Volume osmotic flows of non-homogeneous electrolyte solutions through horizontally mounted membrane

Results of an experimental study of volume osmotic flows in a single-membrane osmotic-diffusive cell, which contains a horizontal, microporous, symmetrical polymer membrane separating water and binary or ternary electrolyte solutions are presented. In the experimental set-up, water was placed on one side of the membrane. The opposite side of the membrane was exposed to binary or ternary solutions. As binary solutions, aqueous potassium chloride or ammonia solutions were used, whereas potassium chloride in 0.25 mol x l(-1) aqueous ammonia solution or ammonia in 0.1 mol x l(-1) aqueous potassium chloride solution were used as ternary solutions. Two (A and B) configurations of a single-membrane osmotic-diffusive cell in a gravitational field were studied. In configuration A, water was placed in a compartment above the membrane and the solution below the membrane. In configuration B the position of water and solution was reversed. Furthermore, the effect of amplification of volume osmotic flows of electrolyte solutions in the single-membrane osmotic-diffusive electrochemical cell was demonstrated. The thermodynamic models of the flux graviosmotic and amplification effects were developed, and the volume flux graviosmotic effect for configurations A and B of a single-membrane osmotic-diffusive cell was calculated. The results were interpreted within the conventional instability category, increasing the diffusion permeability coefficient value for the system: concentration boundary layer/membrane/concentration boundary layer.     

The single file model for the diffusion of ions through a membrane

The model proposed by A. L. Hodgkin and R. D. Keynes (Jour. of Physiol.,128, 61–88, 1955) for the diffusion of potassium through the nerve membrane is extended to cover an arbitrary number of species of ions with charges not necessarily the same. One type of interference is also investigated.     

Imaging calcium entering the cytosol through a single opening of plasma membrane ion channels: SCCaFTs--fundamental calcium events

Recently, it has become possible to record the localized fluorescence transient associated with the opening of a single plasma membrane Ca(2+) permeable ion channel using Ca(2+) indicators like fluo-3. These Single Channel Ca(2+) Fluorescence Transients (SCCaFTs) share some of the characteristics of such elementary events as Ca(2+) sparks and Ca(2+) puffs caused by Ca(2+) release from intracellular stores (due to the opening of ryanodine receptors and IP(3) receptors, respectively). In contrast to intracellular Ca(2+) release events, SCCaFTs can be observed while simultaneously recording the unitary channel currents using patch-clamp techniques to verify the channel openings. Imaging SCCaFTs provides a way to examine localized Ca(2+) handling in the vicinity of a channel with a known Ca(2+) influx, to obtain the Ca(2+) current passing through plasma membrane cation channels in near physiological solutions, to localize Ca(2+) permeable ion channels on the plasma membrane, and to estimate the Ca(2+) currents underlying those elementary events where the Ca(2+) currents cannot be recorded. Here we review studies of these fluorescence transients associated with caffeine-activated channels, L-type Ca(2+) channels, and stretch-activated channels. For the L-type Ca(2+) channel, SCCaFTs have been termed sparklets. In addition, we discuss how SCCaFTs have been used to estimate Ca(2+) currents using the rate of rise of the fluorescence transient as well as the signal mass associated with the total fluorescence increase.     

Experimental study of osmosis through a collodion membrane

Experiments were carried out on a collodion membrane in order to study the factors that determine direction and magnitude of net flow of water across a membrane permeable to the solvent and to some of the solutes present. The solutes used were all non-ionic. When only one solute was present and there was no difference of hydrostatic pressure across the membrane, water flowed toward the side where its vapor pressure was lower, but the rate of transfer depended upon the nature of the solute: for a given difference in osmolality across the membrane, the rate increased with the molecular volume of the solute and reached its maximum with the solute to which the membrane was impermeable. These results led to the experimental demonstration that in the presence of two or more solutes of different molecular volumes, of which one at least can diffuse through the barrier, the net transfer of water can take place against its vapor pressure gradient. Some of the physicochemical and physiological implications of the data are discussed.     

Markov process characterization of a single membrane gating site

We present a method by which the expected macroscopic kinetic behavior of a membrane patch containing many independent gating sites can be determined analytically from a Markov formulation of the probabilistic nature of a single gating site. This method is applicable to any kinetic system with a small number of reactants and subject to the principle of detailed balance. Using this method calculations are carried out based on the concept of aggregation gating involving a voltage-dependent reversible first-order reaction followed by a voltage-independent reversible aggregation process taking place within the microscopic area of the gating site. The basic macrokinetic properties of the potassium and sodium conductance system are derived from a hypothetical aggregation gating site consisting of four molecules. These results are quantitatively consistent with the average of a large number of individual sample paths generated by Monte Carlo simulations for the same model and the same parameters.     

