Elevation of intracellular calcium levels in outer hair cells by trimethyltin

Outer hair cells are thought to modulate sensitivity of the peripheral auditory system to sound and they are frequently targets of injury by ototoxicants. These cells show an unusual motility that is thought to be critical to their physiological role and it is known that persistent contraction can be stimulated by an enhancement in intracellular calcium concentration ([Ca 2+]i). Trimethyltin (TMT) disrupts functioning in the outer hair cell in vivo and produces a persistent contraction of these cells in vitro. Experiments were designed to determine whether TMT can alter [Ca 2+]i levels in isolated outer hair cells maintained in primary culture and to determine the source of such an increase. Parallel positive and negative controls were included to ensure that cell depolarization using K + did produce the expected enhancement of [Ca 2+]i and that exogenous glutamate stimulation produced no shift in [Ca 2+]i. Outer hair cells do not have glutamate receptors. The [Ca 2+]i of isolated outer hair cells from guinea pigs were monitored dynamically using the calcium-sensitive fluorescent dye, Fura 2, after application of TMT (30 μM −1 mm), KCl (70 mm) or glutamate (100 μM). Cells exposed to TMT show a slow but persistent [Ca 2+]i increase that is observed with TMT concentrations as low as 30 μM and is maximal at 100 μM. The elevation in [Ca 2+]i cannot be attenuated by using low calcium medium or by pretreating the cells with nifedipine, an L-type Ca 2+-channel blocker. This suggests strongly that elevation of [Ca 2+]i in outer hair cells by TMT is not mediated by Ca 2+ channels or by a general impairment in the cells' ability to exclude extracellular Ca 2+ but, rather, results from release of calcium from intracellular stores.