Free Water Diffusion Research Articles

Gel formation in polymeric composites for modification of fibrous materials

The possibility of using oxidized nucleotides and nuclosides as cross-linking reagents for preparing chitosan hydrogels with a broad range of applications was examined. It was found that the ratio of dialdehyde to chitosan amine and the reaction medium pH affected gel formation in aqueous acetic acid solutions of chitosan in the presence of the cross-linking reagents glutaraldehyde and oxidized nucleic acid derivatives. IR spectra of chitosan and its derivatives were recorded and characterized. Immobilization of the proteolytic enzyme trypsin in chitosan films on fibrous materials was studied. Fibrous materials are modified by hydrogels based on biocompatible polymers in order to produce new materials for medicine, pharmacology, biotechnology, and ecology [1, 2]. Hydrogels are polymeric systems that are capable of retaining up to several thousand percent of water but have lost the ability to flow. Covalent cross-linking of polymers in aqueous solution by bifunctional reagents forms a continuous gel network that provides strength and simultaneously free diffusion of water. The properties of hydrogels can be used to create new materials for biotechnology (cell cultivation), biomedicine (tissue engineering), pharmacology (controlled release drug systems), and medicine (wound dressings) [3]. One of the most promising polymers for creating such materials is the biocompatible and biodegradable polysaccharide chitosan, which contains reactive groups in almost every elementary unit. Glutaraldehyde (GA) is used as a cross-linking reagent and is capable of reacting with chitosan amines by forming azomethine bonds. However, because GA can undergo aldol and crotonic condensation [4], the products of its reaction with chitosan contain C=C double bonds and carbonyls that can be toxic upon contact with living tissues. Herein the possibility of cross-linking chitosan with oxidized nucleic acid derivatives such as nuclosides consisting of ribose and a purine or pyrimidine base and nucleotides containing also a phosphate was studied in order to prepare

The Nuclear Pore Complex and Nuclear Transport

Internal membrane bound structures sequester all genetic material in eukaryotic cells. The most prominent of these structures is the nucleus, which is bounded by a double membrane termed the nuclear envelope (NE). Though this NE separates the nucleoplasm and genetic material within the nucleus from the surrounding cytoplasm, it is studded throughout with portals called nuclear pore complexes (NPCs). The NPC is a highly selective, bidirectional transporter for a tremendous range of protein and ribonucleoprotein cargoes. All the while the NPC must prevent the passage of nonspecific macromolecules, yet allow the free diffusion of water, sugars, and ions. These many types of nuclear transport are regulated at multiple stages, and the NPC carries binding sites for many of the proteins that modulate and modify the cargoes as they pass across the NE. Assembly, maintenance, and repair of the NPC must somehow occur while maintaining the integrity of the NE. Finally, the NPC appears to be an anchor for localization of many nuclear processes, including gene activation and cell cycle regulation. All these requirements demonstrate the complex design of the NPC and the integral role it plays in key cellular processes.

Temperature dependence of syringomycin E channel functioning in planar lipid bilayers

327 We have studied conductivity and dwell time of ion channels produced by antifungal lipodepsipeptide syringomycin E (SRE) in planar lipid bilayers in the temperature range of 10 − 50 ° C. Membranes were formed from either phosphatidylethanolamine or from an equimomolar mixture of phosphatidylserine and phosphatidylethanolamine; membrane bathing solution contained 0.4 M LiCl or 0.4 M NaCl, pH 6. We have established that a 10 ° C increase in temperature produces ~1.5-fold increase in the single-channel conductance and ~3-fold decrease in the mean dwell time of a single channel. These changes correspond to ~7 kcal/mol activation energy for ion transport through the SRE channel. This value exceeds the activation energy barrier of the free ion diffusion in water by a factor of two (~3.3 kcal/mol). Higher value of the activation energy of ion transport in the SRE channel suggests specific interactions between the permeating ions and the channel walls; this suggestion is also supported by our earlier observation of the difference between SRE channel conductance and the conductance of the equivalent water volume. The decrease of the channel dwell time with the increase of temperature may be related to dissociation of the SRE aggregates (6 − 7 lipodepsipeptide molecules) forming the conducting channel.

