The ability of D1-like receptors to inhibit proximal tubular luminal Na+/H+ exchanger (NHE) activity is much less in immature than in mature animals. In rats, the decreased effect is not due to NHE, adenylyl cyclase enzyme or guanine nucleotide levels. Rather, it is due to decreased coupling efficiency between the D1 receptor to the 6-protein complex which is developmentally regulated. These may be due to age-related differences in distribution or quantity of 6-protein subtypes, D1-like receptor subtypes (DIA and DIB), or factors that influence the coupling of D1 receptors to 6 proteins. The major objective of this competitive renewal is to determine the mechanism(s) that regulates the maturation of this coupling mechanism. There are 3 specific aims. To test the hypothesis that G-protein beta/gamma subunits mediate the D1-receptor-mediated inhibition of nephron segment specific effect on renal tubular sodium transport, we will determine: (a) G-protein subunit expression in specific nephron segments, (b)the role of beta/gamma subunits on second messenger- independent-regulated renal Na+ transport, and (c) the proportional contribution of second messenger dependent and independent D1-mediated Na+ transport (luminal NHE and basolateral Na-HCO3 co-transport activity). We will also test the hypothesis that D1-receptor-linked G-proteins are developmentally regulated and under hormonal influence (e.g. glucocorticoids). 2. To test the hypothesis that there are age-related differences in D1 receptor subtype and distribution, we will determine the polar, regional, and nephron segment distribution of these receptors by immunohistochemistry and quantify D1 receptor subtype (mRNA and protein). 3. To test the hypothesis that there is developmental regulation of factors that influence D1 receptor/G-protein coupling, we will study proteins (e.g. 6 protein-coupled receptor kinases (GRK), phosducin) that regulate D1 receptor/G-protein coupling in fetal, immature, and mature animals. These studies may shed light on the maturation of D1 receptor/G- protein coupling, an abnormality of which is important in the pathogenesis of genetic hypertension.

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