Angiotensin II regulation of renal sodium transporters as identified by interference with AT1 receptor action

Angiotensin II (A II) directly regulates sodium absorption in the proximal nephron, and indirectly regulates it in the distal nephron. It is not known whether regulation is by changes in protein abundance or changes in cellular distribution of renal sodium transporters. To identify direct actions of AII, A II levels were increased by feeding rats a low-sodium diet, then in the experimental group, AII action was blocked by candesartan. The abundance of major sodium transporters was assessed by semiquantitative immunoblotting and cellular localization of transporters was determined by immunocytochemistry. In mice, A II action was blocked by using animals in which the gene coding for the AII receptor, AT1a, was deleted (knock-out).; In the proximal tubule of candesartan-treated rats, abundance of the sodium-phosphate cotransporter (NaPi2) and the sodium-bicarbonate cotransporter (NBC1) decreased significantly. In brush border membrane isolates, abundance of the sodium-proton exchanger (NHE3) decreased, although abundance of NHE3 was not changed in whole tissue homogenates. Candesartan treatment significantly decreased abundance of the alpha subunit of the epithelial sodium channel (alpha ENaC) of the distal nephron, while beta (beta ENaC) and gamma (gamma ENaC) subunits increased. Immunocytochemistry substantiated the increases in beta and gamma ENaC subunits. When aldosterone, a downstream effector of A II in the distal nephron, was blocked with spironolactone (400 mg/kg/day) NCC and alpha ENaC decreased. The addition of candesartan treatment further decreased NBC1, NCC and alpha ENaC abundance in rat kidney. In AT1a knock-out mouse kidneys compared to wild type, abundance of alpha ENaC and NCC decreased while abundance of beta and gamma ENaC increased.; In summary, A II regulation of renal sodium transporters may be mediated by direct action on the abundance of the alpha ENaC subunit and NBC1, as well as the cellular distribution of NHE3. Spironolactone treatment largely eliminated the increased abundance of beta and gamma ENaC subunits and NCC, indicating aldosterone regulation. However, AT1 receptor blockade in rats and inactivation in mice demonstrated that abundance of the alpha ENaC subunit is regulated, at least in part, by direct action of A II.