Renal ammoniagenesis and acid excretion in the dogfish, Squalus acanthias

Renal ammoniagenesis and acid excretion were investigated in normal dogfish (Squalus acanthias) and dogfish made acidotic by HCl injection (0.65 meq X kg-1). After acid loading, renal ammonia excretion doubled, rising from 0.11 to 0.25 mueq X h-1, and titratable acid output increased from 28.6 to 44.9 mueq X h-1. Trimethylamine excretion averaged 23.2 mueq X h-1 and did not change in response to the acidosis. During the first 24 h postinjection, the increase in renal acid excretion accounted for the elimination of 15% of the acid load. In vitro studies with kidney slices demonstrated that the dogfish kidney has the capacity to synthesize ammonia from a number of amino acids including glutamine, glutamate, alanine, aspartate, and glycine, with the greatest ammonia production resulting from glutamine. The relatively high glutamine concentration in the kidney, compared with the blood, suggested a high renal capacity for glutamine synthesis. In renal homogenates, enzymatic activities for both the deamidation (glutaminase) and synthesis (glutamine synthetase) of glutamine were investigated. The Michaelis constant (Km) values for the two enzymes were found to be almost equal (4.48 mM glutamine and 4.33 mM glutamate) and at the same levels as the substrate concentrations in the kidney (3.69 mM glutamine and 3.36 mM glutamate). Subcellular localization revealed that both enzymes occur predominantly in the mitochondria. The activities of glutaminase and glutamine synthetase in the renal mitochondria suggest the presence of a substrate cycle that could be modulated to increase ammonia production during acidosis.