Regulation of Cochlear Fluids pH: (II) The Role of Bicarbonate

Bicarbonate provides the dominant pH buffer of the cochlear fluids. In the present study, perilymph bicarbonate concentration was manipulated during which endocochlear potential, cochlear fluids pH and cochlear sensitivity to sounds were monitored. Perilymphatic perfusion of medium containing elevated HCO3, at higher pH than normal, produced a decrease in endolymph pH. Similarly, perfusion of nominally HCO3-free solution, with lower pH than normal, produced an increase in endolymph pH. When pH changes were induced with HEPES-buffered media, endolymphatic and perilymphatic pH changed in the same direction. The observed endolymph pH changes induced by perilymphatic HCO3 manipulations therefore depend on HCO3, rather than pH changes. These findings can be explained by the generation of localized changes of CO 2 in the cochlear fluids associated with bicarbonate homeostasis. These data demonstrate some key properties of the CO2/ HCO3 buffer system and suggest the likely role for carbonic anhydrase in tissues of the lateral wall. In practical terms, the data also suggest that the pH stability of inner ear fluids cannot be adequately maintained when bicarbonate-altered media are perfused, especially when pH buffers other than HCO3 are used.