Diminished natriuretic response to dopamine in old rats is due to an impaired D-1-like receptor-signaling pathway

Background. Dopamine (DA) causes natriuresis and diuresis, which results fr om activation of D-1-like receptor (D1R) located on proximal tubules. Earli er, we reported that DA failed to inhibit Na,K-ATPase in proximal tubules o f old Fischer 344 rats. The present study was designed to investigate the f unctional consequence of this phenomenon. Methods. Measurements of the functional (natriuretic and diuretic) response to intravenously infused DA and SKF 38393 (D1R agonist) in adult (6 month) and old (24 month) Fischer 344 rats were taken. Biochemical measurements w ere carried out to determine the potential defects in D1R and its signaling pathway in proximal tubules of old rats. Results. We found that intravenous infusion of DA and SKF 38393 caused natr iuresis and diuresis in adult rats, but this response was blunted in old ra ts. In the isolated proximal tubules, DA and SKF 38393 inhibited Na,H-excha nger (NHE) in adult rats, however, this inhibition was attenuated in old ra ts. Radioligand binding revealed approximately 46% reduction in D1R binding sites in brush border membranes (BBMs) in old compared with adult rats. SK F 38393 stimulated [S-35]GTP gamma S binding in BBM in adult rats, but not in old rats, suggesting an impaired D1R-G protein coupling. DA and SKF 3839 3 stimulated adenylyl cyclase (AC) activity in adult but not in the old rat s. Forskolin and NaF stimulated AC activity in a comparable manner in adult and old rats, indicating no defect in AC and G proteins. DA and SKF 38393 failed to stimulate protein kinase A (PKA) activity in proximal tubules of old rats. Dibutyryl-cAMP-mediated PKA activation was also absent in old rat s. Conclusions. A decrease in D1R binding sites, a coupling defect with G prot eins, and a defect in PKA activation lead to diminished DA-mediated inhibit ion of NHE in old rats, which may contribute to the blunted natriuretic res ponse to DA in these animals.