Ultrastructural evidence for protection of the outer hair cells of the inner ear during intense noise exposure by application of the organic calcium channel blocker diltiazem

Outer hair cells could be protected during intense noise exposure by the ap plication of the calcium channel blocker diltiazem given before and after n oise treatment. After various experimental approaches, the ultrastructural morphology was analysed for the different animal populations in the basal p art of the second turn of the cochlea, which was the most destroyed area af ter an acute noise trauma caused by a gun shot (sound pressure at the ear d rum 156 dB, frequency maximum between 4 and 6 kHz). Compared to untreated c ontrol specimens (experimental animal group I), the outer hair cells in the basal part of the second turn of the cochlea were mostly destroyed without any diltiazem application but after intense noise exposure (group II) or w hen the calcium channel blocker was given only before (group III) or only a fter an intense noise exposure (group IV). Only cellular debris of these re ceptor cells could be identified within this region of the cochlea at the e lectron-microscopic level. After diltiazem application before and after the intensive noise exposure (group V), the general morphology of the tissue w as much better conserved and almost no destroyed outer hair cells were foun d. But nevertheless, the electron-microscopic analysis revealed that the ce llular fine structure was altered. Compared to untreated control specimens, small invaginations were sometimes seen in the basolateral membranes of ou ter hair cells, and also the cuticular structures of the Deiters cells form ing the lamina reticularis bulged into the outer hair cells. To get more in formation about possible physiological alterations, the amounts of precipit able calcium within the outer hair cells were determined by an image-proces sing system for the five different populations (groups I-V). The values obt ained were compared to the calcium content in specimens which had received diltiazem without any additional noise exposure (group VI) and with an anim al population which was exposed to a pure tone for 5 min at 90 dB (group VI I). The results are discussed in respect of outer hair cell protection prev enting morphological damage and/or physiological alterations.