Studies of ionic currents in the isolated vestibular hair cell of the chick

Abstract

The ionic currents in enzymatically isolated vestibular hair cells of the chick were studied by a whole-cell-clamp variation of the patch voltage clamp, and by single channel recording. At membrane potentials more negative than -80 mV the hair cell showed anomalous rectification, and at potentials more positive than -40 mV large outward K currents were observed in normal saline. The outward K current decreased at large positive potentials, showing an N-shaped I-V relation. The outward K current was carried mostly through the Ca-activated K channel. K currents through the anomalous rectifier channel showed a decay in normal saline. This decay was eliminated reversibly in Na-free saline when the isotonic KCl-EGTA solution was used as the internal medium. However, a fast decay was still observed in Na-free high-K external solution when isotonic CaCl-EGTA was used as the internal medium. An increase in [K]o decreased the decay rate of the inward K current. The single-channel conductance of the anomalous rectifier channel was 50 pS in 160 mM-K saline and 23 pS in 40 mM-K saline. In 100 mM-Ca, -Sr and -Ba salines a large inward current was observed. At positive potentials the inward current carried by Ca and Sr ions showed significant decay; the current became outward at large positive potentials. Since the decay of the inward current was eliminated when 100 microM-quinine was added to the bathing medium, it was probably due to the activation of some Ca-activated K conductance which remained even with isotonic CsCl-EGTA internal medium. The activation kinetics of the Ca channel were studied in 100 mM-Ba solution at low temperatures (9-13 degrees C). From a comparison of the time constants of activation with the time constants of the tail currents, it was concluded that the Ca channel follows Hodgkin-Huxley-type m2 kinetics. A slow component that deviated from m2 kinetics was frequently observed at relatively large positive potentials. The steady-state fluctuations of Ba current showed a power density spectrum reasonably well fitted by a sum of two Lorentzian functions. The spectrum has a low-frequency component which indicates kinetics close to the macroscopic activation process of the Ca channel and a high-frequency component that indicates very fast flickering kinetics operating in the Ca channel.