Showing papers on "Membrane published in 1988"

Muscarinic activation of ionic currents measured by a new whole-cell recording method

Abstract: A new method is described as an alternative to whole-cell recording in order to prevent "wash-out" of the muscarinic response to acetylcholine (ACh) in rat lacrimal gland cells. The membrane of a cell-attached patch is permeabilized by nystatin in the patch pipette, thus providing electrical continuity between the pipette and the cytoplasm of the cell without the loss or alteration of cytoplasmic compounds necessary for the maintenance of the response to ACh. With normal whole-cell recording in these cells, the response to ACh, seen as the activation of Ca-activated K and Cl currents, lasts for approximately 5 min. With the nystatin method, the response is not diminished after 1 h. Nystatin, applied extracellularly, is shown to cause a rapid and reversible increase of membrane conductance to cations. In the absence of wash-out, we were able to obtain dose-response curves for the effect of ACh on Ca-activated K currents. An increase of [ACh] caused an increase in the K current, with apparent saturation at concentrations above approximately 1 microM ACh. The delay between ACh application and the activation of K current was inversely related to [ACh] and reached a minimum value of 0.7-1.0 s at high [ACh].

Structural and Functional Roles of Glycosyl-Phosphatidylinositol in Membranes

Abstract: Glycosylated forms of phosphatidylinositol, which have only recently been described in eukaryotic organisms, are now known to play important roles in biological membrane function. These molecules can serve as the sole means by which particular cell-surface proteins are anchored to the membrane. Lipids with similar structures may also be involved in signal transduction mechanisms for the hormone insulin. The utilization of this novel class of lipid molecules for these two distinct functions suggests new mechanisms for the regulation of proteins in biological membranes.

Novel amphiphilic nucleic acid conjugates

Abstract: Novel oligonucleotide conjugates are provided, where oligonucleotides are joined through a linking arm to a hydrophobic moiety. The resulting conjugates are more efficient in membrane transport, so as to be capable of crossing the membrane and effectively modulating a transcriptional system. In this way, the compositions can be used in vitro and in vivo, for studying cellular processes, protecting mammalian hosts from pathogens, and the like.

Channel-forming properties of cecropins and related model compounds incorporated into planar lipid membranes

Abstract: Cecropins, positively charged antibacterial peptides found in the cecropia moth, and synthetic peptide analogs form large time-variant and voltage-dependent ion channels in planar lipid membranes in the physiological range of concentration. Single-channel conductances of up to 2.5 nS (in 0.1 M NaCl) were observed, which suggests a channel diameter of 4 nm. Channels formed by the peptides cecropin AD and MP3 had a permeability ratio of Cl-/Na+ = 2:1 in 0.1 M NaCl. A comparative study of the three cecropins, cecropins A, B, and D, and of six synthetic analogs allowed determination of structural requirements for pore formation. Shorter amphipathic peptides did not form channels, although they adsorbed to the bilayer. A flexible segment between the N-terminal amphipathic region and the C-terminal more hydrophobic region of the peptide was required for the observation of a time-variant, voltage-dependent conductance. Cecropin AD was the most effective voltage-dependent pore-forming peptide and was also the most potent antibacterial peptide against several test organisms. A positive surface charge or cholesterol in the bilayer reduced the conductances caused by cecropin AD or MP3 by at least 5-fold. This behavior is consistent with the known insensitivity of eukaryotic cells to cecropins. Our observations suggest that the broad antibacterial activity of cecropins is due to formation of large pores in bacterial cell membranes.

Lipid dynamics in fish: aspects of absorption, transportation, deposition and mobilization.

Abstract: 1. Aspects of lipid metabolism, including absorption and depositional processes, appear quite different in fish as compared to homeothermic vertebrates. 2. Dietary lipids in fish are absorbed as fatty acids and as triacylglycerols aggregated into chylomicra particles. 3. Interorgan transport of lipids, like that of mammals, consists of an exogenous (dietary) loop and an endogenous loop. 4. Fish store lipids among several depot organs, including mesenteric membranes, liver and muscle. 5. Several fast-acting and slow-acting agents modulate depot lipid mobilization. 6. Mobilized lipids may be transported in the serum as free fatty acids bound to specific carrier proteins.

Protein translocation across membranes

Abstract: Many newly synthesized proteins must be translocated across a membrane to reach their final destinations. Translocation requires a signal on the protein itself, a loose conformation of the protein, energy, and receptor-like components in the cytosol and on the target membrane.

