Susceptibility of the endocochlear potential to pH and osmolarity changes in the perilymph of the cochlea in the guinea pig.

The effects of artificial perilymph at various pH levels and osmolarities on the endocochlear potential (EP) were investigated in a guinea pig model. In 47 ears, the mean EP was 74.5 +/- 0.5 mV in the second turn of the cochlea before perilymphatic perfusion. The artificial perilymph with a pH ranging from 5 to 9 and tonicity less than 1.25 (osmolarity, 345 mosmol/l) did not alter the EP when testings were recorded for at least 1 h. However, after the start of a perfusion with a pH less than 4, the EP declined gradually but recovered to the pre-perfusion level rapidly when the perfusion was stopped. The artificial perilymph with a pH over 10 decreased the EP considerably but showed no recovery, even after cessation of the perfusion. Tonicity more than 1.35 (osmolarity, 372.6 mosmol/l) also decreased the EP, with this decrease paralleling further increases in the tonicity used. From these data, we have concluded that a perilymph perfusate with a pH from 5 to 9 and an osmolarity less than 350 mosmol/l can be used in animal testing without any noxious effects on the EP.