The selectivity of the hair cell's mechanoelectrical-transduction channel promotes Ca2+ flux at low Ca2+ concentrations.
The mechanoelectrical-transduction channel of the hair cell is permeable to both monovalent and divalent cations. Because Ca2+ entering through the transduction channel serves as a feedback signal in the adaptation process that sets the channel's open probability, an understanding of adaptation requires estimation of the magnitude of Ca2+ influx. To determine the Ca2+ current through the transduction channel, we measured extracellular receptor currents with transepithelial voltage-clamp 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 much more receptor current than did either K+ or Na+. For higher cation concentrations, however, the flux of Na+ or K+ through the transduction channel exceeded that of Ca2+. For mixtures of Ca2+ and monovalent cations, the receptor current displayed an anomalous mole-fraction effect, which indicates that ions interact while traversing the channel's pore. These results demonstrate not only that the hair cell's transduction channel is selective for Ca2+ over monovalent cations but also that Ca2+ carries substantial current even at low Ca2+ concentrations. At physiological cation concentrations, Ca2+ flux through transduction channels can change the local Ca2+ concentration in stereocilia in a range relevant for the control of adaptation.
Lumpkin
E A
EA
Howard Hughes Medical Institute and Laboratory of Sensory Neuroscience, The Rockefeller University, 1230 York Avenue, New York, NY 10021-6399, USA.
Marquis
R E
RE
Hudspeth
A J
AJ
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000378:metabolism*; D005071:Evoked Potentials; D006198:Hair Cells, Auditory / Q000378:metabolism* / Q000502:physiology; D011188:Potassium / Q000378:metabolism; D011892:Rana catesbeiana; D015398:Signal Transduction; D012964:Sodium / Q000378:metabolism
D015220:Calcium Channels; D012964:Sodium; D011188:Potassium; D002118:Calcium