Effects of solvent and solute drag on transmembrane diffusion

The present study compares and quantitates both solvent drag and solute drag forces in a system with both heteropore and homopore membranes. It is shown that tracer solute permeability can be increased if solution flow or driver solute flux is in the direction of tracer diffusion. Either force can decrease tracer permeability if the force can decrease tracer permeability if the force is opposite to the direction of tracer diffusion. The two forces can be additive or one force may reduce the effect of the other force. In the particular system quantitated, solute drag is shown to be some 300 times more effective than solvent drag on a mole-to-mole basis. The use of a number of solute pairs on other homopore and heteropore membranes confirms the finding that the two drag forces can be analyzed or manipulated in a variety of systems.     

Further Observations on Asymmetrical Solute Movement across Membranes

The permeability of frog skin under the influence of urea hyperosmolarity has been studied. Flux ratio asymmetry has been demonstrated again for tracer mannitol. The inhibitors DNP, CN^-, and ouabain have been used to eliminate active sodium transport and it was found that urea hyperosmolarity produces asymmetrical mannitol fluxes on frog skins having no short-circuit current. These findings suggest that flux ratio asymmetry is due to solute interaction and is unrelated to sodium transport. Studies with a synthetic membrane show clearly that bulk flow of fluid can produce a "solvent drag" effect and change flux ratios. When bulk flow is blocked and solute gradients allowed their full expression, then solute interaction "solute drag" is easily demonstrable in a synthetic system.     

Role of solute drag in intestinal transport

This study presents experiments related to the role of solvent drag and solute drag in the transmembrane movement of nonelectrolytes in a perfused rat intestine preparation. Conditions were chosen to simulate the effects of luminal hyperosmolarity on the permeability of tracer solutes. Data are presented on net water flux, transepithelial potentials, and lumen-to-blood and blood-to-lumen tracer solute movements during control electrolyte perfusion and after making the perfusate hyperosmotic. The results indicate that both solvent drag and solute drag can play significant roles in the transepithelial movement of solute and solute permeabilities in the rat ileum preparation. It is suggested that the potential roles of solvent drag and solute drag should be accounted for or considered during the characterization of the mechanisms of biological membrane function.     

Inhibition of Mung Bean UDP-Glucose: (1-->3)-beta-Glucan Synthase by UDP-Pyridoxal: Evidence for an Active-Site Amino Group

UDP-pyridoxal competitively inhibits the Ca²⁺-, cellobiose-activated (1→3)-β-glucan synthase activity of unfractionated mung bean (Vigna radiata) membranes, with a K_i of 3.8 ± 0.7 micromolar, when added simultaneously with the substrate UDP-glucose in brief (3 minute) assays. Preincubation of membranes with UDP-pyridoxal and no UDP-glucose, however, causes progressive reduction of the V_max of subsequently assayed enzyme and, after equilibrium is reached, 50% inhibition occurs with 0.84 ± 0.05 micromolar UDP-pyridoxal. This progressive inhibition is reversible provided that the UDP-pyridoxylated membranes are not treated with borohydride, indicating formation of a Schiff's base between the inhibitor and an enzyme amino group. Consistent with this, UDP-pyridoxine is not an inhibitor. The reaction of (1→3)-β-glucan synthase with UDP-pyridoxal is stimulated strongly by Ca²⁺ and, less effectively, by cellobiose or sucrose, and the enzyme is protected against UDP-pyridoxal by UDP-glucose or by other competitive inhibitors, implying that modification is occurring at the active site. Pyridoxal phosphate is a less potent and less specific inhibitor. Latent (1→3)-β-glucan synthase activity inside membrane vesicles can be unmasked and rendered sensitive to UDP-pyridoxal by the addition of digitonin. Treatment of membrane proteins with UDP-[³H]pyridoxal and borohydride labels a number of polypeptides but labeling of none of these specifically requires Ca²⁺ and sucrose; however, a polypeptide of molecular weight 42,000 is labeled by UDP-[³H]pyridoxal in the presence of Mg²⁺ and copurifies with (1→3)-β-glucan synthase activity.     

