Endolymph calcium increases with time after surgical induction of hydrops in guinea-pigs

The ionized Ca²⁺ concentration in cochlear endolymph is normally extremely low. Previous studies have shown that endolymph Ca²⁺ levels become elevated when measured at long intervals after endolymphatic hydrops is surgically induced. The present study was designed to investigate how rapidly endolymph Ca²⁺ increases following endolymphatic duct ablation. Hydropic animals were tested at either 4 days, 4 weeks, 8 weeks or 16 weeks after surgery. In each animal endolymph Ca²⁺ and endocochlear potentials were measured in all four cochlear turns using double-barreled Ca²⁺-sensitive electrodes. Cochlear sensitivity was assessed using compound action potential thresholds. Our results confirm that hydropic animals show an elevation of endolymph Ca²⁺ and a reduction of EP which is initially small, but becomes more pronounced at longer times after surgery. At 16 weeks endolymph Ca²⁺ was increased by an average factor of 2.0 in the basal turn and 7.5 in the fourth turn. These findings suggest that endolymph Ca²⁺ changes may not be the primary factor responsible for hydrops generation, but probably contribute to cochlear dysfunction in later phases of hydrops. For some experimental groups, the elevation of AP threshold was more closely correlated with endolymph Ca²⁺ level than it was with endolymph volume. Endolymph Ca²⁺ changes must therefore be considered in order to account for dysfunction in the hydropic cochlea.