Lack of Effect of Carbonic Anhydrase Inhibition on Direct Measurements of Endolymph Bicarbonate

In the past, sodium, potassium, and chloride have been measured in endolymph directly, but bicarbonate has been measured only indirectly. We sampled endolymph directly while monitoring endocochlear potentials in normal and methazolamide-treated guinea pigs. Bicarbonate was determined in samples by use of a method that depends on reduction of NADH to NAD linked to malate formation from oxaloacetate. In 11 normal animals, the bicarbonate in endolymph was 20.2 m M ± 4.4 m M (mean ± standard deviation); in six of these, plasma bicarbonate was 23.1 m M ± 3.5 m M. Nine animals treated with methazolamide (carbonic anhydrase inhibitor) had an endolymph bicarbonate of 19.5 m M ± 3.9 m M; plasma bicarbonate in five of these was 25 m M ± 3.2 m M. Carbonic anhydrase inhibition did not significantly affect endolymphatic bicarbonate levels.

Variation of Endocochlear Po2 and Cochlear Potentials by Breathing Carbon Dioxide

The effect of carbon dioxide on oxygen tension in the endolymph was determined by the micropolarographic technique. Different concentrations (5% and 10% CO2) and different exposure times (3, 5, and 20 minutes) were investigated. The highest levels of Po2 in the endolymph (101.7, 93.9 and 69.5 mm Hg) were accomplished by respiration of 10% CO2, 90% O2, for 20, 5, and 3 minutes consecutively. The lowest Po2 increase, 50.7 mm Hg was observed after breathing 5% CO2, 90% O2 for 20 minutes. Extreme hypercapnia caused an increase of endocochlear potentials (EP) in all groups. In the second group EP increased from +79.3 to +84.9 and in all groups they had returned to the pretreatment level after CO2 discontinuation. These results support the theory that carbonic anhydrase participates in the generation of EP. At the same time that EP increased, cochlear microphonics declined and opposite after the breathing mixture was discontinued. The results permit the conclusion that high levels of Po2 in endolymph is achievable even with short periods of respiration with high CO2 mixture, and suggest the role of carbonic anhydrase during EP generation.

Carbonic anhydrase in the generation of cochlear potentials

The experiments on 18 guinea pigs were divided into two groups and each group was arranged in such a way that the effect of hypercapnia (generated by breathing 10% CO2-90% O2) was investigated with and without inhibition of carbonic anhydrase by methazolamide, 25 mg/kg, in the first group and acetazolamide, 50 mg/kg, in the second group, administered intravenously. The endocochlear potentials (EP) and endocochlear PO2 were recorded by microelectrodes introduced into the scala media, and cochlear microphonics (CM) were monitored by a silver-wire electrode from the round window. In the first exposure to hypercapnia (20-40 min) EP increased about + 6 mV. At the same time CM decreased; the reason for this is not yet known. During the second period of hypercapnia (80-100 min) when carbonic anhydrase was inhibited with methazolamide and acetazolamide, EP did not elevate as during the first period when carbonic anhydrase was not inhibited. In this work, under specific conditions, it was observed for the first time that carbonic anhydrase affects the generation of EP.