Water Transport Kinetics Research Articles

Water Distribution in Multilayers of Weak Polyelectrolytes

The water localization in thin polyelectrolyte multilayers assembled from poly(acrylic acid) and poly(allylamine hydrochloride) was investigated with neutron reflectivity in an atmosphere of controlled humidity and with bulk water. Water was found to be distributed asymmetrically within the multilayer and to localize preferentially at the polymer surface. The diffusion of water into the multilayer did not completely penetrate to the substrate, but instead there appeared to be an exclusion zone near the Si substrate. These results help to explain previous observations of anomalous water transport kinetics in weak polyelectrolyte systems.

Isolation and characterization of theXenopusoocyte plasma membrane: a new method for studying activity of water and solute transporters

The intact Xenopus laevis oocyte is a useful model system for studying expressed water and solute transporters but suffers from a number of limitations, most notably large unstirred layers and other intracellular diffusion barriers. To overcome these, we have developed a method for isolating plasma membrane vesicles from oocytes. This approach facilitates more precise control of the intravesicular environment and virtually eliminates the problem of unstirred layers in kinetic experiments. The isolation procedure results in 50.6-fold enrichment of the plasma membrane marker alkaline phosphodiesterase compared with the homogenate. Markers of late endosomes/lysosomes and mitochondria were not enriched, and the endoplasmic reticulum was enriched only modestly. Permeabilities of native plasma membrane to water and urea were 8.1 x 10(-4) and 5.6 x 10(-7) cm/s, respectively, values that are sufficiently low to classify them as barrier membranes. Phospholipid analysis by mass spectrometry showed the membrane, not including cholesterol, to be rich in phosphatidylcholine (35.8 mole percent), sphingomyelin (25.8 mole percent), and phosphatidylinositol (6.8 mole percent). Cholesterol concentration was 20.7 mole percent. Membrane vesicles isolated from oocytes expressing aquaporin-1 exhibited fourfold higher water permeability in stopped-flow experiments. Oocytes expressing mouse urea transporter A3 (UT-A3) exhibited 7.5-fold faster phloretin-inhibitable urea transport compared with water-injected controls. There was no difference in water permeability between these membrane vesicles, suggesting that UT-A3 is not a water carrier. In conclusion, we describe an improved method for the isolation of the oocyte plasma membrane that will allow the study of water and solute transport kinetics as well as substrate selectivity in heterologously expressed proteins.

Ectomycorrhizas increase apoplastic water transport and root hydraulic conductivity in Ulmus americana seedlings

Summary The extent to which water channel transport is responsible for the observed increases in root water flow of ectomycorrhizal plants is reported here. To examine the contribution of water channel transport to root hydraulic conductance, temperatures in the range 4–20°C and mercuric chloride (HgCl2) were used to study the kinetics of water transport in ectomycorrhizal and nonmycorrhizal roots of American elm (Ulmus americana) seedlings. Hydraulic conductance declined with decreasing temperatures in both mycorrhizal and nonmycorrhizal seedlings. However, hydraulic conductance and conductivity were higher in the mycorrhizal than the nonmycorrhizal roots at all temperatures studied. Mercuric chloride had a relatively greater impact on root hydraulic conductance in nonmycorrhizal than mycorrhizal roots and activation energy for root hydraulic conductance was significantly higher in mycorrhizal than nonmycorrhizal plants. The results suggest that ectomycorrhizal hyphae increase hydraulic conductance of roots by decreasing water flow resistance of the apoplast rather than by water channel‐mediated transport. The high rates of hydraulic conductance at low root temperatures might be important to plants growing in cold soils and under other challenging environmental conditions that inhibit metabolism and limit water transport.

