Regulation of Cochlear Fluids pH: (I) The Role of Carbon Dioxide

ABSTRACT

Although bicarbonate and hydrogen ions (pH) are known to play significant roles in the physiology of the cochlea, the details of these interrelationships have only received limited attention. Since bicarbonate ions represent the predominant pH buffer, one factor which profoundly affects pH in the cochlea is the prevailing level of carbon dioxide. In the present study, endocochlear potential (EP), endolymph pH (PH_e) and scala vestibuli perilymph pH (PH_v) were monitored in the second tum of the guinea pig cochlea during manipulations of the systemic CO₂ level. Hypercapnea and hypocapnea were induced by varying the CO₂ content of inspired air while the animal was slightly hyperventilated. Hypercapnea decreased PH_e and PH_v and increased the EP, while hypocapnea produced opposite results. In separate studies, endolymph potassium concentration (K⁺_e) was found to be decreased by hypercapnea and increased by hypocapnea. No systematic changes of endolymph Cl^-or Na⁺ were found. CO₂-induced EP, pH and K_e changes were virtually abolished by intravenous administration of the carbonic anhydrase inhibitor, acetazolamide. These findings support the view that HCO₃^-/CO₂ buffering of pH plays a prominent role in the pH regulation of cochlear fluids and may play a key role in endolymph electrolyte homeostasis.

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