On the frequency limit and phase of outer hair cell motility: effects of the membrane filter

Whole-cell voltage clamp and displacement-measuring photodiode techniques were used to study electrophysiological and mechanical properties of the guinea pig outer hair cell (OHC). OHCs demonstrate a voltage-mechanical response (V-M) function that can be fit by a two state Boltzmann relation, where the cell normally rests near the hyperpolarizing saturation region (-70 to -90 mV). The voltage at half-maximal length change (Vh) is depolarized relative to the resting potential, and this ensures that for symmetrical sinusoidal voltage stimulation about the resting potential, AC and DC mechanical responses will be generated. Analysis of OHC motility using pure tone voltage bursts from 11 to 3200 Hz demonstrates both AC and DC mechanical responses. By exploiting the frequency-dependent current-voltage phase separation that is characteristic of an RC-dominated system under voltage clamp, it is demonstrated that OHC motility follows the phase of AC transmembrane voltage and not that of current. For voltage stimulation across frequencies in the acoustic range, the motility cutoff frequency corresponds to the cutoff frequency of the imposed transmembrane voltage. Frequency cutoffs approaching 1 kHz have been measured but are clamp time constant limited. These observations are congruent with the voltage dependency hypothesis of OHC motility. In addition, the DC component of the mechanical response is shown to be frequency independent, but to decrease in magnitude disproportionately compared to the AC component as the magnitude of the driving voltage decreases. This is predicted from the form of the V-M function, whose level dependent DC nonlinearity is a consequence of the resting potential being displaced from Vh. The net effect is that the mechanical DC: AC ratio approaches zero for small AC voltages. Taken together, these findings question the ability of the OHC mechanical response to influence organ of Corti micromechanics at high acoustic frequencies where a tuned amplification of basilar membrane motion is hypothesized.