Kinetics of moisture transfer during air drying of blanched and/or osmotically dehydrated mango

The effect of previous blanching and/or osmotic dehydration (at atmospheric pressure or in vacuum) with glucose syrups on the kinetics of water transport during the first falling rate period of air drying of mango at 60°C was investigated. Both pre-drying treatments decreased strongly the effective moisture diffusivity ( D eff) calculated with Fick's second law. For osmotically concentrated fruit at atmospheric pressure, the increase in glucose concentration of the immersion solution decreased the drying rate. D eff values were similar for mangoes predehydrated to a w 0.97 by vacuum or atmospheric osmosis, but previous blanching slightly decreased the D eff value for vacuum treated slices compared with those osmotically dehydrated to the same a w at atmospheric pressure. The air drying behaviour of blanched and/or osmotically dehydrated mangoes was ascribed to glucose uptake during the impregnation step, volume shrinking, low modification of cell wall resistance to water flux by the pretreatments and/or gelatinization of starch and denaturation of protein – carbohydrate mucilage.

Thermal, mechanical, and diffusional properties of nylon 6/ABS polymer blends: Compatibilizer effect

AbstractThe thermal, mechanical, and water absorption properties of blends of nylon 6 (PA6) and acrylonitrile‐butadiene‐styrene copolymer (ABS) with and without the compatibilizer maleic anhydride (MAH) were studied. Polymers were melt‐blended using a twin screw extruder, and injection molded into sheets. Tensile and impact properties, hardness, heat deflection resistance, and dimensional stability were enhanced by the incorporation of MAH. Synergistic effects were observed for tensile elongation and flexural properties. The melting temperature and the thermal stability were not significantly affected by the incorporation of MAH. The mechanical property enhancement by the introduction of compatibilizer was explained by the formation of a micro‐domain structure in the blends. The equilibrium water uptake increased with increasing concentration of PA6, and the diffusion coefficient was determined from the water transport kinetics at different temperatures. Activation energy was extracted from the temperature dependence of the diffusion coefficient. No compatibilizer effect was observed in the swelling behavior.

Age-related Changes in the Kinetics of Water Transport in Normal Human Lenses

Magnetic resonance microscopy (MRM) has been used to study the kinetics of water transport in human eye lenses. Fresh lenses obtained from the Queensland Eye Bank were incubated at 34.5°C in artificial aqueous humour (AAH) containing nutrients and metabolites similar to those that are present in vivo. MR images were acquired over approximately a 20 hr period following replacement of H2O based AAH with deuterium oxide (D2O) based AAH. NMR signal intensity from the lenses decreased with time corresponding to a decrease in concentration of H2O within the lenses. A statistically significant correlation (P<0.001) was found between the rate of NMR signal loss from the lens nuclei and increasing age of the lenses.The results show that as lenses age, there is a reduction in the rate at which water and presumably also water soluble low molecular weight metabolites, can enter the cells of the lens nucleus via the epithelium and cortex. A decrease in the rate of transport of water, nutrients and anti-oxidants (e.g. glutathione) would be expected to lead to progressive oxidative damage to lenses with age, and may ultimately contribute to presbyopia and senile nuclear cataract.

Kinetics of water transport in eel intestinal vesicles.

Brush border membrane vesicles, BBMV, from eel intestinal cells or kidney proximal tubule cells were prepared in a low osmolarity cellobiose buffer. The osmotic water permeability coefficient P(f) for eel vesicles was not affected by pCMBS and was measured at 1.6 x 10(-3) cm sec(-1) at 23 degrees C, a value lower than 3.6 x 10(-3) cm sec(-1) exhibited by the kidney vesicles and similar to published values for lipid bilayers. An activation energy E(a) of 14.7 Kcal mol(-1) for water transport was obtained for eel intestine, contrasting with 4.8 Kcal mol(-1) determined for rabbit kidney proximal tubule vesicles using the same method of analysis. The high value of E(a), as well as the low P(f) for the eel intestine is compatible with the absence of water channels in these membrane vesicles and is consistent with the view that water permeates by dissolution and diffusion in the membrane. Further, the initial transient observed in the osmotic response of kidney vesicles, which is presumed to reflect the inhibition of water channels by membrane stress, could not be observed in the eel intestinal vesicles. The P(f) dependence on the tonicity of the osmotic shock, described for kidney vesicles and related to the dissipation of pressure and stress at low tonicity shocks, was not seen with eel vesicles. These results indicate that the membranes from two volume transporter epithelia have different mechanisms of water permeation. Presumably the functional water channels observed in kidney vesicles are not present in eel intestine vesicles. The elastic modulus of the membrane was estimated by analysis of swelling kinetics of eel vesicles following hypotonic shock. The value obtained, 0.79 x 10(-3) N cm(-1), compares favorably with the corresponding value, 0. 87 x 10(-3) N cm(-1), estimated from measurements at osmotic equilibrium.

