Low endolymph calcium concentrations in deafwaddler2J mice suggest that PMCA2 contributes to endolymph calcium maintenance

In vertebrates, transduction of sound into an electrochemical signal is carried Out by hair cells that rely on calcium to perform specialized functions. The apical surfaces of hair cells are surrounded by endolymphatic fluid containing calcium at concentrations that must be maintained by active transport. The mechanism of this transport is unknown, but an ATP-dependent pump is believed to participate. Mutation of the Atp2b2 gene that encodes plasma membrane calcium ATPase type 2 (PMCA2) produces the deaf, ataxic mouse: deafwaddler(2J) (dfw(2J)). We hypothesized that PMCA2 might transport calcium into the endolymph and that dfw(2J) mice would have low endolymph calcium concentrations, possibly contributing to their deafness and ataxia. First, using immunocytochemistry, we demonstrated that PMCA2 is present in control mice inner and outer hair cell stereocilia where it could pump calcium into the endolymph and that PMCA2 is absent in dfw(2J) stereocilia. Second, using an aspirating microelectrode and calcium-sensitive fluorescent. dye, we found that dfw(2J) mice endolymph calcium concentrations are significantly lower than those of control mice. These findings suggest that PMCA2, located in hair cell stereocilia, contributes significantly to endolymph calcium maintenance.