title: Physiological and molecular ontogeny of branchial and extra-branchial urea excretion in posthatch rainbow trout (Oncorhynchus mykiss).
abstract: All teleost fish produce ammonia as a metabolic waste product. In embryos, ammonia excretion is limited by the chorion, and fish must detoxify ammonia by synthesizing urea via the ornithine urea cycle (OUC). Although urea is produced by embryos and larvae, urea excretion (J(urea)) is typically low until yolk sac absorption, increasing thereafter. The aim of this study was to determine the physiological and molecular characteristics of J(urea) by posthatch rainbow trout (Oncorhynchus mykiss). Following hatch, whole body urea concentration decreased over time, while J(urea) increased following yolk sac absorption. From 12 to 40 days posthatch (dph), extra-branchial routes of excretion accounted for the majority of J(urea), while the gills became the dominant site for J(urea) only after 55 dph. This represents the most delayed branchial ontogeny of any process studied to date. Urea transporter (UT) gene expression in the gills and skin increased over development, consistent with increases in branchial and extra-branchial J(urea). Following exposure to 25 mmol/l urea, the accumulation and subsequent elimination of exogenous urea was much greater at 55 dph than 12 dph, consistent with increased UT expression. Notably, UT gene expression in the gills of 55 dph larvae increased in response to high urea. In summary, there is a clear increase in urea transport capacity over posthatch development, despite a decrease in OUC activity.
authors.affiliation: Department of Biology, McMaster University, Hamilton, Ontario, Canada; and Department of Zoology, University of British Columbia, Vancouver, British Columbia, Canada azimmer@zoology.ubc.ca.
authors.forename: Alex M
authors.initials: AM
authors.lastname: Zimmer
authors.affiliation: Department of Biology, McMaster University, Hamilton, Ontario, Canada; and Department of Zoology, University of British Columbia, Vancouver, British Columbia, Canada.
authors.forename: Chris M
authors.initials: CM
authors.lastname: Wood
mesh_terms: D000367:Age Factors; D000818:Animals; D001692:Biological Transport; D029941:Fish Proteins / Q000235:genetics / Q000378:metabolism*; D005880:Gills / Q000196:embryology / Q000254:growth & development / Q000378:metabolism*; D007814:Larva / Q000378:metabolism; D026901:Membrane Transport Proteins / Q000235:genetics / Q000378:metabolism*; D017686:Oncorhynchus mykiss / Q000196:embryology / Q000235:genetics / Q000254:growth & development / Q000378:metabolism*; D012867:Skin / Q000196:embryology / Q000254:growth & development / Q000378:metabolism*; D013997:Time Factors; D014508:Urea / Q000378:metabolism*; D000096982:Urea Transporters
chemical_list: D029941:Fish Proteins; D026901:Membrane Transport Proteins; D014508:Urea; D000096982:Urea Transporters
keywords: early life stage; larval development; nitrogen excretion; urea excretion; urea excretion (Jurea); urea transporter