title: Rh proteins and NH4(+)-activated Na+-ATPase in the Magadi tilapia (Alcolapia grahami), a 100% ureotelic teleost fish.
abstract: The small cichlid fish Alcolapia grahami lives in Lake Magadi, Kenya, one of the most extreme aquatic environments on Earth (pH ~10, carbonate alkalinity ~300 mequiv l(-1)). The Magadi tilapia is the only 100% ureotelic teleost; it normally excretes no ammonia. This is interpreted as an evolutionary adaptation to overcome the near impossibility of sustaining an NH3 diffusion gradient across the gills against the high external pH. In standard ammoniotelic teleosts, branchial ammonia excretion is facilitated by Rh glycoproteins, and cortisol plays a role in upregulating these carriers, together with other components of a transport metabolon, so as to actively excrete ammonia during high environmental ammonia (HEA) exposure. In Magadi tilapia, we show that at least three Rh proteins (Rhag, Rhbg and Rhcg2) are expressed at the mRNA level in various tissues, and are recognized in the gills by specific antibodies. During HEA exposure, plasma ammonia levels and urea excretion rates increase markedly, and mRNA expression for the branchial urea transporter mtUT is elevated. Plasma cortisol increases and branchial mRNAs for Rhbg, Rhcg2 and Na(+),K(+)-ATPase are all upregulated. Enzymatic activity of the latter is activated preferentially by NH4(+) (versus K(+)), suggesting it can function as an NH4(+)-transporter. Model calculations suggest that active ammonia excretion against the gradient may become possible through a combination of Rh protein and NH4(+)-activated Na(+)-ATPase function.
authors.affiliation: Department of Biology, McMaster University, Hamilton, ON, Canada. woodcm@mcmaster.ca
authors.forename: Chris M
authors.initials: CM
authors.lastname: Wood
authors.forename: C Michele
authors.initials: CM
authors.lastname: Nawata
authors.forename: Jonathan M
authors.initials: JM
authors.lastname: Wilson
authors.forename: Pierre
authors.initials: P
authors.lastname: Laurent
authors.forename: Claudine
authors.initials: C
authors.lastname: Chevalier
authors.forename: Harold L
authors.initials: HL
authors.lastname: Bergman
authors.forename: Adalto
authors.initials: A
authors.lastname: Bianchini
authors.forename: John N
authors.initials: JN
authors.lastname: Maina
authors.forename: Ora E
authors.initials: OE
authors.lastname: Johannsson
authors.forename: Lucas F
authors.initials: LF
authors.lastname: Bianchini
authors.forename: Geraldine D
authors.initials: GD
authors.lastname: Kavembe
authors.forename: Michael B
authors.initials: MB
authors.lastname: Papah
authors.forename: Rodi O
authors.initials: RO
authors.lastname: Ojoo
mesh_terms: D000251:Adenosine Triphosphatases / Q000378:metabolism*; D000641:Ammonia / Q000494:pharmacology*; D000818:Animals; D002118:Calcium / Q000097:blood; D027682:Cation Transport Proteins / Q000378:metabolism*; D004781:Environmental Exposure; D004789:Enzyme Activation / Q000187:drug effects; D004912:Erythrocytes / Q000378:metabolism; D029941:Fish Proteins / Q000235:genetics / Q000378:metabolism*; D005786:Gene Expression Regulation / Q000187:drug effects; D007150:Immunohistochemistry; D007477:Ions / Q000097:blood; D008274:Magnesium / Q000097:blood; D008562:Membrane Glycoproteins / Q000235:genetics / Q000378:metabolism*; D008954:Models, Biological; D010101:Oxygen Consumption / Q000187:drug effects; D010802:Phylogeny; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D017210:Tilapia / Q000097:blood / Q000235:genetics / Q000378:metabolism*; D014508:Urea / Q000378:metabolism*
chemical_list: D027682:Cation Transport Proteins; D029941:Fish Proteins; D007477:Ions; D008562:Membrane Glycoproteins; D012333:RNA, Messenger; D000641:Ammonia; D014508:Urea; D000251:Adenosine Triphosphatases; C021838:sodium-translocating ATPase; D008274:Magnesium; D002118:Calcium
keywords: Alcolapia grahami; Na+,K+-ATPase; ammonia transport; gills; high alkalinity; high environmental ammonia; urea transport