Light scattering by erythrocytes under conditions of osmotic imbalance in a cell-surrounding medium system

A model for changes in the sizes of erythrocytes under conditions of osmotic imbalance in a hypoosmotic medium is proposed assuming free diffusion of water through the erythrocyte membrane. Equations relating the minimum and maximum osmotic resistances to the shape of an erythrocyte in an isotonic medium are obtained. A numerical simulation is used to determine how the optical attenuation coefficient varies as a result of the swelling and hemolysis of erythrocytes; this provides a foundation for a new optical method for determining the osmotic resistance of erythrocytes. This method does not require exerting a force on the erythrocytes, is distinguished by simplicity and high speed, and can be implemented using standard spectrophotometers.

Association between visual degeneration of intervertebral discs and the apparent diffusion coefficient

The value of apparent diffusion coefficient (ADC) measurements in intervertebral disc has been studied because ADC provides an estimate of free diffusion of unbound water and could be used as a quantitative tool to estimate degenerative changes. However, the challenging nature of diffusion imaging of spine and limited numbers of subjects in earlier studies has produced contradictory findings. We aimed to determine the relation between ADC and visual degenerative changes in lumbar intervertebral discs in a sufficiently large homogeneous study group. Lumbar spines of 228 volunteer middle-aged men were MR imaged at 1.5 T including anatomic and diffusion-weighted imaging. ADC values, T2 signal intensity and height, and width of the three lowest lumbar intervertebral discs were measured and disc degeneration visually graded. The calculated average ADC of 530 measured discs was 2.01 x 10(-3) mm(2)/s+/-0.29 (+/-S.D.). The reduction in ADC between visually normal and moderately degenerated discs was 4%. Severely degenerated discs showed 5% larger ADC values than normal discs, presumably due to free water in cracks and fissures of those discs. T2 signal intensity of the disc was significantly correlated with the ADC values, whereas other measured parameters did not show correlation. There was no evident difference in ADC between the studied anatomic lumbar levels. Because there is considerable overlap between ADC values of normal and degenerated discs, we conclude that ADC measurements of intervertebral discs, at least with current technology, have limited clinical value.

Kinetic Model for Ca2+-induced Permeability Transition in Energized Liver Mitochondria Discriminates between Inhibitor Mechanisms

Cytotoxicity associated with pathophysiological Ca(2+) overload (e.g. in stroke) appears mediated by an event termed the mitochondrial permeability transition (mPT). We built and solved a kinetic model of the mPT in populations of isolated rat liver mitochondria that quantitatively describes Ca(2+)-induced mPT as a two-step sequence of pre-swelling induction followed by Ca(2+)-driven, positive feedback, autocatalytic propagation. The model was formulated as two differential equations, each directly related to experimental parameters (Ca(2+) flux/mitochondrial swelling). These parameters were simultaneously assessed using a spectroscopic approach to monitor multiple mitochondrial properties. The derived kinetic model correctly identifies a correlation between initial Ca(2+) concentration and delay interval prior to mPT induction. Within the model's framework, Ru-360 (a ruthenium complex) and Mg(2+) were shown to compete with the Ca(2+)-stimulated initiation phase of mPT induction, consistent with known inhibition at the phenomenological level of the Ca(2+) uniporter. The model further reveals that Mg(2+), but not Ru-360, inhibits Ca(2+)-induced effects on a downstream stage of mPT induction at a site distinct from the uniporter. The analytical approach was then applied to promethazine, an FDA-approved drug previously shown to inhibit both mPT and ischemia-reperfusion injury. Kinetic analysis revealed that promethazine delayed mPT induction in a manner qualitatively distinct from that of lower concentrations of Mg(2+). In summary, we have developed a kinetic model to aid in the quantitative characterization of mPT induction. This model is consistent with/informative about the biochemistry of several mPT inhibitors, and its success suggests that this kinetic approach can aid in the classification of agents or targets that modulate mPT induction.

Estimation of the diffusion coefficient of water evolved during the non-isothermal dehydration of Australian sedimentary opal

The dehydration of an opal specimen was investigated by thermogravimetric analysis (TG) in powder and bulk forms. The change in geometry resulted in a significant difference in the temperature range in which dehydration occurred with peak temperatures in the differential TG (DTG) curve for the hand ground opal at 203°C and for the bulk opal at 340°C. This difference was attributed to time taken for diffusion of free water in the bulk opal to the specimen surface prior to evolution as a registered mass loss. A model was proposed to account for the diffusion of water and was used to estimate the diffusion coefficient.

