Gastric type H+, K+-ATPase in the cochlear lateral wall is critically involved in formation of the endocochlear potential

Cochlear endolymph has a highly positive potential of approximately + 80 mV known as the endocochlear potential (EP). The EP is essential for hearing and is maintained by K+ circulation from perilymph to endolymph through the cochlear lateral wall. Various K+ transport apparatuses such as the Na+, K+-ATPase, the Na+-K+-2Cl(-) cotransporter, and the K+ channels Kir4.1 and KCNQ1/KCNE1 are expressed in the lateral wall and are known to play indispensable roles in cochlear K+ circulation. The gastric type of the H+, K+-ATPase was also shown to be expressed in the cochlear lateral wall (Lecain E, Robert JC, Thomas A, and Tran Ba Huy P. Hear Res 149: 147-154, 2000), but its functional role has not been well studied. In this study we examined the precise localization of H+, K+-ATPase in the cochlea and its involvement in formation of EP. RT-PCR analysis showed that the cochlea expressed mRNAs of gastric alpha(1)-, but not colonic alpha(2)-, and beta-subunits of H+, K+-ATPase. Immunolabeling of an antibody specific to the alpha 1 subunit was detected in type II, IV, and V fibrocytes distributed in the spiral ligament of the lateral wall and in the spiral limbus. Strong immunoreactivity was also found in the stria vascularis. Immunoelectron microscopic examination exhibited that the H+, K+-ATPase was localized exclusively at the basolateral site of strial marginal cells. Application of Sch-28080, a specific inhibitor of gastric H+, K+-ATPase, to the spiral ligament as well as to the stria vascularis caused prominent reduction of EP. These results may imply that the H+, K+-ATPase in the cochlear lateral wall is crucial for K+ circulation and thus plays a critical role in generation of EP.