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

Abstract

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, airbreathing 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 airbreathing fish Pangasianodon hypophthalmus in normoxic and hypoxic water at 28-30°C. Contrary to previous studies, we show that this air-breathing fish has a pronounced ability to regulate extracellular pH (pHe) during hypercapnia,with completemetabolic compensation of pHe within 72h of exposure to hypoxic hypercapnia with CO2 levels above 34 mmHg. The high capacity for pHe 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.