Urea and ethylene glycol-facilitated transport systems in the human red cell membrane. Saturation, competition, and asymmetry

The equilibrium exchange of [14C]urea and ethylene glycol was measured using a new type of fast flow system. Approximately equal volumes of saline and air were mixed to form a segmented fluid stream into which 14C-loaded red cells are injected. The stream flows through three filter chambers which allow sampling of the 14C in the extracellular fluid at three time points. The chambers are designed so that they do not disrupt the segmented bubble pattern. The alternating air and saline segments prevent laminar dispersion in the flowing stream and ensure good mixing at the injection and sampling sites. The equilibrium exchange of both urea and ethylene glycol showed saturation kinetics. The maximum permeability (Po) measured in the limit of zero solute concentration is 1.6 X 10(-3) cm/s for urea and 4.8 X 10(-4) cm/s for ethylene glycol (T = 23 degrees C). The apparent dissociation constant (Km) was 218 mM for urea and 175 mM for ethylene glycol. The Po for thiourea is 2.3 X 10(-6) cm/s and the Km is 19 mM. Urea and thiourea inhibit the transport of each other and the inhibition constant (KI) is approximately equal to the Km for both compounds. 53 other analogues of urea were screened for their inhibition of urea or thiourea transport. Several analogues [e.g., 1-(3,4-dichloro-phenyl)-2-thiourea] had a KI in the range of 0.03 mM. The affinity of the inhibitor increased as it was made more hydrophobic. The urea analogues did not significantly inhibit the ethylene glycol or osmotic permeability. Glycerol inhibited ethylene glycol permeability with a KI of 1,200 mM.     

Selectivity of cations and nonelectrolytes for acetylcholine-activated channels in cultured muscle cells

The selectivity of acetylcholine (A-Ch)-activated channels for alkali cations, organic cations, and nonelectrolytes in cultured muscle cells has been studied. To test the effect of size, charge, and hydrogen- binding capacity of permeant molecules on their permeability, we have obtained the selectivity sequences of alkali cations, compared the permeability of pairs of permeant molecules with similar size and shape but differing in charge, and studied the permeability of amines of different hydrogen bonding capacity. ACh-activated channels transport alkali cations of small hydration radii and high mobility. The molecules with positive charge and (or) a hydrogen-bond donating moiety are more permeable than the ones without. On the other hand, several nonelectrolytes, i.e., ethylene glycol, formamide, and urea, do have a small, but measurable, permeability through the channels. These results are consistent with a model that ACh-activated channel is a water- filled pore containing dipoles or hydrogen bond accepting groups and a negative charged site with a pK of 4.8.     

Response of the Frog Skin to Steady-State Voltage Clamping : I. The shunt pathway

Properties of the shunt pathway (a pathway in parallel to the Na transport system) in frog skin have been examined. The permeability of this shunt to urea increases markedly when the skin is depolarized to -100 mv (inside negative) but hyperpolarization to +100 mv produces no change in urea permeability compared to short-circuit conditions. The permeability increase at depolarizing potentials is dependent on the external solute concentration and is considerably reduced by the presence of external Ca. Neither urea permeability nor its response to changes in potential difference are affected by complete inhibition of Na transport by ouabain. In ouabain-poisoned skins, movements of Na, K, Cl, and mannitol through the shunt change in parallel with urea movements. Ion fluxes under these conditions and their response to potential can be described by the constant field equation. The selectivity of the shunt is in the order Cl > urea > K > Na > mannitol and this order does not appear to be affected by the absolute magnitude of the shunt permeability. Arguments are presented suggesting that the pathway is mainly between cells and that its permeability may be affected by cell swelling.     