Chloride Transport in Porous Lipid Bilayer Membranes

This paper describes dissipative Cl_- transport in "porous" lipid bilayer membranes, i.e., cholesterol-containing membranes exposed to 1–3 x 10^-7 M amphotericin B. P_DCl (cm·s^-1), the diffusional permeability coefficient for Cl^-, estimated from unidirectional ³⁶Cl^- fluxes at zero volume flow, varied linearly with the membrane conductance (Gm, Ω^-1·cm^-2) when the contributions of unstirred layers to the resistance to tracer diffusion were relatively small with respect to the membranes; in 0.05 M NaCl, P_DCl was 1.36 x 10^-4 cm·s^-1 when Gm was 0.02 Ω^-1·cm^-2. Net chloride fluxes were measured either in the presence of imposed concentration gradients or electrical potential differences. Under both sets of conditions: the values of P_DCl computed from zero volume flow experiments described net chloride fluxes; the net chloride fluxes accounted for ~90–95% of the membrane current density; and, the chloride flux ratio conformed to the Ussing independence relationship. Thus, it is likely that Cl^- traversed aqueous pores in these anion-permselective membranes via a simple diffusion process. The zero current membrane potentials measured when the aqueous phases contained asymmetrical NaCl solutions could be expressed in terms of the Goldman-Hodgkin-Katz constant field equation, assuming that the P_DNa/P_DCl ratio was 0.05. In symmetrical salt solutions, the current-voltage properties of these membranes were linear; in asymmetrical NaCl solutions, the membranes exhibited electrical rectification consistent with constant-field theory. It seems likely that the space charge density in these porous membranes is sufficiently low that the potential gradient within the membranes is approximately linear; and, that the pores are not electrically neutral, presumably because the Debye length within the membrane phase approximates the membrane thickness.     

Transmembrane Protein Diffusion in Gel-Supported Dual-Leaflet Membranes

Tools to measure transmembrane-protein diffusion in lipid bilayer membranes have advanced in recent decades, providing a need for predictive theoretical models that account for interleaflet leaflet friction on tracer mobility. Here we address the fully three-dimensional flows driven by a (nonprotruding) transmembrane protein embedded in a dual-leaflet membrane that is supported above and below by soft porous supports (e.g., hydrogel or extracellular matrix), each of which has a prescribed permeability and solvent viscosity. For asymmetric configurations, i.e., supports with contrasting permeability, as realized for cells in contact with hydrogel scaffolds or culture media, the diffusion coefficient can reflect interleaflet friction. Reasonable approximations, for sufficiently large tracers on low-permeability supports, are furnished by a recent phenomenological theory from the literature. Interpreting literature data, albeit for hard-supported membranes, provides a theoretical basis for the phenomenological Stokes drag law as well as strengthening assertions that nonhydrodynamic interactions are important in supported bilayer systems, possibly leading to overestimates of the membrane/leaflet viscosity. Our theory provides a theoretical foundation for future experimental studies of tracer diffusion in gel-supported membranes.     

Theoretical Analysis of Net Tracer Flux Due to Volume Circulation in a Membrane with Pores of Different Sizes : Relation to solute drag model

When osmotic pressure across an artificial membrane, produced by a permeable electrically neutral solute on one side of it, is balanced by an external pressure difference so that there is no net volume flow across the membrane, it has been found that there will be a net flux of a second electrically neutral tracer solute, present at equal concentrations on either side of the membrane, in the direction that the "osmotic" solute diffuses. This has been ascribed to solute-solute interaction or drag between the tracer and the osmotic solutes. An alternative model, presented here, considers the membrane to have pores of different sizes. Under general assumptions, this "heteroporous" model will account for both the direction of net tracer flux and the observed linear dependence of unidirectional tracer fluxes on the concentration of the osmotic solute. The expressions for the fluxes of solutes and solvent are mathematically identical under the two models. An inequality is derived which must be valid if the solute interaction model and/or the heteroporous model can account for the data. If the inequality does not hold, then the heteroporous model alone cannot explain the data. It was found that the inequality holds for most published observations except when dextran is the osmotic solute.     

The Lantibiotic Nisin Induces Transmembrane Movement of a Fluorescent Phospholipid

Nisin is a pore-forming antimicrobial peptide. The capacity of nisin to induce transmembrane movement of a fluorescent phospholipid in lipid vesicles was investigated. Unilamellar phospholipid vesicles that contained a fluorescent phospholipid (1-acyl-2-{6-[(7-nitro-2-1,3-benzoxadiazol-4-yl)amino]caproyl}-sn-glycero-3-phosphocholine) in the inner leaflet of the bilayer were used. Nisin-induced movement of the fluorescent phospholipid from the inner leaflet to the outer leaflet of the membrane reached stable levels, which were dependent on the concentration of nisin added. The rate constant k of this nisin-induced transmembrane movement increased with the nisin concentration but was not dependent on temperature within the range of 5 to 30°C. In contrast, the rate constant of movement of fluorescent phospholipid from vesicle to vesicle strongly depended on temperature. The data indicate that nisin transiently disturbs the phospholipid organization of the target membrane.     

