Changes in secretion kinetics during maturation. Secretion kinetics was investigated by measuring ΔC_m to depolarizations (to −5 mV) of different duration applied in random order. A plots the mean C_m responses recorded in the perforated-patch configuration in IHCs from the different stages (n = 13 cells for P0; n = 10 for P6; n = 9 for P10; and n = 28 for IHCs from hearing mice). B displays the first 30 msec of stimulation with higher resolution. C compares the ΔC_m of whole-cell experiments on IHCs from P6 and hearing mice in which we added a mixture of 5 mm Ca²⁺-free EGTA and 5 mmCa²⁺-loaded EGTA to the pipette (n = 9 cells for P6; n = 27 for IHCs from P14–P25). The solid lines represent double exponential fits to the data to estimate size and release time constant of the readily releasable pool (see text).