1. Under whole-cell voltage clamp, the effects of initial voltage conditions and membrane tension on gating charge and voltage-dependent capacitance were studied in human embryonic kidney cells (TSA201 cell line) transiently transfected with the gene encoding the gerbil protein prestin. Conformational changes in this membrane-bound protein probably provide the molecular basis of the outer hair cell (OHC) voltage-driven mechanical activity, which spans the audio spectrum. 2. Boltzmann characteristics of the charge movement in transfected cells were similar to those reported for OHCs (Q max = 0.99 ± 0.16 pC, z = 0.88 ± 0.02; n = 5, means ± S.E.M.). Unlike that of the adult OHC, the voltage at peak capacitance (V pkcm) was very negative (-74.7 ± 3.8 mV). Linear capacitance in transfected cells was 43.7 ± 13.8 pF and membrane resistance was 458 ± 123 MΩ. 3. Voltage steps from the holding potential preceding the measurement of capacitance-voltage functions caused a time- and voltage-dependent shift in V pkcm. For a prepulse to -150 mV, from a holding potential of 0 mV, V pkcm shifted 6.4 mV, and was fitted by a single exponential time constant of 45 ms. A higher resolution analysis of this time course was made by measuring the change in capacitance during a fixed voltage step and indicated a double exponential shift (τ 0 = 51.6 ms, τ 1 = 8.5 s) similar to that of the native gerbil OHC. 4. Membrane tension, delivered by increasing pipette pressure, caused a positive shift in V pkcm. A maximal shift of 7.5 mV was obtained with 2 kPa of pressure. The effect was reversible. 5. Our results show that the sensitivity of prestin to initial voltage and membrane tension, though present, is less than that observed in adult OHCs. It remains possible that some other interacting molecular species within the lateral plasma membrane of the native OHC amplifies the effect of tension and prior voltage on prestin's activity.
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