High capacity for extracellular acid-base regulation in the air-breathing fish Pangasianodon hypophthalmus

The evolution of accessory air-breathing structures is typically associated with reduction of the gills, although branchial ion transport remains pivotal for acid-base and ion regulation. Therefore, air-breathing fishes are believed to have a low capacity for extracellular pH regulation during a respiratory acidosis. In the present study, we investigated acid-base regulation during hypercapnia in the air-breathing fish Pangasianodon hypophthalmus in normoxic and hypoxic water at 28-30 degrees C. Contrary to previous studies, we show that this air-breathing fish has a pronounced ability to regulate extracellular pH (pH(e)) during hypercapnia, with completemetabolic compensation of pH(e) within 72 h of exposure to hypoxic hypercapnia with CO2 levels above 34 mmHg. The high capacity for pH(e) regulation relies on a pronounced ability to increase levels of HCO3- in the plasma. Our study illustrates the diversity in the physiology of air-breathing fishes, such that generalizations across phylogenies may be difficult.