ACOUSTIC OVERSTIMULATION INCREASES OUTER HAIR CELL CA2+ CONCENTRATIONS AND CAUSES DYNAMIC CONTRACTIONS OF THE HEARING ORGAN

The dynamic responses of the hearing organ to acoustic overstimulation mere investigated using the guinea pig isolated temporal bone prepara tion. The organ was loaded with the fluorescent Ca2+ indicator Fluo-3, and the cochlear electric responses to low-level tones were recorded through a microelectrode in the scala media. After overstimulation, th e amplitude of the cochlear potentials decreased significantly. In som e cases, rapid recovery was seen with the potentials returning to thei r initial amplitude. In 12 of 14 cases in which overstimulation gave a decrease in the cochlear responses, significant elevations of the cyt oplasmic [Ca2+] in the outer hair cells were seen. [Ca2+] increases ap peared immediately after terminating the overstimulation, with partial recovery taking place in the ensuing 30 min in some preparations. Suc h [Ca2+] changes were not seen in preparations that were stimulated at levels that did not cause an amplitude change in the cochlear potenti als. The overstimulation also gave rise to a contraction, evident as a decrease of the width of the organ of Corti. The average contraction in 10 preparations was 9 mu m (SE 2 mu m). Partial or complete recover y was seen within 30-45 min after the overstimulation. The [Ca2+] chan ges and the contraction are likely to produce major functional alterat ions and consequently are suggested to be a factor contributing strong ly to the loss of function seen after exposure to loud sounds.