Rh proteins and NH4+-activated Na+-ATPase in the Magadi tilapia (Alcolapia grahami), a 100% ureotelic teleost fish

Adenosine Triphosphatases Fish Proteins Ions 0301 basic medicine 0303 health sciences Erythrocytes Membrane Glycoproteins Environmental Exposure Immunohistochemistry Models, Biological high environmental ammonia high alkalinity ammonia transport urea transport gills Na+,K+-ATPase Alcolapia grahami Enzyme Activation 03 medical and health sciences Oxygen Consumption Gene Expression Regulation Ammonia Animals Urea Calcium Magnesium RNA, Messenger 14. Life underwater Cation Transport Proteins Phylogeny Tilapia
DOI: 10.1242/jeb.078634 Publication Date: 2013-07-24T23:45:59Z
ABSTRACT
SUMMARYThe 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.
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