The effect of endocochlear potential suppression upon susceptibility to acoustic trauma

In the present study we investigated the influence of decreased endocochlear potential (EP) on acoustic trauma. Guinea pigs with decreased EP following i.v. administration of furosemide (FUR) and normal EP given physiological saline solution (PSS) i.v. instead of FUR were exposed to 2 kHz pure tone ranging from 110 to 130 dB SPL for 5 min. The input-output curves of the compound action potential (CAP) before acoustic overstimulation were compared with those at 2 h after the exposure. In 125 and 130 dB SPL exposure there was no difference in threshold shift between the FUR and PSS groups. In 115 and 120 dB SPL exposure, however, the threshold shift of the FUR group was significantly smaller than that of the PSS group. There was no significant difference between the two groups in 110 dB SPL exposure. These results indicate that the threshold shift of the CAP is mainly due to excessive vibration of the basilar membrane in the greater stimulus groups (125-130 dB SPL) but that the energy exhaustion of the hair cells to a certain extent participates in the threshold shift when the stimulus is milder (115-120 dB SPL). It is concluded that susceptibility to acoustic trauma of a milder intensity (115-120 dB SPL) is reduced when the energy consumption rate of the organ of Corti is decreased by EP suppression.