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

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.

Oxygen Acquisition of the Reedfish, Erpetoichthys Calabaricus

“…Measurements of V ()) in our study were begun after fish had been exposed to air for 2 h. Exposure to air produces an initial increase in the oxygen uptake of the bimodial breathers Clarias batrachus, Anabas testudineus and Heteropneustes fossilis (Singh, 1976). In general, it appears that few species of freshwater, air-breathing fish can sustain aquatic levels of oxygen consumption during air exposure (Singh, 1976;Burggren & Haswell, 1979). Graham (1976) found that a majority of amphibious marine teleosts (e.g.…”

“…The increased frequency and depth of gill breathing observed in reedfish upon return to their aquaria following 6-8 h of terrestrial exposure probably served to assist the excretion of carbon dioxide accumulated during air exposure. Burggren & Haswell (1979) examined the role of carbonic anhydrase in aerial carbon dioxide excretion in the air-breathing anabantid, Trichogaster trichoptenis. A correlation between the presence of this enzyme and the ability to maintain a high respiratory quotient during air exposure was established.…”

“…Since the lungs of the reedfish have little ability to excrete carbon dioxide (Burggren & Haswell, 1979), decreases in gill ventilation would be expected to produce respiratory acidosis. However, the skin of the reedfish is important in respiration (Sacca & Burggren, 1982), and carbon dioxide excretion via this site probably serves to ameliorate acidosis.…”