ANTIDIURETIC-HORMONE RESTORES THE ENDOLYMPHATIC LONGITUDINAL K+ GRADIENT IN THE BRATTLEBORO RAT COCHLEA

In the cochlea, endolymph is hyperosmotic to plasma and perilymph. To test the hypothesis that antidiuretic hormone is involved in the modul ation of endolymph secretion, the electrochemical composition of cochl ear fluids, endolymph and perilymph, was studied in three groups of an aesthetized rats: control Long Evans rats, homozygous Brattleboro rats that are genetically deprived of antidiuretic hormone, and Brattlebor o rats that were treated with antidiuretic hormone (dDAVP, 0.5 mu g/10 0 g body weight/24 h during 8 days). Endolymph was sampled from the sc ala media at each turn of the cochlea and perilymph from the scala ves tibuli. In Long Evans rats, the endocochlear potential, the endolympha tic K+ and Cl- concentrations decreased from base to apex of the cochl ea as previously reported in guinea pigs and Sprague Dawley rats. In B rattleboro rats, the endocochlear potential and the Cl- concentration gradients were still present, whereas the K+ concentration gradient wa s absent. This K+ gradient was restored by the administration of dDAVP , which increased the K+ concentration at the base of the cochlea. Thi s work indicates that the K+ secretion in endolymph, and thus the osmo lality, may be locally modulated by the antidiuretic hormone, probably via V-2 receptors.