Voltage dependence of adaptation and active bundle movement in bullfrog saccular hair cells

Hair cells of the bullfrog sacculus adapt to maintained displacement stimuli in a manner that suggests an active regulation of the tension stimulus reaching transduction channels. We have examined adaptation in dissociated hair cells by whole-cell patch-clamp recording and video microscopy. Adaptation was present in these cells, and it depended on extracellular calcium. The adaptation rate--as well as the position of the resting current-displacement curve--also depended on membrane potential, suggesting that calcium passes into the cytoplasm to reach its site of action. After abrupt hyperpolarization, the adaptation rate increased within milliseconds, suggesting that the calcium site is within a few micrometers of the ion channels through which calcium enters. The voltage dependence of the resting current-displacement curve, together with the "gating springs" hypothesis for transduction, predicts movement of the bundle away from the kinocilium when the cell is depolarized. This was observed.