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S McLaughlin

Publications and source records attributed to S McLaughlin.

At least 109 records · Page 6Linked to original sources
PubMed1986

An experimental test of new theoretical models for the electrokinetic properties of biological membranes. The effect of UO2++ and tetracaine on the electrophoretic mobility of bilayer membranes and human erythrocytes.

For a large smooth particle with charges at the surface, the electrophoretic mobility is proportional to the zeta potential, which is related to the charge density by the Gouy-Chapman theory of the diffuse double layer. This classical model adequately describes the dependence of the electrophoretic mobility of phospholipid vesicles on charge density and salt concentration, but it is not applicable to most biological cells, for which new theoretical models have been developed. We tested these new models experimentally by measuring the effect of UO2++ on the electrophoretic mobility of model membranes and human erythrocytes in 0.15 M NaCl at pH 5. We used UO2++ for these studies because it should adsorb specifically to the bilayer surface of the erythrocyte and should not change the density of fixed charges in the glycocalyx. Our experiments demonstrate that it forms high-affinity complexes with the phosphate groups of several phospholipids in a bilayer but does not bind significantly to sialic acid residues. As observed previously, UO2++ adsorbs strongly to egg phosphatidylcholine (PC) vesicles: 0.1 mM UO2++ changes the zeta potential of PC vesicles from 0 to +40 mV. It also has a large effect on the electrophoretic mobility of vesicles formed from mixtures of PC and the negative phospholipid phosphatidylserine (PS): 0.1 mM UO2++ changes the zeta potential of PC/PS vesicles (10 mol % PS) from -13 to +37 mV. In contrast, UO2++ has only a small effect on the electrophoretic mobility of either vesicles formed from mixtures of PC and the negative ganglioside GM1 or erythrocytes: 0.1 mM UO2++ changes the apparent zeta potential of PC/GM1 vesicles (17 mol % GM1) from -11 to +5 mV and the apparent zeta potential of erythrocytes from -12 to -4 mV. The new theoretical models suggest why UO2++ has a small effect on PC/GM1 vesicles and erythrocytes. First, large groups (e.g., sugar moieties) protruding from the surface of the PC/GM1 vesicles and erythrocytes exert hydrodynamic drag. Second, charges at the surface of a particle (e.g., adsorbed UO2++) exert a smaller effect on the mobility than charges located some distance from the surface (e.g., sialic acid residues).

Electrophoresis↗
PubMed1985

The interaction of calcium with gangliosides in bilayer membranes.

We studied the binding of calcium to bilayer membranes formed from mixtures of phosphatidylcholine and mono-, di-, or trisialoganglioside by measuring its effect on the electrophoretic mobility of multilamellar vesicles and the conductance of planar bilayers. In 0.001 M monovalent salt solutions the surface potential of the membranes is large and micromolar concentrations of calcium have a significant effect on the mobility and conductance. In 0.1 M monovalent salt solutions the surface potential is small and millimolar concentrations of calcium are required to affect these parameters. The strong apparent binding of calcium we observed at low ionic strength could be due to the nonspecific accumulation of calcium in the electrical diffuse double layer. To distinguish between this nonspecific effect and binding of calcium to the membrane, we substituted dimethonium for calcium. Dimethonium is a divalent cation that screens negative charges but does not bind to lipids. We also examined the effect of replacing phosphatidylcholine by monoolein: calcium binds to phosphatidylcholine but not to monoolein. We describe our electrophoretic mobility results by combining the Poisson-Boltzmann and Navier-Stokes equations with the Langmuir adsorption isotherm. We conclude that calcium binds weakly to gangliosides with an intrinsic association constant of less than 100 M-1, which is similar to the association constant of calcium with phospholipids.

Binding Sites↗
PubMed1985

Interaction of gentamicin and spermine with bilayer membranes containing negatively charged phospholipids.

