RENAL AMMONIAGENESIS AND ACID-BASE REGULATION IN FISHES

Renal ammonia excretion and production was studied in selected fish species. In vivo experiments were designed to evaluate the capacity of the fish to increase renal ammonia and titratable acid excretion during metabolic acidosis. In vitro studies investigated the source of renal ammonia, the pathways involved in ammoniagenesis, and the factors possibly regulating renal ammonia production in normal and acidotic fish.; The renal response to acidosis was examined in the dogfish (Squalus acanthias) and the goldfish (Carassius auratus). Following induction of acidosis by acid injection into the caudal vessel (dogfish) or by acidification (pH 4.0) of the aquarium water (goldfish), both species showed a 2-fold increase in urinary excretion of ammonia and titratable acid; the rise in renal ammonia output indicated a stimulation of renal ammonia production. In the dogfish, the increase in renal acid excretion accounts for the elimination of 15% of the acid load. For the goldfish, these data suggest the potential importance of the kidneys in the acid-base regulation of fish inhabiting acidic environments.; The in vitro studies demonstrated that both species have the capacity for renal ammonia synthesis. Data from experiments of ammonia production by isolated renal tubules (goldfish) and kidney slices (dogfish) along with plasma and kidney amino acid analyses indicate that for both species glutamine and aspartate serve as the major precursors for ammoniagenesis in intact kidney cells. The presence of the enzymes involved in the production of ammonia from these amino acids was verified by assaying activities in cell-free kidney homogenates. The control of ammonia production appears to vary for these two species. For the dogfish, the simultaneous activities of glutaminase and glutamine synthetase in the renal mitochondria suggest the presence of a substrate cycle between ammonia plus glutamate and glutamine which could modulate the production of NH(,3) during acidosis. In the goldfish kidney, the very low glutamine synthetase levels preclude the operation of such a cycle.