Figure 9.

Determination of Ca²⁺ sensitivity of exocytosis in VHC-I and IHCs. A, B, From the experiments of Figure 8, mean Δ_Cm responses at each UV flash were plotted against corresponding [Ca²⁺]_i for VHC-I (A) and IHCs (B). A′, B′, Insets, Δ_Cm after two UV flashes (fifth flash: Δ_Cm5 and sixth flash: Δ_Cm6) in control mice. Δ_Cm responses in Otof ^−/− VHC-I and Otof ^−/− IHCs show a very poor Ca²⁺ dependence (red lines). A′, B′, The black dashed lines indicate the Δ_Cm plateau after each UV flash. Data points were fitted with a Hill sigmoidal function. C, The instantaneous rate of the Δ_Cm fast component after each UV flash was plotted against [Ca²⁺]_i for VHC-I (dark blue) and IHCs (light blue). C′, Inset, The method used to measure the instantaneous rate of the fast component. We used a linear function to fit the first 20 ms of the capacitance response (red lines). As in A and B, we used a Hill sigmoidal function to fit these data points. D, The second slow component of the Δ_Cm response was also plotted against [Ca²⁺]_i. D′, Inset, The method used. We linearly fitted the first 50 ms (black lines) after the deflection point of the response (arrowhead). Data points were fit with a Hill sigmoidal function. E, The instantaneous rate of the fast Δ_Cm responses deeply decreased at [Ca²⁺]_i >8 μm. Data points were fitted with a decreasing sigmoidal function to obtain K_i (constant of inhibition) of exocytosis, which we considered as the K_D for endocytosis.