Matrix diffusion coefficients in volcanic rocks at the Nevada test site: Influence of matrix porosity, matrix permeability, and fracture coating minerals

Diffusion cell experiments were conducted to measure nonsorbing solute matrix diffusion coefficients in forty-seven different volcanic rock matrix samples from eight different locations (with multiple depth intervals represented at several locations) at the Nevada Test Site. The solutes used in the experiments included bromide, iodide, pentafluorobenzoate (PFBA), and tritiated water ( 3HHO). The porosity and saturated permeability of most of the diffusion cell samples were measured to evaluate the correlation of these two variables with tracer matrix diffusion coefficients divided by the free-water diffusion coefficient ( D m/ D ⁎). To investigate the influence of fracture coating minerals on matrix diffusion, ten of the diffusion cells represented paired samples from the same depth interval in which one sample contained a fracture surface with mineral coatings and the other sample consisted of only pure matrix. The log of ( D m/ D ⁎) was found to be positively correlated with both the matrix porosity and the log of matrix permeability. A multiple linear regression analysis indicated that both parameters contributed significantly to the regression at the 95% confidence level. However, the log of the matrix diffusion coefficient was more highly-correlated with the log of matrix permeability than with matrix porosity, which suggests that matrix diffusion coefficients, like matrix permeabilities, have a greater dependence on the interconnectedness of matrix porosity than on the matrix porosity itself. The regression equation for the volcanic rocks was found to provide satisfactory predictions of log( D m/ D ⁎) for other types of rocks with similar ranges of matrix porosity and permeability as the volcanic rocks, but it did a poorer job predicting log( D m/ D ⁎) for rocks with lower porosities and/or permeabilities. The presence of mineral coatings on fracture walls did not appear to have a significant effect on matrix diffusion in the ten paired diffusion cell experiments.

Application of the Poisson-Nernst-Planck Theory with Space-Dependent Diffusion Coefficients to KcsA

The Poisson-Nernst-Planck electrodiffusion theory serves to compute charge fluxes and is here applied to the ion current through a protein channel. KcsA was selected as an example because of the abundance of experimental and theoretical data. The potassium channels MthK and KvAP were used as templates to define two open channel models for KcsA. Channel boundary surfaces and protein charge distributions were defined according to atomic radii and partial atomic charges. To establish the sensitivity of the results to these parameters, two different sets were used. Assigning the potassium diffusion coefficients equal to the value for free-diffusion in water (1.96 × 10 −9 m 2/s), the computed currents overestimated the experimental data. Ion distributions inside the channel suggest that the overestimate is not due to an excess of charge shielding. A good agreement with the experimental data was achieved by reducing the potassium diffusion coefficient inside the channel to 1.96 × 10 −10 m 2/s, a value of substantial motility but nonetheless in accord with the intuitive notion that the channel has a high affinity for the ions and therefore slows them down. These results are independent of the open channel model and the parameterization adopted for atomic radii and partial atomic charges. The method offers a reliable estimate of the channel current with low computational effort.

Molecular Dynamics Simulations of Glutamate Diffusion in Synaptic Cleft

Diffusion of transmitters in the synaptic cleft critically influences synaptic efficacy by affecting both the amplitude and the time course of quantal events, but the value of the diffusion constant is speculative. In this study, we use molecular dynamics simulations to determine how the spatial confinement and membrane charges affect the diffusion constants of glutamate- and water as well as general properties of their diffusion. The synaptic cleft is represented as the space enclosed by two single-wall carbon sheets. Both water and especially glutamate are concentrated near the pore wall, where the concentration of glutamate can reach 30-50 times the mean value and the concentration of water can reach 2-8 times the mean value. Such spatial profiles of glutamate contradict the classical notions of diffusion on which both continuous and Monte Carlo simulations are built. The layering of glutamate- and water molecules suggests that the interfacial glutamate-cleft wall (or water-cleft wall) interactions may critically regulate their diffusion in the cleft. Indeed, the effective longitudinal diffusion constant of glutamate is steeply dependent on the cleft width, but only when the cleft is very narrow (< 5 nm). Therefore, even for a cleft as narrow as at the glutamatergic synapse in the central nervous system, the effective diffusion constant of glutamate will not be much lower than free diffusion in the bulk solution due to confinement. The effective diffusion constant of water is considerably less sensitive to cleft width over the same range of cleft widths than is glutamate, but is also higher than that of glutamate. Finally, the layering of glutamate and water and their effective diffusion constants are largely independent of how the cleft wall is charged. In conclusion, in the confined space of the synaptic cleft, glutamate is layered near the wall. Consequently, its diffusion constant becomes dependent on the cleft width. However, the diffusion of glutamate is slower than its free diffusion in water only if the cleft is very narrow. If the width of the cleft is consistent with that determined by morphometric studies in the central nervous system, glutamate diffusion should not be slowed by confinement and is thus likely to be similar to that in free solution.

