Higher basal serine phosphorylation of D1A receptors in proximal tubules of old Fischer 344 rats

Dopamine (DA) and D(1)-like receptor agonists promote an increase in Na excretion by means of activation of the D(1)-like receptor signaling cascade and subsequent inhibition of the Na/H exchanger and Na-K-ATPase in renal proximal tubules. Recently, our laboratory reported that DA and the D(1)-like receptor agonist failed to inhibit Na-K-ATPase activity in old Fischer 344 rats because of uncoupling of D(1A) receptors from G proteins and that this abnormality led to a diminished natriuretic response to DA in old Fischer 344 rats. In this study, we have tested the hypothesis that the mechanism of this uncoupling may be an altered phosphorylation of D(1A) receptors in old rats. In experiments performed in renal cortical slices, both DA and SKF-38393, a D(1)-like receptor agonist, increased the serine phosphorylation of D(1A) receptors in adult (6 mo) but not old (24 mo) rats. Interestingly, the basal serine phosphorylation of D(1A) receptors was higher in old than in adult rats. Competition ligand binding ([(3)H]SCH-23390) experiments on the D(1)-like receptor in adult and old rats with fenoldopam, a D(1)-like receptor agonist, revealed the presence of two affinity states of the receptors. There was a rightward shift in the agonist displacement of the ligand in old compared with adult rats, as reflected in the IC(50) values (adult vs. old, 7.46 x 10(-9) +/- 2.26 vs. 7.93 x 10(-7) +/- 1.33 M). Also, there was a reduction in agonist affinity in the low-affinity receptors in old compared with adult rats (IC50, adult vs. old, 5.67 x 10(-5) +/- 1.33 vs. 12.60 x 10(-5) +/- 6.50 M). Moreover, the abundance of D(1A) receptor proteins was similar to47% lower in the membranes of old compared with adult rats. We speculate that higher basal serine phosphorylation of D(1A) receptors may have rendered the D(1A) receptor uncoupled from G protein, leading to a reduced agonist affinity state and thus diminished natriuretic response to DA in old rats.