Effect of Sucrose on Phase Behavior of Membranes in Intact Pollen of Typha latifolia L., as Measured with Fourier Transform Infrared Spectroscopy

In previous studies, we have shown that the temperature dependent vibrational frequency of the CH₂ stretch in hydrocarbons in intact pollen grains can be recorded with Fourier transform infrared spectroscopy and used to measure phase transition temperatures (T_m) in these hydrocarbons. Circumstantial evidence was provided that the major contribution to the signal seen in these samples was from membrane phospholipids, and that sucrose in the dry pollen grains reduced T_m of those phospholipids. In the current study, we clarify why a major constituent of the pollen grains, neutral lipids contained in discrete lipid droplets, does not contribute significantly to the signal. Further, we have isolated membranes from the pollen and show that T_m in the isolated membranes rises from −6°C in the hydrated membranes to 58°C when the membranes are dried without the addition of sucrose. However, when the isolated membranes are dried in the presence of increasing amounts of sucrose, T_m fell steadily, reaching a minimal value of 31°C, a figure in good agreement with that seen in the intact pollen grains. The amount of sucrose required to depress T_m maximally in these membranes is also apparently in agreement with that found in the intact pollen, suggesting that sucrose depresses T_m in the pollen.     

Identification of three porins in the outer membrane of Bacteroides fragilis

Abstract Four outer membrane proteins were purified to homogeneity from isolated outer membranes of Bacteroides fragilis ; three ( M _r 51000, 92000 and 125 000) had pore-forming activity in reconstituted liposomes as determined by swelling assay. Membrane vesicles containing the M _rmr 55 000 outer membrane protein showed no detectable pore-forming activity. The three B. fragilis porins formed pores that allowed the penetration of uncharged saccharides of M _r lower than 340–400, even though the efficiency of solute diffusion showed slight differences. The diffusion rates of glucose through the porins appeared to be lower than those through Escherichia coli porins.     

ELECTRON MICROSCOPE AND EXPERIMENTAL INVESTIGATIONS OF THE NEUROFILAMENTOUS NETWORK IN DEITERS'' NEURONS : Relationship with the Cell Surface and Nuclear Pores

The assembly of filamentous elements and their relations to the plasma membrane and to the nuclear pores have been studied in Deiters' neurons of rabbit brain. Electron microscopy of thin sections and of ectoplasm spread preparations have been integrated with physicochemical experiments and differential interference microscopy of freshly isolated cells. A neurofilamentous network extends as a continuous, three-dimensional, semilattice structure throughout the ectoplasm, the "plasma roads," and the perinuclear zone of the perikaryon. This space network consists of ~90-Å wide neurofilaments arranged in fascicles which are interconnected by an exchange of neurofilaments. The neurofilaments consist of intercoiled ~20-Å wide unit-filaments and are associated through cross-associating filaments with other neurofilaments of the fascicle and with microfilaments. The ~20–50-Å wide microfilaments display intimate associations with the plasma membrane and with the nuclear pores. Electron microscopy of thin sections from glycerinated and heavy meromyosin-treated Deiters' neurons shows that actin-like filaments are present in the pre- and postsynaptic regions of synapses terminating on these neurons. It is proposed that the neurofilamentous space network serves a transducing function by linking plasma membrane activities with the genetic machinery of the neuron.     

STUDIES OF EXCITABLE MEMBRANES : I. Macromolecular Specializations of the Neuromuscular Junction and the Nonjunctional Sarcolemma

The neuromuscular junctions and nonjunctional sarcolemmas of mammalian skeletal muscle fibers were studied by conventional thin-section electron microscopy and freeze-fracture techniques. A modified acetylcholinesterase staining procedure that is compatible with light microscopy, conventional thin-section electron microscopy, and freeze-fracture techniques is described. Freeze-fracture replicas were utilized to visualize the internal macromolecular architecture of the nerve terminal membrane, the chemically excitable neuromuscular junction postsynaptic folds, and the electrically excitable nonjunctional sarcolemma. The nerve terminal membrane is characterized by two parallel rows of 100–110-Å particles which may be associated with synpatic vesicle fusion and release. On the postsynpatic folds, irregular rows of densely packed 110–140-Å particles were observed and evidence is assembled which indicates that these large transmembrane macromolecules may represent the morphological correlate for functional acetylcholine receptor activity in mammalian motor endplates. Differences in the size and distribution of particles in mammalian as compared with amphibian and fish postsynaptic junctional membranes are correlated with current biochemical and electron micrograph autoradiographic data. Orthogonal arrays of 60-Å particles were observed in the split postsynaptic sarcolemmas of many diaphragm myofibers. On the basis of differences in the number and distribution of these "square" arrays within the sarcolemmas, two classes of fibers were identified in the diaphragm. Subsequent confirmation of the fiber types as fast- and slow-twitch fibers (Ellisman et al. 1974. J. Cell Biol. 63[2, Pt. 2]:93 a. [Abstr.]) may indicate a possible role for the square arrays in the electrogenic mechanism. Experiments in progress involving specific labeling techniques are expected to permit positive identification of many of these intriguing transmembrane macromolecules.     

