FIGURE 5.

Relation of charge movement and OHC displacement. A and B, simultaneous measurement of NLC and motility in the presence of salicylate is shown. In the same experimental configuration as in Fig. 2, NLC and OHC motility were simultaneously measured in the presence of salicylate. NLC and motility were simultaneously recorded with one cycle of 2-Hz, 100-mV amplitude sinusoidal voltage stimulus superimposed with two higher frequency stimuli (390.6 and 781.2 Hz, 10-mV amplitude) at time points indicated by the solid triangles, while the membrane capacitance at 0 mV was constantly monitored during the measurements (A, inset). pF, picofarads. Accompanied by the reduction of the prestin-dependent charge movement, OHC displacement decreased. The same colors were used to show matching simultaneous NLC motility recordings. For determining prestin-dependent charge movement (q), C_lin and C_sa were first determined by the three-state and two-state-Csa Boltzmann models. Subsequently the area between NLC curves and the C_lin (or C_sa) was calculated. OHC displacement (d) was determined by the maximum displacement observed. C and D, shown is a summary of the q-d relation. The measurements shown in A and B were performed on different OHCs (n = 8). The d values were plotted against the q values determined by three-state model (C) or the two-state-Csa model (D) for each OHC. Different colors represent different OHCs. The solid lines are Deming's regression lines. Average r² values from eight OHCs with S.D. are shown in each figure. E, computer simulations of NLC motility responses in the presence and absence of salicylate explain how positive d intercepts can be seen at q = 0 if m₁/m₂ < 1. The following values were used: α₁ = α₂ = 0.04 mV⁻¹ (with and without salicylate), V_pk1 = −75 mV, V_pk2 = −70 mV (without salicylate), V_pk1 = −150 mV, V_pk2 = 0 mV (with salicylate), m₁/m₂ = 0.55. F, using the d-q data shown in C and Equations 8, 9, and 10, m₂ values were determined for each OHC (n = 8). The same colors are used for matching. G, the two-step electromechanical coupling mechanism of prestin is shown. z₁ and z₂ are apparent valences calculated from α₁ and α₂, determined by the three-state model (Fig. 4F). The positive signs are assigned for the rightward transitions. K₁₂ and K₂₃ are voltage-dependent equilibrium constants defined in the text, which increase with depolarizing voltage stimuli, and vice versa.