Ca²⁺-dependent exocytosis can be evoked by depolarizations long before the onset of hearing.A shows [Ca²⁺]_i(top panel) and C_m(bottom panel) of an IHC from a P2 mouse at low time resolution. In response to long depolarizations (200 and 500 msec), sizable increases in C_m (exocytosis) and in [Ca²⁺]_i were observed. Shorter depolarizations resulted in much smaller responses (arrow, 50 msec). The subsequent decline inC_m most likely reflects endocytic membrane retrieval. B, C_m (top panel) and membrane current (bottom panel) of an IHC from a P6 mouse in response to 100 msec long depolarizations to −5 mV in the presence or absence of extracellular Ca²⁺ (Ca²⁺-free Ringer containing 2 mm Ca²⁺-free EGTA and 3 mm MgCl₂).C_m increments were inhibited by abolition of the Ca²⁺ influx and restored on readdition of Ca²⁺. C, Representative Ca²⁺ currents (bottom panel) and C_m increments (top panel) of IHCs from different developmental stages to a 50 msec long depolarization to −5 mV. Baseline capacitance values are indicated below each trace.