Osmotic Pressure Gradient Research Articles

An electrodiffusive neuron-extracellular-glia model for exploring the genesis of slow potentials in the brain.

Within the computational neuroscience community, there has been a focus on simulating the electrical activity of neurons, while other components of brain tissue, such as glia cells and the extracellular space, are often neglected. Standard models of extracellular potentials are based on a combination of multicompartmental models describing neural electrodynamics and volume conductor theory. Such models cannot be used to simulate the slow components of extracellular potentials, which depend on ion concentration dynamics, and the effect that this has on extracellular diffusion potentials and glial buffering currents. We here present the electrodiffusive neuron-extracellular-glia (edNEG) model, which we believe is the first model to combine compartmental neuron modeling with an electrodiffusive framework for intra- and extracellular ion concentration dynamics in a local piece of neuro-glial brain tissue. The edNEG model (i) keeps track of all intraneuronal, intraglial, and extracellular ion concentrations and electrical potentials, (ii) accounts for action potentials and dendritic calcium spikes in neurons, (iii) contains a neuronal and glial homeostatic machinery that gives physiologically realistic ion concentration dynamics, (iv) accounts for electrodiffusive transmembrane, intracellular, and extracellular ionic movements, and (v) accounts for glial and neuronal swelling caused by osmotic transmembrane pressure gradients. The edNEG model accounts for the concentration-dependent effects on ECS potentials that the standard models neglect. Using the edNEG model, we analyze these effects by splitting the extracellular potential into three components: one due to neural sink/source configurations, one due to glial sink/source configurations, and one due to extracellular diffusive currents. Through a series of simulations, we analyze the roles played by the various components and how they interact in generating the total slow potential. We conclude that the three components are of comparable magnitude and that the stimulus conditions determine which of the components that dominate.

MO694SWELLING OF PERITONEAL TISSUE DURING PERITONEAL DIALYSIS: COMPUTATIONAL ASSESSMENT USING POROELASTIC THEORY

Abstract Background and Aims Experimental studies and computational modeling show increased hydration of peritoneal tissue close to peritoneal surface after intraperitoneal (ip) administration of hypertonic dialysis fluid. This overhydration - due to fluid inflow from peritoneal cavity (driven by increased intraperitoneal pressure) and from blood (due to high interstitial concentration of osmotic agent diffusing from the cavity) - may lead to tissue swelling, as observed in experiments and in disturbed physiological conditions. We estimated the degree of swelling using linear poroelastic theory with fluid and solute transport parameters obtained from clinical studies. Method The spatially distributed model of peritoneal transport was extended by equations for tissue deformation and stress derived from linear poroelastic theory. The model describes also fluid and osmotic agent flows across tissue and capillary wall. We assumed that transport and deformation occur across a layer of tissue with initial intact width L0 and deformed width L; the deformation is described as the ratio L/L0. Transport parameters are assumed as average values estimated for intact tissue by Stachowska-Pietka (2019). As tissue stiffness (Lame coefficient) for muscle is not known, we examined stiffness ranging from 110 mmHg (connective tissue; interstitium) to 700 mmHg (solid tumor). We assumed that for initial periods of peritoneal dialysis when osmotic pressure of dialysis fluid is high: 1) osmotic pressure gradient across the capillary wall prevails over the combined Starling forces, 2) spatial profile of osmotic agent concentration in tissue (interstitial fluid) can be approximated by exponential function with the penetration depth ΛS. The model yields an equation for L/L0 to be solved numerically, but an approximated closed formula also works well for typical dialysis conditions. Results The model predicts that swelling of peritoneal tissue depends on factors such as tissue stiffness, tissue width, solute penetration depth, and transport parameters for tissue and capillary wall, and on the forces that induce fluid transport: intraperitoneal pressure and the increment of osmolality of dialysis fluid over plasma osmolality. Examples of L/L0 yielded by the model - with use of glucose 1.36% dialysis fluid and for two levels of ip hydrostatic pressure (Pip) - are shown. In Figure, left panel, for L0 = 1 cm representing human abdominal muscle, and solute diffusional penetration ΛS=ΛD=0.055 cm, or lower, as due to diffusion against fluid flow, ΛS=ΛD/2=0.027 cm, is plotted versus the tissue stiffness; the dialysis fluid with glucose 1.36% is applied (osmolality increment of 60 mmol/L at the beginning of peritoneal dwell, Waniewski et al, 1996) and Pip is 15 mmHg. As stiffness of abdominal and bowel muscles may be expected around 300 mm Hg, swelling might be up to 15%; it decreases with lower ip hydrostatic and osmotic pressures. Hypothetical dialysis at Pip = 0 (isobaric with interstitial fluid) would reduce swelling by factor 2, see Figure, right panel. The depth of osmotic agent penetration into the tissue impacts tissue hydration and swelling, see Figure 1 for L/L0 with twice reduced ΛS. The model and its approximation by the closed formula provide practically the same outcomes for clinical peritoneal dialysis, see Figure 1, but some discrepancy between them may occur for thin tissue, as rat abdominal wall. The approximate formula for L/L0 works well if ΛS is much shorter than L0. Nevertheless, for high degree of swelling a nonlinear theory should be constructed. Conclusion In peritoneal dialysis, exposure of peritoneal tissue to hypertonic dialysis fluid at increased hydrostatic pressure contributes to overhydration and swelling (by 5-15% after fluid infusion) of the tissue. The extent by which this swelling may contribute to changes in peritoneal tissue structure and function warrants further studies.