Diffusion of carbon dioxide through lipid bilayer membranes: effects of carbonic anhydrase, bicarbonate, and unstirred layers.

Abstract: Diffusion of (14)C-labeled CO(2) was measured through lipid bilayer membranes composed of egg lecithin and cholesterol (1:1 mol ratio) dissolved in n-decane. The results indicate that CO(2), but not HCO(3-), crosses the membrane and that different steps in the transport process are rate limiting under different conditions. In one series of experiments we studied one-way fluxes between identical solutions at constant pCO(2) but differing [HCO(3-)] and pH. In the absence of carbonic anhydrase (CA) the diffusion of CO(2) through the aqueous unstirred layers is rate limiting because the uncatalyzed hydration-dehydration of CO(2) is too slow to permit the high [HCO(3-)] to facilitate tracer diffusion through the unstirred layers. Addition of CA (ca. 1 mg/ml) to both bathing solutions causes a 10-100-fold stimulation of the CO(2) flux, which is proportional to [HCO(3-)] over the pH range 7-8. In the presence of CA the hydration- dehydration reaction is so fast that CO(2) transport across the entire system is rate limited by diffusion of HCO(3-) through unstirred layers. However, in the presence of CA when the ratio [HCO(3-) + CO(3=)]:[CO(2)] more than 1,000 (pH 9-10) the CO(2) flux reaches a maximum value. Under these conditions the diffusion of CO(2) through the membrane becomes rate limiting, which allows us to estimate a permeability coefficient of the membrane to CO(2) of 0.35 cm s(-1). In a second series of experiments we studied the effects of CA and buffer concentration on the net flux of CO(2). CA stimulates the net CO(2) flux in well buffered, but no in unbuffered, solutions. The buffer provides a proton source on the upstream side of the membrane and proton sink on the downstream side, thus allowing HCO(3-) to facilitate the net transport of CO(2) through the unstirred layers.

A mechanism of liposome electroformation

Abstract: External electric fields can induce or prevent liquid swelling and liposome formation on solid surfaces. These effects depend on the type of lipid and surface, the medium parameters (temperature, osmolarity, ionic strength), the dried lipid layer thickness, the type and parameters of the electric field (dc or ac, amplitude, frequency, current), the situation of the lipid (on the very electrode surface or an another surface) and the time of exposure. This paper presents new data and theoretical estimates which allow us to suggest a possible mechanism of liposome electroformation. The new experimental results with negatively charged egg lecithin (EggL−) and neutral synthetic phosphatidylcholine (PC) are that (1) cholesterol in mixtures with EggL− inhibits liposome formation, (2) sodium chloride leads to a decrease in the size and number of liposomes; liposomes do not form in solution more concentrated than 10 mM NaCl, (3) dextran also decreases the size and number of liposomes; they do not form in solution of dextran concentration higher than 2.5 mM, (4) dc electric fields can overcome the effect of dextran and lead to swelling even in 2.5 mM dextran solutions, (5) dc fields are most effective if applied in the first 30 s of lipid swelling, the increase of the period of the field action does not lead to a significant effect on the liposome yield, (6) a similar effect was also observed with (PC), but the critical period of time was a bit shorter — 10 s. The balance of forces acting on a lamellae of hydrating lipid shows that a possible pathway of liposome formation can include at least three basic stages: (1) separation of interacting membranes — the hydration and electrostatic interactions yield the main driving forces for this process, (2) instability of bending which can result from negative membrane tension due to surface and line tension and (3) the bending itself and closing of the membranes — the kinetics of this process and the instability of bending largely determine the liposome size distribution. External electric fields can affect any of these stages by at least two mechanisms: (1) direct electrostatic interaction and (2) redistribution of the counter-ions between the membranes. The interplay between the van der Waals, hydration and electrostatic forces is the major determinant of the mechanisms of liposome electroformation.

Enhancement of uphill transport by a double carrier membrane system.

