Mechano‐electrical transduction currents in isolated vestibular hair cells of the chick.

Abstract

Properties of a mechano-electrical transduction channel were studied in enzymatically dissociated chick vestibular hair cells by using a whole-cell recording variation of the patch voltage-clamp technique. The apical hair bundle was stimulated by a glass rod which moved along a 1-dimensional axis when stimulated by either a triangular or a trapezoidal command voltage. The motion of the glass rod was monitored optically using a photodiode. In response to triangular stimuli, the hair cell generated a current of triangular wave form with occasional step-like spikey or zigzag-appearing events. Control experiments confirmed that the current was generated only when the hair bundle was displaced towards the tallest stereocilium. The mechano-sensitive current was blocked by streptomycin and by neomycin. The blockage by streptomycin was clearly voltage dependent: the reduction of the current became larger with hyperpolarization of the membrane. Apparently the positively charged antibiotic molecules plug the mechanically gated channels. The mechano-sensitive current recorded was identified as the machano-electrical transduction (met) current. The permeability of the met channel to various monovalent cations was determined from reversal potential measurements. Since a CsCl-EGTA [ethzlene glycol bio(.beta.-amino ethyl ether) N,N,N''-tetracitic acid] intracellular medium was used, all permeabilities [p] were calcualted relative to PCs. The sequence of permeabilities was Li > Na .gtoreq. K .gtoreq. Rb > Cs > choline > choline > TMA [tetra methylammonium] > TEA [tetran ethylammonium]. External Ca ions were indispensable for the recording of transduction current and Sr ions could replace Ca ions without loss of the transduction activity. The minimum [Ca]o for stable generation of the met current was 20 .mu.M in Cs saline. The addition of 50-200 .mu.M-Ca to the isotonic Ba saline could maintain the met current. The met current was observed in isotonic Ca and in Sr salines. Isotonic Ba, Mg and Mn salines were enriched with 1-2 mM-Ca to generate the met current. The permeabilites of the divalent cations relative to Cs were calculated from the reversal potentials, and the sequence of permeabilities among divalent cations was Ca > Sr > Ba > Mn > Mg. Step-like met currents were observed in Cs saline. The smallest step amplitude with clear resolution had a conductance of 49.7 .+-. 4.5 pS (mean .+-. S.D., n = 7 cells). This is likely to be an elementary met channel conductance. The permeability coefficient of Cs ions calculated from the above conductance was 7.04 x 10-14 cm3/s. The met potential was observed in current-clamp experiments and the amplitude was 24 mV at -43 mV when using CsCl-EGTA intracellular medium. The potential change was smooth and indicated strong low-pass filtering by the membrane. The cell''s input capacitance ranged from 4.5 to 7.4 pF and the membrane time constant, calculated for voltages between -50 and -70 mV, was .apprx. 10 ms. An approximately linear transduction of a mechanical force into an electrical current is discussed in relation to the morphological features of the hair bundle.