An integrated microfluidic concentration gradient generator for mechanical stimulation and drug delivery

Mechanical stimuli, including fluid shear stress, osmotic pressure gradient, and extracellular matrix stiffness, significantly affect cellular interactions with drugs in biological structures. This paper introduces an integrated concentration gradient generator (CGG) capable of providing cell monolayers with these stimuli and demonstrates its design, fabrication, and quantification procedures. The proposed multi-layer chip consists of a CGG integrated with a membrane-based cell culture chamber (MCCC) and two bubble trappers for removal of micro-bubbles. The CGG provides cultured cells in the MCCC with four different concentrations of desirable inlet drug/chemical reagents. The MCCC is able to impose adjustable shear stresses, as well as osmotic pressure gradients on cell monolayers. The stiffness of the extracellular matrix (ECM) is also accommodating by utilizing a proper membrane in the MCCC. A numerical simulation based on the finite element method (FEM) is employed to design and optimize the integrated device, and then, the chip's performance is quantified using the experimental data. Finally, the biocompatibility of the proposed device is investigated by dynamic culturing of human lung cancer cells (A549 cell line) on the chip.

Taste Sensor Using Ionic Polymer Metal Composite

The usage of ionic polymer metal composite (IPMC) being an electroactive polymer is prevalent in different engineering applications. IPMC are synthetic composite nanomaterials which show artificial muscle behavior under applied voltage or electric impetus. When an electrical potential is applied across the two membranes, the positively charged conjugated and hydrated cations migrate toward the negative electrode. This migration creates an osmotic pressure gradient across the membrane; hence, the strip gets deformed or bent. On the other hand, IPMC strips are capable of generating electrical potential under the action of stretching or bending (thereby acting as a sensor). In this letter, the authors report the pragmatic application of the IPMC strip to study a taste sensor. The sensing characteristics of IPMC strips are applied to sense five general tastes (sweet, sour, bitter, salty, and spicy). Primarily, IPMCs are immersed into individual solutions and kept for 15 min for doping action. Thus, different solutions having distinct concentration of ions will lead to distinct identifications. In this present investigation, the authors have instigated the IPMC strip as a sensor; when an IPMC strip is bent or deformed, an analogous voltage in the range of millivolts is generated across the membranes. Second, clammy IPMCs are bent in the clockwise and again in the counterclockwise direction (10°) to record the corresponding voltage. A set of experimental data was collected, which divulged unique characteristics for different solutions pertinent to different tastes. Thus, IPMC strips are able to recognize different taste senses.

Combining Nanoparticle Shape Modulation and Polymersome Technology in Drug Delivery

This paper highlights the potential benefits of using self-assembled polymeric nanoparticles of various shapes to enhance drug uptake. First, we highlight the growth and development of the polymersome, using a liposome as a blueprint for amphiphilic codelivery. Then, we focus on the advantages of nanoparticle elongation, drawing from the field of solid nanoparticles, as opposed to self-assembled vesicles which have not yet been extensively explored in shape-modulated drug delivery applications. Notably, regardless of the material used in the solid nanoparticle systems, more elongated shapes lead to greater cellular uptake, decreased interaction with the reticuloendothelial system macrophages, and increased circulation times. Finally, we highlight the methods currently being developed to modulate polymersome shape, thus providing a drug delivery system with the benefits derived from amphiphilicity and elongated structures. Current methods employed to modulate polymersome shape involve osmotic pressure gradients, solvent switching, and the use of cross-linking agents. Although these methods are successful in modulating polymersome shapes and the benefits of elongated nanoparticles in therapeutic targeting are clear, these methods have not yet been explored for applications in drug delivery.

