Evidence for distinct vascular and tubular urea transporters in the rat kidney

Facilitated urea transport has been demonstrated in several mammalian tissues, including those of the collecting ducts and red blood cells. Two urea transporters have been recently cloned: UT2, expressed in rabbit inner medullary collecting ducts, and HUT11, expressed in human erythrocytes. Because of significant identify (63%) between these two transporters, and because HUT11 is also expressed in the human kidney, they could represent the same transporter with species-related differences in their sequences. In the study presented here, two different cDNA fragments, corresponding to the rat equivalents (rUT2 and rUT11) of the two previously cloned urea transporters, were isolated by reverse transcription-polymerase chain reaction. These rat probes were used for Northern analysis of RNA extracted from rat tissues. From the following findings, the results show that rUT2 and rUT11 are two distinct urea transporters: (1) The two cDNA fragments isolated in the rat exhibit different sequences; (2) The mRNA for rUT2 is found exclusively in the kidney, with two transcripts (3.2- and 4.4-kilobase (kb)), whereas rUT11 (only one transcript, 4.2 kb) is present in the brain, spleen, kidney, and testis; (3) In the kidney, the inner stripe of the outer medulla expresses rUT11 mRNA and the short transcript of rUT2, whereas the inner medulla expresses rUT11 and the two rUT2 transcripts; (4) In hydronephrotic kidneys that have completely lost their tubular epithelium but have intact vasculature, rUT2 transcripts are no longer expressed, whereas expression of rUT11 is intensified; (5) Experimental chronic alterations in urine concentrating activity induced different changes in the expression of rUT2 and rUT11.