Regulation of the proximal tubular sodium/proton exchanger NHE3 in rats with puromycin aminonucleoside (PAN)-induced nephrotic syndrome.

Excessive proteinuria due to loss of glomerular permselectivity in nephrotic syndrome can cause disturbances in renal salt and water handling with edema formation. Apart from oncotic and hydrostatic mechanisms associated with hypoalbuminemia, primary derangements in renal tubular sodium transport may contribute to the pathogenesis of nephrotic edema. Whereas there is evidence for an increase of cortical collecting duct sodium reabsorption in nephrotic rats, it remains controversial whether proximal tubule sodium transport may also be activated in this condition. The regulation of the cortical Na/H exchanger NHE3, the main pathway for Na reabsorption in the proximal tubule (PT), was investigated in rats with puromycin aminonucleoside (PAN)-induced nephrotic syndrome. PAN rats developed reduced GFR, severe proteinuria, and sodium retention within 3 d. After 10 d, immunoblots of brush border vesicles revealed a decreased abundance of NHE3 in nephrotic animals. However, the Na/H antiporter activity in the same vesicle preparations was not significantly altered. Antiporter activity normalized for NHE3 protein was increased by 88% in nephrotic animals (P = 0.025). Immunohistochemistry with the same polyclonal antibody as for immunoblots revealed a decrease of NHE3 abundance in PT. In contrast, immunoreactivity for the monoclonal antibody 2B9, which specifically recognizes the non-megalin-associated, transport-competent pool of NHE3, was higher in PAN-treated rats than in controls. In conclusion, increased sodium reabsorption might be associated with a shift of NHE3 from an inactive pool to an active pool, thus contributing to sodium retention in a state of proteinuria.