Reflection coefficient and permeability of urea and ethylene glycol in the human red cell membrane

The reflection coefficient (sigma) and permeability (P) of urea and ethylene glycol were determined by fitting the equations of Kedem and Katchalsky (1958) to the change in light scattering produced by adding a permeable solute to a red cell suspension. The measurements incorporated three important modifications: (a) the injection artifact was eliminated by using echinocyte cells; (b) the use of an additional adjustable parameter (Km), the effective dissociation constant at the inner side of the membrane; (c) the light scattering is not directly proportional to cell volume (as is usually assumed) because refractive index and scattering properties of the cell depend on the intracellular permeable solute concentration. This necessitates calibrating for known changes in refractive index (by the addition of dextran) and cell volume (by varying the NaCl concentration). The best fit was for sigma = 0.95, Po = 8.3 X 10(-4) cm/s, and Km = 100 mM for urea and sigma = 1.0, Po = 3.9 X 10(-4) cm/s, and Km = 30 mM for ethylene glycol. The effects of the inhibitors copper, phloretin, p- chloromercuriphenylsulfonate, and 5,5'-dithiobis (2-nitro) benzoic acid on the urea, ethylene glycol, and water permeability were determined. The results suggest that there are three separate, independent transport systems: one for water, one for urea and related compounds, and one for ethylene glycol and glycerol.     

Actions of external hypertonic urea, ADH, and theophylline on transcellular and extracellular solute permeabilities in frog skin

Increases in transepithelial solute permeability were elicited in the frog skin with external hypertonic urea, theophylline, and vasopressin (ADH). In external hypertonic urea, which is known to increase the permeability of the extracellular (paracellular) pathway, the unidirectional transepithelial fluxes of Na (passive), K, Cl, and urea increased substantially while preserving a linear relationship to each other. The same linear relationship was also observed for the passive Na and urea fluxes in regular Ringer and under stimulation with ADH or 10 mM theophylline, indicating that their permeation pathway was extracellular. A linear relationship between Cl and urea fluxes could be demonstrated if the skins were separated according to their open circuit potentials; parallel lines were obtained with increasing intercepts on the Cl axis as the open circuit potential decreased. The slopes of the Cl vs. urea lines were not different from that obtained in external hypertonic urea, indicating that this relationship described the extracellular movement of Cl. The intercept on the ordinate was interpreted as the contribution from the transcellular Cl movement. In the presence of 0.5 mM theophylline or 10 mU/ml of ADH, mainly the transcellular movement of Cl increased, whereas 10 mM theophylline caused increases in both transcellular and extracellular Cl fluxes. These and other data were interpreted in terms of a possible intracellular control of the theophylline-induced increase in extracellular fluxes. The changes in passive solute permeability were shown to be independent of active transport. The responses of the active transport system, the transcellular and paracellular pathways to theophylline and ADH could be explained in terms of the different resulting concentrations of cyclic 3'-5'-AMP produced by each of these substances in the tissue.     

Decrease of transport of some polyols in sickle cells

This paper reports the results of kinetic studies on the inward net-flux of small non-electrolytes (ethylene glycol, glycerol and erythritol) in sickle cells as compared to normal erythrocytes. Net transport rates were evaluated by turbidimetric measurements for ethylene glycol and glycerol and by hematocrit monitoring for erythritol. A 2-fold and 4-fold reduction in the permeability coefficient for ethylene glycol and glycerol, respectively, were found in sickle cells as compared to normal erythrocytes. In contrast, no significant changes in erythritol transport kinetics were observed. The dependence of glycerol permeability on temperature, pH and oxygenation is the same in both types of cells. A significant correlation was observed between glycerol permeability and cell density only for sickle cells. The results indicate that irreversible modifications of membrane proteins, responsible for the glycerol and ethylene glycol transport, do occur in sickle cells.     

Action of forskolin on non-electrolyte permeability across the frog skin as compared to that of vasopressin and isoprenaline

Forskolin, a natural diterpene activating the adenyl cyclase in a receptor-independent manner, increases symmetrically both transepithelial fluxes of urea and erithrytol through the frog skin. The effect is dose-dependent, being 5 X 10(-6) M the dose necessary to obtain the maximal action. Forskolin-induced permeabilization is inversely proportional to the molecular weight of water soluble molecules (urea greater than erythritol greater than mannitol); also the permeability of a mainly lipid soluble molecule, i.e. antipyrine, is slightly increased by the diterpene. The permeability pattern is more similar to that induced by isoprenaline as compared to that elicited by vasopressin. Differently from what occurs in other tissues, small doses of forskolin (10(-8) M) are unable to potentiate the actions of vasopressin and isoprenaline on urea permeability across the frog skin. Moreover, the maximal action of forskolin is not additive with the maximal ones of isoprenaline and vasopressin.     

