Renal urea transporters. Direct and indirect regulation by vasopressin.
Urea is the most abundant urinary solute and is excreted in urine at a much higher concentration than in other body fluids. Urea concentration is achieved in the kidney through complex urea movements between blood vessels and renal tubules, which involve facilitated urea transport. Three major urea transporters expressed in the kidney have been cloned, UT-A1, UT-A2 and UT-B1, the first two derived from the same gene by differential transcription. These membrane proteins enable facilitated diffusion of urea through specific parts of the nephron (UT-A) and through renal vasculature (UT-B) in the medulla. UT-A1 is localised in the terminal part of the inner medullary collecting ducts and accounts for the vasopressin-dependent increase in urea permeability of this segment. UT-A2 is found in the descending thin limbs of Henle's loops. UT-B1 is expressed in the endothelium of the descending vasa recta supplying blood to the renal medulla, and in red cells. All three urea transporters are primarily involved in the process of intrarenal urea recycling, which enables the establishment, and prevents the dissipation, of a high concentration of urea in the inner medulla. This is an essential feature for producing a concentrated urine and thus for water economy in mammals. Vasopressin, upon binding to V2 receptors in the inner medullary collecting ducts, increases urea permeability through activation of UT-A1 molecules, thus enabling urea to diffuse into the inner medullary interstitium. Urea then taken up in ascending vasa recta is returned to the inner medulla via UT-A2 and UT-B1 by countercurrent exchange. These latter two urea transporters are not influenced acutely by vasopressin, but UT-A2 expression is markedly increased in the descending thin limbs of the loops of Henle after sustained exposure to vasopressin or its V2 agonist dDAVP. This effect is indirect because vasopressin receptors are lacking in the descending limbs. The acute direct and delayed indirect actions of vasopressin on renal urea transporters will increase medullary urea accumulation and thus the ability of the kidney to conserve water. Atrial natriuretic peptide inhibits the vasopressin-dependent increase in urea permeability in the inner medullary collecting ducts. The interruption of urea recycling probably contributes to the natriuresis. Impairing in this way the capacity of the kidney to concentrate urea enhances its capacity to concentrate sodium in the urine.