The Detailed Localization Pattern of Na+/K+/2Cl− Cotransporter Type 2 and Its Related Ion Transport System in the Rat Endolymphatic Sac

Abstract

The endolymphatic sac (ES) is a part of the membranous labyrinth. ES is believed to perform endolymph absorption, which is dependent on several ion transporters, including Na+/K+/2Cl− cotransporter type 2 (NKCC-2) and Na+/K+-ATPase. NKCC-2 is typically recognized as a kidney-specific ion transporter expressed in the apical membrane of the absorptive epithelium. NKCC-2 expression has been confirmed only in the rat and human ES other than the kidney, but the detailed localization features of NKCC-2 have not been investigated in the ES. Thus, we evaluated the specific site expressing NKCC-2 by immunohistochemical assessment. NKCC-2 expression was most frequently seen in the intermediate portion of the ES, where NKCC-2 is believed to play an important role in endolymph absorption. In addition, NKCC-2 expression was also observed on the apical membranes of ES epithelial cells, and Na+/K+-ATPase coexpression was observed on the basolateral membranes of ES epithelial cells. These results suggest that NKCC-2 performs an important role in endolymph absorption and that NKCC-2 in apical membranes and Na+/K+-ATPase in basolateral membranes work coordinately in the ES in a manner similar to that in renal tubules.