Modulation of water transport in human red cells: effect of urea

Urea and urine concentrating ability: new insights from studies in mice

Urea is the most abundant solute in the urine in humans (on a Western-type diet) and laboratory rodents. It is far more concentrated in the urine than in plasma and extracellular fluids. This concentration depends on the accumulation of urea in the renal medulla, permitted by an intrarenal recycling of urea among collecting ducts, vasa recta and thin descending limbs, all equipped with specialized, facilitated urea transporters (UTs) (UT-A1 and 3, UT-B, and UT-A2, respectively). UT-B null mice have been recently generated by targeted gene deletion. This review describes 1) the renal handling of urea by the mammalian kidney; 2) the consequences of UT-B deletion on urinary concentrating ability; and 3) species differences among mice, rats, and humans related to their very different body size and metabolic rate, leading to considerably larger needs to excrete and to concentrate urea in smaller species (urea excretion per unit body weight in mice is 5 times that in rats and 23 times that in humans). UT-B null mice have a normal glomerular filtration rate but moderately reduced urea clearance. They exhibit a 30% reduction in urine concentrating ability with a more severe defect in the capacity to concentrate urea (50%) than other solutes, despite a twofold enhanced expression of UT-A2. The urea content of the medulla is reduced by half, whereas that of chloride is almost normal. When given an acute urea load, UT-B null mice are unable to raise their urinary osmolality, urine urea concentration (Uurea), and the concentration of non-urea solutes, as do wild-type mice. When fed diets with progressively increasing protein content (10, 20, and 40%), they cannot prevent a much larger increase in plasma urea than wild-type mice because they cannot raise Uurea. In both wild-type and UT-B null mice, urea clearance was higher than creatinine clearance, suggesting the possibility that urea could be secreted in the mouse kidney, thus allowing more efficient excretion of the disproportionately high urea load. On the whole, studies in UT-B null mice suggest that recycling of urea by countercurrent exchange in medullary vessels plays a more crucial role in the overall capacity to concentrate urine than its recycling in the loops of Henle.

Suppression of red cell diffusional water permeability by lipophilic solutes

The inhibitory effect of a series of neutral lipophilic solutes (methanol, ethanol, n-propanol, iso-propanol, n-butanol, iso-amylalcohol, n-hexanol, diethylether, nitrobenzene, and pyridine) on the diffusional water permeability (Pd, tot) of bovine erythrocyte membrane at 25 degrees C was studied in comparison to that of p-chloromercuri benzoate (pCMB). Permeability data were obtained by measuring the transmembrane diffusional water exchange time tau(exch) using an 1H-T2 NMR technique. Maximal inhibition by approximately 50% of Pd, tot was produced by 2 mM pCMB which completely blocked the membrane water channels in 20 min, hence suggesting the channel-to-lipid diffusional water permeability ratio of about 1:1. Furthermore, the maximal inhibitory effect of pCMB in combination with the lipophilic solutes was lower than that of pCMB alone. As pCMB does not interfere with the lipid bilayer, and provided that it blocks the water channels in solute presence as well, this confirms that the solutes induce an increase in the lipid-mediated background water permeability contribution (Pd, lipid) by the formation of aqueous leaks in the membrane hydrophobic barrier. However, faster but less efficient in permeability inhibition than pCMB (either alone or combined with solutes) were the lipophilic solutes alone. Taken together, the results indicate that the lipophilic solutes suppress the membrane total permeability Pd, tot by two opposing effects: a reduction of its channel-mediated part (Pd, channel) to the extent exceeding that of a simultaneous Pd, lipid increase. The inhibitory potency of the solutes tested appears to be correlated with their solubility in the membrane medium.

Osmometric and permeability characteristics of human placental/umbilical cord blood CD34+ cells and their application to cryopreservation

