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The full-length cDNA sequence of a putative urea transporter (IfUl) of the facilitated diffusion UT-A type has been cloned from the African lungfish Protopterus annectens. The IFUTcDNA is 1990bp in length and its open reading frame encodes a 409 amino acid long protein, with a calculated molecular mass of 44,723 Da. The sequence is closest to those of amphibians (∼65% amino acid homology), followed by mammals and elasmobranchs (∼60%), and then teleosts (∼50%). IfUT was clearly expressed in gill, kidney, liver, skeletal muscle and skin. Upon re-immersion in water after 33days of air exposure ('terrestrialization'), lungfish exhibited a massive rise in urea-N excretion which peaked at 12-30h with rates of 2000-5000μmol-N kg-1 h -1 (versus normal aquatic rates of <130μmol-Nkg -1h-1) and persisted until 70h. This appears to occur mainly through the skin. Total 'excess' urea-N excretion amounted to ∼81,000-91,000 μmol-N kg-1 over 3 days. By real-time PCR, there was no difference in IfUT expression in the ventral abdominal skin between aquatic ammoniotelic controls and terrestrialized lungfish immediately after return to water (0h), and no elevation of urea-N excretion at this time. However, skin biopsies revealed a significant 2.55-fold elevation of IfUT expression at 14h, coincident with peak urea-N excretion. At 48h, there was no longer any significant difference in IFUT mRNA levels from those at 0 and 14h, or from aquatic fed controls. In accordance with earlier studies, which identified elevated urea-N excretion w'athe skin of P. dolloi with pharmacology typical of UT-A carriers, these results argue that transcriptional activation of a facilitated diffusion type urea transporter (IfUT) occurs in the skin during re-immersion. This serves to clear the body burden of urea-N accumulated during terrestrialization.

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Journal of Experimental Biology

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