The effect of tanning agents on the permeability of the toad bladder to water and solutes

Summary Previous studies with phloretin have shown that the movement of urea and other solutes across the toad bladder can be inhitited with no effect on osmotic water flow, active sodium transport, or the movement of ethanol and ethylene glycol. These findings have suggested that a vasopressin-sensitive carrier is involved in the transport of solutes such as urea across the luminal membrane of the epithelial cell. The present paper describes the effect of two agents other than phloretin: tannic acid and chromate, on water and solute movement across the bladder. The pattern of action of these two agents resembles that of phloretin, and supports our earlier findings of the independence of solute and water movement. The effect of chromate on urea movement is seen only in the presence of vasopressin, and only if chromate is added prior to vasopressin. Chromate also proves to be an irreversible inhibitor of urea movement. The implications of these findings are discussed. In view of the known interactions of both agents with proteins, it is suggested that carrier-mediated transport of urea proceeds across a protein component of the membrane.Presented in part at the 57th annual meeting, Federation of American Societies for Experimental Biology, Atlantic City, April 1973.     

Effect of metabolic inhibitors on vasopressin-stimulated transport systems in the toad bladder

Vasopressin increases the permeability of receptor cells to water and, in tissues such as toad bladder, to solutes such as urea. While cyclic AMP appears to play a major role in mediating the effects of vasopressin, there is evidence that activation of the water permeability system and the urea permeability system involves separate pathways. In the present study, we have shown that inhibitors of oxidative metabolism (rotenone, dinitrophenol, and methylene blue) selectively inhibit either vasopressin-stimulated water flow or vasopressin-stimulated urea transport. There was no inhibition, however, when exogenous cyclic AMP was substituted for vasopressin, and little to no inhibition when the potent analogue 8-bromoadenosine 3′,5′-cyclic monophosphate (8-Br-cAMP) was employed. Rotenone had no effect on adenylate cyclase activity or cyclic AMP levels within the cell; dinitrophenol decreased adenylate cyclase activity minimally. Additional studies with vinblastine and nocodazole, inhibitors of microtubule assembly, demonstrated an inhibition of vasopressin and cyclic AMP-stimulated water flow but showed no effect on urea transport. We would conclude that water and urea transport, as examples of hormone-stimulated processes, have different links to cell metabolism, and that in addition to cyclic AMP, a non-nucleotide pathway may be involved in the action of vasopressin.     

Permeability properties of the Bufo bufo bladder as affected by isoprenaline and vasopressin

Isoprenaline, a beta adrenergic agonist, strongly increases both transepithelial fluxes across the urinary bladder of Bufo bufo; this effect is dose dependent, 10(-6)M being necessary for the maximal action. This effect is less selective than that of vasopressin: the ratio J urea/J thiourea is 3.8 under isoprenaline and 30.4 under vasopressin treatment. Both hormones differently affect the permeability of a mainly liposoluble molecule, i.e. antipyrine: vasopressin increases antipyrine permeability, while isoprenaline decreases it. Moreover diethylpyrocarbonate treatment of the luminal membrane strongly inhibits vasopressin effect on urea permeability leaving unmodified that of isoprenaline. However, the actions of both hormones are not additive. These results allows to assume that the tissue has a feedback mechanism which inhibits other hormonal action while the bladder is stimulated by a particular hormone.     

Selective fixation with glutaraldehyde of ADH-induced urea permeability sites in toad bladder

