Two mechanisms for transducer adaptation in vertebrate hair cells

Deflection of the hair bundle atop a sensory hair cell modulates the open p robability of mechanosensitive ion channels. In response to sustained defle ctions, hair cells adapt. Two fundamentally distinct models have been propo sed to explain transducer adaptation. Both models support the notion that c hannel open probability is modulated by calcium that enters via the transdu ction channels, Both also suggest that the primary effect of adaptation is to shift the deflection-response [I(X)] relationship in the direction of th e applied stimulus, thus maintaining hair bundle sensitivity. The models di ffer in several respects. They operate on different time scales: the faster on the order of a few milliseconds or less and the slower on the order of 10 ms or more. The model proposed to explain fast adaptation suggests that calcium enters and binds at or near the transduction channels to stabilize a closed conformation. The:model proposed to explain the slower adaptation suggests that adaptation is mediated by an active, force-generating process that regulates the effective stimulus applied to the transduction channels , Here we discuss the evidence in support of each model and consider the po ssibility that both may function to varying degrees in hair cells of differ ent species and sensory organs.