter paper and counted in a scintillation counter. Pilot experiment determined the optimal incubation time, temperature, and effective ligand concentration range. An estimate of specific binding was determined by using an excess (10μM) of unlabeled SCH 23390.

Specific binding of ³H-SCH 23390 in rat lung sections was significantly higher in the fen group (42% fen group, vs. 0.8% control. p = 0.0001 by ANOVA). This experiment suggests that binding characteristics of receptors can be altered by pharmacologic manipulation. These studies may increase our understanding of the relationship between functional and anatomic aspects of the dopaminergic receptor system within the pulmonary vasculature.