Comparison of Pressure-Retarded Osmosis Performance between Pilot-Scale Cellulose Triacetate Hollow-fiber and Polyamide Spiral-Wound Membrane Modules.

Pressure-retarded osmosis (PRO) has recently received attention because of its ability to generate power via an osmotic pressure gradient between two solutions with different salinities: high- and low-salinity water sources. In this study, PRO performance, using the two pilot-scale PRO membrane modules with different configurations—five-inch cellulose triacetate hollow-fiber membrane module (CTA-HF) and eight-inch polyamide spiral-wound membrane modules (PA-SW)—was evaluated by changing the draw solution (DS) concentration, applied hydrostatic pressure difference, and the flow rates of DS and feed solution (FS), to obtain the optimum operating conditions in PRO configuration. The maximum power density per unit membrane area of PA-SW at 0.6 M NaCl was 1.40 W/m2 and 2.03-fold higher than that of CTA-HF, due to the higher water permeability coefficient of PA-SW. In contrast, the maximum power density per unit volume of CTA-SW at 0.6 M NaCl was 4.67 kW/m3 and 6.87-fold higher than that of PA-SW. The value of CTA-HF increased to 13.61 kW/m3 at 1.2 M NaCl and was 12.0-fold higher than that of PA-SW because of the higher packing density of CTA-HF.

Salt and water profiles in surface dry-salting goat’s cheese during ripening

In order to determine the evolution of sodium chloride and water distribution profiles, surface dry-salting goat cheese was analyzed during a four-week ripening period. The concentration profiles of sodium chloride and water were analyzed according to the height and diameter of the cheese pieces and the response surface analysis was applied for the interpretation. Water distribution was higher on the outside of the pieces until the second week of ripening due to water movement caused by osmotic pressure gradients after NaCl uptake, after which there was surface dehydration. In the sodium chloride profile there was an initially rapid salt uptake by inward diffusion in the surface layers, which decreased as ripening advanced. Although the cheeses were turned over regularly, the salt and water distributions were unequal on the top and bottom surfaces in the early stages and vertical distribution was favored due to the gravitational effect. At the end of the study period the concentration of NaCl in the vertical direction was in equilibrium, while small differences persisted in the horizontal direction. The prediction of the evolution of the proposed models indicates that equilibrium of NaCl concentration would be reached after 35 days ripening.

Mechanics of biofilms formed of bacteria with fimbriae appendages.

Gram-negative bacteria, as well as some Gram-positive bacteria, possess hair-like appendages known as fimbriae, which play an important role in adhesion of the bacteria to surfaces or to other bacteria. Unlike the sex pili or flagellum, the fimbriae are quite numerous, with of order 1000 fimbriae appendages per bacterial cell. In this paper, a recently developed hybrid model for bacterial biofilms is used to examine the role of fimbriae tension force on the mechanics of bacterial biofilms. Each bacterial cell is represented in this model by a spherocylindrical particle, which interact with each other through collision, adhesion, lubrication force, and fimbrial force. The bacterial cells absorb water and nutrients and produce extracellular polymeric substance (EPS). The flow of water and EPS, and nutrient diffusion within these substances, is computed using a continuum model that accounts for important effects such as osmotic pressure gradient, drag force on the bacterial cells, and viscous shear. The fimbrial force is modeled using an outer spherocylinder capsule around each cell, which can transmit tensile forces to neighboring cells with which the fimbriae capsule collides. We find that the biofilm structure during the growth process is dominated by a balance between outward drag force on the cells due to the EPS flow away from the bacterial colony and the inward tensile fimbrial force acting on chains of cells connected by adhesive fimbriae appendages. The fimbrial force also introduces a large rotational motion of the cells and disrupts cell alignment caused by viscous torque imposed by the EPS flow. The current paper characterizes the competing effects of EPS drag and fimbrial force using a series of computations with different values of the ratio of EPS to bacterial cell production rate and different numbers of fimbriae per cell.

