Mechanoelectrical transducer current in cochlear IHCs. A, B, Saturating mechanotransducer currents recorded from an apical and basal mouse IHC, respectively, exposed to 1.3 mm Ca²⁺ (middle) and an endolymphatic Ca²⁺ concentration (0.3 mm: bottom). The driver voltage (DV) signal to the fluid jet is shown above the traces (positive deflection of the DV is inhibitory). Note that mechanotransducer current amplitude was larger and its fraction activated at rest was increased in the presence of endolymph-like Ca²⁺. In 0.3 mm Ca²⁺ the mechanotransducer current was abolished by 0.2 mm DHS. IHC holding potential was −84 mV. C, D, Maximum amplitude (C) and resting open probability (D) of the mechanotransducer current in perilymph-like (1.3 mm) and endolymph-like (0.3 mm: see Materials and Methods for details) Ca²⁺ concentrations. The resting open probability was calculated by dividing the mechanotransducer current available at rest (the difference between the current level before the stimulus, indicated by the dashed line, and the current level on the negative phase of the stimulus when all channels were closed) by the maximum peak-to-peak mechanotransducer current.