Mammalian prestin is a weak Cl − /HCO 3 − electrogenic antiporter

Key points•  Outer hair cells of the mammalian cochlea are cells which amplify the incoming sound using mechanisms based on prestin, a molecular actuator related to a family of chloride–bicarbonate exchangers. •  It has not been clear so far whether prestin has any bicarbonate–chloride exchange properties, often being described as an ‘incomplete transporter’. •  Here we show, using a pH probe linked to prestin in an expression system, that prestin can transport bicarbonate at low rates and acts as an electrogenic transporter for chloride. •  The high expression level of prestin in mammalian outer hair cells thus accounts for a number of previous observations of the cells’ internal pH regulation, and may indicate an additional role for prestin in homeostatic regulation of cochlear amplification.Abstract  The lateral membrane of mammalian cochlear outer hair cells contains prestin, a protein which can act as a fast voltage‐driven actuator responsible for electromotility and enhanced sensitivity to sound. The protein belongs to the SLC26 family of transporters whose members are characterised as able to exchange halides for SO 4 2− or HCO 3 − yet previous analyses of mammalian prestin have suggested that such exchange functions were minimal. Here anion transport is investigated both in guinea‐pig outer hair cells (OHCs) and in an expression system where we employ a sensitive intracellular pH (pH i ) probe, pHluorin, to report HCO 3 − transport and to monitor the small pH i changes observable in the cells. In the presence of extracellular HCO 3 − , pH i recovered from an acid load 4 times faster in prestin‐transfected cells. The acceleration required a chloride gradient established by reducing extracellular chloride to 2 m m . Similar results were also shown using BCECF as an alternative pH i sensor, but with recovery only found in those cells expressing prestin. Simultaneous electrophysiological recording of the transfected cells during bicarbonate exposure produced a shift in the reversal potential to more negative potentials, consistent with electrogenic transport. These data therefore suggest that prestin can act as a weak Cl − /HCO 3 − antiporter and it is proposed that, in addition to participating in wide band cochlear sound amplification, prestin may also be involved in the slow time scale (>10 s) phenomena where changes in cell stiffness and internal pressure have been implicated. The results show the importance of considering the effects of the endogenous bicarbonate buffering system in evaluating the function of prestin in cochlear outer hair cells.