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A Ring

Publications and source records attributed to A Ring.

At least 37 records · Page 2Linked to original sources
PubMed1992

Monitoring the surface tension of lipid membranes by a bubble method.

A new method for the determination of lipid membrane surface tension was developed. Advantages of this method are that it allows for multiple measurements on a single membrane, is fast and direct not requiring empirical corrections, may be applied for dynamical surface-tension measurements and may be used with thin films and asymmetrical electrolytes. The pressure and radius of a bubble are measured. A piezoresistive sensor is used to minimize the transducer compliance. By moulding the sensor to a brass plate a resolution of 0.025 mm H2O (0.25 Pa) is obtained. The bubble is filmed using a videocamera and the radius of the bubble determined with the aid of a microcomputer. Data for monoolein/hexadecane in potassium chloride solutions and a cooling curve are presented and compared with previous results.

Lipid Bilayers↗
PubMed1992

Influence of ion occupancy and membrane deformation on gramicidin A channel stability in lipid membranes.

The average lifetime of gramicidin A channels in monoolein/decane bilayer membranes was measured. The results support the hypothesis of channel stabilization by ion occupancy. The effects of electric field and salt concentration are consistent with the expected effects on both occupancy and membrane compression. The lifetime in asymmetric solutions with divalent cation blockers on one side of the membrane shows a voltage dependence such that the lifetime decreases for positive voltages applied from the blocking side and increases for negative voltages. This result strongly supports the occupancy hypothesis. The lifetime increases with permeant ion concentration, and at the one molar level it also increases with voltage. The voltage dependence of lifetime for a low concentration of permeant ion depends on the total salt level. The results for these conditions are consistent with the assumption that membrane compression also influences the lifetime, even for the "soft" solvent-containing membrane considered here. It is proposed that the channel nearest neighbor lipids need not be fixed in a plane at the channel end. Using a liquid crystal model it may then be shown that surface tension is the major component of the membrane deformation free energy, which may explain the significant effects of the membrane compression on the lifetime.

Biophysical Phenomena↗
PubMed1991

The Association of Teachers of Preventive Medicine's recommendations for postgraduate education in prevention.

The Guide to Clinical Preventive Services, prepared in 1989 by the U.S. Preventive Services Task Force, assesses the effectiveness of 169 types of preventive interventions. In 1990, the Association of Teachers of Preventive Medicine formed a panel to review the guide and recommend ways it could be used to enhance both undergraduate and postgraduate medical education. This paper outlines the panel's recommendations of the types of knowledge and attitudes on which postgraduate medical education in prevention should be built. Detailed recommendations are presented, based on the summary findings of the guide, for residency education in prevention. Implementation of these recommendations will integrate preventive services into the continuum of medical care. These recommendations are presented to achieve the goal of educating physicians to approach the total patient, putting the patient's health rather than the disease process in the forefront of primary medical care.

Education, Medical, Graduate↗
PubMed1988

Evaluation of surface tension and ion occupancy effects on gramicidin A channel lifetime.

The surface tension of glycerylmonooleate-hexadecane lipid bilayer membranes and the lifetime of gramicidin A channels were measured at various concentrations of the surrounding solutions. For HCl the surface tension is essentially constant at approximately 5 mN/m up to approximately 1 M, whereas the average lifetime increases approximately 40-fold. At higher concentrations the surface tension decreases markedly. For CsCl the surface tension is constant up to about 1 M then increases with salt level. The average lifetime in this case increases about sixfold. In both cases the lifetime levels off and even decreases at higher salt levels. The increase in lifetime observed with ion activity is therefore qualitatively different from, and not explained by, the established dependence of lifetime on membrane properties (Elliot, J.R., D. Needham, J.P. Dilger, and D.A. Haydon. 1983. Biochim. Biophys. Acta. 735:95-103). We have previously proposed that ion occupancy is a determinant of channel stability, and to test this hypothesis the voltage dependence of channel lifetime was measured in asymmetrical solutions. For the case of a potassium chloride solution on one side of the membrane and a hydrogen chloride solution, on the other, the voltage dependence of the lifetime is asymmetrical. The asymmetry is such that when the electrical field is applied in the direction of the chemical gradient for each of the ions, the channel lifetime approaches, at increasing field strengths, that of a symmetrical solution of the respective ion. The voltage dependence of the surface tension, on the other hand, is negligible for the range of voltages used. These results, and the earlier findings that the order of the lifetimes for the alkali cations generally agree with the order of the permeability selectivity of the gramicidin A channel, support the hypothesis that ion occupancy is a major factor determining the lifetime of gramicidin A channels. The effects of multivalent blockers and osmotic agents were also tested. Ba2", La3+,and Mg2" decrease the lifetime and conductance markedly. Sucrose and urea increase the lifetime and decrease the conductance. The voltage dependence of the lifetime in symmetrical solutions was examined. Contrary to previous reports it was found that the lifetimes for K+, Cs', and H+ are voltage dependent. For 0.5 M HCI the lifetime decreases monotonically by .60% at 150 mV, and for 0.5 M KCI the lifetime increases by -60% at 200 mV. Below 10 mM there is no effect of voltage for H+, K+, and Cs+. These effects of blockers, osmotic agents, and voltage on the lifetime, as well as the lack of effect of voltage at low salt levels, are consistent with the occupancy hypothesis.

