MOLECULAR-CLONING AND CHARACTERIZATION OF THE VASOPRESSIN-REGULATED UREA TRANSPORTER OF RAT-KIDNEY COLLECTING DUCTS

Absorption of urea in the renal inner medullary collecting duct (IMCD) contributes to hypertonicity in the medullary interstitium which, in turn, provides the osmotic driving force for water reabsorption. This mechanism is regulated by vasopressin via a cAMP-dependent pathway and activation of a specialized urea transporter located in the apical me mbrane. We report here the cloning of a novel urea transporter, design ated UT1, from the rat inner medulla which is functionally and structu rally distinct from the previously reported kidney urea transporter UT 2. UT1 expressed in Xenopus oocytes mediated passive transport of urea that was inhibited by phloretin and urea analogs but, in contrast to UT2, was strongly stimulated by cAMP agonists. Sequence comparison rev ealed that the coding region of UT1 cDNA contains the entire 397 amino acid residue coding region of UT2 and an additional 1,596 basepair-st retch at the 5' end. This stretch encodes a novel 532 amino acid resid ue NH2-terminal domain that has 67% sequence identity with UT2. Thus, UT1 consists of two internally homologous portions that have most like ly arisen by gene duplication. Studies of the rat genomic DNA further indicated that UT1 and UT2 are derived from a single gene by alternati ve splicing. Based on Northern analysis and in situ hybridization, UT1 is expressed exclusively in the IMCD, particularly in its terminal po rtion. Taken together, our data show that UT1 corresponds to the previ ously characterized vasopressin-regulated urea transporter in the apic al membrane of the terminal IMCD which plays a critical role in renal water conservation.