Calcium Balance and Mechanotransduction in Rat Cochlear Hair Cells

Abstract

Auditory transduction occurs by opening of Ca2+-permeable mechanotransducer (MT) channels in hair cell stereociliary bundles. Ca2+clearance from bundles was followed in rat outer hair cells (OHCs) using fast imaging of fluorescent indicators. Bundle deflection caused a rapid rise in Ca2+that decayed after the stimulus, with a time constant of about 50 ms. The time constant was increased by blocking Ca2+uptake into the subcuticular plate mitochondria or by inhibiting the hair bundle plasma membrane Ca2+ATPase (PMCA) pump. Such manipulations raised intracellular Ca2+and desensitized the MT channels. Measurement of the electrogenic PMCA pump current, which saturated at 18 pA with increasing Ca2+loads, indicated a maximum Ca2+extrusion rate of 3.7 fmol·s−1. The amplitude of the Ca2+transient decreased in proportion to the Ca2+concentration bathing the bundle and in artificial endolymph (160 mM K+, 20 μM Ca2+), Ca2+carried 0.2% of the MT current. Nevertheless, MT currents in endolymph displayed fast adaptation with a submillisecond time constant. In endolymph, roughly 40% of the MT current was activated at rest when using 1 mM intracellular BAPTA compared with 12% with 1 mM EGTA, which enabled estimation of the in vivo Ca2+load as 3 pA at rest. The results were reproduced by a model of hair bundle Ca2+diffusion, showing that the measured PMCA pump density could handle Ca2+loads incurred from resting and maximal MT currents in endolymph. The model also indicated the endogenous mobile buffer was equivalent to 1 mM BAPTA.