Toad bladders exposed to vasopressin (ADH) and then fixed on the mucosal surface with 1% glutaraldehyde were highly permeable to water and to urea compared to control bladders fixed in the absence of hormone. When identical conditions of fixation were were used, but the concentration of glutaraldehyde was decreased to 0.25%, the ADH-induced increase in membrane permeability to urea was preserved whereas water permeability was not. About 74% of the hormone-induced urea permeability sites were preserved by glutaraldehyde and were stable to changes in temperature as suggested by a constant value for the activation energy of urea movement of 5.4 kcal/mole (4-33 degrees C). In other studies bladders were exposed at low temperatures to 0.17% glutaraldehyde applied either to the serosal or the mucosal surface. The ADH-induced increase in membrane permeability to urea, bulk water, and tritiated water was well preserved with serosal fixation, but not with mucosal fixation. The observation that the urea pathway can be selectively preserved with 0.25% glutaraldehyde applied to the mucosa indicates that this structure is more accessible and (or) more sensitive to low-dose glutaraldehyde than is the ADH-induced water pathway. The observation that glutaraldehyde is more effective in stabilizing the ADH-induced urea channels from the serosal than from the mucosal surface indicates that these channels are not fixed at the extracellular surface of the apical plasma membrane. It appears, rather, that glutaraldehyde exerts its effects from an intracellular position, where it cross-links components of the urea channels at the cytoplasmic surface of the apical membrane and (or) inactivates the intracellular machinery responsible for the removal or dispersal of the ADH-induced urea permeability sites.     

A study on cryoprotectant solution suitable for vitrification of rat two-cell stage embryos

The present study was performed to develop a suitable cryoprotectant solution for cryopreservation of rat two-cell stage embryos. First, we examined the cell permeability of several cryoprotectants; propylene glycol had the fastest permeability compared to dimethyl sulfoxide, ethylene glycol, and glycerol. Embryos were then exposed to a solution containing propylene glycol to evaluate its effects on fetal development. As the development was similar to that of fresh embryos, P10 (10% v/v propylene glycol in PB1) was used as a pretreatment solution. Next, the effects of the vitrification solution components (sucrose, propylene glycol, ethylene glycol, and Percoll) were examined by observing the vitrification status; 10% v/v propylene glycol, 30% v/v ethylene glycol, 0.3 mol sucrose, and 20% v/v Percoll in PB1 (PEPeS) was the minimum essential concentration for effective vitrification without the formation of ice crystals or freeze fractures.     

Effect of cryoprotectors on the structural state of liposomes cooled to minus 25 degrees C

Cryoprotectors (propylene glycol), ethylene glycol, polyethylene glycol-1500 and dimethyl sulphoxide) are studied for their effect on permeability of liposomes for incorporated molecules of 5,5-dithiobis-2-nitrobenzoic acid (DTNB) under cooling within a temperature range from 0 degree C to -25 degrees C. A similarity is found in the way of ethylene glycol and propylene glycol, dimethyl sulphoxide and polyethylene glycol-1500 effect on the liposome permeability way. Cooling in the presence of ethylene glycol and propylene glycol causes changes in liposome permeability with a local maximum at -18 degrees C. In the medium with 2M NaCl and ethylene glycol, liposomes were resistant to cooling. Dimethyl sulphoxide and polyethylene glycol-1500 induced a two-phase kinetics of changes in liposome permeability, the first phase being within the 0 = -9 degrees C and the second--within -9--25 degrees C temperature ranges. The found differences are supposed to be associated with the effect of the cryoprotective compounds on the lipid crystallization in a lower-temperatures range.     

Action of photoreactive analogs of vasopressin in toad bladder

A series of analogs of vasopressin with photoreactive groups in positions 1, 2, 3, 4, 8 or 9 of the nonapeptide sequence have been studied for their effects on water and urea permeability of the isolated toad urinary bladder. Compounds with photoreactive groups in positions 3 or 8 bound covalently to receptors as judged by a persistent increase in water and urea permeability following UV irradiation, prevention of photolabeling by incubation in the presence of vasopressin, and a persistent increase in membrane-bound adenylate cyclase activity. Some analogs were inactive in the dark, but became active and bound covalently to receptors during photolysis. Other analogs were inhibitors or agonists in the dark, but did not bind to receptors following UV irradiation. Time course studies with photolabelled bladders showed a stable urea flux for 4 hr in the absence of osmotic water flow. However, in the presence of water flow urea flux was initially enhanced (solvent drag effect) and later retarded (diminished urea permeability). Binding of photoaffinity analogs to receptors was not diminished with acidification of the serosal bathing medium, lowering of the bath temperature from 21 degrees C to 4 degrees C or with addition of prostaglandin E1. However, the capacity of photoreactive analogs to effect an increase in transmural water flux, once the analog was bound covalently to receptors, was markedly diminished under these conditions.     

