Proceedings of The Physiological Society
University of Bristol (2005) J Physiol 567P, C126
Fast adaptation and hair bundle mechanics in rat outer hair cells
Kennedy, Helen; Crawford, Andrew; Fettiplace, Robert;
1. Physiology, University of Bristol, Bristol, United Kingdom. 2. Physiology, University of Cambridge, Cambridge, United Kingdom. 3. Physiology, University of Wisconsin, Madison, WI, USA.
The properties of the mechanotransducer (MET) channels and how they influence the mechanical properties of the hair bundle have been amply documented in hair cells of lower vertebrates. In the turtle, transduction exhibits a fast adaptation with a time constant varying inversely with the hair cell characteristic frequency (CF) (Ricci et al., 2003). Fast adaptation reflects calcium-dependent reclosure of the MET channels, which can in turn elicit a mechanical reaction that moves the hair bundle. To ascertain whether measurements in the turtle provide a good paradigm for mammals, we have studied the properties of hair cell MET currents in mammalian outer hair cells. Currents were recorded at room temperature 19-22°C from hair cells in acutely isolated cochleae of humanely killed rats between postnatal days 6 and 12. The tectorial membrane was removed and the upper surface of the hair cell epithelium was separately perfused through a 100 μm pipette with a saline containing (2.8 mM) or endolymph-like (0.05 mM) CaCl2. Hair bundles were deflected with a glass pipette driven by a high-speed piezoelectric actuator, and currents were measured under whole-cell voltage clamp (Kennedy et al., 2003). In response to a step displacement of the hair bundle, MET currents activated with a time course indistinguishable from the mechanical stimulus, but then adapted with a time constant of less than 0.1 ms. As in turtle, both the size and adaptation rate of the MET current depended on external calcium concentration. Comparison of two cochlear locations with CFs of 4 and 14 kHz showed that cells with higher CF had on average larger MET currents and faster adaptation to those with low CF. At high CF and 2.8 mM external calcium adaptation time constants as fast as 50μs were recorded. At more physiological calcium levels of 0.5mM the time constant was reversibly slowed to 90 μs, but the speed with which the MET current developed was too fast to measure, even at room temperature. Fast activation and adaptation kinetics are consistent with the higher frequencies encoded by the rat cochlea. We also examined the mechanical properties of outer hair cell bundles using force?stimulation with flexible fibers whose motion was determined by imaging on a dual photodiode. The force-displacement relationship of the hair bundle was non-linear over the range where the MET channels were gated and in some cells showed evidence of a negative slope region similar to that observed in frog saccular hair cells. Such non-linearity was sensitive to external calcium and developed with a time course similar to fast adaptation (Kennedy et al., 2005). We conclude that fast adaptation in outer hair cells functions on a time scale appropriate for cycle-by-cycle regulation at the hair cell CF, and we suggest it is linked to force generation the hair bundle that may contribute to the cochlear amplifier.
Where applicable, experiments conform with Society ethical requirements