DISSOCIATION BETWEEN THE CALCIUM-INDUCED AND VOLTAGE-DRIVEN MOTILITY IN COCHLEAR OUTER HAIR-CELLS FROM THE WALTZING GUINEA-PIG

The waltzing guinea pig, possessing an hereditary progressive deafness, shows pathology to the actin-bearing structures within the hair cells of the organ of Corti. In particular, the affected structures include the stereocilia, the cuticular plate and, as shown in the present study, swollen and disorganized subsurface cisternae. To test whether this pathology affected outer hair cell motility, cells were isolated from waltzing guinea pigs and their age-matched controls and were subjected to either electrical or chemical stimulation. Visual detection thresholds and the magnitude of the electrically-induced length changes were equivalent for both groups. However, when intracellular calcium was increased with either the calcium ionophore, ionomycin or Ca2+/ATP (under permeabilized conditions with DMSO), length changes were significantly reduced for the outer hair cells from waltzing guinea pigs compared to the controls. The average percent length increase induced by 10 muM ionomycin for the outer hair cells from control animals was 2.3 +/- 1.7 whereas for postnatal day 4 waltzing guinea pigs it was 1.3 +/- 1.7. Postnatal day 7 and 10 waltzing guinea pigs responded with significantly smaller percent length changes. The intracellular concentration of ionic calcium increased similarly for both groups after the application of ionomycin as revealed with the indicator fluo-3. In the permeabilized cells in the presence of Ca2+/ATP, control cells responded with a percent length change of 3.5, whereas, age-matched waltzing outer hair cells responded with barely detectable length changes. When the osmolarity of the external medium was reduced, both the control and waltzing outer hair cells responded with a length change that was proportional to the change in osmolarity, indicating the capability of passive length changes. In summary, the voltage-dependent motility of isolated outer hair cells from the waltzing guinea pigs is intact, whilst the slow calcium-dependent motility is abnormal. As a result of these findings, the outer hair cells from the waltzing guinea pig allow separations between the slow, metabolically dependent length changes induced by chemical stimulation and the fast, metabolically independent length changes induced by electrical stimulation.