The voltage-sensitive motor protein and the Ca2+-sensitive cytoskeleton in developing rat cochlear outer hair cells

Cochlear outer hair cells (OHCs) possess a unique fast voltage‐driven motility associated with a voltage‐sensitive motor protein embedded in the basolateral membrane. This mechanism is believed to underlie the cochlear amplification in mammals. OHCs also have a Ca2+/calmodulin‐dependent mechanical pathway which involves a submembranous circumferential cytoskeleton. The purpose of this study was to compare the functional appearance of the voltage‐sensitive motor proteins with that involving the Ca2+‐sensitive cytoskeleton during postnatal development of rat OHCs. We demonstrate that whole‐cell electromotility and Ca2+‐voked mechanical responses, by ionomycin, develop concomitantly after postnatal day 5 (P5). These two mechanical properties also develop simultaneously in OHCs isolated from two‐week‐old cultures of P0‐P1 organs of Corti. This excludes the participation of neural innervation in the postnatal maturation of the OHCs' motile properties. In addition, we show that the expression of the membranous voltage‐sensitive motor protein precedes, by several days, the appearance of whole‐cell electromotility. The concomitant development of whole‐cell electromotility and Ca2+‐sensitive motility, both in vivo and in vitro, underlines the cytoskeleton as an important factor in the functional organization of the voltage‐sensitive motor proteins within the plasma membrane.