The mode of action of vasopressin: membrane microstructure and biological transport

Vasopressin affects a variety of cell systems. This review is focused on permeability changes induced by vasopressin in tight epithelia such as the collecting duct of the mammalian kidney and the skin and the bladder of anurans. These vasopressin effects are discussed with reference to current concepts and models of the microstructure of the plasma membrane. The transport of three major chemical species--Na, urea and water--is analyzed. In each instance, the hormone appears to activate selective membrane pathways situated at the rat-limiting barrier of the epithelium, i.e., the apical membrane. Available data suggest that two intra-cellular messengers -- cAMP and calcium -- plan a key role in the coupling between stimulus (receptor occupancy) and biological effect (permeability change). The enhancement of Na transport (natriferic effect) depends on the opening and/or the insertion of Na channels, the biophysical and biochemical characteristics of which have been investigated by fluctuation analysis and by means of several chemical blockers of Na transport, particularly the amiloride molecule and its congeners. Likewise, the finding of inhibitors and activators of urea transport, which do not cause any appreciable change in Na or water permeability, led to the notion of selective urea channels or pores. Finally, the enhancement of water transport (hydrosmotic effect) possibly results from the insertion in the apical membrane of water channels already present in vesicular cytoplasmic structures. The restructuring of the apical membrane underlying the transition from a low to a higher state of water permeability is very likely related to the appearance of intramembrane particle aggregates detectable with the freeze-fracture technique in epithelia exposed to vasopressin. The putative water channels (or pores) appear to be so narrow that trans-apical water movement is constrained to single-file diffusion. Recent data also suggest that, in addition to cAMP, microtubules and microfilaments, the calmodulin-Ca complex is a major element in the hydrosmotic effect of vasopressin.     

Transport kinetics and selectivity of HpUreI, the urea channel from Helicobacter pylori

Helicobacter pylori's unique ability to colonize and survive in the acidic environment of the stomach is critically dependent on uptake of urea through the urea channel, HpUreI. Hence, HpUreI may represent a promising target for the development of specific drugs against this human pathogen. To obtain insight into the structure-function relationship of this channel, we developed conditions for the high-yield expression and purification of stable recombinant HpUreI. Detergent-solubilized HpUreI forms a homotrimer, as determined by chemical cross-linking. Urea dissociation kinetics of purified HpUreI were determined by means of the scintillation proximity assay, whereas urea efflux was measured in HpUreI-containing proteoliposomes using stopped-flow spectrometry to determine the kinetics and selectivity of the urea channel. The kinetic analyses revealed that urea conduction in HpUreI is pH-sensitive and saturable with a half-saturation concentration (or K(0.5)) of ~163 mM. The extent of binding of urea by HpUreI was increased at lower pH; however, the apparent affinity of urea binding (~150 mM) was not significantly pH-dependent. The solute selectivity analysis indicated that HpUreI is highly selective for urea and hydroxyurea. Removing either amino group of urea molecules diminishes their permeability through HpUreI. Similar to urea conduction, diffusion of water through HpUreI is pH-dependent with low water permeability at neutral pH.     

Investigation of matrix effects of urine on a molecularly imprinted solid-phase extraction

This study investigates matrix effects on a molecularly imprinted solid-phase extraction (MISPE) method developed for the clean-up of diphenyl phosphate (a hydrolysis product of the commonly used flame retardant and plasticizer, triphenyl phosphate) in urine samples. The influence of potentially interfering compounds that naturally occur in urine was examined with respect to extraction recovery, repeatability and selectivity. The components tested were NaCl, urea, creatinine and hippuric acid. The imprinted polymer was prepared using 2-vinylpyridine as the functional monomer, ethylene glycol dimethacrylate as crosslinker and a structural analogue of the analyte as the template molecule. The recovery of diphenyl phosphate from water standards was over 90% using MISPE, compared to less than 25% using a non-imprinted SPE (NISPE) counterpart. The selectivity of MISPE compared to NISPE was achieved in a wash step with a basic modifier in methanol. The recovery and repeatability of the MISPE method were affected most by NaCl in the tested concentrations, while urea, creatinine and hippuric acid had no significant influence. NaCl most likely weakens the binding during the loading of the sample. This effect could be suppressed by diluting the sample with a citrate buffer at pH 4.0.     