The transplantation of placental/cord blood-derived HPC (e.g., CD34+ cells) has become a useful treatment for a broad spectrum of malignant and nonmalignant diseases. The ability to cryopreserve this cell type with high efficiency adds considerable flexibility to cord blood transplantation. The purpose of this study was to develop an understanding of the fundamental cryobiologic factors of these cells, including the osmotic/permeability characteristics, and to use a theoretical approach to optimize freezing procedures. To that end, biophysical parameters, including the osmotically inactive cell volume (Vb), hydraulic conductivity (Lp), and cryoprotectant permeability coefficient (P(CPA)) for DMSO and propylene glycol were measured using a modified Coulter Counter (Coulter Electronics, Inc., Hialeah, FL) at 22 degrees C. In addition, the osmotic tolerance of PCB CD34+ cells was assessed using a colony-forming assay. These experimentally determined parameters were used in a mathematical model to predict optimal cryoprotectant addition and removal procedures. The results demonstrate a Vb of 0.32 x V(iso), an average Lp of 0.17 +/- 0.03 (microm/min/atm +/- SD), and a PCPA of 0.94 +/- 0.004 or 1.0 +/- 0.004 cm/min (x10(-3)) for DMSO or propylene glycol, respectively. No significant difference was determined between the two cryoprotectants used. The osmotic tolerance limits were determined to be 200 and 600 mOsm/kg (1.29 and 0.62 x V(iso), respectively). These results indicate potential benefits of modifications to the widely used method of Rubinstein et al. Proc Natl Acad Sci USA 92:10119-10122, 1995) for cord blood CD34+ cell cryopreservation. As opposed to Rubinstein's method in which DMSO is added to cooled cell suspensions over a 15-min interval, our data indicate that better results may be obtained by introducing and removing the cryoprotectant at ambient temperature over 5 min both to increase viability by avoiding unnecessary risks from osmotic shock and to simplify the protocol. In addition, substitution of propylene glycol for DMSO may be of benefit during the actual freezing and thawing process.

Membrane permeability modeling: Kedem-Katchalsky vs a two-parameter formalism

The analysis of experiments for the purpose of determining cell membrane permeability parameters is often done using the Kedem-Katchalsky (KK) formalism (1958). In this formalism, three parameters, the hydraulic conductivity (Lp), the solute permeability (Ps), and a reflection coefficient (final sigma), are used to characterize the membrane. Sigma was introduced to characterize flux interactions when water and solute (cryoprotectant) cross the membrane through a common channel. However, the recent discovery and characterization of water channels (aquaporins) in biological membranes reveals that aquaporins are highly selective for water and do not typically cotransport cryoprotectants. In this circumstance, sigma is a superfluous parameter, as pointed out by Kedem and Katchalsky. When sigma is unneeded, a two-parameter model (2P) utilizing only Lp and Ps is sufficient, simpler to implement, and less prone to spurious results. In this paper we demonstrate that the 2P and KK formalism yield essentially the same result (Lp and Ps) when cotransporting channels are absent. This demonstration is accomplished using simulation techniques to compare the transport response of a model cell using a KK or 2P formalism. Sigma is often misunderstood, even when its use is appropriate. It is discussed extensively here and several simulations are used to illustrate and clarify its meaning. We also discuss the phenomenological nature of transport parameters in many experiments, especially when both bilayer and channel transport are present.

Chill sensitivity and cryoprotectant permeability of dechorionated zebrafish embryos, Brachydanio rerio

The zebrafish (Brachydanio rerio) was used as a model for basic studies of the chilling sensitivity, permeability and toxicity of cryoprotectants. In both intact and dechorionated embryos, early-stage embryos (1.25, 1.5, 1.75, and 2 h) were more susceptible (P < 0.05) to chilling injury at 0 degrees C than late-stage embryos (50, 75, and 100% epiboly and three-somite stage). Moreover, enzymatic removal of the chorion did not alter (P > 0.05) this pattern of sensitivity to chilling. Eight-hour zebrafish embryos tolerated short-term exposures to temperatures ranging from 4 to 23 degrees C for 3.5 h with no detrimental developmental effects. The permeability of dechorionated embryos to cryoprotectants was examined by measuring the kinetics of volumetric change at various developmental stages (16 cells to six somites or ca. 1.25 to 14 h postfertilization) at 28.5 degrees C. The dechorionated zebrafish embryo is composed of two complex cellular compartments (i.e., a large yolk and the developing blastoderm). From 40 to 100% epiboly, the volumes of yolk and blastoderm remained constant, ca. 82 and 18%, respectively. However, these volumes changed rapidly after epiboly. For example, at the six-somite stage, the yolk composed 61% of the total volume, whereas the blastoderm composed 39%. When three- and six-somite embryos were placed in 1.5 and 2.0 M cryoprotectants (dimethyl sulfoxide and propylene glycol), osmometric measurement of volume changes indicated no permeation of the cryoprotectants. However, some permeation was observed for six-somite embryos immersed in a 2.0 M methanol solution, but not for 3-somite embryos. For up to 30 min at room temperature, these cryoprotectant solutions were toxic to zebrafish embryos; however, 1.5 M glycerol and ethylene glycol solutions were. We conclude that the complex nature of the zebrafish embryo reduces the effectiveness and predictive value of light microscopical measurements for cryoprotectant permeability studies.

