Effects of CO2/HCO3- in perilymph on the endocochlear potential in Guinea pigs

Abstract

The effect of CO2/HCO3- on the endocochlear potential (EP) was examined by using both ion-selective and conventional microelectrodes and the endolymphatic or perilymphatic perfusion technique. The main findings were as follows: (i) A decrease in the EP from ∼+75 to ∼+35 mV was produced by perilymphatic perfusion with CO 2/HCO3--free solution, which decrease was accompanied by an increase in the endolymphatic pH (ΔpHe, ∼0.4). (ii) Perilymphatic perfusion with a solution containing 20 mM NH 4Cl produced a decrease in the EP (ΔEP, ∼20 mV) with an increase in the pHe (ΔpHe, ∼0.2), whereas switching the perfusion solution from the NH4Cl solution to a 5% CO2/25 mM HCO3- solution produced a gradual increase in the EP to the control level with the concomitant recovery of the pH2. (iii) The perfusion with a solution of high or low HCO 3- with a constant CO2 level within 10 min produced no significant changes in the EP. (iv) Perfusion of the perilymph with 10 μg/ml nifedipine suppressed the transient asphyxia-induced decrease in EP slightly, but not significantly. (v) By contrast, the administration of 1 mu;g/ ml nifedipine via the endolymph inhibited significantly the reduction in the EP induced by transient asphyxia or perilymphatic perfusion with CO 2/HCO3--free or 20 mM NH4Cl solution. These findings suggest that the effect of CO2 removal from perilymphatic perfusion solution on the EP may be mediated by an increase in cytosolic Ca2+ concentration induced by an elevation of cytosolic pH in endolymphatic surface cells.