Localization of dopamine-1 receptors along the microdissected rat nephron

Dopamine exerts numerous actions on the kidney but the precise location of its receptor subtypes along the nephron is unknown. Using a microassay we determined the specific binding of 125I-Sch 23982, a specific and selective dopamine-1 (DA1) receptor antagonist, to microdissected glomeruli and tubule segments. Binding of 125I-Sch 23982 in the proximal convoluted tubule (PCT) was time- and concentration dependent, saturable and reversible. The linear Scatchard plot of saturation experiments suggested binding to a single site with an apparent Kd of 16.7 nM and Bmax of 0.4 fmol.mm-1 in the PCT, and 6.2 nM and 0.1 fmol.mm-1 in the cortical collecting tubule (CCT). Mapping of DA1 binding sites along the nephron revealed their presence in each of the segments examined, albeit in markedly different concentrations: the highest specific binding was measured in PCT followed by the pars recta. Binding was less in the distal nephron, and least in the medullary and cortical thick ascending limb. Modest binding was also detected in glomeruli. In cortical collecting tubules competition studies with unlabeled dopamine and probes for DA1 (Sch 23390, fenoldopam), DA2 (domperidone, S-sulpiride), serotonergic (serotonin, ketanserin, mianserin), and alpha-(phentolamine) and beta-(propranolol) adrenergic receptors indicated a rank-order potency for displacement of 125I-Sch 23982 binding, consistent with labeling of DA1 receptors. Dopamine inhibited Na/K-ATPase both in PCT and CCT, an effect duplicated in the latter segment by the DA1 agonist fenoldopam, and blocked by the DA1 antagonist Sch23390.