Regulation of cochlear hair cell function by intracellular calcium stores.

Introduction: Mammalian hearing depends on the dual mechanosensory and motor functions of cochlear hair cells. Both these functions may be regulated by Ca²⁺ release from intracellular stores. However, it is still unclear how exactly intracellular Ca²⁺ release may affect either hair cell mechano-electrical transduction (MET) or prestin-dependent electromotility in outer hair cells (OHCs).

Methods: Here, we used photo-activatable (caged) compounds to generate fast increases of either Ca²⁺ or inositol-3-phosphate (IP₃) in the cytosol of young postnatal rodent auditory hair cells, thereby stimulating either Ca²⁺- or IP₃- induced releases of Ca²⁺ from intracellular stores. Fast Ca²⁺ imaging was used to monitor propagation of Ca²⁺ signals along the length of a hair cell. To access potential physiological role(s) of intracellular Ca²⁺ releases, we used whole cell patch clamp to record: i) OHC voltage-dependent capacitance, a known electrical correlate of prestin-based electromotility, and ii) MET currents evoked by stereocilia bundle deflections with fluid-jet. In the latter experiments, changes of mechanical stiffness of the hair bundles were also quantified from video recordings of stereocilia movements.

Results: Ca²⁺ uncaging at the OHC apex initiated Ca²⁺ wave propagating to the base of the cell with subsequent Ca²⁺ build-up there. Ca²⁺ uncaging at the OHC base generated long-lasting and apparently self-sustained Ca²⁺ responses, further confirming Ca²⁺-induced Ca²⁺ release in the OHC basal region. Photoactivated IP₃ initiated a slow increase of cytosolic Ca²⁺ ([Ca²⁺] _i ) throughout the whole OHC, confirming the presence of slow-activated IP₃-gated Ca²⁺ stores in OHCs. Interestingly, Ca²⁺ uncaging produced no effects on OHC voltage-dependent capacitance. In an OHC, the rise of [Ca²⁺] _i is known to decrease axial stiffness of the cell and may modulate the stiffness of mechanosensory stereocilia bundles. To separate these two phenomena, we explored the potential effects of intracellular Ca²⁺ release on mechanical properties of stereocilia bundles in cochlear inner hair cells (IHCs). Ca²⁺ uncaging at the apex of an IHC caused a long-lasting increase in mechanical stiffness of stereocilia bundle without any changes in the amplitude or deflection sensitivity of the MET current.

Discussion: We concluded that the most likely physiological role of IP₃-gated Ca²⁺ release at the apex of the cell is the regulation of hair bundle stiffness. In contrast, Ca²⁺-induced Ca²⁺ release at the base of OHCs seems to regulate axial stiffness of the cells and its hyperpolarization in response to efferent stimuli, without direct effects on the OHC prestin-based membrane motors.