Two Components of Transducer Adaptation in Auditory Hair Cells

Mechanoelectrical transducer currents in turtle auditory hair cells adapted to maintained stimuli via a Ca2+-dependent mechanism characterized by two time constants of ∼1 and 15 ms. The time course of adaptation slowed as the stimulus intensity was raised because of an increased prominence of the second component. The fast component of adaptation had a similar time constant for both positive and negative displacements and was unaffected by the myosin ATPase inhibitors, vanadate and butanedione monoxime. Adaptation was modeled by a scheme in which Ca2+ ions, entering through open transducer channels, bind at two intracellular sites to trigger independent processes leading to channel closure. It was assumed that the second site activates a modulator with 10-fold slower kinetics than the first site. The model was implemented by computing Ca2+diffusion within a single stereocilium, incorporating intracellular calcium buffers and extrusion via a plasma membrane CaATPase. The theoretical results reproduced seve...