LONG-TERM REGULATION OF RENAL UREA TRANSPORTER PROTEIN EXPRESSION IN RAT

To test the hypothesis that the abundance of the apical urea transport er of the inner medullary collecting duct (IMCD) is regulated in vivo by factors associated with altered water balance, immunoblots of rat i nner medullary membrane fractions were probed with rabbit polyclonal a ntibodies against the renal urea transporter (RUT) gene product. In in ner medullas of Brattleboro rats, which manifest severe chronic water diuresis, a 117-kD band was seen, in addition to the previously descri bed 97-kD band. These two bands were detectable by antibodies directed against two different regions of the RUT sequence. When Brattleboro r ats were treated with a 5-d infusion of arginine vasopressin (AVP) by osmotic minipump, the 117-kD band was markedly diminished, whereas the 97-kD band was unchanged. Simultaneous infusion of the diuretic agent furosemide prevented the AVP-induced decrease in the 117-kD band. In AVP-infused Sprague Dawley rats, the 117-kD band was barely perceptibl e. However, when AVP treated rats were infused with furosemide for 5 d , the 117-kD band was markedly accentuated, whereas the 97-kD band was unchanged. The abundance of the 117-kD RUT protein in the renal papil la was inversely correlated with dietary protein intake. Further immun oblotting studies revealed that the 117-kD protein is heavily expresse d in IMCD cells and not in non-collecting duct components of the inner medulla, and is present in low-density microsome fractions from inner medulla. From this study, the following conclusions can be made: (I) The collecting duct urea transporter is present in at least two forms (97 and 117 kD) in the IMCD. (2) The expression level of the 117-kD ur ea transporter protein is regulated and is inversely correlated with m edullary osmolality and urea concentration, but does not correlate wit h circulating AVP level. (3) Although AVP regulates RUT function on a short-term basis, long-term changes in AVP levels do not increase RUT abundance.