The preparation of acetic acid for use in otic drops and its effect on endocochlear potential and pH in inner ear fluid.

The ototoxicity of an otic drop preparation containing 2% acetic acid and 3% propylene glycol (VoSol, Denver Chemical Co., Humacao, PR) was investigated according to measurements of endocochlear potential (EP) and inner ear fluid pH. The application of this preparation to the round window membrane for 30 minutes caused a depression in EP from 80.5 +/- 2.5 mV (mean +/- SD; n = 6) to 11.7 +/- 7.7 mV, and lowered inner ear fluid pH from 7.55 +/- 0.09 to 5.06 +/- 0.19 (n = 6) in perilymph and from 7.52 +/- 0.07 to 5.88 +/- 0.63 (n = 6) in endolymph. Two percent acetic acid produced similar changes after 30 minutes: EP was reduced from 83.0 +/- 2.2 mV to 34.0 +/- 2.9 mV and endolymphatic pH from 7.49 +/- 0.04 to 6.83 +/- 0.21 (n = 4). However, the application of artificial perilymph of pH 4 titrated with HCl induced no significant changes in either EP or endolymphatic pH. We suggest that the mechanisms of ototoxicity in the otic drop preparation are Na+ and K+-ATPase inhibition, and that such inhibition is due to the intracellular acidification of strial cells resulting from the penetration of acetic acid across the cell membrane, and to the direct and synergistic actions of propylene glycol.