Independence of water and solute pathways in human RBCs.

We have investigated the permeability of the human red blood cell to four di-hydroxy alcohols, 1,2PD (1,2 propanediol), 1,3PD (1.3 propanediol), 1,4BD (1,4 butanediol), and 2,3BD (2,3 butanediol), and to water by using a recently developed ESR stopped-flow method which is free from artifacts found in light scattering methods. Numerical solutions of the Kedem-Katchalsky equations fit to experimental data yielded the following permeability coefficients: P1,2PD = 3.17 x 10(-5) cm sec-1, P1,3PD = 1.75 x 10(-5) cm sec-1, P1,4BD = 2.05 x 10(-5) cm sec-1, P2,3BD = 7.32 x 10(-5) cm sec-1. Reflection coefficients (sigma) were evaluated by comparing data fit with assumed values of sigma = 0.6, 0.8 and 1.0. In all four cases the best fit was obtained with sigma = 1.0. Treatment of cells with PCMBS (para-chloro mercuri-benzene-sulfonate) was followed by a large (> 10-fold) decrease in water permeability with virtually no change in alcohol permeability. We conclude that these alcohols do not permeate the water channels to any significant extent, and discuss some of the problems in light scattering measurements of reflection coefficients that could lead to erroneous values for sigma.

Temperature- and concentration-dependence of urea permeability of human erythrocytes determined by NMR.

The temperature- and concentration-dependence of [13C]urea self-exchange across the human red cell membrane has been determined by NMR measurements of T1 (spin-lattice) relaxation times. T1 for intracellular label is 17 s, which is much longer than the urea exchange time across the cell membrane (about 0.5 s). T1 for urea in extracellular solution is quenched with 17 mM of impermeable Mn2+ in less than 2 ms. Hence the observed T1 (corrected for intracellular decay) is a measure of urea exchange across the cell membrane. The method is tested by showing both PCMBS and increasing concentrations of urea lengthen T1. Urea exchange permeability, defined as Purea = flux/conc, can be described by Purea = Vmax/(K1/2 + conc). Studies of temperature-dependence showed that activation energies were strongly dependent on both temperature and concentration. However, this apparently anomalous behavior was resolved into two well-behaved functions, K1/2 and Vmax, with linear Arrhenius plots and apparent 'activation energies' of 15.5 and 12.4 kcal/mol, respectively. These were used to construct an equation for calculating Purea at any concentration and temperature. Assuming a simple channel model with single binding, K1/2 becomes the dissociation equilibrium constant for the site with delta H degree = 15.5 kcal/mol and delta S degree = 51.8 cal/(mol.deg); dissociation is entropically driven.

The permeability of the human red cell to deuterium oxide (heavy water).

Using a stopped-flow device, the osmotic water permeability of human red cells to D2O and H2O was studied as a function of temperature and under the influence of the sulfhydryl reagent paracholoromercuribenzene sulfonic acid (PCMBS), an inhibitor of water transport. The ratio, permeability (D2O)/permeability (H2O) at each temperature can be predicted simply by assuming that permeability varies inversely with macroscopic viscosity. When water permebility is inhibited with PCMBS, this dependency on viscosity vanishes; the inhibited permeabilities in D2O and H2O are indistinguishable.