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

A novel type of urea transporter, UT-C, is highly expressed in proximal tubule of seawater eel kidney. Am J Physiol Renal Physiol 288: F455-17465, 2005. First published September 21, 2004; doi:10.1152/ajprenal. 00296.2004.-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 (similar to35 %), 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.