Gap junctions amplify spatial variations in cell volume in proliferating tumor spheroids

Sustained proliferation is a significant driver of cancer progression. Cell-cycle advancement is coupled with cell size, but it remains unclear how multiple cells interact to control their volume in 3D clusters. In this study, we propose a mechano-osmotic model to investigate the evolution of volume dynamics within multicellular systems. Volume control depends on an interplay between multiple cellular constituents, including gap junctions, mechanosensitive ion channels, energy-consuming ion pumps, and the actomyosin cortex, that coordinate to manipulate cellular osmolarity. In connected cells, we show that mechanical loading leads to the emergence of osmotic pressure gradients between cells with consequent increases in cellular ion concentrations driving swelling. We identify how gap junctions can amplify spatial variations in cell volume within multicellular spheroids and, further, describe how the process depends on proliferation-induced solid stress. Our model may provide new insight into the role of gap junctions in breast cancer progression.

A computational study on osmotic chemotaxis of a reactive Janusbot

We explore the chemotaxis of an elliptical double-faced Janus motor (Janusbot) stimulated by a second-order chemical reaction on the surfaces, aA + bB → cC + dD, inside a microfluidic channel. The self-propulsions are modeled considering the full descriptions of hydrodynamic governing equations coupled with reaction–diffusion equations and fluid–structure interaction. The simulations, employing a finite element framework, uncover that the differential rate kinetics of the reactions on the dissimilar faces of the Janusbot help in building up enough osmotic pressure gradient for the motion as a result of non-uniform spatiotemporal variations in the concentrations of the reactants and products around the particle. The simulations uncover that the mass diffusivities of the reactants and products along with the rates of forward and backward reactions play crucial roles in determining the speed and direction of the propulsions. Importantly, we observe that the motor can move even when there is no difference in the total stoichiometry of the reactants and products, (a + b) = (c + d). In such a scenario, while the reaction triggers the motion, the difference in net-diffusivities of the reactants and products develops adequate osmotic thrust for the propulsion. In contrast, for the situations with a + b ≠ c + d, the particle can exhibit propulsion even without any difference in net-diffusivities of the reactants and products. The direction and speed of the motion are dependent on difference in mass diffusivities and reaction rate constants at different surfaces.

Features of the composition of certain complex hyperosmolar infusion solutions

Objective. To justify the use of complex hyperosmolar solutions.
 Results and discussion. Today, the question of the qualitative composition and quantitative relationship between the various components of infusion-transfusion therapy remains controversial. Hypertonic solutions in various concentrations of NaCl were first used for “low-volume resuscitation” in patients with hemorrhagic shock. Administration of these solutions results in a temporary osmotic pressure gradient between the blood plasma and the interstitial space, causing fluid to move into the intravascular space, increasing cardiac output, blood pressure, and improving microcirculation. Recently, combinations of hypertonic NaCl with colloids, as well as multicomponent infusion drugs are increasingly used. These are, in particular, the drugs Rheosorbilact, Xylate, Lactoxyl, Sorbilact, Lactoprotein with sorbitol, Gecoton developed at the Institute of Blood Pathology and Transfusion Medicine of the National Academy of Medical Sciences of Ukraine. Preparations based on polyhydric alcohols of sorbitol or xylitol, alkalizing agents of sodium lactate or sodium acetate and electrolytes. Rheosorbilact, Xylate, Lactoxyl are basic solutions for detoxification therapy, including in patients with diabetes. Sorbilact used as an osmodiuretic and for the treatment of patients in the postoperative period. Protein-salt solution based on donor albumin Lactoprotein with sorbitol is a mean for intensive care in emergencies. Colloid-hyperosmolar infusion solution Gecoton based on hydroxyethyl starch is used as a mean for low-volume resuscitation in the prehospital stage, including combat victims. Specially selected substances comprising the combined drugs affect different parts of the pathological process and have several advantages compared to mono-drugs. The pharmacological action of such solutions is due to the mutual potentiation of the action of its components. This provides hemodynamic effect and can reduce the dosage of each component separately, reducing the likelihood of side effects. The use of multicomponent hyperosmolar solutions can significantly reduce the total volume of fluid administered to patients.
 Conclusions. Complete hyperosmolar drugs can be recommended for routine and auxiliary fluid therapy and treatment for emergency conditions.

