Bk. Kishore et al., ULTRAMICRODETERMINATION OF VASOPRESSIN-REGULATED UREA TRANSPORTER PROTEIN IN MICRODISSECTED RENAL TUBULES, American journal of physiology. Renal, fluid and electrolyte physiology, 41(4), 1997, pp. 531-537
The vasopressin-regulated urea transporter (VRUT) is a 97-kDa protein
(also called ''UT-1'') responsible for facilitated urea transport acro
ss the apical plasma membrane of inner medullary collecting duct (IMCD
) cells. To determine the abundance of VRUT protein in collecting duct
cells of the rat, we designed a sensitive fluorescence-based enzyme-l
inked immunosorbent assay capable of detecting <5 fmol of VRUT protein
. In collecting duct segments, measurable VRUT was found in microdisse
cted IMCD segments but not in other portions of the collecting duct. I
n the mid-IMCD, the measured level averaged 5.3 fmol/mm tubule length,
corresponding to similar to 5 million copies of VRUT per cell. Thus V
RUT is extremely abundant in the IMCD, accounting, in part, for the ex
tremely high urea permeability of this segment. Feeding a low-protein
diet (8% protein) markedly decreased urea clearance but did not alter
the quantity of VRUT protein in the IMCD. Thus increased urea transpor
t across the collecting duct with dietary protein restriction is not a
consequence of increased expression of VRUT. Based on urea fluxes mea
sured in the IMCD and our measurements of the number of copies of VRUT
, we estimate a turnover number of greater than or equal to 0.3-1 x 10
(5) s. In view of the large magnitude of this value and previously rep
orted biophysical properties of urea transport in collecting ducts, we
hypothesize that the VRUT may function as a channel rather than a car
rier.