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 approximately 1200 microN.m-1 occurred when bundles were bathed in solutions containing 250 microM Ca2+, approximately the concentration found in frog endolymph. For cells exposed to solutions with sufficient chelator capacity to reduce the Ca2+ concentration below approximately 100 nM, hair-bundle stiffness fell to approximately 200 microN.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.
Marquis
R E
RE
Howard Hughes Medical Institute and Laboratory of Sensory Neuroscience, The Rockefeller University, 1230 York Avenue, New York, NY 10021-6399, USA.
Hudspeth
A J
AJ
D008267:Acoustic Maculae / Q000187:drug effects*; D000222:Adaptation, Physiological; D000818:Animals; D001696:Biomechanical Phenomena; D002118:Calcium / Q000494:pharmacology*; D002923:Cilia / Q000187:drug effects; D004305:Dose-Response Relationship, Drug; D018069:Hair Cells, Vestibular / Q000187:drug effects*; D008564:Membrane Potentials / Q000187:drug effects; D018583:Pliability / Q000187:drug effects*; D011892:Rana catesbeiana; D015398:Signal Transduction
D002118:Calcium