Expression of the Na+-K+-2Cl- cotransporter in the rat endolymphatic sac

The endolymphatic sac (ES) is a part of the membranous labyrinth that contains the cochlea, vestibular organs, and semicircular canals, and is believed to absorb endolymphatic fluid. Na⁺–K⁺–2Cl⁻ (NKCC) is a cotransporter that occurs as two isoforms (NKCC-1 and NKCC-2). Especially, NKCC-2 is suggested to participate in ES endolymph absorption. In the present study, the expression and cellular localization of NKCC-1 and NKCC-2 in the rat ES were examined by RT-PCR and in situ hybridization, respectively. The findings indicate that both NKCC-1 and NKCC-2 are expressed in the rat ES and suggest that NKCC is involved in ES homeostasis. NKCC-2 may be particularly involved in endolymph absorption. This is the first report confirming NKCC expression in the ES.