Abstract: We report a preliminary study on double carrier membrane systems that mimic, in principle, the function of ATPase, although chemical compounds involved are totally different. It shown that the transport of ions with a double carrier membrane system is facilitated more efficiently than the conventional symport system

Multiplex DNA Sequencing

Abstract: The increasing demand for DNA sequences can be met by replacement of each DNA sample in a device with a mixture of N samples so that the normal throughput is increased by a factor of N. Such a method is described. In order to separate the sequence information at the end of the processing, the DNA molecules of interest are ligated to a set of oligonucleotide "tags" at the beginning. The tagged DNA molecules are pooled, amplified, and chemically fragmented in 96-well plates. The resulting reaction products are fractionated by size on sequencing gels and transferred to nylon membranes. These membranes are then probed as many times as there are types of tags in the original pools, producing, in each cycle of probing, autoradiographs similar to those from standard DNA sequencing methods. Thus, each reaction and gel yields a quantity of data equivalent to that obtained from conventional reactions and gels multiplied by the number of probes used. To date, even after 50 successive probings, the original signal strength and the image quality are retained, an indication that the upper limit for the number of reprobings may be considerably higher.

Glucose electrode and method of determining glucose

Abstract: An electrode assembly for polarographic assay of glucose in solution having a laminated membrane, the outer membrane being from 1 to 20 μm thick and having a pore size of 10 to 125 Å, the inner membrane having a thickness of 2 to 4 μm, and bonded to the outer membrane with immobilized glucose oxidase. Rapid assays are carried out by measuring the current difference before equilibrium is attained and at least 5 seconds after initial contact of sample with the outer membrane.

Molecular dynamics simulation of a smectic liquid crystal with atomic detail

Abstract: A molecular dynamics simulation of a sodium–decanoate/decanol/water system is reported. The system is treated in full atomic detail, with the exception of CH2 and CH3 groups that are considered to be ‘‘united atoms,’’ and is a refinement of a previous model membrane [Mol. Phys. 11, 1 (1983)]. The long‐range Coulomb interactions were included specifically. The order parameters of the chain units of the lipids and diffusion constants of components in the system calculated from the simulation agree well with those reported in experiments on this model membrane. The overall structure of the membrane shows considerable disorder, with a broad lipid–water interface, extending over approximately 1 nm. The distribution of the components is such that an almost complete charge cancellation occurs throughout the system, which is in contradiction with the generally assumed electrical double layer structure for membranes. A counterion condensation of 70% is observed. Both the translational and the rotational motions of water are slowed down compared to bulk water. The penetration of water into the hydrocarbon region of the membrane is substantial. Pair correlations of various atom pairs, and dihedral statistics and transition rates of the dihedrals in the lipids are reported. The distributions of chain segments of the lipids, of water molecules, and of sodium ions are compared with theoretical predictions.

Increased amounts of HMG-CoA reductase induce "karmellae": a proliferation of stacked membrane pairs surrounding the yeast nucleus.

Abstract: Overproduction of the enzyme 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase in yeast resulted in striking morphological effects on the structure of intracellular membranes. Specifically, stacks of paired membranes closely associated with the nuclear envelope were observed in strains that over-produced the HMG1 isozyme, one of two isozymes for HMG-CoA reductase in yeast. These nuclear-associated, paired membranes have been named "karmellae." In strains that overproduced the HMG1 isozyme, HMG-CoA reductase was present in the karmellar layers. At mitosis, karmellae were asymmetrically segregated: the mother cells inherited all of the karmellae and the daughter cells inherited none. A membranous structure of different morphology was occasionally found in cells that overproduced the HMG2 isozyme. These observations further establish the existence of cellular mechanisms that monitor the levels of membrane proteins and compensate for changes in these levels by inducing synthesis of particular types of membrane.

Changes induced in the permeability barrier of the yeast plasma membrane by cupric ion.

Abstract: A specific effect of Cu2+ eliciting selective changes in the permeability of intact Saccharomyces cerevisiae cells is described. When 100 microM CuCl2 was added to a cell suspension in a buffer of low ionic strength, the permeability barrier of the plasma membranes of the cells was lost within 2 min at 25 degrees C. The release of amino acids was partial, and the composition of the amino acids released was different from that of those retained in the cells. Mostly glutamate was released, but arginine was mainly retained in the cells. Cellular K+ was released rapidly after CuCl2 addition, but 30% of the total K+ was retained in the cells. These and other observations suggested that Cu2+ caused selective lesions of the permeability barrier of the plasma membrane but did not affect the permeability of the vacuolar membrane. These selective changes were not induced by the other divalent cations tested. A novel and simple method for differential extraction of vacuolar and cytosolic amino acid pools by Cu2+ treatment was established. When Ca2+ was added to Cu2+-treated cells, a large amount of Ca2+ was sequestered into vacuoles, with formation of an inclusion of a Ca2+-polyphosphate complex in the vacuoles. Cu2+-treated cells also showed enhanced uptake of basic amino acids and S-adenosylmethionine. The transport of these substrates showed saturable kinetics with low affinities, reflecting the vacuolar transport process in situ. With Cu2+ treatment, selective leakage of K+ from the cytosolic compartment appears to create a large concentration gradient of K+ across the vacuolar membrane and generates an inside-negative membrane potential, which may provide a driving force of uptake of positively charged substances into vacuoles. Cu2+ treatment provides a useful in situ method for investigating the mechanisms of differential solute pool formation and specific transport phenomena across the vacuolar membrane.

