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

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 NH 3 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 Supplementary material available online at

Adaptations of a deep sea scavenger: High ammonia tolerance and active NH 4 + excretion by the Pacific hagfish ( Eptatretus stoutii )

“…Some ammonia tolerant fishes including the Lake Magadi tilapia, the gulf toadfish (Opsanus beta) and climbing catfish use the ornithine-urea cycle to detoxify ammonia (Mommsen and Walsh, 1989;Randall et al, 1989;Saha and Ratha, 2007;Wood et al, 2013). At least in the Magadi tilapia and the gulf toadfish, the excretion of urea is promoted by facilitated urea transport proteins found mainly in the gills (Walsh and Smith, 2001;Walsh et al, 2003) but there is emerging evidence that other fishes including the Pacific hagfish also use urea transport proteins (Braun and Perry, 2010).…”

Ammonia excretion inCaenorhabditis elegans: mechanism and evidence of ammonia transport of the Rhesus protein CeRhr-1

“…For the basolateral localized Na + /K + -ATPase, it was shown in 1960 by Jens Skou that this enzyme does accept NH 4 + as a substrate by replacing K + ions (Skou, 1960) and thereby is capable of actively pumping NH 4 + from the body fluids into the respective ammonia-transporting epithelial cell. The direct participation of this pump in the ammonia transport mechanism has now been identified for numerous systems, including those in gills of crustaceans (Furriel et al, 2004;Masui et al, 2002;Weihrauch et al, 1998;Weihrauch et al, 1999) and fish (Mallery, 1983;Nawata et al, 2010a;Wood et al, 2013), frog skin (Cruz et al, 2013), mammalian kidney (Garvin et al, 1985;Wall and Koger, 1994) and intestine (Worrell et al, 2008). The second pump often, if not always, involved in the ammonia transport processes is the V-ATPase.…”

Rh protein expression in branchial neuroepithelial cells, and the role of ammonia in ventilatory control in fish

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