Molecular characterization of an elasmobranch urea transporter

Marine elasmobranch fishes retain relatively high levels of urea to balance the osmotic stress of living in seawater. To maintain osmotic balance and reduce the energetic costs of making urea, it is important for these animals to minimize urea excretion to the environment. We have isolated a novel 2.2-kb cDNA from Squalus acanthias (spiny dogfish shark) kidney encoding a 380-amino acid hydrophobic protein (ShUT) with 66% identity to the rat facilitated urea transporter protein UT-A2. Injection of ShUT cRNA into Xenopus oocytes induced a 10-fold increase in C-14-labeled urea uptake, inhibitable by phloretin (0.35 mM). ShUT mRNA is expressed in kidney and brain. Related mRNA species are found in liver, blood, kidney, gill, intestine, muscle, and rectal gland. This is the first facilitated urea transporter to be identified in a marine fish. We propose that the ShUT protein is involved in urea reabsorption by the renal tubules of the dogfish shark, which in turn minimizes urea loss in the urine.