Calcium Balance and Mechanotransduction in Rat Cochlear Hair Cells

Abstract

Auditory transduction occurs by opening of Ca(2+)-permeable mechanotransducer (MT) channels in hair cell stereociliary bundles. Ca(2+) clearance from bundles was followed in rat outer hair cells (OHCs) using fast imaging of fluorescent indicators. Bundle deflection caused a rapid rise in Ca(2+) that decayed after the stimulus, with a time constant of about 50 ms. The time constant was increased by blocking Ca(2+) uptake into the subcuticular plate mitochondria or by inhibiting the hair bundle plasma membrane Ca(2+) ATPase (PMCA) pump. Such manipulations raised intracellular Ca(2+) and desensitized the MT channels. Measurement of the electrogenic PMCA pump current, which saturated at 18 pA with increasing Ca(2+) loads, indicated a maximum Ca(2+) extrusion rate of 3.7 fmol x s(-1). The amplitude of the Ca(2+) transient decreased in proportion to the Ca(2+) concentration bathing the bundle and in artificial endolymph (160 mM K(+), 20 microM Ca(2+)), Ca(2+) 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 Ca(2+) load as 3 pA at rest. The results were reproduced by a model of hair bundle Ca(2+) diffusion, showing that the measured PMCA pump density could handle Ca(2+) loads incurred from resting and maximal MT currents in endolymph. The model also indicated the endogenous mobile buffer was equivalent to 1 mM BAPTA.

Calcium entry into stereocilia drives adaptation of the mechanoelectrical transducer current of mammalian cochlear hair cells

“…This finding is consistent with Ca 2+ entry driving adaptation as demonstrated in hair cells from lower vertebrates (9,10). Additional evidence for the presence of MET current adaptation is the channels' increased resting open probability when Ca 2+ influx into the stereocilia is decreased (9,12,28,29), which was evident in OHCs at +99 mV ( Fig. 1 A and E; see also Fig.…”

“…To probe the direct dependence of MET current adaptation on intracellular Ca 2+ , recordings from mouse cochlear hair cells were performed by using either low, endolymphatic extracellular Ca 2+ (0.04 mM: see Materials and Methods) or different intracellular concentrations of the fast Ca 2+ chelator 1,2-Bis (2-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid (BAPTA). Extracellular Ca 2+ is known to block the current through the MET channels with a half-blocking concentration of 1 mM (12,29). We found that reducing extracellular Ca 2+ from 1.3 mM ( Fig.…”

“…Fits to the data points were as follows: at -81 mV: I max = −1,024 pA, a 1 = 0.044 nm −1 , a 2 = 0.015 nm −1 , x 1 and x 2 = 51 nm; at +99 mV: I max = 1,744 pA, and the other parameters were as at −81 mV, except for and that a reduced entry, in low Ca 2+ conditions, significantly increases the open probability of the MET channels (9,12,28,29). Different from 1.3 mM Ca 2+ (Fig.…”

Striated organelle, a cytoskeletal structure positioned to modulate hair-cell transduction

“…Such subcuticular aggregation of mitochondria with large volumes and surface areas, and "tethering" (34,35) indicate the need for tight control of calcium homeostasis or that the SO, SRs, and other structures in the apical part of the hair cell may have high energetic requirements. Recent experiments in cochlear outer hair cells demonstrated that apical mitochondria can act to block Ca 2+ diffusion into the hair-cell soma (36). In addition, mitochondrial crista structure has recently been related to the high metabolic rate of synapses at the calyx of Held (33).…”

“…K + depolarization induces not only reversible shape changes in type I hair cells (39)(40)(41), but also a rise in cytosolic Ca 2+ concentration (42) that could trigger such changes. Rising Ca 2+ levels have been suggested as a potential substrate for mechanotransduction (3), with the large SO-associated mitochondria potentially playing a role (36,43) by promoting reuptake of Ca 2+ after mechanotransduction. A prior study (44) has shown that in the presence of local elevated calcium levels, fodrin (α-2 spectrin) can change from a diffuse distribution within the cytoplasm to a submembranous position, or a patch.…”