Effects of extracellular Ca2+ concentration on hair-bundle stiffness and gating-spring integrity in hair cells

When a hair cell is stimulated by positive deflection of its hair bundle, increased tension in gating springs opens transduction channels, permitting cations to enter stereocilia and depolarize the cell, Ca2+ is thought to be required in mechanoelectrical transduction, for exposure of hair bundles to Ca2+ chelators eliminates responsiveness by disrupting tip links, filamentous interstereociliary connections that probably are the gating springs, Ca2+ also participates in adaptation to stimuli by controlling the activity of a molecular motor that sets gating-spring tension, Using a flexible glass fiber to measure hair-bundle stiffness, we investigated the effect of Ca2+ concentration on stiffness before and after the disruption of gating springs, The stiffness of intact hair bundles depended nonmonotonically on the extracellular Ca2+ concentration; the maximal stiffness of approximate to 1200 mu LN . m(-1) occurred when bundles were bathed in solutions containing 250 mu M Ca2+, approximately the concentration found in frog endolymph, For cells exposed to solutions with sufficient chelator capacity to reduce the Ca2+ concentration below approximate to 100 nM, hair-bundle stiffness fell to approximate to 200 mu N . m(-1) and no longer exhibited Ca2+-dependent changes. Because cells so treated lost mechanoelectrical transduction, we attribute the reduction in bundle stiffness to tip-link disruption, The results indicate that gating springs are not linearly elastic; instead, they stiffen with increased strain, which rises with adaptation-motor activity at the physiological extracellular Ca2+ concentration.