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Rh proteins and NH4+-activated Na+-ATPase in the Magadi tilapia (Alcolapia grahami), a 100% ureotelic teleost fish.
MedLine Citation:
PMID:  23885087     Owner:  NLM     Status:  In-Data-Review    
Abstract/OtherAbstract:
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:
Chris M Wood; C Michele Nawata; Jonathan M Wilson; Pierre Laurent; Claudine Chevalier; Harold L Bergman; Adalto Bianchini; John N Maina; Ora E Johannsson; Lucas F Bianchini; Geraldine D Kavembe; Michael B Papah; Rodi O Ojoo
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Publication Detail:
Type:  Journal Article    
Journal Detail:
Title:  The Journal of experimental biology     Volume:  216     ISSN:  1477-9145     ISO Abbreviation:  J. Exp. Biol.     Publication Date:  2013 Aug 
Date Detail:
Created Date:  2013-07-25     Completed Date:  -     Revised Date:  -    
Medline Journal Info:
Nlm Unique ID:  0243705     Medline TA:  J Exp Biol     Country:  England    
Other Details:
Languages:  eng     Pagination:  2998-3007     Citation Subset:  IM    
Affiliation:
Department of Biology, McMaster University, Hamilton, ON, Canada, L8S 4K1.
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