Chronotropic response to hyperosmotic solutions of isolated rat atria

The changes in spontaneous rate of isolated rat atria in response to increased extracellular osmotic pressure were examined using sucrose, urea and several polyhydroxyalcohols (mannitol, glycerol and ethylene glycol) as test solutes. Sucrose, mannitol and urea induced a fall in atrial rate, which was transient with the last compound. On the other hand, media made hyperosomotic by addition of glycerol or ethylene glycol increased the beating frequency. Sucrose effect was not affected by low extracellular calcium, nifedipine or atropine. Glycerol-induced increase in atrial rate was a calcium-dependent mechanism sensitive to nifedipine. Thus, positive chronotropic effect occurs in the rat atria only with certain diffusible solutes which probably promote calcium entry. The response to pure osmotic change, resulting from changes in concentration of ions within the cell as water moves out, is a negative chronotropic effect.     

Variation in the effects of antidiuretic hormone on the isolated skin of the toad, Scaphiopus couchi.

The rate of active sodium transport as measured by short-circuit current across the isolated skin of the toad, Scaphiopus couchi, was elevated following vasopressin (0.2 units/ml) or arginine vasotocin (0.1 units/ml) treatment of skins from active animals at all times of the year tested. Skins from dormant animals showed no such elevation at any time of the year. The rate of active sodium transport was elevated following treatment with dibutyryl cyclic AMP (2.5mM) plus theophylline (10 mM) in all skins tested. The hydraulic conductivity of isolated skins from both active and dormant animals showed no significant change following treatment with vasopressin (0.2 units/ml) or arginine vasotocin (0.1 units/ml except on the first day following emergence from dormancy in the field. A correlation was, therefore, observed between the occurrence of a hydroosmotic response to antidiuretic hormones and the seasonal exposure of S. couchi to standing water. A small but significant elevation of hydraulic conductivity was observed across the skins of dormant toads following treatment with dibutyryl cyclic AMP (2.5 mM) plus theophylline (10 mM) whereas a substantial elevation was observed with the skins of active animals.     

Osmotic tolerance and membrane permeability characteristics of rhesus monkey (Macaca mulatta) spermatozoa

Biophysical characteristics of the plasma membrane, such as osmotic sensitivity and water and cryoprotectant permeability are important determinants of the function of spermatozoa after cryopreservation. A series of experiments was conducted with rhesus macaque spermatozoa at 23 degrees C to determine their: (1) cell volume and osmotically inactive fraction of the cell volume; (2) permeability coefficients for water and the cryoprotectants dimethyl sulfoxide, glycerol, propylene glycol, and ethylene glycol; (3) tolerance to anisosmotic conditions; and (4) motility after a one step addition and removal of the four cryoprotectants. An electronic particle counter and computer aided semen analysis were used to determine the cell volume and permeability coefficients, and motility, respectively. Rhesus spermatozoa isosmotic cell volume was 27.7+/-3.0 microm3 (mean+/-SEM) with an osmotically inactive cell fraction of 51%. Hydraulic conductivity in the presence of dimethyl sulfoxide, glycerol, propylene glycol, and ethylene glycol was 1.09+/-0.30, 0.912+/-0.27, 1.53+/-0.53, and 1.94+/-0.47 microm/min/atm, respectively. Cryoprotectant permeability was 1.39+/-0.31, 2.21+/-0.32, 3.38+/-0.63, and 6.07+/-1.1 (x10(-3)cm/min), respectively. Rhesus sperm tolerated all hyposmotic exposures. However, greater than 70% motility loss was observed after exposure to solutions of 600 mOsm and higher. A one step addition and removal of all four cryoprotectants did not cause significant motility loss. These data suggest that rhesus sperm are tolerant to hyposmotic conditions, and ethylene glycol may be the most appropriate cryoprotectant for rhesus sperm cryopreservation, as it has the highest permeability coefficient of the tested cryoprotectants.