IONIC SELECTIVITY OF THE MECHANOELECTRICAL TRANSDUCTION CHANNELS IN THE HAIR-CELLS OF THE FROG SACCULUS

In the isolated saccular macula of Rana esculenta extracellular hair c ell receptor currents evoked by mechanical stimulation of the otolithi c membrane were recorded under transepithelial voltage clamp condition s. The ionic selectivity of the mechano-electrical transduction channe ls of the hair cells was determined by examining the effects of differ ent concentrations of Ca2+ and K+ in the apical solution on the transe pithelial voltage at which the extracellular receptor current was zero (V-rev). Changing the concentration of Ca2+ from 0.26 mM to 0.026 and to 2.6 mM at a constant K+ concentration caused changes in V-rev of - 15+/-7 mV (mean+/-SD; n = 9) and 20+/-6 mV (n = 13), respectively. The relative ionic permeabilities of the transduction channels were estim ated from a modified Goldman, Hodgkin and Katz equation, assuming that 80%, of the transepithelial resistance is located in the apical membr anes of the hair cells. The permeability of the transduction channels for Ca2+ was found to be two orders of magnitude larger than that for K+, The measured effects on V-rev of changing the concentration of Kat constant ionic strength and at different constant Ca2+ concentratio ns were well predicted by the same equation. These results indicate th at the transduction channels of the frog saccular hair cells are highl y selective to Ca2+.