Tissue distribution of UT-A and UT-B mRNA and protein in rat.

Mammalian urea transporters are facilitated membrane transport proteins belonging to two families, UT-A and UT-B. They are best known for their role of maintaining the renal inner medullary urinary concentrating gradient. Urea transporters have also been identified in tissues not typically associated with urea metabolism. The purpose of this study was to survey the major organs in rat to determine the distribution of UT-A and UT-B mRNA transcripts and protein forms and determine their cellular localization. Five kidney subregions and 17 extrarenal tissues were screened by Northern blot analysis using two UT-A and three UT-B probes and by Western blot analysis using polyclonal COOH-terminal UT-A and UT-B antibodies. Immunohistochemistry was performed on 16 extrarenal tissues using the same antibodies. In kidney, we detected mRNA transcripts and protein bands consistent with previously-identified UT-A and UT-B isoforms, as well as novel forms. We found that UT-A mRNA and protein are widely expressed in extrarenal tissues in various forms that are different from the known isoforms. We determined the cellular localization of UT-A and UT-B in these tissues. We found that both UT-A and UT-B are ubiquitously expressed as numerous tissue-specific mRNA transcripts and protein forms that are localized to cell membranes, cytoplasm, or nuclei.