Thermomechanical and transition properties of dimyristoylphosphatidylcholine/cholesterol bilayers.

Abstract: Mixtures of dimyristoylphosphatidylcholine (DMPC) and cholesterol (Chol) have been used to examine the effects of cholesterol on the chain crystallization transitions and thermomechanical properties in phospholipid bilayer membranes. The mechanical properties--elastic moduli and level of tension at membrane rupture--were derived from micropipet pressurization of giant single-walled vesicles. Also, the micropipet method allowed temperature-dependent area transitions to be measured at constant membrane tension. X-ray diffraction measurements were made on selected lipid/cholesterol mixtures. Wide-angle patterns and electron density profiles were used to measure bilayer thickness as an indication of chain tilt and fluidity. Vesicle area versus temperature plots showed that the main acyl chain crystallization transition of DMPC broadened and shifted to higher temperatures. Both above and below the broad transition, the elastic area compressibility modulus, K, was greatly increased with cholesterol addition. The value for the 1:1 DMPC/Chol complex was found to be approximately 700 dyn/cm, comparable to that for DMPC in the L beta' phase. However, for all concentrations above 12.5 mol % (which was weakly solid), vesicle bilayers behaved as surface liquids with no surface shear rigidity even at temperatures well below the DMPC phase transition. Area changes over the broadened transitions were reduced by cholesterol and disappeared with the addition of 50 mol % to leave the thermal area expansivity at 1.3 X 10(-3)/degrees C. These area changes are consistent with separate formation of a 1:1 DMPC/Chol complex that does not condense plus residual free lipid and lipid loosely associated with the 1:1 complex that freezes normally.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for reduced Cl- and increased Na+ permeability in cystic fibrosis human primary cell cultures.

Abstract: 1. Employing a primary cell culture system and intracellular microelectrodes, we quantitated and compared the Na+ and Cl- pathways in apical membranes of normal and cystic fibrosis (CF) human airway epithelia. 2. Like the transepithelial difference (PD) in situ, the PD of CF epithelia in culture (-27 +/- 4 mV, mean +/- S.E.M.; n = 28) exceeded the PD of normal epithelia (-10 +/- 1 mV; n = 22). The raised PD principally reflected an increase in the rate of active transport (equivalent short circuit, Ieq) for CF epithelia (61 +/- 9 microA cm-2) as compared with normal epithelia (23 +/- 3 microA cm-2). No significant differences in transepithelial resistance were detected. 3. As indicated by ion replacement studies (gluconate for Cl-), the apical membrane of normal cells exhibits an apical membrane Cl- conductance (GCl) that can be activated by isoprenaline. CF cells do not exhibit an apical membrane GCl, nor can a GCl be activated by isoprenaline. 4. CF cells exhibited a larger amiloride-sensitive Ieq and amiloride-sensitive apical membrane conductance (GNa) than normal cells. Further, the amiloride-sensitive Ieq was increased by isoprenaline in CF but not normal airway epithelia. 5. Equivalent circuit analysis yielded evidence for a more positive electromotive force (EMF) across the apical membrane and a more negative EMF across the basolateral membrane of CF cells as compared with normal cells. Baseline resistances of the apical (Ra) and basolateral (Rb) membranes did not differ for normal and CF cells. 6. Estimates of the resistance of the paracellular path to ion flow (Rs) by equivalent circuit analysis or ion substitution detected no differences in Rs between CF and normal cells. 7. We conclude that abnormalities in both cellular Cl- permeability (reduced) and Na+ permeability (increased) are characteristic of the cultured CF respiratory epithelial cell. These data suggest that a defect in the regulation of apical membrane permeabilities is a central feature of this disease.