Water transport in red blood cell membranes

Water transport in human red cells: effects of 'non-inhibitory' sulfhydryl reagents

The water diffusional permeability of human red blood cells following exposure to various sulfhydryl group (SH) reagents have been studied using a nuclear magnetic resonance technique. Exposure of red blood cells up to 12 mM N-ethylmaleimide (NEM) or 10 mM 5,5'-dithio-bis(2-nitrobenzoic acid) (DTNE) alone does not affect water diffusion. In contrast, when DTNB treatment follows a preincubation of the cells with NEM, a small (18% at 37 degrees C) but significant inhibition of water permeability occurs. The NEM and DTNB treatment of the cells caused no change of the cell shape and volume or of the cell water volume. Consequently, the inhibition observed after NEM and DTNB treatment has a real significance.

The effects of the extracellular manganese concentration and variation of the interpulse delay time in the CPMG sequence on water exchange time across erythrocyte membranes

There has been broad disagreement in the literature regarding the dependence of water exchange times (Te) across erythrocyte membranes studied by the 1H-NMR Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence on extracellular Mn2+ concentration. While some workers saw no change in Te with Mn2+, others reported a 35-50% decrease in Te with this extracellular paramagnetic relaxation agent. We present 1H-NMR evidence that a 30-50% change in Te can be produced by interdependence of the interpulse delay time of the CPMG pulse sequence and the external Mn2+ concentration. Such a large dependency is interpreted in terms of the diffusional effect as a major source. However, it is shown experimentally that if a large number of refocusing pi pulses are used, the observed transverse relaxation times are unaffected by Mn2+. Under these conditions excellent agreement of Te obtained in our study (13.0 +/- 0.64 ms (N = 36) at 21 degrees C) and that of 12.8 +/- 3.6 ms at 20-23 degrees C reported by the radiotracer method was found. Our findings suggest new and important implications for evaluating the previous reports of the 1H-NMR CPMG method concerning the [Mn2+] effect in the decrease of Te, and provide conditions where studies of water transport across erythrocyte membranes using this magnetic resonance method can be used with confidence.

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.

Water transport across red-cell membranes following reductive methylation of the major transmembrane protein, Band 3: implications to increased divalent cation membrane permeability

1H-T-NMR methods in conjunction with normally membrane-impermeable Mn2+ were used to study the effect of reductive methylation of specific lysine residues of Band 3, the major transmembrane protein, on water permeability. At 21 degrees C, the water apparent transverse relaxation time (T2) was decreased by nearly 16% (p less than .00001) for cells with modified Band 3. Atomic absorption measurements of control and methylated cells showed an increased level of Mn2+ in the erythrocyte cytosol following methylation. This increased level of this paramagnetic relaxation agent is sufficient to relax interior water protein to the values obtained. Thus, following specific methylation of band 3, increased membrane permeability to divalent cations is observed. The results are discussed with reference to possible conformation changes of Band 3 following methylation, and the findings are interpreted be mean that the conformation of Band 3 has influence on cation permeability to erythrocyte membranes.

The basal permeability to water of human red blood cells evaluated by a nuclear magnetic resonance technique

The characteristics of water diffusional permeability (P) of human red blood cells were studied on isolated erythrocytes by a doping nuclear magnetic resonance technique. In order to estimate the basal permeability the maximal inhibition of water diffusion was induced by exposure of red blood cells to p-chloromercuribenzene sulfonate (PCMBS) under various conditions (concentration, duration, temperature). The lowest values of P were around 0.7 X 10(-3) cm s-1 at 10 degrees C, 1.2 X 10(-3) cm s-1 at 15 degrees C, 1.4 X 10(-3) cm s-1 at 20 degrees C, 1.8 X 10(-3) cm s-1 at 25 degrees C, 2.1 X 10(-3) cm s-1 at 30 degrees C and 3.5 X 10(-3) cm s-1 at 37 degrees C. The mean value of the activation energy of water diffusion (Ea,d) was 25 kJ/mol for control and 43.7 kJ/mol for PCMBS--inhibited erythrocytes. The values of P and Ea,d obtained after induction of maximal inhibition of water diffusion by PCMBS can be taken as references for the basal permeability to water of the human red blood cell membrane.