Regulation of Urea Transport by Adrenal Steroids

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Terrestrial life would be miserable without the ability to concentrate the urine. Glucocorticoids are the primary catabolic hormone that causes protein breakdown and ureagenesis. Urea excretion into the urine is the primary mechanism for eliminating this excess nitrogen from the body. A urea transporter is critically important to the theories proposed to explain the physiologic processes occurring when urine is concentrated. Only 14 years ago, evidence for such a transporter was largely speculative, but we provided experimental evidence that there is facilitated urea transport in the kidney inner medullary collecting duct. Subsequently, cDNA isoforms for facilitated urea transporters were cloned from kidney medulla (UT-A1, UT-A2, UT-A3, UT-A4), testis (UT-A5), and erythrocytes (UT-B). Recently, we cloned the rat and human genes for the UT-A family of urea transporters and showed that there are 2 promoters: promoter I, located 5' to exon 1, that controls transcription of UT-A1, UT-A3, and UT-A4; and promoter II, located in intron 12, that controls transcription of UT-A2. Our group has also made major progress in understanding the long-term regulation of urea transport by studying 5 animal models associated with an impaired urine concentrating ability. Our studies led to the surprising result that facilitated urea permeability and UT-A1 protein abundance are increased during in vivo conditions associated with a reduced urine concentrating ability and plasma vasopressin level. We also found that UT-A1 protein abundance is decreased during in vivo conditions associated with increased glucocorticoids.