Contribution of the plasmalemma to Ca2+ homeostasis in hair cells

Calcium influx through transduction channels and efflux via plasmalemmal Ca 2+-ATPases (PMCAs) are known to contribute to calcium homeostasis and modul ate sensory transduction in vertebrate hair cells. To examine the relative contributions of apical and basolateral pathways, we analyzed the calcium d ynamics in solitary ciliated and deciliated guinea pig type I and type II v estibular hair cells. Whole-cell patch-clamp recordings demonstrated that t hese cells had resting potentials near -70 mV and could be depolarized by 1 0-20 mV by superfusion with high potassium. Fura-2 measurements indicated t hat ciliated type [I cells and deciliated cells of either type had low basa l [Ca2+](i), near similar to 90 nM, and superfusion with high potassium led to transient calcium increases that were diminished in the presence of Ca2 + channel blockers. In contrast, measurements of type I ciliated cells, hai r cells with large calyceal afferents, were associated with a higher basal [Ca2+](i) of similar to 170 nM. High-potassium superfusion of these cells i nduced a paradoxical decrease in [Ca2+](i) that was augmented in the presen ce of Ca2+ channel blockers. Optical localization of dihydropyridine bindin g to the kinocilium suggests that they contain L-type calcium channels, and as a result apical calcium influx includes a contribution from voltage-dep endent ion channels in addition to entry via transduction channels localize d to the stereocilia. Eosin block of PMCA significantly altered both [Ca2+] (i) baseline and transient responses only in ciliated cells suggesting that ,in agreement with immunohistochemical studies, PMCA is primarily localized to the bundles.