AUTORADIOGRAPHIC LOCALIZATION OF DOPAMINE DA-1 RECEPTORS IN THE RAT RENAL VASCULATURE USING [H-3] SCH 23390 AS A LIGAND

1 The localization of the selective dopamine (DA) DA-1 receptor antagonist [H-3]-SCH 23390 was studied in sections of rat kidney using combined radioreceptor binding and autoradiographic techniques to identify the target of DA-1 receptors in renal vasculature. 2 Binding data were consistent with the labelling by [H-3]-SCH 23390 of a single population of high affinity DA-1 sites. The apparent dissociation constant value (Kd) was 4.2 nM and the maximum binding capacity value (Bmax) was 140.6 +/- 6.5 fmol mg-1 tissue. Selective DA-1 or mixed DA-1 and DA-2 receptor antagonists or agonist were able to antagonize dose-dependently [H-3]-SCH 23390 binding to sections of rat kidney. DA-2 receptor selective antagonists were ineffective. 3 Light microscope autoradiography using sections exposed to nuclear emulsion for 2 weeks revealed development of specific silver grains only within proximal cortical tubules. Longer exposure times (10 weeks) caused the development of specific silver grains in proximal and distal cortical tubules and in the medial layer of intrarenal arteries. 4 Quantitative analysis of the density of silver grains within the cortical tubules and the medial layer of intrarenal arteries revealed a significantly higher density of binding sites in proximal cortical tubules than in the vasculature or in distal cortical tubules. No significant differences in the density of silver grains in the medial layer of interlobar arcuate or cortical radial arteries were observed. 5 These results suggest that, using adequate ligand concentrations and exposure times for autoradiography, both tubular and vascular DA-1 receptors can be visualized in the rat kidney. The observation of a higher density of proximal tubules DA-1 receptors is consistent with the hypothesis of a more important role of DA-1 receptors in the control of natriuresis than of renal haemodynamics.