The electromotility of outer hair cells (OHCs) in the organ of Corti allows enhanced sensitivity and frequency selectivity in mammalian hearing. There is good evidence that voltage sensitive motor complex, localised in the basolateral membrane and containing a novel motor protein’ prestin’, is responsible for this phenomenon. Prestin is a member of a superfamily (SLC26) of anion-bicarbonate exchangers which are expressed in a wide range if tissues. We have investigated the transport of chloride by prestin chloride indicator 6-Methoxy-N-ethylquinolinium iodide (MEQ, Molecular Probes). As a voltage dependant motor intracellular anions are thought to act as the sensor (Oliver et al. 2001).

OHCs were isolated from the cochlea of the guinea pig following humane cervical dislocation. Cells were bathed in hepes buffered perilymph containing (mM): Cl, 155; Na, 144; K, 4.6; Mg, 1.5; Ca, 1.5; hepes, 5; pH 7.3, 320 mOsm. MEQ (3 mM) was loaded into the cells via the patch pipette using conventional intracellular medium containing 30 mM KCl and gluconate as the substitute anion. The intracellular fluorescent dye is reversibly quenched by chloride. Using in vitro calibration, we estimate the IC₅₀ for Cl quench to have been 8.7 mM. MEQ quench was virtually independent of gluconate (IC₅₀, 200 mM). When cells were voltage clamped at -40 mV and equilibrated with MEQ, the imaged fluorescence levels increased when an extracellular solution containing 0 Cl (gluconate as substituting anion) was perfused around the OHC lateral membrane, indicating a reduction of intracellular chloride. Simultaneous recordings showed a small increase in outward current through the cell membrane. Cells shortened by about 4 % as the volume increased. The increase in fluorescence was not due to an increase in illumination path length as fluorescence changes also occurred in cells collapsed by hypo-osmotic intracellular solution. Salicylate, known to block electromotility, eliminated ionic current changes on zero chloride solution exposure suggesting chloride is transported through the prestin motor complex. The data is consistent with prestin being a low efficacy integral membrane transporter whose motor function is a consequence of the anion transport cycle.

This work was supported by the Wellcome Trust. NT held a vacation studentship of the Physiological Society.