Differential motile response of isolated inner and outer hair cells to stimulation by potassium and calcium ions.

Inner and outer hair cells were mechanically isolated from the guinea pig cochlea and subjected to stimuli known to induce shape changes in outer hair cells. Depolarization by 70 mM KCl which causes osmotic swelling of outer hair cells also swelled inner hair cells by approximately 8% of their volume. The application of the calcium ionophore ionomycin which induces cortical contractions and elongation of outer hair cells, did not affect the shape of inner hair cells. Since ionomycin increased free intracellular calcium levels in both inner and outer hair cells, the results demonstrate that inner hair cells do not possess the mechanisms necessary for a contractile response to calcium. Thus, calcium is a specific regulator of outer hair cell motility making this mechanism a likely physiological modulator of a transduction feedback process.