Annular Anionic Lipids Stabilize the Integrin αIIbβ3 Transmembrane Complex

Cationic membrane-proximal amino acids determine the topology of membrane proteins by interacting with anionic lipids that are restricted to the intracellular membrane leaflet. This mechanism implies that anionic lipids interfere with electrostatic interactions of membrane proteins. The integrin αIIbβ3 transmembrane (TM) complex is stabilized by a membrane-proximal αIIb(Arg⁹⁹⁵)-β3(Asp⁷²³) interaction; here, we examine the influence of anionic lipids on this complex. Anionic lipids compete for αIIb(Arg⁹⁹⁵) contacts with β3(Asp⁷²³) but paradoxically do not diminish the contribution of αIIb(Arg⁹⁹⁵)-β3(Asp⁷²³) to TM complex stability. Overall, anionic lipids in annular positions stabilize the αIIbβ3 TM complex by up to 0.50 ± 0.02 kcal/mol relative to zwitterionic lipids in a headgroup structure-dependent manner. Comparatively, integrin receptor activation requires TM complex destabilization of 1.5 ± 0.2 kcal/mol, revealing a sizeable influence of lipid composition on TM complex stability. We implicate changes in lipid headgroup accessibility to small molecules (physical membrane characteristics) and specific but dynamic protein-lipid contacts in this TM helix-helix stabilization. Thus, anionic lipids in ubiquitous annular positions can benefit the stability of membrane proteins while leaving membrane-proximal electrostatic interactions intact.     

POROUS SUBSTRUCTURE OF THE GLOMERULAR SLIT DIAPHRAGM IN THE RAT AND MOUSE

The highly ordered, isoporous substructure of the glomerular slit diaphragm was revealed in rat and mouse kidneys fixed by perfusion with tannic acid and glutaraldehyde. The slit diaphragm was similar in both animal species and appeared as a continuous junctional band, 300–450 Å wide, consistently present within all slits formed by the epithelial foot processes. The diaphragm exhibited a zipper-like substructure with alternating, periodic cross bridges extending from the podocyte plasma membranes to a central filament which ran parallel to and equidistant from the cell membranes. The dimensions and spacing of the cross bridges defined a uniform population of rectangular pores approximately 40 by 140 Å in cross section and 70 Å in length. The total area of the pores was calculated to be about 2–3% of the total surface area of the glomerular capillaries. Physiological data indicate that the glomerular filter functions as if it were an isoporous membrane which excludes proteins larger than serum albumin. The similarity between the dimensions of the pores in the slit diaphragm and estimates for the size and shape of serum albumin supports the conclusion from tracer experiments that the slit diaphragm may serve as the principal filtration barrier to plasma proteins in the kidney.     

Transport across homoporous and heteroporous membranes in nonideal, nondilute solutions. II. Inequality of phenomenological and tracer solute permeabilities.

The phenomenological solute permeability (omega p) of a membrane measures the flux of solute across it when the concentrations of the solutions on the two sides of the membrane differ. The relationship between omega p and the the conventionally measured tracer permeability (omega T) is examined for homoporous and heteroporous (parallel path) membranes in nonideal, nondilute solutions and in the presence of boundary layers. In general, omega p and omega T are not equal; therefore, predictions of transmembrane solute flux based on omega T are always subject to error. For a homoporous membrane, the two permeabilities become equal as the solutions become ideal and dilute. For heteroporous membranes, omega p is always greater than omega T. An upper bound on omega p- omega T is derived to provide an estimate of the maximum error in predicted solute flux. This bound is also used to show that the difference between omega P and omega T demonstrated earlier for the sucrose-Cuprophan system can be explained if the membrane is heteroporous. The expressions for omega P developed here support the use of a modified osmotic driving force to describe membrane transport in nonideal, nondilute solutions.     