We measured the electrophoretic mobility of multilamellar phospholipid vesicles, the 31P NMR spectra of both sonicated and multilamellar vesicles, and the conductance of planar bilayer membranes to study the binding of spermine and gentamicin to membranes. Spermine and gentamicin do not bind significantly to the zwitterionic lipid phosphatidylcholine. We measured the concentrations of gentamicin and spermine that reverse the charge on vesicles formed from a mixture of phosphatidylcholine and either phosphatidylserine or phosphatidylinositol. From these measurements, we determined that the intrinsic association constants of the cations with these negative lipids are all about 10 M-1. This value is orders of magnitude lower than the apparent binding constants reported in the literature by other groups because the negative electrostatic surface potential of the membranes and the resultant accumulation of these cations in the aqueous diffuse double layer adjacent to the membranes have not been explicitly considered in previous studies. Our main conclusion is that the Gouy-Chapman-Stern theory of the aqueous diffuse double layer can describe surprisingly well the interaction of gentamicin and spermine with bilayer membranes formed in a 0.1 M NaCl solution if the negative phospholipids constitute less than 50% of the membrane. Thus, the theory should be useful for describing the interactions of these cations with the bilayer component of biological membranes, which typically contain less than 50% negative lipids. For example, our results support the suggestion of Sastrasinh et al. [Sastrasinh, M., Krauss, T. C., Weinberg, J. M., & Humes, H. D. (1982) J. Pharmacol. Exp. Ther. 222, 350-358] that phosphatidylinositol is the major binding site for gentamicin in renal brush border membranes.

Electric Conductivity↗
PubMed1985

Electro-osmosis and the reabsorption of fluid in renal proximal tubules.

The lateral intercellular spaces (LIS) are believed to be the final common pathway for fluid reabsorption from the renal proximal tubule. We postulate that electrogenic sodium pumps in the lateral membranes produce an electrical potential within the LIS, that the lateral membranes bear a net negative charge, and that fluid moves parallel to these membranes because of Helmholtz-type electro-osmosis, the field-induced movement of fluid adjacent to a charged surface. Our theoretical analysis indicates that the sodium pumps produce a longitudinal electric field of the order of 1 V/cm in the LIS. Our experimental measurements demonstrate that the electrophoretic mobility of rat renal basolateral membrane vesicles is 1 micron/s per V/cm, which is also the electro-osmotic fluid velocity in the LIS produced by a unit electric field. Thus, the fluid velocity in the LIS due to electro-osmosis should be of the order of 1 micron/s, which is sufficient to account for the observed reabsorption of fluid from renal proximal tubules. Several experimentally testable predictions emerge from our model. First, the pressure in the LIS need not increase when fluid is transported. Thus, the LIS of mammalian proximal tubules need not swell during fluid transport, a prediction consistent with the observations of Burg and Grantham (1971, Membranes and Ion Transport, pp. 49-77). Second, the reabsorption of fluid is predicted to cease when the lumen is clamped to a negative voltage. Our analysis predicts that a voltage of -15 mV will cause fluid to be secreted into the Necturus proximal tubule, a prediction consistent with the observations of Spring and Paganelli (1972, J. Gen. Physiol., 60:181).

Animals↗
PubMed1985

Wound infection in elective biliary surgery: controlled trial using one dose cephamandole.

In a prospective randomized double-blind trial using a 1 g single dose of cephamandole versus placebo, given 1 h before surgery, the wound infection rate after elective surgery for gallbladder stones in 200 consecutive cases was 11%, being 15% in the placebo group and 7% in the cephamandole group (chi 2 = 4.03; P less than 0.05). The average hospital stay was 7.7 days in the absence of wound infection and 13.6 days in the presence of wound infection. Contaminated bile was significantly positively related to wound infection, and cephamandole significantly protected the culture-positive group from wound infection.

Adult↗
PubMed1984

Bilayer membranes containing the ganglioside GM1: models for electrostatic potentials adjacent to biological membranes.

Although the Gouy-Chapman-Stern theory of the aqueous diffuse double layer describes well the electrostatic potential adjacent to negatively charged phospholipid bilayer membranes, it does not describe adequately the zeta potential of biological membranes: the zeta potential of an erythrocyte is about half the value predicted from the theory by using the known density of negatively charged sialic acid residues. To investigate the factors responsible for this low electrophoretic mobility, we formed membranes from mixtures of the zwitterionic lipid phosphatidylcholine, PC, and the glycolipid galactosyl-N-acetylgalactosaminyl(N-acetylneuraminyl) -galactosylglucosylceramide, GM1. This glycolipid differs from phospholipids in two respects. First, the negative charge on GM1 is located about 1 nm from the surface, which tends to increase the electrophoretic mobility of vesicles. Second, the head group of GM1 contains five sugar groups that exert a hydrodynamic drag, which tends to decrease the mobility of the vesicles. In a decimolar monovalent salt solution, where the Debye length is about 1 nm, the electrophoretic mobility of the PC-GM1 vesicles is about half the mobility of PC-phosphatidylserine or PC-phosphatidylglycerol vesicles of equivalent composition. In addition, conductance measurements with planar bilayer membranes as well as 31P nuclear magnetic resonance and fluorescence measurements with sonicated vesicles indicate that the potential at the surface of PC-GM1 membranes is about half the value measured for PC-phosphatidylserine membranes in a 0.1 M monovalent salt solution.

