Transport characteristics of urea transporter-B

Abstract

UT-B represents the major urea transporter in erythrocytes, in addition to being expressed in kidney descending vasa recta, brain, spleen, ureter, bladder, and testis. Expression of urea transporter UT-B confers high urea permeability to mammalian erythrocytes. Erythrocyte membranes are also permeable to various urea analogues, suggesting common transport pathways for urea and structurally similar solutes. UT-B is highly permeable to urea and the chemical analogues for-mamide, acetamide, methylurea, methylformamide, ammonium carbamate, and acrylamide, each with a P s 5.0 × 10- 6 cm/s at 10 °C. The amides formamide, acetamide, acrylamide, and butyramide effciently diffuse across lipid bilayers. The urea analogues dimethylurea, acryalmide, methylurea, thiourea, and methylformamide inhibit UT-B-mediated urea transport by >60% by a pore-blocking mechanism. UT-B is also a water channel in erythrocytes and has a single-channel water permeability that is similar to aquaporin-1. Whether UT-B is an NH3 channel still needs further study. Urea permeability (P ure a) in erythrocytes differs between different mammals. Carnivores (dog, fox, cat) exhibit high P ur ea. In contrast, herbivores (cow, donkey, sheep) show much lower P ur ea. Erythrocyte P ure a in human and pig (omnivores) was intermediate. Rodents and lagomorphs (mouse, rat, rabbit) have P ure a intermediate between carnivores and omnivores. Birds that do not excrete urea and do not express UT-B in their erythrocytes have very low values. In contrast to Purea, water permeability is relatively similar in all mammals studied. This chapter will provide information about the transporter characteristics of UT-B.