Endocochlear potential and scala media oxygen tension during partial anoxia.

The endocochlear potential and the scala media oxygen tension were measured from the basal coil of the guinea pig cochlea during levels of partial respiratory anoxia. This was accomplished by simultaneously positioning two electrodes (for endocochlear potential and PO2) into the endolymph by a transbasilar membrane approach, in order to minimize blood capillary damage. The intracochlear pO2 was stable until the respiratory oxygen concentration was reduced to 16 per cent on the average. Thereafter, a further reduction of oxygen in the respiratory mix reduced both the endolymph pO2 and the endocochlear potential. The coupling between the endocochlear potential and endolymph pO2 is proportional over a 70 to 80 per cent reduction in endolymph pO2, and at this point the endocochlear potential is reduced by 50 per cent. For partial levels of respiratory oxygen that are not too low (e.g., > 12 per cent oxygen) it is possible to maintain partial levels of intracochlear pO2 for at least six minutes. During this time period the endocochlear potential is also held at a stable but reduced level.