Interpretation of injection–withdrawal tracer experiments conducted between two wells in a large single fracture

Tracer experiments conducted using a flow field established by injecting water into one borehole and withdrawing water from another are often used to establish connections and investigate dispersion in fractured rock. As a result of uncertainty in the uniqueness of existing models used for interpretation, this method has not been widely used to investigate more general transport processes including matrix diffusion or advective solute exchange between mobile and immobile zones of fluid. To explore the utility of the injection–withdrawal method as a general investigative tool and with the intent to resolve the transport processes in a discrete fracture, two tracer experiments were conducted using the injection–withdrawal configuration. The experiments were conducted in a fracture which has a large aperture (>500 μm) and horizontally pervades a dolostone formation. One experiment was conducted in the direction of the hydraulic gradient and the other in the direction opposite to the natural gradient. Two tracers having significantly different values of the free-water diffusion coefficient were used. To interpret the experiments, a hybrid numerical–analytical model was developed which accounts for the arcuate shape of the flow field, advection–dispersion in the fracture, diffusion into the matrix adjacent to the fracture, and the presence of natural flow in the fracture. The model was verified by comparison to a fully analytical solution and to a well-known finite-element model. Interpretation of the tracer experiments showed that when only one tracer, advection–dispersion, and matrix diffusion are considered, non-unique results were obtained. However, by using multiple tracers and by accounting for the presence of natural flow in the fracture, unique interpretations were obtained in which a single value of matrix porosity was estimated from the results of both experiments. The estimate of porosity agrees well with independent measurements of porosity obtained from core samples. This suggests that: (i) the injection–withdrawal method is a viable tool for the investigation of general transport processes provided all relevant experimental conditions are considered and multiple conservative tracers are used; and (ii) for the conditions of the experiments conducted in this study, the dominant mechanism for exchange of solute between the fracture and surrounding medium is matrix diffusion.

Interpretation of injection?withdrawal tracer experiments conducted between two wells in a large single fracture

Tracer experiments conducted using a flow field established by injecting water into one borehole and withdrawing water from another are often used to establish connections and investigate dispersion in fractured rock. As a result of uncertainty in the uniqueness of existing models used for interpretation, this method has not been widely used to investigate more general transport processes including matrix diffusion or advective solute exchange between mobile and immobile zones of fluid. To explore the utility of the injection–withdrawal method as a general investigative tool and with the intent to resolve the transport processes in a discrete fracture, two tracer experiments were conducted using the injection–withdrawal configuration. The experiments were conducted in a fracture which has a large aperture (>500 μm) and horizontally pervades a dolostone formation. One experiment was conducted in the direction of the hydraulic gradient and the other in the direction opposite to the natural gradient. Two tracers having significantly different values of the free-water diffusion coefficient were used. To interpret the experiments, a hybrid numerical–analytical model was developed which accounts for the arcuate shape of the flow field, advection–dispersion in the fracture, diffusion into the matrix adjacent to the fracture, and the presence of natural flow in the fracture. The model was verified by comparison to a fully analytical solution and to a well-known finite-element model. Interpretation of the tracer experiments showed that when only one tracer, advection–dispersion, and matrix diffusion are considered, non-unique results were obtained. However, by using multiple tracers and by accounting for the presence of natural flow in the fracture, unique interpretations were obtained in which a single value of matrix porosity was estimated from the results of both experiments. The estimate of porosity agrees well with independent measurements of porosity obtained from core samples. This suggests that: (i) the injection–withdrawal method is a viable tool for the investigation of general transport processes provided all relevant experimental conditions are considered and multiple conservative tracers are used; and (ii) for the conditions of the experiments conducted in this study, the dominant mechanism for exchange of solute between the fracture and surrounding medium is matrix diffusion.

