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First published online December 10, 2003
Journal of Experimental Biology 207, 347-356 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.00773

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A facilitative urea transporter is localized in the renal collecting tubule of the dogfish Triakis scyllia

Susumu Hyodo^1,*, Fumi Katoh², Toyoji Kaneko² and Yoshio Takei¹

¹ Laboratory of Physiology, Ocean Research Institute, University of Tokyo, 1-15-1 Minamidai, Nakano, Tokyo 164-8639, Japan
² Center for International Cooperation, Ocean Research Institute, University of Tokyo, 1-15-1 Minamidai, Nakano, Tokyo 164-8639, Japan

^* Author for correspondence (e-mail: hyodo{at}ori.u-tokyo.ac.jp)

Accepted 20 October 2003

Reabsorption of filtered urea by the kidney tubule is essential for retaining high levels of urea in body fluids of marine elasmobranchs. To elucidate the mechanisms of urea reabsorption, we examined the distribution of a facilitative urea transporter (UT) in the kidney of the dogfish Triakis scyllia. We isolated a cDNA encoding a UT that is homologous to the facilitative UT cloned from another dogfish species, Squalus acanthias. The Triakis UT mRNA is abundantly expressed in the kidney, while low levels of expression were detected in the brain and liver. In the dogfish kidney, each nephron makes four turns and traverses repeatedly between bundle zone and sinus zone. In the bundle zone, the resulting five tubular segments are arranged in a countercurrent loop fashion. Immunohistochemistry using specific antibodies raised against the cloned UT revealed that, among the nephron segments, the UT is expressed exclusively in the final segment of the bundle zone, i.e. in the collecting tubule of the Triakis kidney. In contrast to the limited localization of UT, the transport enzyme Na⁺/K⁺-ATPase is distributed in the basolateral membrane of numerous tubular segments both in the sinus zone and the bundle zone. However, in the collecting tubule, Na⁺/K⁺-ATPase immunoreactivity was not detected. The present study suggests that the collecting tubule is responsible for the reabsorption of urea in the marine elasmobranch kidney. Other countercurrent segments may contribute to production of a driving force for facilitative diffusion of urea through the UT.

Key words: urea transporter, urea reabsorption, kidney, bundle zone, collecting tubule, Na⁺/K⁺-ATPase, facilitative diffusion, elasmobranch, dogfish, Triakis scyllia

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