Calcium permeation of the turtle hair cell mechanotransducer channel and its relation to the composition of endolymph

Authors


Corresponding author R. Fettiplace: Department of Neurophysiology, 273 Medical Sciences Building, 1300 University Avenue, Madison, WI 53706, USA Email: fettiplace@neurophys.wisc.edu

Abstract

  • 1Recordings of mechanoelectrical transducer currents were combined with calcium imaging of hair bundles in turtle auditory hair cells located near the high-frequency end of the cochlea. The external face of the hair bundles was perfused with a range of Ca2+ concentrations to study the quantitative relationship between Ca2+ influx and transducer adaptation.
  • 2With Na+ as the monovalent ion, the peak amplitude of the transducer current decreased monotonically as the external [Ca2+] was raised from 25 μM to 20 mM. When Na+ was replaced with the impermeant Tris the transducer current increased with external [Ca2+]. These results indicate that Ca2+ can both permeate and block the transducer channels. The Ca2+ concentration for half-block of the monovalent current was 1 mM.
  • 3To quantify the Ca2+ influx, the fraction of transducer current carried by Ca2+ was measured using the change in bundle fluorescence in cells loaded with 1 mM Calcium Green-1. The fluorescence change was calibrated by substituting an impermeable monovalent ion to render Ca2+ the sole charge carrier.
  • 4In the presence of Na+, the fractional Ca2+ current was ≈10 % in 50 μM Ca2+, a concentration similar to that in endolymph, which bathes the hair bundles in vivo. The amount of Ca2+ entering was dependent on the identity of the monovalent ion, and was larger with K+, suggesting that the transducer channel is a multi-ion pore.
  • 5Over a range of ionic conditions, the rate of transducer adaptation was proportional to Ca2+ influx indicating that adaptation is driven by a rise in intracellular [Ca2+].
  • 6Shifts in the current-displacement function along the displacement axis in different external Ca2+ concentrations were predictable from variation in the resting Ca2+ influx. We suggest that changes in the resting open probability of the transducer channels adjust the entry of Ca2+ to keep its concentration constant at an internal site.
  • 7The results demonstrate that endolymph containing high K+, 50 μM Ca2+ and low Mg2+ concentrations, maximizes the transducer current while still allowing sufficient Ca2+ entry to drive adaptation. The hair cell mechanotransducer channel, in its permeation and block by Ca2+, shows behaviour similar to the voltage-gated Ca2+ channel and the cyclic nucleotide-gated channel.

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