Ethanol- and acetonitrile-induced inhibition of water diffusional permeability across bovine red blood cell membrane

The effect of 0-3% (v/v) ethanol and acetonitrile on water diffusional permeability of bovine and chicken red blood cells (RBCs) was studied using a pulse 1H-T2 NMR technique. Transmembrane water diffusional exchange times, tau exch, of 9.2 +/- 0.46 ms and 18.3 +/- 1.0 ms were determined for bovine and chicken erythrocytes at 27.5 degrees C, respectively. Arrhenius activation energies Ea of water diffusion were 20.4 and 35.8 kJ mol-1. Ethanol, and acetonitrile being 2-fold more effective, markedly increased both tau exch and Ea in bovine RBC as compared to the well-known mercurial inhibitor of water channels, p-chloromercuribenzene sulfonate. Chicken RBCs that have no protein water channels, were found to be completely insensitive for either agent. It was suggested that ethanol and acetonitrile partitioning into the lipid phase of bovine RBC membrane affects the permeability of CHIP28 water channel but not the lipid confined water diffusion. The results suggest that the inhibition of transmembrane movement of water via CHIP28 channels might be involved in the anti-hemolytic action of anaesthetics such as ethanol.

Cloning and characterization of the vasopressin-regulated urea transporter

Urea is the principal end product of nitrogen metabolism in mammals. Movement of urea across cell membranes was originally thought to occur by lipid-phase permeation, but recent studies have revealed the existence of specialized transporters with a low affinity for urea (Km > 200 mM)2. Here we report the isolation of a complementary DNA from rabbit renal medulla that encodes a 397-amino-acid membrane glycoprotein, UT2, with the functional characteristics of the vasopressin-sensitive urea transporter previously described in in vitro-perfused inner medullary collecting ducts. UT2 is not homologous to any known protein and displays a unique pattern of hydrophobicity. Because of the central role of this transporter in fluid balance and nitrogen metabolism, the study of this protein will provide important insights into the urinary concentrating mechanism and nitrogen balance.

Transport of drugs through human erythrocyte membranes: pH dependence of drug transport through labeled human erythrocytes in the presence of band 3 protein inhibitor

To reveal the role of the band 3 anion transport protein of the erythrocyte membrane in drug transport through the membrane, the possible effects of inhibitors of anion transport on the permeability of some anionic drugs were examined. The amounts of these drugs that permeated varied markedly with the pH of the outer medium around human erythrocytes that contained the band 3 protein inhibitor 4,4'-diisothiocyanostilbene-2,2'-disulfonate (DIDS). In the pH range 7.0-8.0, the membrane permeability of drugs through DIDS-treated erythrocytes was affected by a slight pH change (0.2), whereas that through the intact erythrocytes was not pH dependent. These results suggest that the band 3 protein acts not only as a channel for the transport of anions or some anionic drugs but also as protection for the transport system from changes in the pH of the outer medium.

Transport parameters in the human red cell membrane: solute-membrane interactions of hydrophilic alcohols and their effect on permeation

A systematic study has been made of the three coefficients that describe the human red cell membrane transport of a series of short straight-chain hydrophilic alcohols: the permeability coefficient, omega i, the reflection coefficient, sigma i, and the hydraulic conductivity, Lp. Ethylene glycol transport is saturable with Km = 220 +/- 50 mM; there is a second, low-affinity, ethylene glycol site which inhibits water transport (K = 570 +/- 140 mM, max. inhib. = 90 +/- 10%). sigma eth gly = 0.71 +/- 0.04 which is significantly less than 1 (n = 6, P less than 0.001), as are sigma i for six other alcohols (n = 23), thus providing strong thermodynamic evidence that water and these alcohols cross the red cell membrane, at least in part, in an aqueous channel. The solute/membrane frictional coefficient, fsm, for all seven alcohols has been determined and found to decrease monotonically as membrane permeability increases. The red cell membrane has been perturbed by treatments with phenylglyoxal and BS3 (bis(succinimidyl suberate]; these treatments are accompanied by correlated modulation of both ethylene glycol and urea permeability. In one set of experiments in control cells, urea permeability is correlated with water permeability; and, in another set, ethylene glycol permeability is correlated with water permeability. All of these observations support the proposition that the urea class of solutes, the ethylene glycol class of solutes and water all cross the membrane through the same aqueous pore. A schematic model of the red cell pore, consistent with the experimental observations, is presented.