Motility of outer hair cells as an active, actin-mediated process

The cochlea of the inner ear is no longer regarded exclusively as a passive mechanical signal analyser. Active mechanical processes are thought to influence the vibration pattern of the basilar membrane. The source of these active mechanical processes is sought in the outer hair cells (OHC). These auditory sensory cells contain actin and actin-associated structural elements. Recently, isolated OHC have been demonstrated to be capable of reversible longitudinal contractions in vitro, whose molecular mechanism could not be documented, however. Furthermore, following an increase in the intracellular Ca2+ level, isolated OHC showed longitudinal motile responses in the presence of ATP. We now report that the observed Ca2+/ATP induced mechanical events in OHC are active processes, that are driven by the hydrolytic energy of ATP. Moreover, motile responses are frequently associated with a movement of the cuticular plate, including the stereociliary bundle. Cytochalasin B, inorganic bisphosphate, trifluoperazine and antiactin inhibit the motile response. Thus, our results show that outer hair cells possess actin-dependent Ca2+ and calmodulin-controlled motor properties, so that potentially they can actively adjust the mechanical properties of the basilar membrane during auditory stimulation.