Characterization of potassium permeability of cochlear duct by perilymphatic perfusion of barium

The relative transepithelial "permeabilities" of the cochlear duct to K, Na, and Cl were investigated so as to identify the K-selective tissues and to determine the cellular origin of this selectivity. Single-ion substitutions were made for K, Na, and Cl with the impermeant species N-methyl-D-glucamine (NMDG) for K and Na and gluconate or sulfate for Cl in perilymph. Transepithelial potential changes were relatively slow and small for Na and Cl substitutions. However, either K for Na or K for NMDG substitutions demonstrated a pronounced K selectivity (rapid changes of electrical potential) of only the sensory-cell tissue (organ of Corti). The response to the K for Na substitution was most clearly seen after electrogenic K transport was inhibited by ischemia while the sensory cells were metabolically sustained via perilymphatic perfusion. Under this condition, perfusion of a medium containing 154 mM K gluconate reduced the negative potential (typically -25 to -40 mV) to within a few millivolts of zero. In a control medium, perilymphatic barium (0.5-5 mM) produced qualitatively similar effects, suggesting that this K selectivity is localized primarily at the basolateral membrane of the sensory cells rather than at the junctional complexes.