Renal function and acid–base regulation in two Amazonian erythrinid fishes: Hoplias malabaricus, a water breather, and Hoplerythrinus unitaeniatus, a facultative air breather
The function of the kidney in ion, water, and acid excretion was investigated in two erythrinid fishes, the water-breathing Hoplias malabaricus and the facultative air-breathing Hoplerythrinus unitaeniatus. Chronic catheterization of the urinary papilla and the dorsal aorta provided information on the urinary parameters and blood acid–base status. By monitoring total flow of urine, pH, and concentrations of Na+, Cl−, ammonia, titratable acidity, and lactate, the total renal flux of water, various ions, and total acid was computed. The kidneys of both species were found capable of acidifying urine, creating gradients of up to 620:1 for H+ ion, and contributing substantially to steady-state acid excretion. There was no significant increase in lactate or total acid efflux from urine during postoperative (metabolic) acidosis. Respiratory (hypercapnic) acidosis caused a compensatory increase in blood HCO3−, and an increase in branchial Na+ uptake (presumably by Na–H exchange), but no change in ammonia excretion. There was no renal response in one Hoplias to hypercapnia, but an increased acid excretion in one Hoplerythrinus. The behavior of the urinary excretion system appears in various respects similar to the higher vertebrates. There was no obvious correlation between renal parameters and air breathing in these two species.