Cross-linked high amylose starch for controlled release of drugs: recent advances

Cross-linked high amylose starches have been developed as excipients for the formulation of controlled-release solid dosage forms for the oral delivery of drugs. Advantages of this new class of excipients include cost-effectiveness, readily accessible industrial manufacturing technology, high active ingredient core loading and the possibility of achieving a quasi zero-order release for most drugs. In addition to the latter, other features distinguish cross-linked high amylose starches from other excipients used to prepare hydrophilic matrices. Among these are the absence of erosion, the limited swelling and the fact that increasing cross-linking degrees results in increased water uptake rate, drug release rate and equilibrium swelling. Thus the goal of the present study was to gain some insights into the mechanism of drug release control by matrices of cross-linked high amylose starch. Water transport kinetics and dimensional changes were studied in matrices placed in water at 37°C by an image analysis technique. The results show that in the first 5 min, a gel layer is formed at the surface of the tablet, after which the gel front seems to halt its progression toward the center of the tablet. Water continues to diffuse through the front and to invade the core. As a consequence, this latter swells, with a predominance for radial swelling. Equilibrium swelling is reached over 3 days, when the water concentration in the tablet becomes homogeneous and the whole tablet gelifies. Solid-state 13C-NMR were acquired on cross-linked high amylose starch powders, tablets and hydrated tablets with varying cross-linking degrees. They show a predominance of the V-type single helix arrangement of amylose in the dry state irrespective of the cross-linking degree. Upon hydration, the homologues with a low cross-linking degrees show a transition from the V to the B-type double helix arrangement. It is therefore hypothesized that the capacity of amylose to undergo the V to B transition is an important factor in controlling water transport and drug release rate. Finally applications to different drugs are reviewed briefly. They illustrate the versatility of this technology as generic versions of zero order OROS drug (Efidac) and Fickian release conventional matrices (Voltaren SR) were developed and successfully tested in pilot clinical studies to be bioequivalent to the references. These studies further showed that cross-linked high amylose starch matrices have the lowest inter-subject variability among the systems tested and show a total absence of food effect.

Analysis of repeated measurement data using the nonlinear mixed effects model

Davidian, M. and Giltinan, D.M., 1993. Analysis of repeated measurement data using the nonlinear mixed effects model. Chemometrics and Intelligent Laboratory Systems, 20: 1–24. Situations in which repeated measurements are taken on each of several individual items arise in many areas. These include assay development, where concentration—response data are available for each assay run in a series of assay experiments; pharmacokinetic analysis, where repeated blood concentration measurements are obtained from each of several subjects; and growth or decay studies, where growth or decay are measured over time for each plant, animal, or some other experimental unit. In these situations the model describing the response is often nonlinear in the parameters to be estimated, as is the case for the four-parameter logistic model, which is frequently used to characterize concentration—response relationships for radioimmunoassay enzyme-linked immunosorbent assay. Furthermore, response variability typically increases with level of response. The objectives of an analysis vary according to the application: for assay analysis, calibration of unknowns for the most recent run may be of interest; in pharmacokinetics, characterization of drug disposition for a patient population may be the focus. The nonlinear mixed effects (NME) model has been used to describe repeated measurement data for which the mean response function is nonlinear. In this tutorial, the NME model is motivated and described, and several methods are given for estimation and inference in the context of the model. The methods are illustrated by application to examples from the fields of water transport kinetics, assay development, and pharmacokinetics.

