CELLULAR AND SUBCELLULAR-LOCALIZATION OF THE VASOPRESSIN-REGULATED UREA TRANSPORTER IN RAT-KIDNEY

The renal urea transporter (RUT) is responsible for urea accumulation in the renal medulla, and consequently plays a central role in the uri nary concentrating mechanism. To study its cellular and subcellular lo calization, we prepared affinity-purified, peptide-derived polyclonal antibodies against rat RUT based on the cloned cDNA sequence. Immunobl ots using membrane fractions from rat renal inner medulla revealed a s olitary 97-kDa band. Immunocytochemistry demonstrated RUT labeling of the apical and subapical regions of inner medullary collecting duct (I MCD) cells, with no labeling of outer medullary or cortical collecting ducts. Immunoelectron microscopy directly demonstrated labeling of th e apical plasma membrane and of subapical intracellular vesicles of IM CD cells, but no labeling of the basolateral plasma membrane. Immunobl ots demonstrated RUT labeling in both plasma membrane and intracellula r vesicle-enriched membrane fractions from inner medulla, a subcellula r distribution similar to that of the vasopressin-regulated water chan nel, aquaporin-2.In the outer medulla, RUT labeling was seen in termin al portions of short-loop descending thin limbs. Aside from IMCD and d escending thin limbs, no other structures were labeled in the kidney. These results suggest that: (i) the RUT provides the apical pathway fo r rapid, vasopressin-regulated urea transport in the IMCD, (ii) collec ting duct urea transport may be increased by vasopressin by stimulatio n of trafficking of RUT-containing vesicles to the apical plasma membr ane, and (iii) the rat urea transporter may provide a pathway for urea entry into the descending limbs of short-loop nephrons.