Alkanes↗
PubMed1988

Modulation of gramicidin A open channel lifetime by ion occupancy.

The hypothesis that the gramicidin A channel stability depends on the level of ion occupancy of the channel was used to derive a mathematical model relating channel lifetime to channel occupancy. Eyring barrier permeation models were examined for their ability to fit the zero-voltage conductance, current-voltage, as well as lifetime data. The simplest permeation model required to explain the major features of the experimental data consists of three barriers and four sites (3B4S) with a maximum of two ions occupying the channel. The average lifetime of the channel was calculated from the barrier model by assuming the closing rate constant to be proportional to the probability of the internal channel sites being empty. The link between permeation and lifetime has as its single parameter the experimentally determined averaged lifetime of gramicidin A channels in the limit of infinitely dilute solutions and has therefore no adjustable parameters. This simple assumption that one or more ions inside the channel completely stabilize the dimer conformation is successful in explaining the experimental data considering the fact that this model for stabilization is independent of ion species and configurational occupancy. The model is used to examine, by comparison with experimental data, the asymmetrical voltage dependence of the lifetime in asymmetrical solutions, the effects of blockers, and the effects of elevated osmotic pressure.

Cations, Monovalent↗
PubMed1987

[Assistant and AIDS].

Explore the source record for details and available documents.

Acquired Immunodeficiency Syndrome↗
PubMed1986

Brief closures of gramicidin A channels in lipid bilayer membranes.

Brief closures, so called flickers, gramicidin A channels were observed for glycerol monooleate/n-decane membranes for cesium chloride and hydrochloric acid solutions. The flickers, similar in nature to the flickers observed for physiological channels, were of the order of 1 ms and the interval between flickers was of the order of 50 ms. The flicker-duration and interval between flickers both decrease with voltage. The field dependence of the flickers is consistent with the hypothesis that the membrane forms a dimple when accomodating a dimer in the membrane and that the monomers, on breaking up, are associated over displacements of the order of 2 nm. For similar measurements for glycerol monoleate/hexadecane membranes only rare occurrences of flickers were observed. It is suggested that the flicker phenomenon is governed by the physical and chemical properties of the membrane and the influence of membrane thickness and interfacial free energy is emphasized.

Alkanes↗
PubMed1985

Possible aetiology of oral cancer in Papua New Guinea.

Oral cancer is the most common cancer seen in Papua New Guinea. Although there is a broad geographical correlation with betel chewing, there have been no analytical epidemiological studies and the aetiology remains poorly understood. A greater understanding of several putative risk factors and how they may interact is needed prior to any attempt to modify traditional customs. Although current strategies for primary prevention in India are based on the assumption that the risk of betel chewing is related to the tobacco in the quid, this cannot explain oral cancer in Papua New Guinea. In the latter, special epidemiological and laboratory investigations are needed to provide a scientific basis for future prevention.

Alcohol Drinking↗
PubMed1982

Linear network representation of multistate models of transport.

By introducing external driving forces in rate-theory models of transport we show how the Eyring rate equations can be transformed into Ohm's law with potentials that obey Kirchhoff's second law. From such a formalism the state diagram of a multioccupancy multicomponent system can be directly converted into linear network with resistors connecting nodal (branch) points and with capacitances connecting each nodal point with a reference point. The external forces appear as emf or current generators in the network. This theory allows the algebraic methods of linear network theory to be used in solving the flux equations for multistate models and is particularly useful for making proper simplifying approximation in models of complex membrane structure. Some general properties of linear network representation are also deduced. It is shown, for instance, that Maxwell's reciprocity relationships of linear networks lead directly to Onsager's relationships in the near equilibrium region. Finally, as an example of the procedure, the equivalent circuit method is used to solve the equations for a few transport models.

Biological Transport↗