Dependence of endolymph pH on perilymph bicarbonate concentration

Y.L. Ma, A.N. Salt and M. Suzuki

Abstract

In previous studies, we found endolymph pH to be highly sensitive to respiratory CO2 status. Hypercapnea decreased endolymph pH while hypocapnea elevated pH. When the perilymphatic scalae of the cochlea are perfused with CO2-equilibrated artificial perilymph, we consistently observe an elevation in endolymph pH. We have found that one factor which contributes significantly to perfusion-induced endolymph pH changes is the concentration of bicarbonate in the perfusion medium. Since bicarbonate provides the dominant pH buffering in the perfusate, solutions of different bicarbonate content, equilibrated to physiological CO2 levels (approximately 5 %), will vary in pH, with pH increasing as bicarbonate increases. When the cochlea was perfused with CO2-equilibrated high-bicarbonate medium (50 mM HCO3), which has a higher pH than normal (pH 7.65), endolymph pH was decreased by an average of 0.05 (n=4). Conversely, perfusion with a nominally HCO3-free solution, caused endolymph pH to rise by an average of 0.11 (n=6). Both high- and low-bicarbonate manipulations generally reduced the endocochlear potential and elevated cochlear action potential thresholds. The observation that the change in endolymph pH is opposite in direction to the change in perilymph pH suggests that mechanisms of bicarbonate or pH homeostasis are involved. Far smaller endolymph pH changes were measured when comparable perilymph pH changes were induced by perfusing HCO3-free, HEPES-buffered solutions of varying pH. This supports the view that the observed endolymph pH changes with HCO3-modified media may be related to perilymph bicarbonate homeostasis. One explanation of the observations is that when perilymph bicarbonate is high, the secretion of acid to neutralize the excess perilymph bicarbonate, would release additional CO2, which, by rapidly equilibrating across membranous boundaries, would produce an acidification of endolymph. Conversely, under low-HCO3 conditions, the generation of bicarbonate from the reaction of CO2 and water, catalyzed by carbonic anhydrase, would reduce CO2 levels, resulting in an alkalinization of endolymph. These studies demonstrate that in the cochlea there are strong interrelationships between CO2, bicarbonate and fluid pH, in which the homeostasis of bicarbonate plays a highly prominent role.

This study was supported by NIH/NIDCD, grant number DC01368

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