Effects of topical sodium potassium ATPase inhibitors upon endocochlear potential in chinchilla.

Sodium orthovanadate (vanadate) is a recently discovered inhibitor of the enzyme sodium potassium adenosine triphosphatase, which has properties similar to ouabain in peripheral tissues. The effects of topical application of vanadate and ouabain to the round window membrane of the chinchilla were studied using endocochlear potential (EP) and action potential as measures of cochlear function. The EP demonstrated an initial elevation followed by a secondary decline after topical application of 2 mmol vanadate to the round window membrane. During the EP elevation, the resistance was unchanged, but the action potential response declined, with a greater effect on responses to 8-kHz tone bursts than to 2-kHz tone bursts. Thus, it does not appear that the initial increase of endocochlear potential that was observed was due to a change in the resistance of the cochlear duct; rather, some other mechanism must be involved. The round window topical application of potentially ototoxic drugs accompanied by cochlear function monitoring appears to be a useful method for testing drugs with marked systemic effects such as ouabain or vanadate and is simpler to perform than perilymphatic or vascular perfusion.