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

Abstract

Recordings 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 Ca 2+ concentrations to study the quantitative relationship between Ca 2+ influx and transducer adaptation. With Na+ as the monovalent ion, the peak amplitude of the transducer current decreased monotonically as the external [Ca 2+] was raised from 25 μM to 20 mM. When Na+ was replaced with the impermeant Tris the transducer current increased with external [Ca 2+]. These results indicate that Ca 2+ can both permeate and block the transducer channels. The Ca 2+ concentration for half‐block of the monovalent current was 1 mM. To quantify the Ca 2+ influx, the fraction of transducer current carried by Ca 2+ 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 Ca 2+ the sole charge carrier. In the presence of Na+, the fractional Ca 2+ current was ≈10 % in 50 μM Ca 2+, a concentration similar to that in endolymph, which bathes the hair bundles in vivo. The amount of Ca 2+ 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. Over a range of ionic conditions, the rate of transducer adaptation was proportional to Ca 2+ influx indicating that adaptation is driven by a rise in intracellular [Ca 2+]. Shifts in the current‐displacement function along the displacement axis in different external Ca 2+ concentrations were predictable from variation in the resting Ca 2+ influx. We suggest that changes in the resting open probability of the transducer channels adjust the entry of Ca 2+ to keep its concentration constant at an internal site. The results demonstrate that endolymph containing high K+, 50 μM Ca 2+ and low Mg 2+ concentrations, maximizes the transducer current while still allowing sufficient Ca 2+ entry to drive adaptation. The hair cell mechanotransducer channel, in its permeation and block by Ca 2+, shows behaviour similar to the voltage‐gated Ca 2+ channel and the cyclic nucleotide‐gated channel.