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 Ca release from intracellular stores. However, it is still unclear how exactly intracellular Ca 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 Ca or inositol-3-phosphate (IP) in the cytosol of young postnatal rodent auditory hair cells, thereby stimulating either Ca- or IP- induced releases of Ca from intracellular stores. Fast Ca imaging was used to monitor propagation of Ca signals along the length of a hair cell. To access potential physiological role(s) of intracellular Ca 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

Ca uncaging at the OHC apex initiated Ca wave propagating to the base of the cell with subsequent Ca build-up there. Ca uncaging at the OHC base generated long-lasting and apparently self-sustained Ca responses, further confirming Ca-induced Ca release in the OHC basal region. Photoactivated IP initiated a slow increase of cytosolic Ca ([Ca] ) throughout the whole OHC, confirming the presence of slow-activated IP-gated Ca stores in OHCs. Interestingly, Ca uncaging produced no effects on OHC voltage-dependent capacitance. In an OHC, the rise of [Ca] 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 Ca release on mechanical properties of stereocilia bundles in cochlear inner hair cells (IHCs). Ca 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 IP-gated Ca release at the apex of the cell is the regulation of hair bundle stiffness. In contrast, Ca-induced Ca 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.