INCREASED SERINE-PHOSPHORYLATION OF THE D_1A RECEPTOR IN RENAL PROXIMAL TUBULE CELLS IN SPONTANEOUS HYPERTENSION. • 2218

We have suggested that a defect in the coupling of the D₁ receptor to G-protein/enzyme complex in renal proximal tubules is involved in the pathogenesis of spontaneous hypertension. The D_1A receptor subtype may be involved since hypertension develops in mice mutant for the D_1A receptor allele. However, we have found no mutation of the D_1A receptor gene in the spontaneously hypertensive rat (SHR) or Dahl salt-sensitive rat suggesting that the D_1A receptor defect may be due to post-translational modification. Since the renal D₁ receptor resembles the phenomenon of desensitization, and desensitization occurs as a result of receptor phosphorylation, we studied the phosphorylation of the D_1A receptor in renal proximal tubules from SHR and its normotensive control, the Wistar-Kyoto rat (WKY). Vehicle alone or fenoldopam, a D₁ agonist, 1μg/kg/min, was infused for 5-10′ into the renal artery via the right suprarenal artery of anesthetized rats. Renal cortical homogenates (n=5) were then immunoprecipitated with anti-phosphoserine antibodies and immunoblotted with anti D_1A antibodies. Basal serine-phosphorylated D_1A receptor was greater in SHR (21.0±1.5 density units, DU) than in WKY(7.4±2.9 DU) (P<0.01 t-test); the D₁ agonist increased serine-phosphorylated D_1A receptor in WKY (55.0±2.8 DU) but not in SHR (17.0±6.3 DU) (P<0.01 t-test) (n=5/group). The“hyper” phosphorylation of the D_1A receptor in the SHR was not the result of homologous desensitization because similar studies were obtained when the rats (n=3) were injected with carbidopa (40 μmol/kg) 24, 18, and 1 hr prior to study to decrease renal dopamine production. Moreover, the decreased ability of D₁ agonist to stimulate adenylyl cyclase and phospholipase C (n=5-8) and the “hyper” phosphorylation (n=3) of the D_1A receptor persist in immortalized proximal tubular cells from SHR (dopamine synthesis does not occur because of the absence of DOPA, the substrate for renal dopamine synthesis). The mechanism of “hyper” phosphorylation of the D_1A receptor in renal proximal tubules from SHR is not known. However, we have recently found that protein phosphatase 2A activity in cytosol but not in membranes of renal proximal tubules is decreased in the SHR compared to the WKY; protein phosphatases are important in the resensitization process. We conclude that a defect in the post-translational modification of the D_1A receptor is important in the pathogenesis of spontaneous hypertension.