Volume regulation in cochlear outer hair cells.

Many cells placed in a hypotonic medium initially swell and then rapidly undergo a regulatory volume decrease (RVD) to return towards original volume. Re-exposure to the isotonic solution results in the cells shrinking followed by a regulatory volume increase (RVI). Previous studies have shown that isolated outer hair cells (OHCs) placed in a hypotonic medium swell and maintain this shape until returned to the original medium. We re-examined this apparent lack of cell volume regulation in OHCs. OHCs were isolated from guinea pig cochleae, mechanically dissociated and dispersed, and placed in a Hank's balanced salt solution (HBS). In the cells studied, switching the perfusate to a hypotonic HBS (290-280 mmol/kg) for 15 min resulted in an immediate shortening of the OHCs (i.e., volume increase). In 26% of the cells, this increase was followed by a return to original length during the time the cell was perfused with the hypotonic medium, a RVD. Twelve percent of the cells demonstrating a RVD also displayed a RVI. Omitting collagenase and increasing Ca2+ concentration did not increase the percentage of cells displaying a RVD, while gadolinium (Gd3+, 10 microM) decreased the percentage to zero. This is the first report of isolated OHCs undergoing cell volume regulation.