Urea transporters are critical to the urinary concentrating mechanism in the mammalian kidney. While the kidney in teleost (bony) fish plays an important role in water balance and ion regulation, urea and other nitrogen wastes are mainly excreted across the gill. Gill urea transporters have been well characterized both on a physiological and molecular basis, and are very similar to mammalian kidney urea transporters on an amino acid level. However,despite physiological evidence suggesting the presence of urea transporters in fish kidney, attempts at finding a kidney urea transporter using molecular probes based on gill urea transporters had failed. The existence of kidney urea transporters in fish remained an enigma. Until now...

Using an innovative approach, A.C. Mistry's team decided to challenge the general belief that urea transporters may not be expressed in physiologically significant levels in fish kidney; they undertook an extensive genome database mining expedition. Their search revealed a novel nucleotide sequence in the puffer fish (Takifugu) genome that encoded a protein distantly related to known urea transporters. Through mRNA amplification, the gene product (UT-C) was found to be present in both puffer fish and eel kidney. The team was able to determine the presence of eel UT-C (eUT-C) in the basolateral membranes of eel proximal tubule kidney cells using immunohistochemistry and in situ hybridization. Using northern blot analysis, the team found eUT-C was highly expressed in the eel kidney and weakly expressed in the eel stomach.

But what is the role of eUT-C in the kidney and the stomach, and does it transport urea? Interestingly, when the authors transferred eels from freshwater to seawater, eUT-C was strongly up-regulated both in kidney and in stomach tissues. This up-regulation of eUT-C was confirmed by western blot analysis using antibodies against eUT-C. Increased reabsorption of urea by the kidney via eUT-C in seawater would result in the retention of water and a reduction of urine volume; an effective strategy for seawater fish since they are plagued by continuous water loss. To confirm that eUT-C was responsible for urea uptake, the authors injected Xenopus oocytes with eUT-C cRNA and found increased urea uptake in these oocytes compared with oocytes injected with water. Urea uptake was inhibited by phloretin, a common blocker of facilitated-diffusion urea transporters, providing evidence that eUT-C was likely a urea transporter of this type. However, urea uptake by oocytes was not sensitive to the hormone vasopressin, which is known to stimulate some urea transporters. The team now had hard evidence for the elusive kidney urea transporter in fish.

Based on the findings of this study, the authors suggest a potential interaction between the eel's gill and kidney. Namely, eUT-C in the kidney may be working in combination with the gill urea transporter (eUT) to achieve an efficient urea excretory system. This system would involve the reabsorption of urea by the kidney eUT-C, which would help in the retention of water when in seawater. Urea would then circulate to the gill eUT to be excreted. However,the team does not claim to have all the answers, there is still the question of eUT-C expression in the stomach and its apparent salinity dependence. Alas,tomorrow is another day...

Mistry, A. C., Chen, G., Kato, A., Nag, K., Sands, J. M. and Hirose, S. (
). A novel type of urea transporter, UT-C,highly expressed in proximal tubule of seawater eel kidney.
Am J. Physiol. Renal Physiol.