Possibility of electron tunneling through a nerve membrane

From a quantum mechanical standpoint, electron tunneling may occur in a nerve axon because the nerve membrane (60 to 100 angstroms) is thin enough for the tunnel effect and because the external solution and the axoplasma are redox systems which can provide electrons. Calculations and diagrams of the density-of-states are given to predict theN-shaped current-voltage characteristics which have been observed in the studies of squid giant axons, of the reconstituted liquid membranes and of the marine algae.     

Modulation of chloroplast membrane lipids by homogeneous catalytic hydrogenation

A method is reported for the modification of lipids in situ in chloroplast membrane by which a homogeneous, water-soluble catalyst Pd(QS)2 (QS, sulphonated alizarine; C14H6O7NaS) is incorporated into the thylakoids of isolated chloroplast. The catalyst itself did not affect the photosynthetic activity but caused an extensive loss of unsaturated fatty acids in the presence of hydrogen gas. The polyunsaturated fatty acids were hydrogenated at a faster rate than the monoenoic acids. During hydrogenation the orientational ordering of membrane lipids, as measured with the C-12 positional isomer of spin-labelled stearic acid, displayed a slight increase in agreement with the alterations in membrane composition. Progressive saturation of double bonds of lipids primarily inhibits electron transport between the photosystems followed by the inhibition of electron flow around photosystem II. Photosystem I electron transport was not inhibited even by 50% fatty acid hydrogenation. We suggest that using Pd(QS)2 catalyst for thylakoid hydrogenation offers an excellent technique to study the role of various unsaturated fatty acids in the regulation of membrane fluidity and photosynthetic processes.     

Diffusive and hydraulic transfer of solutes through homogeneous membranes

Rates of hydraulic transport of water, solute permeabilities, and sieving coefficients of homogeneous kappa-carrageenan and bovine serum albumin membranes were measured. These values increased with the water content of membranes. The data show good agreement with the predictions based on the pore model.     

Numerical simulation of a single cell passing through a narrow slit

The narrow slit between endothelial cells that line the microvessel wall is the principal pathway for tumor cell extravasation to the surrounding tissue. To understand this crucial step for tumor hematogenous metastasis, we used dissipative particle dynamics method to investigate an individual cell passing through a narrow slit numerically. The cell membrane was simulated by a spring-based network model which can separate the internal cytoplasm and surrounding fluid. The effects of the cell elasticity, cell shape, nucleus and slit size on the cell transmigration through the slit were investigated. Under a fixed driving force, the cell with higher elasticity can be elongated more and pass faster through the slit. When the slit width decreases to 2/3 of the cell diameter, the spherical cell becomes jammed despite reducing its elasticity modulus by 10 times. However, transforming the cell from a spherical to ellipsoidal shape and increasing the cell surface area by merely 9.3 % can enable the cell to pass through the narrow slit. Therefore, the cell shape and surface area increase play a more important role than the cell elasticity in cell passing through the narrow slit. In addition, the simulation results indicate that the cell migration velocity decreases during entrance but increases during exit of the slit, which is qualitatively in agreement with the experimental observation.     

Equilibrium sampling through membrane based on a single hollow fibre for determination of drug-protein binding and free drug concentration in plasma

The determination of drug-protein binding and free drug concentration in plasma applying the equilibrium sampling through membrane (ESTM) technique has been studied using supported liquid membrane extraction in a single hollow fibre without any membrane carrier. In the extraction setup, the donor phase (plasma or buffer) was placed in the vial, into which was immersed the hollow fibre with the acceptor phase situated in the lumen. This proposed technique was applied to study the drug-protein binding of five local anaesthetics and two antidepressants as model substances, and the influence of the total drug concentration on the drug-protein binding was investigated. The brief theoretical background for determination of the drug-protein binding under equilibrium conditions is described. The developed method shows a new, improved and simple procedure for determination of free drug concentration in plasma and extent of drug-protein binding.     

