THE SELECTIVITY OF THE HAIR-CELLS MECHANOELECTRICAL-TRANSDUCTION CHANNEL PROMOTES CA2+ FLUX AT LOW CA2+ CONCENTRATIONS

The mechanoelectrical-transduction channel of the hair cell is permeab le to both monovalent and divalent cations, Because Ca2+ entering thro ugh the transduction channel serves as a feedback signal in the adapta tion process that sets the channel's open probability, an understandin g of adaptation requires estimation of the magnitude of Ca2+ influx, T o determine the Ca2+ current through the transduction channel, we meas ured extracellular receptor currents with transepithelial voltage-clam p recordings while the apical surface of a saccular macula was bathed with solutions containing various concentrations of K+, Na+, or Ca2+, For modest concentrations of a single permeant cation, Ca2+ carried mu ch more receptor current than did either K+ or Na+, For higher cation concentrations, however, the flux of Naf or K+ through the transductio n channel exceeded that of Ca2+, For mixtures of Ca2+ and monovalent c ations, the receptor current displayed an anomalous mole-fraction effe ct, which indicates that ions interact while traversing the channel's pore. These results demonstrate not only that the hair cell's transduc tion channel is selective for Ca2+ over monovalent cations but also th at Ca2+ carries substantial current even at low Ca2+ concentrations. A t physiological cation concentrations, Ca2+ flux through transduction channels can change the local Ca2+ concentration in stereocilia in a r ange relevant for the control of adaptation.