Physiological and molecular ontogeny of branchial and extra-branchial urea excretion in posthatch rainbow trout (Oncorhynchus mykiss)

Adaptation of the carbamoyl-phosphate synthetase enzyme in an extremophile fish

Tetrapods and fish have adapted distinct carbamoyl-phosphate synthase (CPS) enzymes to initiate the ornithine urea cycle during the detoxification of nitrogenous wastes. We report evidence that in the ureotelic subgenus of extremophile fish Oreochromis Alcolapia, CPS III has undergone convergent evolution and adapted its substrate affinity to ammonia, which is typical of terrestrial vertebrate CPS I. Unusually, unlike in other vertebrates, the expression of CPS III in Alcolapia is localized to the skeletal muscle and is activated in the myogenic lineage during early embryonic development with expression remaining in mature fish. We propose that adaptation in Alcolapia included both convergent evolution of CPS function to that of terrestrial vertebrates, as well as changes in development mechanisms redirecting CPS III gene expression to the skeletal muscle.

Ontogeny of urea and ammonia transporters in mahi-mahi (Coryphaena hippurus) early life stages

The mechanism(s) of ammonia and urea excretion in freshwater fish have received considerable attention; however, parallel investigations of seawater fish, specifically in the early life stages are scarce. The first objective of this study was to evaluate the patterns of ammonia and urea excretion in mahi-mahi (Coryphaena hippurus) up to 102  hours post fertilization (hpf). Similar to other teleosts, mahi embryos are ureotelic before hatch and gradually switch to being ammoniotelic around the time of hatch. The second objective was to characterize mRNA levels of ammonia transporters (Rhag, Rhbg, Rhcg1 and Rhcg2), as well as urea transporter (UT) and sodium hydrogen exchangers (NHE3 and NHE2) during mahi development. As predicted, the mRNA levels of the Rhesus glycoprotein (Rh) genes, especially Rhag, Rhbg and the UT gene were highly consistent with the ontogeny of ammonia and urea excretion rates. Further, the localization of each transporter was examined in larvae collected at 60 and 102 hpf using in situ hybridization. Rhag was expressed in the gills, yolk sac, and operculum. Rhbg was expressed in the gills and upper mouth. Rhcg1 and NHE3 were co-localized in the sub-operculum, and Rhcg2 was expressed in the skin. Together, these results indicate that urea excretion is critical for ammonia detoxification during embryonic development and that Rh proteins are involved in ammonia excretion via gills and yolk sac, possibly facilitated by NHE3.

Ammonia and urea handling by early life stages of fishes

Nitrogen metabolism in fishes has been a focus of comparative physiologists for nearly a century. In this Review, we focus specifically on early life stages of fishes, which have received considerable attention in more recent work. Nitrogen metabolism and excretion in early life differs fundamentally from that of juvenile and adult fishes because of (1) the presence of a chorion capsule in embryos that imposes a limitation on effective ammonia excretion, (2) an amino acid-based metabolism that generates a substantial ammonia load, and (3) the lack of a functional gill, which is the primary site of nitrogen excretion in juvenile and adult fishes. Recent findings have shed considerable light on the mechanisms by which these constraints are overcome in early life. Perhaps most importantly, the discovery of Rhesus (Rh) glycoproteins as ammonia transporters and their expression in ion-transporting cells on the skin of larval fishes has transformed our understanding of ammonia excretion by fishes in general. The emergence of larval zebrafish as a model species, together with genetic knockdown techniques, has similarly advanced our understanding of ammonia and urea metabolism and excretion by larval fishes. It has also now been demonstrated that ammonia excretion is one of the primary functions of the developing gill in rainbow trout larvae, leading to new hypotheses regarding the physiological demands driving gill development in larval fishes. Here, we highlight and discuss the dramatic changes in nitrogen handling that occur over early life development in fishes.