UT-A2: a 55-kDa urea transporter in thin descending limb whose abundance is regulated by vasopressin

The renal urea transporter gene (UT-A) produces different transcripts in th e inner medullary collecting ducts (UT-A1) and thin descending limbs of Hen le's loop (UT-A2), coding for distinct proteins. Peptide-directed rabbit po lyclonal antibodies were used to identify the UT-A2 protein in renal medull a of mouse and rat. In the inner stripe of outer medulla, an antibody direc ted to the COOH terminus of UT-A recognized a membrane protein of 55 kDa. T he abundance of this 55-kDa protein was strongly increased in response to c hronic infusion of the vasopressin analog 1-deamino-[8-D-arginine]vasopress in (DDAVP) in Brattleboro rats, consistent with previous evidence that UT-A 2 mRNA abundance is markedly increased. Immunofluorescence labeling with th e COOH-terminal antibody in Brattleboro rats revealed labeling in the lower portion of descending limbs from short-looped nephrons (in the aquaporin-1 -negative portion of this segment). This UT-A labeling was increased in res ponse to DDAVP. Increased labeling was also seen in descending limbs of lon g-looped nephrons in the base of the inner medulla. These results indicate that UT-A2 is expressed as a 55-kDa protein in portions of the thin descend ing limbs of Henle's loop and that the abundance of this protein is strongl y upregulated by vasopressin.