The cochlea of the inner ear is filled with two different types of fluid: a unique extracellular solution, endolymph, which has a positive potential of ~+80 mV and 150 mM K⁺ ions, and a normal extracellular solution, perilymph. The specialized ionic composition and high potential in endolymph is essential for hearing. The circulation of K⁺ ions from perilymph to endolymph through the cochlear lateral wall is crucially involved in establishment and maintenance of endolymph. We reported previously an inwardly rectifying K⁺ channel, Kir4.1, expressed in stria vascularis in cochlea. The exact molecules responsible for the K⁺ transport, however, have remained largely unknown.

Here we report that an inwardly rectifying K⁺ channel, Kir5.1, is strongly expressed in the spiral ligament of the lateral wall. A specific antibody labelled Kir5.1 in type II and IV fibrocytes in spiral ligament that are exposed to the perilymph in scala tympani. On the other hand, no Kir5.1 immunoreactivity was detected in either type I or III fibrocytes of the ligament. Immuno-electron microscopic examination revealed that Kir5.1 is abundantly localized in the processes of the fibrocytes. Developmental expression of Kir5.1 was paralleled with those of Na⁺,K⁺-ATPase and Na⁺-K⁺-2Cl^– cotransporters, the molecules important for K⁺ transport in the ligament. Furthermore, Kir5.1 also occurs at the fibrocytes of suprastrial zone of the ligament and those of the spiral limbus that face to another perilymphatic space, the scala vestibuli.

These results suggest that the Kir5.1 channel plays a central role in recycling action and homeostasis of K⁺ ions in cochlea that are indispensable for auditory function.