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EDITORIAL FOCUS

A novel type of urea transporter, UT-C, is highly expressed in proximal tubule of seawater eel kidney

¹Department of Biological Sciences, Tokyo Institute of Technology, Yokohama, Japan; and ²Renal Division, Department of Medicine, Emory University School of Medicine, Atlanta, Georgia

Submitted 10 August 2004 ; accepted in final form 18 September 2004

A new type of urea transporter was identified by a database search and shown to be highly expressed in the renal proximal tubule cells of teleosts; proximal tubule-type urea transporters have not been describe previously. We first identified urea transporter-like sequences in the fugu genome and in an EST database of rainbow trout. Based on these pieces of sequence information, we obtained a full-length cDNA for the eel ortholog, consisting of 378 amino acid residues, and named it eUT-C. Although its sequence similarity to the known urea transporters is low (35%), its heterologous expression in Xenopus laevis oocytes indicated that it is a facilitative urea transporter sensitive to phloretin. Its activity is not dependent on Na⁺. Northern blot analysis showed that expression of eUT-C is highly restricted to the kidney, with weak expression in the stomach. In both tissues, eUT-C mRNA was strongly induced when eels were transferred from freshwater to seawater. Immunohistochemistry and in situ hybridization histochemistry revealed proximal tubule cell localization of eUT-C. Taking into account that 1) urea is mainly secreted from the gill where another type of urea transporter (eUT) has been identified and 2) fish excrete a very small volume of urine in seawater, we propose that eUT-C cloned here is a key component working in combination with the gill transporter to achieve an efficient urea excretory system in fish, namely, eUT-C reabsorbs urea from glomerular filtrate and sends it to the gill, through the circulation, for excretion.

gill excretion; salinity-dependent regulation; serum urea concentration; stomach; urea reabsorption

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