Imaging calcium entering the cytosol through a single opening of plasma membrane ion channels: SCCaFTs—fundamental calcium events

Recently, it has become possible to record the localized fluorescence transient associated with the opening of a single plasma membrane Ca²⁺ permeable ion channel using Ca²⁺ indicators like fluo-3. These Single Channel Ca²⁺ Fluorescence Transients (SCCaFTs) share some of the characteristics of such elementary events as Ca²⁺ sparks and Ca²⁺ puffs caused by Ca²⁺ release from intracellular stores (due to the opening of ryanodine receptors and IP₃ receptors, respectively). In contrast to intracellular Ca²⁺ release events, SCCaFTs can be observed while simultaneously recording the unitary channel currents using patch-clamp techniques to verify the channel openings. Imaging SCCaFTs provides a way to examine localized Ca²⁺ handling in the vicinity of a channel with a known Ca²⁺ influx, to obtain the Ca²⁺ current passing through plasma membrane cation channels in near physiological solutions, to localize Ca²⁺ permeable ion channels on the plasma membrane, and to estimate the Ca²⁺ currents underlying those elementary events where the Ca²⁺ currents cannot be recorded. Here we review studies of these fluorescence transients associated with caffeine-activated channels, L-type Ca²⁺ channels, and stretch-activated channels. For the L-type Ca²⁺ channel, SCCaFTs have been termed sparklets. In addition, we discuss how SCCaFTs have been used to estimate Ca²⁺ currents using the rate of rise of the fluorescence transient as well as the signal mass associated with the total fluorescence increase.     

Membrane and secretory forms of mouse membrane cofactor protein (CD46) generated from a single gene through alternative splicing

 A cDNA encoding a new secretory form of mouse membrane cofactor protein (MCP, CD46) was identified additionally to the membrane form cDNA. The secretory MCP, predicted from the cDNA sequence, consisted of the conserved four short consensus repeats (SCRs) plus a four amino acid-stretch. Unlike human MCP which comprises many isoforms, mouse MCP cDNA predicted a single isoform of membrane MCP with cytoplasmic tail 1 (CYT1) and serine/threonine-rich domain C (ST^C). To clarify the genomic origin and monomorphic alteration of these cDNAs, we cloned and analyzed a mouse genomic DNA harboring the full coding sequence of MCP from a 129/SV mouse genomic library. The mouse Mcp was a single gene ∼50 kilobases long. Eleven of the 14 coding exons of the human MCP gene and intron-exon boundary sequences were found to be conserved in the mouse gene. The ST^C homologue but not the ST^A or ST^B homologue in the mouse exons was functional: the latter being due to deletions and lack of consensus sequences for splicing. The sequence equivalent to cytoplasmic tail 2 (CYT2) has not been identified in the Mcp genome. Thus, the three exons (ST^A, ST^B, and probably CYT2) responsible for the polymorphism of human MCP by differential splicing were missing in the mouse Mcp gene. Unlike the case in humans, no Mcp-related genes or pseudogenes were observed in the mouse genome. The single mouse Mcp gene was mapped to the R-positive H5 band of mouse Chromosome 1 by FISH. Strikingly, one alternative exon with 73 base pairs (encoding the four new amino acids and a TGA stop codon) was discovered between the SCRIV and the ST^C exons; alternative splicing causes the generation of the secretory form of mouse MCP. These results on mouse MCP, together with the information concerning other mouse SCR proteins, infer that the regulator of complement activation (RCA) gene cluster is genetically diverged between humans and mice. Received: 22 April 1999 / Revised: 21 June 1999     

The nature of passive flows through tightly folded membranes

Summary Membrane processes such as microvilli and folded structures greatly increase the area for passive transport. A mathematical analysis of flows through folded membrane structures demonstrates that the increase in flows is not proportional to the increase in area. Furthermore, the flows of salt and water do not increase in the same ratio: that of water can be several times the increase in area and that of salt a fraction of the increase in area. A preferential passive flow of water is created by the structure. It is only in the neighborhood of the isotonic state that the effect is significant, but in that region, the effect can be dramatic.A parameter study shows that the effect is most sensitive to the relative dimensions of the folded membrane structure and to the salt permeability. The effect of stirring within the folds is also studied. In the general case, the system of two-dimensional diffusion equations is integrated numerically; an analytical solution is presented for the special case of negligible convection coupling. The calculations show that salt-concentration profiles within the folds establish a distribution of thermodynamic driving forces across the membrane barrier which differs significantly from that found across a plane membrane separating the same bathing solutions. The overall behavior of the flows through folded membrane structures is thus nonlinear.     

Immersion weighing for recording the flows through macroscopic membranes

The continuous record of the apparent weight of a membrane-bounded small vessel immersed in a solution of different density as compared to that inside it, offers a cheap and reliable possibility to measure osmotic flows at 10?5 kg · m^?2 · s^?1 sensitivity. The method equally applies for diffusional (isotopic) water flows.