Integrated function of urea transporters in the mammalian kidney.

Specific urea transporters are responsible for the rapid urea movements occurring in precise medullary structures of the mammalian kidney. Three of them, ensuring facilitated passive transports, have been cloned yet: UT2-long is responsible for the high vasopressin-dependent urea permeability of the terminal inner medullary collecting ducts; UT2-short is located along a short portion of the thin descending limbs of Henle's loops; UT11 is expressed along the descending vasa recta. These three transporters are involved in the accumulation of urea in the medulla, participating to the corticopapillary osmotic gradient required for urine concentration in the presence of antidiuretic hormone. UT2-long enables diffusion of urea in the inner medulla, and UT2-short and UT11 enable the recycling of this urea by counter-exchange. These transporters could also be involved in nitrogen balance by modulation of their expression according to the need for urea excretion (protein-rich diet), or for nitrogen conservation (protein-poor diet). Several other urea transporters, including active transporters responsible for urea secretion or reabsorption, remain to be cloned and characterized.