Calcium buffering in the chick cochlear hair cell active zone

The cochlear hair cell reliably and precisely translates acoustic energy into quantal chemical signals. Calcium influx triggers several processes important to this function, yet how calcium is regulated to subserve these multiple functions within a hair cell is poorly understood. The driving hypothesis of this research was that using exogenous chelators to vary the size of calcium domains would impact various calcium dependent processes differently. Perforated-patch and whole-cell patch recordings were used to study the effects of exogenous buffers on calcium-dependent neurotransmitter release and membrane properties associated with the action of calcium-activated BK channels. Endogenous mobile calcium buffering in the chick was best approximated by submillimolar concentrations of an exogenous buffer with fast calcium binding. However, physiological data and computational modeling indicate that vesicle exocytosis and BK-channel function are affected by distinct properties of mobile calcium buffers based on their relative location to sites of calcium entry. Exocytosis was most sensitive to binding kinetics of calcium buffers. Sites of fusion must be within hundreds of nanometers of active-zone centers. Magnitude of depolarization during electrical resonance depends only on the concentration of calcium buffer, indicating that a subset BK channels reside hundreds of nanometers further from sites of calcium entry. However, electrical resonance frequency is unaffected by either concentration or kinetic of buffers suggesting a tight association between calcium channels and the subset of BK channels that determine frequency. The endogenous mobile buffers can provide distinct zones of varying free Ca 2+ concentrations. These zones play a role in determining the function of the different calcium effectors that reside within them