Erythrocyte permeability to urea and water: comparative study in rodents, ruminants, carnivores, humans, and birds
Mammalian erythrocytes exhibit high urea permeability (Purea) due to UT-B expression in their cytoplasmic membrane. This high Purea allows fast equilibration of urea in erythrocytes during their transit in the hyperosmotic renal medulla. It also allows more urea (in addition to that in plasma) to participate in counter-current exchange between ascending and descending vasa recta, thus improving the trapping of urea in the medulla and improving urine concentrating ability. To determine if Purea in erythrocytes is related to diet and urine concentrating ability, we measured Purea in erythrocytes from 11 different mammals and 5 birds using stopped-flow light scattering. Carnivores (dog, fox, cat) exhibited high Purea (in ×10−5 cm/s, 5.3 ± 0.6, 3.8 ± 0.5 and 2.8 ± 0.7, respectively). In contrast, herbivores (cow, donkey, sheep) showed much lower Purea (0.8 ± 0.2, 0.7 ± 0.2, 1.0 ± 0.1, respectively). Erythrocyte Purea in human (1.1 ± 0.2), and pig (1.5 ± 0.1), the two omnivores, was intermediate. Rodents and lagomorphs (mouse, rat, rabbit) had Purea intermediate between carnivores and omnivores (3.3 ± 0.4, 2.5 ± 0.3 and 2.4 ± 0.3, respectively). Birds that do not excrete urea and do not express UT-B in their erythrocytes had very low values (<0.1 × 10−5 cm/s). In contrast to Purea, water permeability, measured simultaneously, was relatively similar in all mammals. The species differences in erythrocytes Purea most probably reflect adaptation to the different types of diet and resulting different needs for concentrating urea in the urine.