The perception of equilibrium and sound in fish depends on the deflection of hair bundles of hair cell by the otolith. However, the accreting nature of teleostean otoliths poses a problem for maintenance of proper contact between the hair bundle and the otolith surface. Immunocytochemical staining localizes abundant proton-secreting H+-ATPase in the apical membrane of the hair cells. The H+-ATPase-mediated proton secretion into the endolymph causes an approximately 0.4-unit pH decrease, which was quantified by an H+-selective microelectrode. Thus, the hair cells maintain the proper distance from the otolith by neutralizing the alkaline endolymph to retard CaCO3 deposition on the otolith opposite the sensory macula. Carbonic anhydrase, which hydrolyses CO2 and produces HCO 3- and H+, was also localized in the hair cells. Ionocytes showed prominent immunostaining of carbonic anhydrase and Na+-K+-ATPase, indicating its role in transepithelial transport of HCO3- across the membranous labyrinth into the endolymph. Ionocytes form a ring closely surrounding the sensory macula. HCO3- secreted from the ionocytes may serve as a barrier to neutralize H+ diffused from the sensory macula while keeping the endolymph alkaline outside the sensory macula. The ingenious arrangement of ionocytes and hair cells results in a unique sculptured groove, which is a common feature on the proximal surface of all teleostean otoliths.
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