Dissociation of the cochlear microphonics and endocochlear potential after injection of ethacrynic acid.

The cochlear microphonics (CM), endocochlear potential (EP) and summating potential (SP) were simultaneously recorded for 60 min after injection of 30 mg/kg body weight of ethacrynic acid. The EP decreased parallel with the CM for about 20 min after injection, but they differed in the recovery stage. Even when the EP was still negative, the output of CM for 500 Hz acoustic stimulus recovered to its original level and was supernormal. A large SP observed in the recovery stage of CM and EP seemed to suggest that some functional change in the hair cells might be connected with the supernormal CM. Four minutes of anoxia during the supernormal CM decreased CM to only 68% of its level prior to anoxia. A cathodal polarization of the scala media, to change the polarity of the dc potential gradient, reversed the polarity of CM in the anoxic guinea-pig cochlea but not in the guinea-pig cochlea administered ethacrynic acid. These results suggest that supernormal CM observed in this experiment may not be dependent on the dc potential gradient, but due to some functional change in the hair cells after the injection of ethacrynic acid, although CM is essentially dependent on the dc potential gradient between the scala media and the inside of the hair cells.