Charged moieties in the outer hair cell (OHC) molecular motor protein, prestin, are driven by transmembrane voltage to ultimately provide for cochlear amplification. The speed of voltage-dependent conformational switching underlies its ability to influence micromechanics of the cell and the organ of Corti. Corresponding voltage-sensor charge movements in prestin, classically assessed as a voltage-dependent, nonlinear membrane capacitance ( NLC ), have been used to gauge its frequency response. Using megahertz sampling of prestin charge movements, we extend interrogations of prestin performance into the ultrasonic range (up to 120 kHz) and find response magnitude larger than previously reported. We also confirm kinetic model predictions of prestin by directly observing its cut-off frequency under voltage-clamp as the intersection frequency (F is ) of the real and imaginary components of complex NLC (cNLC) , showing values near 19 kHz. At higher frequencies, the imaginary component roll-off exactly tracks that of Abs( cNLC ). The frequency response of prestin displacement current noise determined from the Nyquist relation aligns with this cut-off. On the other hand, previous measures of stationary thermal-driven noise of prestin indicated that the cut-off was several fold greater than that of NLC , in violation of the fluctuation-dissipation theorem. We have attempted to confirm this apparent paradox, but find that low frequency (<10kHz), voltage-dependent 1/f noise, likely due to intrinsic prestin conductance, can limit the accessible bandwidth for stationary noise analysis. Nevertheless, within those bandwidths, frequency response comparisons of stationary measures and Nyquist relation measures are consistent. We conclude that voltage stimulation accurately assesses the spectral limits of prestin activity.
Using megahertz sampling, we extend measures of prestin charge movement into the ultrasonic range and find that the frequency roll-off is less than previously reported. Nevertheless, analysis of complex nonlinear capacitance confirms low-pass behavior, with a characteristic cut-off frequency near 19 kHz. The frequency response of prestin noise garnered by the admittance-based Nyquist relation confirms this cut-off frequency. In conflict with previous results, however, we find a similar low-pass frequency response using direct measures of prestin noise in the absence of voltage stimulation. Our data indicate that voltage perturbation provides an accurate assessment of prestin performance.