Aerial CO2 Excretion in the Obligate Air Breathing Fish Trichogaster Trichopterus: A Role For Carbonic Anhydrase

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Abstract
Total oxygen uptake and carbon dioxide excretion and their partitioning between gills and aerial exchange organs have been measured in an obligate air breathing fish, the blue gourami (Trichogaster trichopterus). Measurements were made during normal bimodal breathing and in air-exposed fish. MO2 increased during air exposure, but the aerial exchange organs or ‘labyrinth’ of Trichogaster could assume a highly effective CO2 excretion in the absence of branchial gas exchange. Hence, aerial MCO2 was greatly increased during air exposure, and the labyrinth gas exchange ratio increased to 0.8 from the value of 0.1 evident during bimodal gas exchange. The large aerial CO2 excretion in Trichogaster, unobtainable in many air breathing fish, was unaffected by the injection of carbonic anhydrase, but was seriously disrupted in the presence of this enzyme's potent inhibitor, acetazolamide. In vitro assay for carbonic anhydrase demonstrated the labyrinth epithelium of the gourami to be rich in this enzyme. It is concluded that highly effective CO2 excretion from the labyrinth organs during air exposure in Trichogaster results from dehydration of plasma bicarbonate to molecular CO2 at the catalysed rate; carbonic anhydrase for this purpose being present in the labyrinth epithelium. The distribution of carbonic anyhdrase in the tissues of other air breathing fishes is determined, and its implications to aerial CO2 excretion discussed.