G(M1) Ganglioside↗
PubMed1984

Unique epidemiology of nosocomial infection in a children's hospital.

A prospective study of the epidemiology of nosocomial infections was undertaken in a large pediatric hospital. Infections were identified during a 12-month period with the use of medical record audits, indices identifying high-risk patients, and analysis of positive cultures and rapid diagnostic tests for viruses, fungi, and bacteria. The attack rate of the entire hospital population surveyed was 4.1 nosocomial infections per 100 patients discharged. In contrast with studies of patients in general hospitals that included comparatively few children, the most common sites of nosocomial infections were the respiratory and gastrointestinal tracts. Staphylococcus aureus was the most commonly isolated pathogen. However, rotavirus was identified more frequently than Escherichia coli, and documented viral infections were more common than infections caused by gram-negative bacilli.

Child↗
PubMed1984

The kinetic mechanism by which CCCP (carbonyl cyanide m-chlorophenylhydrazone) transports protons across membranes.

We demonstrate that a simple kinetic model describes the transport of protons across lipid bilayer membranes by the weak acid CCCP (carbonyl cyanide m-chlorophenylhydrazone). Four parameters characterize this model: the adsorption coefficients of the anionic and neutral forms of the weak acid onto the interface (beta A and beta HA) and the rate constants for the movement of A- and HA across the membrane (kappa A and kappa HA). These parameters were determined by equilibrium dialysis, electrophoretic mobility, membrane potential, membrane conductance, and spectrophotometric measurements. From these equilibrium and steady state measurements on diphytanoyl phosphatidylcholine/chlorodecane membranes we found that beta A = beta HA = 1.4 10(-3) cm, kappa A = 175 s-1 and kappa HA = 12,000 sec-1. These parameters and our model describe our kinetic experiments if we assume that the protonation reactions, which occur at the interfaces, remain at equilibrium. The model predicts a single exponential decay of the current in a voltage-clamp experiment. The model also predicts that the decay in the voltage across the membrane following an intense current pulse of short duration (approximately 50 nsec) can be described by the sum of two exponentials. The magnitudes and time constants of the relaxations that we observed in both voltage-clamp and charge-pulse experiments agree well with the predictions of the model for all values of pH, voltage and [CCCP].

Biological Transport↗
PubMed1983

Dimethonium, a divalent cation that exerts only a screening effect on the electrostatic potential adjacent to negatively charged phospholipid bilayer membranes.

Calcium and other alkaline earth cations change the electrostatic potential adjacent to negatively charged bilayer membranes both by accumulating in the aqueous diffuse double layer adjacent to the membrane and by adsorbing to the phospholipids. The effects of these cations on the electrostatic potential are described adequately by the Gouy-Chapman-Stern theory. We report the results of experiments with ethane-bis-trimethylammonium, a cation that has been termed "dimethonium" or "ethamethonium" in analogy with hexamethonium (hexane-1,6-bis-trimethylammonium) and decamethonium (decane-1,10-bis-trimethylammonium). We examined the effect of dimethonium on the zeta potential of multilamellar vesicles formed from the negative lipid phosphatidylserine (PS) and from 5:1 phosphatidylcholine/phosphatidylserine mixtures in solutions containing 0.1, 0.01 and 0.001 M sodium, cesium, or tetramethylammonium chloride. We also examined the effect of dimethonium on the conductance of planar PS bilayer membranes and the 31P NMR signal from sonicated PS vesicles formed in 0.1 M NaCl. We found no evidence that dimethonium adsorbs specifically to bilayer membranes. All the results, except for those obtained with vesicles of low charge density formed in a solution with a high salt concentration, are consistent with the predictions of the Gouy-Chapman theory. We conclude that dimethonium, which does not have the pharmacological effects of hexamethonium and decamethonium, is a useful divalent cation for physiologists interested in investigating electrostatic potentials adjacent to biological membranes.

Bis-Trimethylammonium Compounds↗
PubMed1983

Large divalent cations and electrostatic potentials adjacent to membranes. A theoretical calculation.

