The voltage responses of hair cells in the basal turn of the guinea-pig cochlea.

1. Intracellularly recorded voltage responses to tones and to current injection were measured from inner (IHC) and outer (OHC) hair cells in the basal turn of the guinea-pig cochlea. 2. The voltage responses of IHCs to tones are increased by negative current and decreased by positive current. For any given current strength, the decrease in the receptor potential amplitude caused by positive current is less than the increase caused by negative current. This was attributed to the voltage-dependent rectification of the membrane conductance. 3. Estimates of the reversal potential of the receptor current were based on extrapolation of the slopes of the current-voltage relations for potentials more negative than -60 mV. The estimated reversal potentials were close to the measured endolymphatic potential. 4. Negative current increased the IHC membrane time constant and increased the transducer driving voltage. When receptor potentials to low-frequency tones were adjusted for constant driving voltage and for the membrane time constant, the positive DC component of the receptor potential was decreased relative to the AC component by current injection but the DC component in response to high-frequency tones near the best frequency of the IHC (16 kHz) was unchanged by negative current. 5. When compensated for constant driving voltage and low-pass filtering due to the basolateral membrane conductance, the proportion of the transducer conductance open in IHCs at rest is increased by negative current. 6. Negative current injection reduced the positive DC components of OHC voltage responses to low-frequency tones and may make them negative. In some cases negative current injection decreased the amplitude of the OHC receptor potential while positive current injection enhanced it. From these observations it is proposed that negative current injection shifts the operating point of the OHC transducer functions towards a positive, saturating region of the relationship and positive current decreases the proportion of the transducer conductance which is open at rest.