Abstract
A family of molecular urea transporters (UTs) has been identified whose members appear to have an exceptionally high transport turnover rate. To test the hypothesis that urea transport involves passage through an aqueous channel, osmotic water permeability was measured in Xenopus oocytes expressing UTs. The UT3 class of urea transporters functioned as efficient water channels. Quantitative measurement of single channel water permeability (pf) using epitope-tagged rat UTs gave pf (in cm3/s x 10(-14)) of 0.14 +/- 0.11 (UT2) and 1.4 +/- 0.2 (UT3), compared with 6.0 and 2.3 for water channels AQP1 and AQP3, respectively. Relative single channel urea permeabilities (purea) were 1.0 (UT2), 0.44 (UT3), and 0.0 (AQP1). UT3-mediated water and urea transport were weakly temperature-dependent (activation energy <4 kcal/mol), inhibited > 75% by the urea transport inhibitor 1,3-dimethylthiourea, but not inhibited by the water transport inhibitor HgCl2. To test for a common water/urea pore, the urea reflection coefficient (sigmaurea) was measured by independent induced osmosis and solvent drag methods. In UT3-expressing oocytes, the time course of oocyte volume in response to different urea gradients (induced osmosis) gave sigmaurea approximately 0.3 for the UT3 pathway, in agreement with sigmaurea determined by the increase in uptake of [14C]urea during osmotic gradient-induced oocyte swelling (solvent drag). In oocytes of comparable water and urea permeability coexpressing AQP1 (permeable to water, not urea) and UT2 (permeable to urea, not water), sigmaurea = 1. These results indicate that UT3 functions as a urea/water channel utilizing a common aqueous pathway. The water transporting function and low urea reflection coefficient of UT3 in vasa recta may be important for the formation of a concentrated urine by countercurrent exchange in the kidney.
Highlights
Several related urea transporters (UTs)1 have been cloned recently
A 391-amino acid urea transporter expressed in erythrocytes was initially cloned from human bone marrow (HUT11, Ref. 6) and subsequently from rat kidney
UT3 was initially recognized as the Kidd antigen (Jk) in erythrocytes (10 –12)
Summary
Several related urea transporters (UTs) have been cloned recently. The UT2 transporter (rat form referred to as rUT2, Ref. 1) was first identified in rabbit by expression cloning [2]. A 391-amino acid urea transporter expressed in erythrocytes was initially cloned from human bone marrow (HUT11, Ref. 6) and subsequently from rat kidney (named UT3, Ref. 7). From the number of Kidd antigen proteins per erythrocyte and the erythrocyte urea permeability, it was estimated that the UT3 turnover rate is 2–15 ϫ 106 urea molecules/s [9, 14] This turnover rate is substantially higher than that of usual solute carriers, suggesting a channel mechanism for urea transport. A key finding was that the reflection coefficient for urea was remarkably less than unity, providing strong evidence for a common water and urea pathway through UT3 These results have important implications regarding urea transporter structure and transporting mechanism and suggest a novel role for UT3 in the urinary concentrating mechanism as a facilitator of urea solvent drag in renal vasa recta
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