We have extended the Gouy-Chapman theory of the electrostatic diffuse double layer by considering the finite size of divalent cations in the aqueous phase adjacent to a charged surface. The divalent cations are modeled as either two point charges connected by an infinitely thin, rigid "rod" or two noninteracting point charges connected by an infinitely thin, flexible "string." We use the extended theory to predict the effects of a cation of length 10 A (1 nm) on the zeta and surface potentials of phospholipid bilayer membranes. The predictions of the rod and string models are similar to one another but differ markedly from the predictions of the Gouy-Chapman theory. Specifically, the extended model predicts that a large divalent cation will have a smaller effect on the potential adjacent to a negatively charged bilayer membrane than a point divalent cation, that the magnitude of this discrepancy will decrease as the Debye length increases, and that a large divalent cation will produce a negative zeta potential on a membrane formed from zwitterionic lipids. These predictions agree qualitatively with the experimental results obtained with the large divalent cation hexamethonium. We discuss the biological relevance of our calculations in the context of the interaction of cationic drugs with receptor sites on cell membranes.

Cations, Divalent↗
PubMed1983

Large divalent cations and electrostatic potentials adjacent to membranes. Experimental results with hexamethonium.

A simple extension of the Gouy-Chapman theory predicts that the ability of a divalent cation to screen charges at a membrane-solution interface decreases significantly if the distance between the charges on the cation is comparable with the Debye length. We tested this prediction by investigating the effect of hexamethonium on the electrostatic potential adjacent to negatively charged phospholipid bilayer membranes. The distance between the two charges of an extended hexamethonium molecule is approximately 1 nm, which is the Debye length in the 0.1 M monovalent salt solutions used in these experiments. Six different experimental approaches were utilized. We measured the electrophoretic mobility of multilamellar vesicles to determine the zeta potential, the line width of the 31P nuclear magnetic resonance (NMR) signal from sonicated vesicles to calculate the change in potential at the phosphodiester moiety of the lipid, and the conductance of planar bilayer membranes exposed to either carriers (nonactin) or pore formers (gramicidin) to estimate the change in potential within the membrane. We also measured directly the effect of hexamethonium on the potential above a monolayer formed from negative lipids, and attempted to calculate the change in the surface potential of a bilayer membrane from capacitance measurements. With the exception of the capacitance calculations, each of the techniques gave comparable results: hexamethonium exerts a smaller effect on the potential than that predicted by the classic screening theory. The results are consistent with the predictions of the extended Gouy-Chapman theory and are relevant to the interpretation of physiological and pharmacological experiments that utilize hexamethonium and other large divalent cations.

Animals↗
PubMed1983

The molecular mechanism of action of the proton ionophore FCCP (carbonylcyanide p-trifluoromethoxyphenylhydrazone).

We propose a simple model that accounts for the ability of the weak acid FCCP (Carbonylcyanide-p-trifluoromethoxyphenylhydrazone) to both transport protons across phospholipid bilayer membranes and uncouple oxidation from phosphorylation in mitochondria. Four parameters are required to characterize this model: the rate constant for the movement of A- across the membrane, kA, the rate constant for the movement of HA across the membrane, kHA, the adsorption coefficient of A- onto the membrane-solution interface, beta A, and the surface pK. These four parameters were determined from kinetic measurements on planar bilayer membranes using the charge-pulse and voltage-clamp techniques. We confirmed the adequacy of the model by determining each of these parameters independently, utilizing equilibrium dialysis, zeta potential, membrane potential, spectrophotometric, and conductance measurements. For a phosphatidylethanolamine bilayer the values of the parameters are kHA = 10(4)S-1, beta A = 3 10(-3) cm, and 6.0 less than pK less than 6.4. As predicted theoretically, the value of KA depends on both the applied voltage, V, and dielectric constant of the membrane, epsilon r; when V approaches zero and the membrane contains chlorodecane (epsilon r congruent to 2.7) kA = 700 s-1. If oxidation is coupled to phosphorylation by means of a delta microH+, and V er congruent to 2.7 for the inner membrane of the mitochondrion, the model predicts that FCCP should exert maximal uncoupling activity at a pH congruent to pK. This prediction agrees with the published experimental results.

Carbonyl Cyanide p-Trifluoromethoxyphenylhydrazone↗
PubMed1982

Electro-osmosis at the surface of phospholipid bilayer membranes.

The electro-osmotic velocity is the velocity of a fluid near an interface produced by an electric field parallel to a surface. The velocity adjacent to fixed phospholipid bilayer membranes was measured by observing the velocity of small vesicles suspended in the fluid. The charge densities of the bilayers ranged from 0 to 1 electronic charge per lipid and experiments were performed at temperatures above and below the transition temperature of the phospholipid bilayer in 1, 10 and 100 mM NaCl solutions. The Helmholtz-Smoluchowski equation correctly predicted the electro-osmotic velocity from the known value of zeta potential of the phospholipid bilayer.

Dimyristoylphosphatidylcholine↗