The Kinetics of Osmotic Transport through Pores of Molecular Dimensions

This paper presents a theoretical analysis of the kinetics of osmotic transport across a semipermeable membrane. There is a thermodynamic connection between the rate of flow under a hydrostatic pressure difference and the rate of flow due to a difference in solute concentration on the two sides. One might therefore attempt to calculate the osmotic transport coefficient by applying Poiseuille's equation to the flow produced by a difference in hydrostatic pressure. Such a procedure is, however, inappropriate if the pores in the membrane are too small to allow molecules to “overtake.” It then becomes necessary to perform a statistical calculation of the transport coefficient, and such a calculation is described in this paper. The resulting expression for the number of solvent molecules passing through a pore per second is J = m D₁ δn₁/l² where m is the number of solvent molecules in the pore, l is the length of the pore, D₁ is the self-diffusion coefficient of the solute, and δn₁ the difference in solvent mole fraction on the two sides of the membrane. This equation is used for estimating the number of pores per unit area of the squid axon membrane; the result is 6 × 10⁹ pores/cm².     

Breakdown of lipid bilayer membranes in an electric field

The behaviour of lipid bilayer membranes, made of oxidized cholesterol, and UO₂²⁺-modified azolectin membranes in a high electric field has been investigated using the voltage clamp method. When a voltage pulse is applied to the membrane of these compositions, the mechanical rupture of the membranes is preceded by a gradual conductance increase which remains quite reversible till a certain moment. The voltage drop at this reversible stage of breakdown leads to a very rapid (characteristic time of less than 5 μs) decrease in the membrane conductance. At repeated voltage pulses of the same amplitude with sufficient intervals between them (approx. 10 s), the current oscillograms reflecting the reversible resistance decrease are well reproduced on the same membrane. The time of attainment of the predetermined level of the membrane conductance is strongly dependent on voltage. At different stages of breakdown we have investigated changes in the conductance of UO₂²⁺-modified membrane after the application of two-step voltage pulses, the kinetics of development of the reversible decrease in the membrane resistance in solutions of univalent and divalent ions, and also the influence of sucrose and hemoglobin on the current evolution. The relationship between the reversible conductance increase, the reversible electrical breakdown [15] and the rupture of membrane in an electric field is discussed. We propose the general interpretation of these phenomena, based on the representation of the potential-dependent appearance in the membrane of pores, the development of which is promoted by an electric field.     

Mechanisms of equinatoxin II-induced transport through the membrane of a giant phospholipid vesicle

Protein equinatoxin II from sea anemone Actinia equina L. was used to form pores in phospholipid membranes. We studied the effect of these pores on the net transmembrane transport of sucrose and glucose by observing single giant (cell-size) vesicles under the phase contrast microscope. Sugar composition in the vesicle was determined by measuring the width of the halo, which appears around the vesicle in the phase contrast image. The transport of sugars was induced when a vesicle, filled with the sucrose solution, was transferred into the isomolar environment of a glucose solution with added equinatoxin II. Typically, a vesicle grew to a critical size, then the membrane broke by bursting and the vesicle shrank, started to grow again, and the whole process was repeated. The consecutive membrane breaks occurred in the same spot. The observed behavior was interpreted by the diffusion flow of the glucose molecules through the equinatoxin II-induced pores and the consequent increase of the vesicle water content. The burst relaxed the critically strained membrane, which then apparently resealed. A mathematical model of the described behavior was developed and was used to obtain the equinatoxin II-induced membrane permeability for the glucose molecules. Its dependence on the equinatoxin II concentration is in agreement with the previous reports.     

Steady State Sodium and Rubidium Effluxes in Pisum sativum Roots

Steady state effluxes of potassium and sodium ions were measured on Pisum sativum var. Alaska root segments excised from seedlings which had grown in a nutrient solution containing the major inorganic ions and either ⁸⁶Rb as a tracer for K or ²²Na as a tracer for Na. Fluxes appeared to be from 2 cellular compartments, a small compartment with a high flux rate and a larger compartment with a slow flux rate. Cell wall exchange fluxes are believed to have been negligible. Efflux rates for 11.3% and 88.7% of cellular potassium ions were 6 × 10⁻⁷ and 1.32 × 10⁻⁷ respectively; rates for 33.7% and 66.3% of cellular sodium ions were 1.48 × 10⁻⁷ and 3.83 × 10⁻⁸ respectively, (equivalents per gram fr wt per hr). The sodium flux measurements, with previous measurements of ionic concentrations and transmembrane potentials, support the theory that sodium is transported actively from Pisum roots.