Mixed-Effects Nonlinear Regression for Unbalanced Repeated Measures

Repeated measures data, such as clinical pharmacokinetic data, growth data, and dose-response data, are often inherently nonlinear with respect to a given response function and are frequently incomplete and/or unbalanced. Nonlinear random-effects models together with a variety of estimation procedures have been proposed for the analysis of such data. This paper is concerned with a straightforward procedure for estimating and comparing the parameters of a generalized mixed-effects nonlinear regression model. The asymptotic properties of the proposed estimators are given and large-sample tests of hypothesis provided. The results are applied to in vitro data on the water transport kinetics of hemodialyzers used in the treatment of patients with chronic renal failure.

Water transport and estimated transmembrane potential during freezing of mouse oocytes

The kinetics of water transport and the changes in transmembrane potential during freezing of mouse oocytes in isotonic phosphate buffered saline (PBS) were simulated using thermodynamic models. The permeability to water at 0 degree C, Lpg, and the activation energy, ELp, of metaphase II mouse oocytes from B6D2F1 mice were determined to be 0.044 +/- 0.008 micron/min-atm and 13.3 +/- 2.5 kcal/mol during freezing at 2 degrees C/min. The inactive cell volume was determined to be 0.214 with a correlation coefficient of 0.995, indicating that the oocytes closely follow the ideal Boyle-van't Hoff relation. The mean value of the oocyte diameter was 79.41 +/- 4.62 microns. These results were used to predict the behavior of mouse oocytes under various freezing conditions. The effect of the cooling rate on the cell volume and cytoplasm undercooling was investigated. The changes in transmembrane potential were also investigated during freezing of mouse oocytes. The computer simulations showed that at the beginning of the freezing process (-1 degrees C), the fast growth of ice in the extracellular solution causes a sharp increase of the membrane potential. It is predicted that the change in membrane potential is substantial for almost all cooling rates. Estimations show that values as high as -90 mV may be reached during freezing. The hyperpolarization of the membrane may cause orientation of the dipoles within the membrane. For membrane proteins with 300 debye dipole moment, the theoretical prediction suggests that the percentage of dipoles aligned with the membrane potential increases from 16% at 0 degrees C prior to freezing to 58% at -8 degrees C after seeding of the external ice followed with a cooling at 120 degrees C/min.

Thermodynamics and kinetics of intracellular ice formation during freezing of biological cells

A physicochemical model based on the modified classical heterogeneous nucleation theory was proposed to analyze the ice formation inside biological cells during freezing. According to this model, intracellular ice formation (IIF) may be catalyzed either by the plasma membrane via the effects of the external ice on the plasma membrane, called surface-catalyzed nucleation (SCN), or by the intracellular particles, called volume-catalyzed nucleation (VCN), depending on the freezing conditions. The model for IIF was coupled with the model describing the kinetics of water transport to predict the thermodynamic state of the cytoplasm. A method to estimate the nucleation frequency from the observed probability of nucleation was suggested. This method was based on the assumption that each cell has the same heteronucleating particles with identical properties to alter the nucleation kinetics in an identical way. This allowed the correlation of the experimental probability of IIF with the nucleation rate. It was suggested that the uncertainties in the experimental conditions could be minimized if cells were cooled at rapid rates to prevent water transport during freezing. The model was applied to mouse oocytes at metaphase II. Correlation of the theory with the experimental kinetics of the cumulative fraction of IIF revealed preexponentional factors of 3.56×108 (m2 s)−1 and 1.84×1050 (m3 s)−1, and exponential factors of 4.6×109 and 1.08×1012 K5 for SCN and VCN, respectively. The contact angle between the internal surface of the plasma membrane and the ice cluster for SCN, θ, was determined to be 35° for freezing in isotonic solutions (285 mosm) increasing linearly to 47° in hypertonic solutions (1035 mosm). SCN was active between ∼−5 and −20 °C in the presence of external ice. It is suggested that the presence of external ice alters the plasma membrane through its chemical, electrical, mechanical, ionic, and thermal effects, to make it an energetically favorable site for heterogeneous nucleation. For VCN, lower and upper limits for θ and R were found to be 60°&amp;lt;θ&amp;lt;100° and 10−7&amp;lt;R&amp;lt;4×10−7 cm, respectively. VCN was active between ∼−30 and −35 °C.

