Presence of plasma membrane-bound Ca2+-ATPase in the secretory epithelia of the inner ear

The Ca2+ concentration of the endolymph is low, around 0.023 mM. Yet, becau se of the positive endocohlear potential, Ca2+ must be actively transported into the endolymphatic space. The mechanisms responsible for the active Ca 2+ transport into the endolymph are not known. In this study, the presence of plasma membrane-bound Ca2+-ATPase (PMCA ATPase) in the endolymph-produci ng, secretory epithelia of the inner ear from guinea pig was investigated w ith immunoblotting and immunohistochemistry. The antibody used was a monocl onal antibody which recognizes an epitope shared by all four known isoforms of PMCA ATPase. With immunoblotting, a band corresponding to PMCA ATPase w as found in the stria vascularis, the ampullary tissue, the utricle and the endolymphatic sac in assays from at least three different batches of tissu e. With immunohistochemistry a strong positive staining reaction for PMCA A TPase could be seen in the stria vascularis and the dark cells of the ampul lary tissue and the utricle. The epithelial cells in the endolymphatic sac showed a moderate positive staining reaction. Accordingly, in this study th e presence of PMCA ATPase was shown in all the endolymph-producing, secreto ry epithelia of the inner ear. These results indicate that PMCA ATPase play s a role in the regulation of the Ca2+ concentration in the endolymph.