Microporosity of Bicontinuous Nanoporous Polymeric Materials, Characterized with Restricted Diffusion

The 1H NMR pulsed-gradient spin−echo method has been used to study the self-diffusion D of the mobile fractions in polymerized bicontinuous microemulsions formed with methyl methacrylate and hydroxyethyl methacrylate, cross-linked with ethylene glycol dimethacrylate, and water containing 10% sodium dodecyl sulfate as surfactant. Measurements were made at 50.5 °C over the bicontinuous range (30−96 wt % water) at diffusion times between 8 and 1000 ms. In addition to the signal from the nondiffusing glassy phase, two distinct components are observed differing in D by 2 orders of magnitude, with an intensity ratio monotonic in water content. Both D values depend weakly on diffusion time, but D attributed to free water increases with water content, whereas D attributed to surfactant decreases with water content in the bicontinuous region. This may be due to a size reduction in the surfactant micelles in samples containing less aqueous phase. Assuming free diffusion of water in the open pores, we extract from the data the reduced permeabilities (i.e., detour and tortuosity), discuss the origin of the slow-D component as surfactant undergoing surface-exchange with the network, and derive an upper limit on the pore size on the order of 1 μm near the midpoint of the bicontinuous concentration domain, in keeping with evidence from SEM imaging; a pore size distribution derived from freezing point depression attests to a large aspect ratio of the porosities.

Osmometric and water-transporting properties of guinea pig cardiac myocytes.

To elucidate the mechanism of water flux across heart cell membranes, osmotically induced volume changes and sarcolemmal water permeability were evaluated in isolated guinea pig ventricular myocytes by videomicroscopic measurements of cell surface dimensions. Superfusion with anisosmotic solution (0.5-4 times normal osmolality) caused a rapid (lt;3 min to new steady state) and reversible cell swelling or shrinkage mainly because of proportional changes in cell width and thickness. The van't Hoff relationship between relative cell volume and the reciprocal of relative osmolality was linear and predicted an apparent osmotically dead space of approximately 35% cell volume. The osmotic water permeability coefficient (P(f)) measured from the time course of cell swelling/shrinkage was approximately 22 microm.s(-1) at 35 degrees C. Arrhenius activation energy (E(a)), a measure of the energy barrier to water flux, was approximately 3.8 kcal.mol(-1) between 11 and 35 degrees C; this value is equivalent to E(a) for free-water diffusion in bulk solution ( approximately 4 kcal.mol(-1)). Treatment with 0.1 mM Hg(2+), a sulfhydryl-oxidizing reagent, reduced P(f) by approximately 90%, and the sulfhydryl-reducing reagent dithiothreitol (10 mM) antagonized the inhibitory action of Hg(2+). E(a) measured from Hg(2+)-treated myocytes (12.3 kcal.mol(-1)) was in the range of that for diffusional water movement through the lipid bilayers (>10 kcal.mol(-1)). Although the observed P(f) is small in magnitude, both the low E(a) and the sulfhydryl-related modifications of P(f) are characteristic of channel-mediated water transport. These data suggest that water channels form a major conduit for water crossing the sarcolemma of guinea-pig heart cells.

Isoflurane alters the recirculatory pharmacokinetics of physiologic markers.

Earlier studies have demonstrated that physiologic marker blood concentrations in the first minutes after administration, when intravenous anesthetics exert their maximum effect, are determined by both cardiac output and its distribution. Given the reported vasodilating properties of isoflurane, we studied the effects of isoflurane anesthesia on marker disposition as another paradigm of altered cardiac output and regional blood flow distribution. The dispositions of markers of intravascular space and blood flow (indocyanine green), extracellular space and free water diffusion (inulin), and total body water and tissue perfusion (antipyrine) were determined in four purpose-bred coonhounds. The dogs were studied while awake and while anesthetized with 1.7%, 2.6%, and 3.5% isoflurane (1.15, 1.7, and 2.3 minimum alveolar concentration, respectively) in a randomized order determined by a Latin square experimental design. Marker dispositions were described by recirculatory pharmacokinetic models based on very frequent early, and less frequent later, arterial blood samples. These models characterize the role of cardiac output and regional blood flow distribution on drug disposition. Isoflurane caused a significant and dose-dependent decrease in cardiac output. Antipyrine disposition was profoundly affected by isoflurane anesthesia, during which nondistributive blood flow was maintained despite decreases in cardiac output, and the balance between fast and slow tissue volumes and blood flows was altered. The isoflurane-induced changes in marker disposition were different than those the authors reported previously for halothane anesthesia, volume loading, or hypovolemia. These data provide further evidence that not only cardiac output but also its peripheral distribution affect early drug concentration history after rapid intravenous administration.