Kinetics of water and ion transport during freezing of mouse oocytes

Kinetics of water and ion transport during freezing of mouse oocytes

Water Transport in the Midrib Tissue of Maize Leaves

WATER MOVEMENT ACROSS PLANT TISSUES OCCURS ALONG TWO PATHS: from cell-to-cell and in the apoplasm. We examined the contribution of these two paths to the kinetics of water transport across the parenchymatous midrib tissue of the maize (Zea mays L.) leaf. Water relations parameters (hydraulic conductivity, Lp; cell elastic coefficient, epsilon; half-time of water exchange for individual cells, T((1/2))) of individual parenchyma cells determined with the pressure probe varied in different regions of the midrib. In the adaxial region, Lp = (0.3 +/- 0.3).10(-5) centimeters per second per bar, epsilon = 103 +/- 72 bar, and T((1/2)) = 7.9 +/- 4.8 seconds (n = seven cells); whereas, in the abaxial region, Lp = (2.5 +/- 0.9).10(-5) centimeters per second per bar, epsilon = 41 +/- 9 bar, and T((1/2)) = 1.3 +/- 0.5 seconds (n = 7). This zonal variation in Lp, epsilon, and T((1/2)) indicates that tissue inhomogeneities exist for these parameters and could have an effect on the kinetics of water transport across the tissue.The diffusivity of the tissue to water (D(t)) obtained from the sorption kinetics of rehydrating tissue was D(t) = (1.1 +/- 0.4).10(-6) square centimeters per second (n = 6). The diffusivity of the cell-to-cell path (D(c)) calculated from pressure probe data ranged from D(c) = 0.4.10(-6) square centimeters per second in the adaxial region to D(c) = 6.1.10(-6) square centimeters per second in the abaxial region of the tissue. D(t) approximately D(c) suggests substantial cell-to-cell transport of water occurred during rehydration. However, the tissue diffusivity calculated from the kinetics of pressure-propagation across the tissue (D(t)') was D(t)' = (33.1 +/- 8.0).10(-6) square centimeters per second (n = 8) and more than 1 order of magnitude larger than D(t). Also, the hydraulic conductance of the midrib tissue (Lp(m) per square centimeter of surface) estimated from pressure-induced flows across several parenchyma cell layers was Lp(m) = (8.9 +/- 5.6).10(-5) centimeters per second per bar (n = 5) and much larger than Lp.These results indicate that the preferential path for water transport across the midrib tissue depends on the nature of the driving forces present within the tissue. Under osmotic conditions, the cell-to-cell path dominates, whereas under hydrostatic conditions water moves primarily in the apoplasm.

Alcohol effects on rapid kinetics of water transport through lipid membranes and location of the main barrier.

The effect of 1-alkanols (from 1-butanol up to 1-dodecanol) on the water permeability of dimyristoylphosphatidylcholine vesicle membranes was studied by measuring the osmotic swelling rate as functions of 1-alkanol concentrations and temperatures above the gel-to-liquid-crystalline phase transition. For 1-butanol and 1-hexanol, the activation energy for water permeation was invariant with the addition of alkanols, whereas for 1-octanol, 1-decanol and 1-dodecanol, the activation energy decreased depending on the alkanol concentration, and the extent of the decrease was larger for alkanol with a longer hydrocarbon chain. These results suggests that hydrocarbon moiety beyond seven or eight carbon atoms from the head group in phospholipid molecules constitutes the main barrier for water permeation through the dimyristoylphosphatidylcholine vesicle membrane. The relative volume change of the vesicle due to osmotic swelling increased with the addition of 1-alkanols. Presumably, the membrane structural strength is weakened by the presence of 1-alkanols in the membrane. Contrary to the dependence of the swelling rate upon the alkanol carbon-chain length, no significant difference in the effect on the relative volume changes was seen among the 1-alkanols. This result suggests that weakening of the membrane structure is caused by perturbation of the membrane/water interface induced by incorporation of 1-alkanols into the membrane.

