DETECTION OF CA2-CELLS( ENTRY THROUGH MECHANOSENSITIVE CHANNELS LOCALIZES THE SITE OF MECHANOELECTRICAL TRANSDUCTION IN HAIR)

A hair cell, the sensory receptor of the internal ear, transduces mech anical stimuli into electrical responses. Transduction results from di splacement of the hair bundle, a cluster of rod-shaped stereocilia ext ending from the cell's apical surface. Biophysical experiments indicat e that, by producing shear between abutting stereocilia, a bundle disp lacement directly opens cation-selective transduction channels. Specif ic models of gating depend on the location of these channels, which ha s been controversial: although some physiological and immunocytochemic al experiments have situated the transduction channels at the hair bun dle's top, monitoring of fluorescence signals from the Ca2+ indicator fura-2 has instead suggested that Ca2+ traverses channels at the bundl e's base. To examine the site of Ca2+ entry through transduction chann els, we used laser-scanning confocal microscopy, with a spatial resolu tion of <1 mu m and a temporal resolution of <2 ms, to observe hair ce lls filled with the indicator fluo-3. An unstimulated hair cell showed a ''tip blush'' of enhanced fluorescence at the hair bundle's top, wh ich we attribute to Ca2+ permeation through transduction channels open at rest. Upon mechanical stimulation, individual stereocilia displaye d increased fluorescence that originated near their tips, then spread toward their bases. Our results confirm that mechanoelectrical transdu ction occurs near stereociliary tips.