Matrix Diffusion of Simple Cations, Anions, and Neutral Species in Fractured Crystalline Rocks

The diffusion of radionuclides into the pore spaces of a rock matrix and the pore properties in fractured crystalline rocks were studied. The work concentrated on the predominant water-conducting fracture system in the host granodiorite of the Kamaishi In Situ Test Site, which consists of fracture fillings and altered granodiorite. Through-diffusion experiments to obtain effective and apparent diffusion coefficients (De and Da, respectively) for Na+, Cs+, HTO, Cl-, and as a function of ionic charge were conducted through the fracture fillings and altered and intact granodiorite. The total porosity ϕ, density, pore-size distribution, and specific surface area of the pores of the rocks were also determined by a water saturation method and Hg porosimetry. The average ϕ is, in the order from highest to lowest, as follows: fracture fillings (5.6%) greater than altered granodiorite (3.2%) greater than intact granodiorite (2.3%), and gradually it decreases into the matrix. The pore sizes of the intact and altered granodiorite range from 10 nm to 200 μm, and the fracture fillings from 50 nm to 200 μm, but almost all pores are found around 0.1 and 200 μm in the fracture fillings. The De values for all species are in the following order: fracture fillings greater than altered granodiorite greater than intact granodiorite, as with the rock porosity. In addition, no effect of ionic charge on De is found. No significant dependence for Da values on the rock porosity is found. The formation factors FF and geometric factors G of the rocks were evaluated by normalizing the free water diffusion coefficient Do for each species. The FF decreased with decreasing rock porosity, and an empirical equation for the rock porosity was derived to be FF = ϕ1.57±0.02. The G values showed a tendency to slightly decrease with decreasing rock porosity, but they were approximately constant (0.12 to 0.19) in this porosity range. This indicates that accessible pores decrease into the rock matrix from the fractures, and diffusion is also reduced into the rock matrix. Furthermore, it was concluded that a simplified model based on FF and Do values is approximately reasonable to predict De.

Channel-Type Crystal Packing in the Very Rare Space Group P4212 with Z' = 3/4: Crystal Structure of the Complex γ-Cyclodextrin–Methanol–n-Hydrate

The crystal structure of the title compound is discussed with focus on its unusual crystal packing mode. The γ-cyclodextrin molecules stack in columns which have internal channels and additional large channels are formed between the columns. Both types of channels are so wide that more or less free diffusion of water and small guest molecules is possible, preventing characterization of the solvent and guest arrangements. Although the space group P4212 (no. 90) and the unit-cell content Z′ = 3\over 4 are extremely rare in general, this is the favorite crystal packing arrangement of γ-cyclodextrin inclusion complexes. This space group has not been observed as yet for other purely organic compounds.

The Biology of Lipids: Integrative and Comparative Perspectives

A unifying feature of lipids is relatively low solubility in aqueous media, yet lipids embody an immensely disparate set of molecules. It can be argued that lipids represent some of the most complex biological molecules. Classes of lipids include hydrocarbons, alcohols, fatty acids, waxes, glycerides, phospholipids, and glycolipids. There are also variations on a theme within a single lipid class. Molecular species that make up a lipid class possess differences that are seemingly subtle, but these differences oftentimes have major consequences for physical properties and ultimately for biological function. The complexities of these molecules (including their composite nature, intricate structure, and daunting nomenclature) have intimidated many a biochemistry student and professor! Not surprisingly, functional diversity is another hallmark of lipid compounds. The wide array of chemical and physical properties of different lipids allow these molecules to perform a variety of roles in biological processes. Polar lipids arranged as bilayers establish barriers to free diffusion of water and solutes, provide a microenvironment for membrane-associated proteins, and organize and maintain an interface between discrete metabolic and functional compartments. Surface lipids are integral to water balance in all terrestrial organisms. Lipids serve as metabolic fuels and signalling molecules (hormones, second messengers, and pheromones). Lipids play specialized roles in many organisms as electrical (myelin) or thermal (blubber) insulators. Surfactant lipids are essential components 1 From the Symposium The Biology of Lipids: Integration of Structure and Function presented at the Annual Meeting of the Society for Integrative and Comparative Biology, 26-30 December 1996, Albuquerque, New Mexico.