Diffusioosmotic and convective flows induced by a nonelectrolyte concentration gradient

Glucose is an important energy source in our bodies, and its consumption results in gradients over length scales ranging from the subcellular to entire organs. Concentration gradients can drive material transport through both diffusioosmosis and convection. Convection arises because concentration gradients are mass density gradients. Diffusioosmosis is fluid flow induced by the interaction between a solute and a solid surface. A concentration gradient parallel to a surface creates an osmotic pressure gradient near the surface, resulting in flow. Diffusioosmosis is well understood for electrolyte solutes, but is more poorly characterized for nonelectrolytes such as glucose. We measure fluid flow in glucose gradients formed in a millimeter-long thin channel and find that increasing the gradient causes a crossover from diffusioosmosis-dominated to convection-dominated flow. We cannot explain this with established theories of these phenomena which predict that both scale linearly. In our system, the convection speed is linear in the gradient, but the diffusioosmotic speed has a much weaker concentration dependence and is large even for dilute solutions. We develop existing models and show that a strong surface-solute interaction, a heterogeneous surface, and accounting for a concentration-dependent solution viscosity can explain our data. This demonstrates how sensitive nonelectrolyte diffusioosmosis is to surface and solution properties and to surface-solute interactions. A comprehensive understanding of this sensitivity is required to understand transport in biological systems on length scales from micrometers to millimeters where surfaces are invariably complex and heterogeneous.

All-Aqueous Liquid Crystal Nanocellulose Emulsions with Permeable Interfacial Assembly.

We report on the formation of water-in-water liquid crystal emulsions with permeable colloidal assemblies. Rodlike cellulose nanocrystals (CNC) spontaneously self-assemble into a helical arrangement with the coexistence of nonionic, hydrophilic polyethylene glycol (PEG) and dextran, whereas the two polymer solutions are thermodynamically incompatible. Stable water-in-water emulsions are easily prepared by mixing the respective CNC/polymer solutions, showing micrometric CNC/PEG dispersed droplets and a continuous CNC/dextran phase. With time, the resulting emulsion demixes into an upper, droplet-lean isotropic phase and a bottom, droplet-rich cholesteric phase. Owing to the osmotic pressure gradient between PEG and dextran phases, target transfer of cellulose nanoparticles occurs across the water/water interface to reassemble into a liquid crystal-in-liquid crystal emulsion with global cholesteric organization. The observed structural, optical, and temporal evolution confirm that the colloidal particles in the two immiscible phases experience short-range interactions and form long-range assemblies across the interface.

Magnetic Sedimentation Velocities and Equilibria in Dilute Aqueous Ferrofluids.

Dilute ferrofluids have important applications in the separation of materials via magnetic levitation. However, dilute ferrofluids pose an additional challenge compared to concentrated ones. Migration of the magnetic nanoparticles toward a magnet is not well counteracted by a buildup of an osmotic pressure gradient, and consequently, homogeneity of the fluid is gradually lost. Here, we investigate this phenomenon by measuring and numerically modeling time-dependent concentration profiles in aqueous ferrofluids in the field of a neodymium magnet and at 10 T in a Bitter magnet. The numerical model incorporates magnetic, frictional, and osmotic forces on the particles and takes into account the polydispersity of the particles and the spatial dependence of the magnetic field. The magnetic sedimentation rate in our most stable ferrofluids can be understood in terms of the magnetophoresis of separate nanoparticles, a best-case scenario when it comes to applications.

Cells in New Light: Ion Concentration, Voltage, and Pressure Gradients across a Hydrogel Membrane.

The ionic compositions of the intra- and extracellular environments are distinct from one another, with K+ being the main cation in the cytosol and Na+ being the most abundant cation outside of the cell. Specific ions can permeate into and out of the cell at different rates, bringing about uneven distribution of charges and development of negative electric potential inside the cell. Each healthy cell must maintain a specific ion concentration gradient and voltage. To account for these functions, various ionic pumps and channels located within the cell membrane have been invoked. In this work, we use a porous alginate hydrogel as a model gelatinous network representing the plant cell wall or cytoskeleton of the animal cell. We show that the gel barrier is able to maintain a stable separation of ionic solutions of different ionic strengths and chemical compositions without any pumping activity. For the Na+/K+ concentration gradient sustained across the barrier, a negative electric potential develops within the K+-rich side. The situation is reminiscent of that in the cell. Furthermore, also the advective flow of water molecules across the gel barrier is restricted, despite the gel’s large pores and the osmotic or hydrostatic pressure gradients across it. This feature has important implications for osmoregulation. We propose a mechanism in which charge separation and electric fields developing across the permselective (gel) membrane prevent ion and bulk fluid flows ordinarily driven by chemical and pressure gradients.

