Concentrating defect in experimental nephrotic syndrome: Altered expression of aquaporins and thick ascending limb Na+ transporters

Abstract

Background. Several pathophysiological states associated with deranged water balance are associated with altered expression and/or intracellular distribution of aquaporin water channels. The possible role of dysregulation of thick ascending limb NaCl transporters, which are responsible for countercurrent multiplication in the kidney, has not been evaluated. Methods. Semiquantitative immunoblotting and immunocytochemistry were carried out in the kidneys of rat with adriamycin-induced nephrotic syndrome and in vehicle- injected control rats. Results. Preliminary studies confirmed the presence of a severe concentrating defeci. Semiquantitative immunoblottng of outer medullary homogenates demonstrated a marked decrease in the abundance of three thick ascending limb Na+ transporters in nephrotid rats, namely the bumetanide-sinsitive Na-K-2Cl cotransporter (BSC-1), the type 3 Na/H exchanger (NHE-3, and the α-subunit of the Na-K-ATPase. These results are predictive of a decrease in the NaCl transport capacity of the medullary thick ascending limb and therefore a decrease in countercurrent multiplication. Immunocytochemistry of outer medullary thin sections demostrated broad (but highly variable) suppression of BSC-1 expression in the outer medullas of adriamycin-nephrotic rats. There was also a large decrease in outer medullary expression of two collecting duct water channels (aquaporin-2 and -3) and the major water channel of the thin descending limb of Henle's loop (aquaporin-1). Conclusion. The concentrating defect in adriamycin-induced nephrotic syndrome in rats is a consequence of multiple defects in water and solute transporter expresssion, which would alter both the generation of medullary interstitial hypertonicity and osmotic equilibration in the collecting duct. Whether a similar widespread defect in transporter expression is present in idiopathic nephrotic syndrome in humans is, at this point, untested.