K+ and Na+ absorption by outer sulcus epithelial cells

Transduction of sound into nerve impulses by hair cells depends on modulation of a current carried primarily by K+ into the cell across apical transduction channels that are permeable to cations. The cochlear function thus depends on active secretion of K+ accompanied by absorption of Na+ by epithelial cells enclosing the cochlear duct. The para-sensory cells which participate in the absorption of Na+ (down to the uniquely low level of 1 mM) were previously unidentified and the existence of a para-sensory pathway which actively absorbs K+ was previously unknown. A relative short circuit current (Isc,probe, measured as the extracellular current density with a vibrating electrode) was directed into the apical side of the outer sulcus epithelium, decreased by ouabain (1 mM), an inhibitor of Na+, K(+)-ATPase, and found to depend on bath Na+ and K+ but on neither Ca2+ nor Cl-. Isc,probe was shown to be an active current by its sensitivity to ouabain. On-cell patch clamp recordings of the apical membrane of outer sulcus cells displayed a channel activity, which carried inward currents under conditions identical to those used to measure Isc,probe. Both Isc,probe and non-selective cation channels (27.4+/-0.6 ps, n = 22) in excised outside-out patches from the apical membrane were inhibited by Gd3+ (1 mM). Ics,prob was also inhibited by 5 mM lidocaine, 1 mM quinine and 500 microM amiloride but not by 10 microM amiloride. These results demonstrate that outer sulcus epithelial cells contribute to the homeostasis of endolymph by actively absorbing Na+ and K+. An entry pathway in the apical membrane was shown to be through non-selective cation channels that were sensitive to Gd3+.