Acetylcholine activates two currents in guinea-pig outer hair cells

1. Whole-cell voltage clamp recordings were made from outer hair cells (OHCs) isolated from the basal and middle regions of the cochlea. The current produced when acetylcholine (ACh) was applied to the basal pole was investigated. 2. Acetylcholine (50-70 mu M) activated an early inward current followed by a late outward current in most cells at -70 mV. The activation of the early current was very rapid, with no delay. 3. The late outward current was a K+ current and was the major component of the total ACh-sensitive current at negative voltages. 4. The ACh-sensitive current was voltage dependent. From tail current measurements, the current was maximal at -55 mV and declined steeply at more positive potentials. On average, 50% of the current was active at -22 mV and 10% was active at 0 mV. This decline was caused by a reduction in the K+ current with depolarization. Between +20 and +40 mV, the major component of the ACh-sensitive current was the early current. 5. The steady-state I-V curve for the ACh-sensitive current was N-shaped, with a peak at -36 mV. The instantaneous I-V curve fur the ACh-sensitive current taken from measurements just after a voltage step was outwardly rectifying and did not show this peak. This difference occurred because the voltage dependence was time dependent. 6. The reversal potential for the early current was estimated to be close to +13 mV, in accordance with it being a non-specific cation current. 7. The K+ current was abolished by the removal of external Ca2+. This effect occurred with no measurable delay. When external Ca2+ was lowered to 0.4 mM, the peak of the steady-state I-V curve for the ACh-sensitive current shifted by UP to -20 mV and its amplitude was reduced. These results suggested that the K+ current was dependent on Ca2+ influx. 8. The inward current usually remained when the K+ current was abolished or greatly reduced by removing external Ca2+ or by dialysing the cells with BAPTA (5-10 mM). 9. Both early and late currents were reversibly blocked by alpha-bungarotoxin (0.2 mu M) and curare (1 mu M). 10. A simple scheme is proposed to account for the response. ACh, binding to a nicotinic receptor, directly gates the early cation current. Part of this current is carried by Ca2+ which once inside the cell leads to the activation of a Ca2+-dependent K+ current.