The permeability of red blood cells to chloride, urea and water

This study extends permeability (P) data on chloride, urea and water in red blood cells (RBC), and concludes that the urea transporter (UT-B) does not transport water. P of chick, duck, Amphiuma means, dog and human RBC to (36)Cl(-), (14)C-urea and (3)H2O was determined under self-exchange conditions. At 25°C and pH 7.2-7.5, PCl is 0.94 × 10(-4)-2.15 × 10(-4) cm s(-1) for all RBC species at [Cl]=127-150 mmol l(-1). In chick and duck RBC, P(urea) is 0.84 × 10(-6) and 1.65 × 10(-6) cm s(-1), respectively, at [urea]=1-500 mmol l(-1). In Amphiuma, dog and human RBC, P(urea) is concentration dependent (1-1000 mmol l(-1), Michaelis-Menten-like kinetics; K1/2;=127, 173 and 345 mmol l(-1)), and values at [urea]=1 mmol l(-1) are 29.5 × 10(-6), 467 × 10(-6) and 260 × 10(-6) cm s(-1), respectively. Diffusional water permeability, Pd, was 0.84 × 10(-3) (chick), 5.95 × 10(-3) (duck), 0.39 × 10(-3) (Amphiuma), 3.13 × 10(-3) (dog) and 2.35 × 10(-3) cm s(-1) (human). DIDS, DNDS and phloretin inhibit PCl by >99% in all RBC species. PCMBS, PCMB and phloretin inhibit P(urea) by >99% in Amphiuma, dog and human RBC, but not in chick and duck RBC. PCMBS and PCMB inhibit Pd in duck, dog and human RBC, but not in chick and Amphiuma RBC. Temperature dependence, as measured by apparent activation energy, EA, of PCl is 117.8 (duck), 74.9 (Amphiuma) and 89.6 kJ mol(-1) (dog). The EA of P(urea) is 69.6 (duck) and 53.3 kJ mol(-1) (Amphiuma), and that of Pd is 34.9 (duck) and 32.1 kJ mol(-1) (Amphiuma). The present and previous RBC studies indicate that anion (AE1), urea (UT-B) and water (AQP1) transporters only transport chloride (all species), water (duck, dog, human) and urea (Amphiuma, dog, human), respectively. Water does not share UT-B with urea, and the solute transport is not coupled under physiological conditions.