Renal acid excretion contributes to acid-base regulation during hypercapnia in air-exposed swamp eel (Monopterus albus)

Abstract

The swamp eel (Monopterus albus) uses its buccal cavity to air-breathe while the gills are strongly reduced. They burrow into mud during the dry season, are highly tolerant of air-exposure, and experience severe hypoxia in both their natural habitat and in aquaculture. To study their ability to compensate for respiratory acidosis, we implanted catheters to sample both arterial blood and urine during hypercapnia (4% CO2) in either water and air, or during whole animal air-exposure. These hypercapnic challenges caused an immediate reduction in arterial pH, followed by progressive compensation through a marked elevation of plasma HCO3− over the course of 72h. There was no appreciable rise in urinary acid excretion in fish exposed to hypercapnia in water, although urine pH was reduced and ammonia excretion did increase. In the air-exposed fish, however, hypercapnia was attended by a large elevation of ammonia in the urine and a large rise in titratable acid excretion. The time course of the increased renal acid excretion overlapped with the time period required to elevate plasma HCO3−, and we estimate that the renal compensation contributed significantly to whole body acid-base compensation.