Toluene disrupts outer hair cell morphometry and intracellular calcium homeostasis in cochlear cells of guinea pigs.

The aromatic hydrocarbon, toluene, has been demonstrated to disrupt auditory system function both in occupational epidemiological and in laboratory animal investigations. This agent, along with several other organic solvents, impairs hearing preferentially at middle frequencies-a finding that distinguishes these agents from the traditional high-frequency impairment observed with ototoxic drugs such as aminoglycoside antibiotics and cisplatin. Prior investigations have identified the outer hair cell as a probable target for toluene exposure, although studies designed to evaluate spiral ganglion cell impairment have not been reported. The purpose of this investigation was to determine directly whether outer hair cells isolated from the guinea pig cochlea show morphological alterations consistent with a toxic response to toluene exposure. Since slow adjustments of outer hair cell length can result from alteration in free intracellular calcium concentration, the effect of toluene on calcium homeostasis was monitored in both outer hair cells and spiral ganglion cells. A dose-response relationship was observed in the extent of outer hair cell shortening produced by toluene with a significant shortening observed at concentrations of 100 microM and higher. By contrast, the nonototoxic solvent, benzene, produced little shortening at 100 microM to 1 mM concentrations. Studies of calcium homeostasis conducted using the fluorescent probe, Fura-2, showed that toluene enhanced free intracellular calcium levels of both outer hair cells and spiral ganglion cells within 5 min of exposure at concentrations of 30 microM and higher. Intracellular calcium levels were elevated only slightly following benzene administration at 1 mM, but not at lower concentrations. Cells cultured in artificial perilymph nominally containing no calcium and those to which EGTA was added still showed a maximal increase in intracellular calcium level when treated with toluene. These data indicate that the elevation in free intracellular calcium levels produced by toluene results from release of calcium from intracellular stores.