Effect of number of layers and deposition time on layer-by-layer (LbL) composite forward osmosis membrane

Forward osmosis (FO) refers to the osmotic process for water transfer through a semipermeable membrane that results in the separation of water from dissolved solutes. This method has been explored due to low fouling propensity and low energy requirement since the driving force is an osmotic pressure gradient between the feed and draw solutions. This research focused on determining the optimum FO membrane by controlling the number of polyelectrolyte bilayers and deposition time of membrane for seawater desalination. Poly(diallyl-dimethylammonium chloride), PDADMAC and poly(sodium 4-styrenesulfonate), PSS were used as active monomers to generate a thin layer on the ultrafiltration membrane support through layer-by-layer (LbL) deposition method. The FO lab-scale experiment was performed by using 1.75 M of sodium sulphate (Na 2 SO 4 ) as draw solution and deionised water (DI) as feed. This research was conducted based on the different number of bilayers (5, 10, and 15 bilayers) and deposition time (5, 10, and 15 min) which affected the water flux. After the best parameters were obtained, treatment on synthetic seawater (35 g/L of NaCl) was conducted, and the results were compared with that of real seawater. Among the modified membranes, it was found that the membrane 5FO15 with 5-bilayers of polyelectrolytes and 15 min of deposition time exhibited the highest water flux (4.75 L/m 2 ∙h). The membrane exhibited a lower water flux for seawater than synthetic seawater due to the existing of other solutes which may contribute to membrane fouling. From this research, the polyelectrolyte membrane exhibited a high potential of treating seawater in Malaysia by FO process which is a good option of water treatment.

Macromolecular Crowding Affects Voltage-Dependent Alamethicin Pore Formation in Lipid Bilayer Membranes.

Macromolecular crowding is known to modulate chemical equilibria, reaction rates, and molecular binding events, both in aqueous solutions and at lipid bilayer membranes, natural barriers that enclose the crowded environments of cells and their subcellular compartments. Previous studies on the effects of macromolecular crowding in aqueous compartments on conduction through membranes have focused on single-channel ionic conduction through previously formed pores at thermodynamic equilibrium. Here, the effects of macromolecular crowding on the mechanism of pore formation itself were studied using the droplet interface bilayer (DIB) technique with the voltage-dependent pore-forming peptide alamethicin (alm). Macromolecular crowding was varied using 8 kDa molecular weight polyethylene glycol (PEG8k) or 500 kDa dextran (DEX500k) in two aqueous droplets on both sides of the bilayer membrane. In general, voltage thresholds for pore formation in the presence of crowders in the droplets decreased compared to their values in the absence of crowders, due to excluded volume effects, water binding by PEG, and changes in the ordering of water molecules and hydrogen-bonding interactions involving the polar lipid headgroups. In addition, asymmetric crowder loading (e.g., PEG8k-DEX500k on either side of the membrane) resulted in transmembrane osmotic pressure gradients that either enhanced or degraded the ionic conduction through the pores.

The time course of calf muscle fluid volume during prolonged running.

Muscle fluid is essential for the biochemistry and the biomechanics of muscle contraction. Here, we provide evidence that muscle fluid volumes undergo significant changes during 75 min of moderate intensity (2.7 ± 0.4 m/s) running. Using MRI measurements at baseline and after 2.5, 5, 10, 15, 45 and 75 min, we found that the volumes of calf muscles (quantified through average cross‐sectional area) in 18 young recreational runners increase (up to 9% in the gastrocnemii) at the beginning and decrease (below baseline levels) at later stages of running. However, the intensity of changes varied between analyzed muscles. We speculate that these changes are induced by muscle activity and dehydration‐related changes in osmotic pressure gradients between intramuscular and extramuscular spaces. These findings highlight the complex nature of muscle fluid shifts during prolonged running exercise.