Diffusional transport of the aminoterminal propeptide of type III procollagen in the interstitium of the globally ischaemic cat myocardium

Local repair after acute myocardial infarction appears to be reflected by levels in serum of the aminoterminal propeptide of type III procollagen (serum-PIIINP). Furthermore, serum-PIIINP has recently been reported to provide information on prognosis after acute myocardial infarction. However, no attention has yet been paid to the resistance to diffusion offered by the myocardial interstitium. We determined the diffudion coefficient of PIIINP in the interstitium of the globally ischaemic interstitium of the cat (D 37′) by means of a ‘true transient diffusion’ method, and compared with the free diffusion in water (D 37). D 37′, (in cm 2 s −1 · 10 −5) was 0.0157 ± 0.0005 (mean ± SEM) ( n = 13), and D 37 was 0.0624 ± 0.0024 ( n = 12). The mean diffusive progression during 20 min of the concentration profile of [ 125I]PIIINP into the tissue was calculated to be 0.19 mm. The D 37′ of albumin is practically identical to the D 37′ of PIIINP, and the myocardium offers a similar resistance to diffusion of PIIINP and albumin, as expressed from the ratio D 37/D 37′ of approximately 4 for both molecules. PIIINP has a molecular weight of 42 000 Da, is rod shaped and has an overall negative charge. These characteristics explain the similarity in diffusion coefficients of PIIINP and albumin, which has a molecular weight of 69 000 Da. Albumin is known to pass the membrane of the continuous capillaries of the heart, making it very likely that direct exchange of PIIINP between interstitium and capillary plasma can also occur. During one hour of interstitial diffusion PIIINP will have traversed a distance calculated to correspond to 15–20 capillaries. Therefore, the results support the concept of serum-PIIINP as a direct marker of events taking place locally in the myocardium following acute myocardial infarction.

Mathematic modeling of human amniotic fluid dynamics

OBJECTIVE: We sought to develop a model quantifying the relative contributions of fetal swallowing and intramembranous flow to amniotic fluid dynamics during human gestation. We then used the model to simulate the impact of absent swallowing on amniotic fluid volume. STUDY DESIGN: The model was developed with published data for normal human amniotic fluid volume and composition, human fetal urine flow rate and composition (11 to 42 weeks), and extrapolated data from ovine lung fluid production. Fetal swallowing and intramembranous flow were calculated with assumptions that (1) swallowed fluid is isotonic to amniotic fluid, (2) intramembranous flow is free water diffusion, and (3) 50% of lung fluid is swallowed. The model was then applied to simulate absent fetal swallowing and variable (0%, 50%) proportions of swallowed lung fluid were used as a representation of esophageal atresia - tracheal fistula variations. RESULTS: Fetal swallowed volume and intramembranous flow linearly increase until 28 to 30 weeks. Daily swallowed volume then exponentially increases to a maximum of 1006 ml/day at term, whereas intramembranous flow continues on a linear trend to reach 393 ml/day at term. With absent swallowing and variable amounts of lung fluid swallowed (0%, 50%), predicted amniotic fluid volume is similar to normal values through 20 weeks, exceeds the 95% confidence interval for normal amniotic fluid volume at 29 to 30 weeks' gestation (~2000 ml), and then exponentially increases. Predicted amniotic fluid osmolality (280 to 257 mOsm/kg) is slightly lower than actual values although within the clinically normal range. CONCLUSIONS: This model indicates that the normal reduction in amniotic fluid volume beginning at 34 weeks results from the marked increase in swallowed volume during the third trimester. Additionally, this model correlates well with the timing of the initial clinical presentation of polyhydramnios observed in some fetuses with conditions that result in absent or reduced swallowing or gastrointestinal atresia. Modeling of amniotic fluid dynamics can predict normal changes in fetal fluid exchange and may aid in understanding of amniotic fluid imbalances. (Am J Obstet Gynecol 1996;175:937-44.)