title: Physiological and molecular responses of the spiny dogfish shark (Squalus acanthias) to high environmental ammonia: scavenging for nitrogen.
abstract: In teleosts, a branchial metabolon links ammonia excretion to Na(+) uptake via Rh glycoproteins and other transporters. Ureotelic elasmobranchs are thought to have low branchial ammonia permeability, and little is known about Rh function in this ancient group. We cloned Rh cDNAs (Rhag, Rhbg and Rhp2) and evaluated gill ammonia handling in Squalus acanthias. Control ammonia excretion was <5% of urea-N excretion. Sharks exposed to high environmental ammonia (HEA; 1 mmol(-1) NH4HCO3) for 48 h exhibited active ammonia uptake against partial pressure and electrochemical gradients for 36 h before net excretion was re-established. Plasma total ammonia rose to seawater levels by 2 h, but dropped significantly below them by 24-48 h. Control ΔP(NH3) (the partial pressure gradient of NH3) across the gills became even more negative (outwardly directed) during HEA. Transepithelial potential increased by 30 mV, negating a parallel rise in the Nernst potential, such that the outwardly directed NH4(+) electrochemical gradient remained unchanged. Urea-N excretion was enhanced by 90% from 12 to 48 h, more than compensating for ammonia-N uptake. Expression of Rhp2 (gills, kidney) and Rhbg (kidney) did not change, but branchial Rhbg and erythrocytic Rhag declined during HEA. mRNA expression of branchial Na(+)/K(+)-ATPase (NKA) increased at 24 h and that of H(+)-ATPase decreased at 48 h, while expression of the potential metabolon components Na(+)/H(+) exchanger2 (NHE2) and carbonic anhydrase IV (CA-IV) remained unchanged. We propose that the gill of this nitrogen-limited predator is poised not only to minimize nitrogen loss by low efflux permeability to urea and ammonia but also to scavenge ammonia-N from the environment during HEA to enhance urea-N synthesis.
authors.affiliation: Bamfield Marine Sciences Centre, 100 Pachena Road, Bamfield, BC, Canada V0R 1B0 Department of Biology, McMaster University, 1280 Main Street, West, Hamilton, ON, Canada L8S 4K1 Department of Physiology, University of Arizona, 1501 N. Campbell Avenue Tucson, AZ 85724, USA cmnawata@email.arizona.edu.
authors.forename: C Michele
authors.initials: CM
authors.lastname: Nawata
authors.affiliation: Bamfield Marine Sciences Centre, 100 Pachena Road, Bamfield, BC, Canada V0R 1B0 Department of Biology and Centre for Advanced Research in Environmental Genomics, University of Ottawa, Ottawa, ON, Canada K1N 6N5.
authors.forename: Patrick J
authors.initials: PJ
authors.lastname: Walsh
authors.affiliation: Bamfield Marine Sciences Centre, 100 Pachena Road, Bamfield, BC, Canada V0R 1B0 Department of Biology, McMaster University, 1280 Main Street, West, Hamilton, ON, Canada L8S 4K1 Department of Zoology, University of British Columbia, #4321-6270 University Boulevard, Vancouver, BC, Canada V6T 1Z4.
authors.forename: Chris M
authors.initials: CM
authors.lastname: Wood
mesh_terms: D000595:Amino Acid Sequence; D000641:Ammonia / Q000097:blood / Q000378:metabolism*; D000818:Animals; D001483:Base Sequence; D018076:DNA, Complementary; D004032:Diet / Q000706:statistics & numerical data; D005880:Gills / Q000502:physiology*; D006023:Glycoproteins / Q000378:metabolism; D007668:Kidney; D008969:Molecular Sequence Data; D009584:Nitrogen / Q000378:metabolism; D006180:Proton-Translocating ATPases; D012623:Seawater / Q000737:chemistry; D000254:Sodium-Potassium-Exchanging ATPase; D048251:Squalus acanthias / Q000378:metabolism*; D014508:Urea / Q000378:metabolism
chemical_list: D018076:DNA, Complementary; D006023:Glycoproteins; D000641:Ammonia; D014508:Urea; D006180:Proton-Translocating ATPases; D000254:Sodium-Potassium-Exchanging ATPase; D009584:Nitrogen
keywords: Elasmobranchs; Gene expression; Gills; Ornithine–urea cycle; PNH3 gradient; Rh glycoproteins; Transepithelial potential; Urea