The Change of Hearing Threshold and Endocochlear Potential by Bafilomycin Delivered to Round Window in Guinea Pigs

Ahn, Joong Ho; Kang, Hun Hee; Chung, Jong Woo

Abstract

BACKGROUND AND OBJECTIVES
Vacuolar type H+-ATPase (V-H+-ATPase) has a role in the regulation of endolymphatic pH and certain cells (including strial marginal cells, inner hair cells and epithelial cells of the endolymphatic sac) may be specialized for this regulation. Bafilomycin is a specific V-H+-ATPase inhibitor affecting inner ear function by controlling the intracytolic pH decrease. We designed the study to analyze the effect of bafilomycin delivered to the inner ear on the hearing threshold measured by auditory brainstem response (ABR) and endolymphatic potential (EP). MATERIALS AND METHOD: For measuring the hearing threshold change, 13 guinea pigs with normal Preyer's reflex and normal ABR were used. Guinea pigs were randomly divided into control group (n=3, 6 ears) and study groups which were subdivided into the following ;1 mM bafilomycin group (n=3, 6 ears), 5 mM bafilomycin group (n=3, 6 ears) and 10 mM bafilomycin group (n=4, 8 ears). The mastoid cavity was opened to expose the round window and HBSS buffer (300 osm) was applied for the control group and bafilomycin with different concentrations were also applied to the round windows of studied guinea pigs. The hearing threshold was measured using ABR before and after the application of appropriate solutions. For measuring of EP, the cochlea helix and round window of guinea pig with normal hearing were defined and a tungsten micro needle was inserted into the endolymphatic space at 2nd turn of guinea pig's cochlea. EP was measured after application of HBSS buffer as control, 1 mM, 5 mM, and 10 mM bafilomycin. RESULTS: In the control group, the hearing threshold was 21.6+/-2.8 dB (mean+/-SD) initially both before and after mastoidectomy and stayed that way all throughout the study. The hearing threshold increased as bafilomycin was applied. For 1 mM of bafilomycin application, the threshold changed from initial 30.0+/-5 dB to 33.3+/-5.7 dB after 2 hours. For 5 mM of bafilomycin application, the threshold changed after 2 hours from initial 30.0+/-5 dB to 50.0+/-0 dB. With 10 mM of bafilomycin application, the threshold changed after 2 hours from initial 27.5+/-6.4 dB to 52.5+/-8.6 dB. For 5 mM and 10 mM bafilomycin group, there was a significant statistical change of hearing threshold (p<0.05). However, there was no meaningful difference between 5 mM and 10 mM group (p=0.88). Initial EP was 85+/-10 mV and was significantly decreased in 5 mM (35 mV) and 10 mM (19.8 mV) bafilomycin groups, but such was not observed in 1 mM bafilomycin and control group. CONCLUSION: We could observe the elevation of hearing threshold and decrease EP after applying bafilomycin on the round windows of guinea pigs and also observed that this change reached a critical point when the concentration of bafilomycin was 5 mM or higher. From these results, we can conclude that bafilomycin does affect the hearing threshold through its mechanism and the degree of damage has a critical point dependent on the concentration of bafilomycin.