Kinetics of water transport in sickle cells

This paper reports the result of stopped-flow studies on differences in the kinetics of osmotic water transport of sickle and normal erythrocytes. The kinetics of inward osmotic water permeability are similar in sickle and normal red blood cells. In contrast, the kinetics of outward water flux are significantly (approx. 38%) decreased in sickle cells. Deoxygenation does not modify the water influx kinetics in either type of cells, but accelerates considerably the rate of water efflux in sickle cells. No significant variation of water transport kinetics was observed in density-separated cell fractions of either type. The results suggest that membrane-associated hemoglobin may decrease the outward water permeability and that in deoxygenated sickle cells the fraction of hemoglobin S near the lipid bilayer is lower than in oxygenated conditions.

Stopped-flow study of anesthetic effect on water-transport kinetics through phospholipid membranes. Interfacial versus lipid core ligands

We have compared ligand effects between polar and apolar anesthetic molecules upon water transport across phospholipid membranes by kinetic analysis of the osmotic swelling rate, using a stopped-flow technique. Chloroform and 1-hexanol were used as interfacial ligands, and carbon tetrachloride and n-hexane were used as their counterparts, representing lipid core action. Because anesthetics transform the solid-gel membrane into a liquid-crystalline state, and because phospholipid membranes display an anomaly in permeability at the phase transition, dimyristoylphosphatidylcholine vesicles were studied at temperatures above the main phase transition to avoid this anomaly. All these molecules increased the osmotic swelling rate. However, a significant difference was observed in the activation energy, ΔE p, between polar and apolar molecules; ΔE p was almost unaltered by the addition of polar molecules (chloroform and 1-hexanol), whereas it was decreased by apolar molecules (carbon tetrachloride and n-hexane). The obtained results were analyzed in terms of the dissolution-diffusion mechanism for water permeation across the lipid membrane. It is suggested that polar molecules affect water permeability by altering the partition of water between the membrane interior and water phase, and apolar molecules affect it by altering both the partition and the diffusion of water within the membrane interior.

Changes in water and salt transport during hydrolysis of cellulose acetate reverse osmosis membranes

Cellulose acetate membranes undergo hydrolytic decomposition which is accelerated at very high or low pH levels. Hydrolysis results in the loss of acetyl groups leading to a sharp decline in membrane performance. Minimum hydrolysis rates occur in the pH range of 4–6. The two parameter transport equation (Merten et al.) gives salt and water permeability coefficients (A and B) which depend on membrane acetyl content. In this paper we have shown both coefficients, calculated from performance changes, to increase exponentially with time. Rate constants calculated from these changes show changes in salt permeability to be an order of magnitude greater than changes in water permeability. The observed discrepancy in salt and water transport kinetics suggests differing mechanisms for these two processes. Evidently, salt-water coupling during transport increases as membrane hydrolysis progresses. Thus, the two parameter transport equations do not apply since they neglect the salt-water coupling phenomenon. This paper discusses the use of other membrane transport models to account for changes in membrane performance as hydrolysis occurs. The relation between acetyl content and hydrolysis rates is also discussed.

Water transport in a cluster of closely packed erythrocytes at subzero temperatures

A one-dimensional model has been developed to describe the kinetics of water transport in a cluster of closely packed cells. For the case of human red blood cells, the intracellular medium has been treated as an ideal, hydrated, nondilute multicomponent electrolyte solution. Results show that the volume flux of water out of the interior cells of the cluster lags behind that of the exterior cells. At any given temperature (or time), the amount of water retained within a cluster of closely packed cells of a given type exceeds (on an overall percentage basis) the amount of water retained within a single isolated cell of the same type. For a given cooling rate the probability of intracellular ice nucleation at any given temperature will therefore be greater for cells in the interior of a cluster, and the survival signature for a cell cluster should peak at a cooling rate which is less than the corresponding optimal value for a single, isolated cell. These results are consistent with experimental observations.

Kinetics of water transport through erythrocyte membranes at subzero temperatures

Kinetics of water transport through erythrocyte membranes at subzero temperatures