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 cell receptor currents evoked by mechanical stimulation of the otolithic membrane were recorded under transepithelial voltage clamp conditions. The ionic selectivity of the mechano-electrical transduction channels of the hair cells was determined by examining the effects of different concentrations of Ca2+ and K+ in the apical solution on the transepithelial 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 estimated from a modified Goldman, Hodgkin and Katz equation, assuming that 80%, of the transepithelial resistance is located in the apical membranes 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 K+ at constant ionic strength and at different constant Ca2+ concentrations were well predicted by the same equation. These results indicate that the transduction channels of the frog saccular hair cells are highly selective to Ca2+.