The Concept of Tonicity: Problems and Resolutions

IntroductionIn this presentation, we address 6 major problems in the current understanding of the concept of tonicity that uses inconsistent descriptions/definitions of tonicity rooted in the problems with the current definitions of osmolarity (conventional osmolarity, see our abstract #2, “The Concept of Osmolarity: Problems and Resolutions” in EB2020). We then resolve these problems using two new concepts we defined in the abstract #2: 1) OC0 (the initial osmotic concentration (the concentration of the impermeant solute particles (SP)) in S‐m‐H2O (a simple osmosis system consisting of a solution (S) compartment and a water compartment separated by a selectively permeable membrane (either water‐permeable (only permeable to water) or SP‐permeable (permeable to water and some solute particle species). 2) S1‐m‐S2 (a composite osmosis system consisting of two solutions separated by a selectively permeable membrane).Methodlogical reasoning.Results1) To understand tonicity, a composite S1‐m‐S2 needs to be deconstructed into two simple osmosis systems that are mirrored (Fig 1). 2) When OC0 is used to eliminate the use of both conventional osmolarity and effective osmolarity (see the abstract #2), the meanings of the two sets of osmolarity and tonicity‐related terms (hyper/iso/hypo‐osmotic and hyper/iso/hypotonic) become consistent: If a solution is hyper/iso/hypo‐osmotic to another solution, it must also be hyper/iso/hypotonic to another solution. 3) Tonicity is a score‐like, relational concept that reflects a comparison between two OC0 in a composite osmosis system (S1‐m‐S2). It does not “belong” to a solution, but between two solutions. Its owner is a given composite osmosis system (S1‐m‐S2). 4) Tonicity can be expressed in multiple ways. 5) Tonicity has three important properties (graphical illustration will be provided in the poster): membrane‐dependency inherited from the membrane‐dependency of OC0 (see the abstract #2), reciprocity, and reference‐solution dependency or relativity. 6) Tonicity has a physical nature (Fig 2): either the ΔOC0 (the concentration gradient between two OC0 across the membrane) or the Δπ(S1‐S2) (the osmotic pressure gradient between S1 and S2). 7) The relationship among CTSP, OC0, and tonicity is illustrated in Fig 2. CTSP is the molar concentration of the total solute particles (TSP) measured in mM, not mOsm/L, defined in the abstract #2.Deconstruction of a composite S1‐m‐S2 into two simple osmosis systems: S1‐m‐H2O + H2O‐m‐S2.Figure 1A panoramic view of the concepts of CTSP, OC0, tonicity as the cause of osmosis (the ΔOC0(SX‐SY)), and tonicity as the effect of osmosis (reflected as the Δπ). Water is symbolized by the rope in the tug‐of‐war analogy. σ: reflection coefficient.Figure 2

Vesicle Budding Induced by the Asymmetric Membrane Insertion of a Surfactant is Limited by an Osmotic Barrier

Due to medical, technical and economical importance, substantial efforts have been made to further understand interactions of surfactants and lipid-bilayers. While approaches such as the well known three-stage-model - describing solubilization of lipid bilayers by surfactants - serve their purpose, complex systems escape its applicability. One such case is where equilibration of surfactant across the bilayer does not occur rapidly after addition from the outside, i.e. surfactant flip does not occur on the observed timescale. In this scenario, stress is induced by non-matching surface area requirements of the inner and outer leaflet. Several mechanisms for the release of this stress have been proposed. 12:0 Lyso-PC and POPC liposomes represent such a system. Here, asymmetry stress stops further lysolipid uptake beyond a certain content and gives rise to originally largely pure lysolipid micelles that equilibrate with the liposomes over days only (“staying out” scenario). However, part of the stress is also relaxed by a limited number of very small exovesicle budding off from the mother vesicle. Here, we present a first quantification of the extent of budding that accompanies “staying out”. This is achieved by separating the budded exovesicles from the mother vesicles using Asymmetric Flow-Field-Flow Fractionation (AF4). We studied the correlation of the extent of budding with osmotic pressure gradients, which affect the sphericity of the liposomes before addition of lysolipid. It suggests that fast budding from liposomes in buffer proceeds under constant entrapped volume conditions and, hence, stops as the mother vesicle becomes spherical. Further budding is slow, probably limited by ion leakage across the membrane. The detailed understanding of the budding processes is valuable both for technical applications and for deeper insight into natural budding-related phenomena such as exo- and endocytosis.