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  • 1
    Electronic Resource
    Electronic Resource
    Effects of calcium, lanthanum, and temperature on the fluidity of spin-labeled human platelets (1980)
    Springer
    The journal of membrane biology 52 (1980), S. 201-219 
    Details
    ISSN: 1432-1424
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary Previous platelet studies have shown that calcium plays important roles in stimulus-secretion coupling, aggregation, and other membrane-associated functions. In addition, lanthanum induces platelet aggregation and the platelet release reaction and also influences platelet responsiveness to various stimuli. The spin-label results presented here suggest that one mechanism through which calcium and lanthanum mediate their effects on platelet functions may be by decreasing the lipid fluidity of the surface membrane. The structure of platelet membrane lipids was examined with the spin-label method. Washed human platelets were labeled with the 5-, 12- and 16-nitroxide stearic acid spin probes. Order parameters which measure the fluidity of the lipid environment of the incorporated probe may be calculated from the electron spin resonance (ESR) spectra of 5-nitroxide stearate [I(12,3)]-labeled cells. Evidence is presented which indicates that these spectra principally reflect properties of the platelet surface membrane lipids. The membrane fluidity increased with temperature for the range 17 to 37 °C. Either calcium or lanthanum additions to intact cells increased the rigidity of the platelet membranes at 37 °C, although the La3+ effect was larger and occurred at lower concentrations than that of Ca2+. For example, addition of 1mm La3+ or 4mm Ca2+ increased the order parameter of I(12,3)-labeled platelets by 4.3±1.7% or 2.1±0.5%. Preliminary studies conducted on purified platelet plasma membranes labeled with I(12,3) indicated that 1mm LaCl3 or 4mm CaCl2 additions similarly decreased the lipid fluidity at 37 °C. The above cation-induced effects on the fluidity of whole platelets were reversed by the use of the divalent cation-chelating agent ethylene glycol-bis-(β-aminoethyl ether)-N,N′-tetra-acetic acid (EGTA). Lastly, lanthanum (0.2–1mm) caused rapid aggregation of platelets which were suspended in a 50-mm Tris buffer pH 7.4 that did not contain adenosine.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01869190
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  • 2
    Electronic Resource
    Electronic Resource
    Thermotropic lipid phase separations in human platelet and rat liver plasma membranes (1983)
    Springer
    The journal of membrane biology 76 (1983), S. 139-149 
    Details
    ISSN: 1432-1424
    Keywords: platelet plasma membranes ; liver plasma membranes ; spin probe ; cholesterol ; lipid phase separation ; platelet acid phosphatase
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary Electron spin resonance (ESR) studies were conducted on human platelet plasma membranes using 5-nitroxide stearate, I(12,3). The polarity-corrected order parameterS and polarity-uncorrected order parametersS(T ‖) andS(T ⊥) were independent of probe concentration at low I(12,3)/membrane protein ratios. At higher ratios,S andS(T ⊥) decreased with increasing probe concentration whileS(T ‖) remained unchanged. This is the result of enhanced radical interactions due to probe clustering. A lipid phase separation occurs in platelet membranes that segregates I(12,3) for temperatures less than 37°C. As Arrhenius plots of platelet acid phosphatase activity exhibit a break at 35 to 36°C, this enzyme activity may be influenced by the above phase separation. Similar experiments were performed on native [cholesterol/phospholipid ratio (C/P)=0.71] and cholesterol-enriched [C/P=0.85] rat liver plasma membranes. At 36°C, cholesterol loading reduces I(12,3) flexibility and decreases the probe ratio at which radical interactions are apparent. The latter effects are attributed to the formation of cholesterol-rich lipid domains, and to the inability of I(12,3) to partition into these domains because of steric hinderance. Cholesterol enrichment increases both the high temperature onset of the phase separation occurring in liver membranes from 28° to 37°C and the percentage of probe-excluding, cholesterolrich lipid domains at elevated temperatures. A model is discussed attributing the lipid phase separation in native liver plasma membranes to cholesterol-rich and-poor domains. As I(12,3) behaves similarly in cholesterol-enriched liver and human platelet plasma membranes, cholesterol-rich and-poor domains probably exist in both systems at physiologic temperatures.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02000614
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  • 3
    Electronic Resource
    Electronic Resource
    The selective effects of charged local anaesthetics on the glucagon- and fluoride-stimulated adenylate cyclase activity of rat-liver plasma membranes (1980)
    New York, N.Y. : Wiley-Blackwell
    Journal of Supramolecular Structure 14 (1980), S. 21-32 
    Details
    ISSN: 0091-7419
    Keywords: glucagon ; adenylate cyclase ; anaesthetics ; membrane bilayer fluidity ; Life Sciences ; Molecular Cell Biology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Biology , Chemistry and Pharmacology , Medicine
    Notes: The cationic local anaesthetics carbocaine and unpercaine were found to increase the fluoride-stimulated adenylate cyclase up to a maximum level; above this maximum level further increases in drug concentration inhibited the enzyme. At concentrations where this activity was stimulated, a fatty acid spin label detected an increase in bilayer fluidity, which, it is suggested, is responsible for the activation of the enzyme. A solubilized enzyme was unaffected by the drugs, a finding consistent with this proposal.These cationic drugs began to inhibit the glucagon-stimulated activity at concentrations where they activated the fluoride-stimulated activity. It is suggested that this is due to their effect on the coupling interaction between the receptor and catalytic unit.The anionic drugs, phenobarbital, pentobarbital, and salicylic acid, all inhibited the fluoride-stimulated enzyme. This may be due in part to a direct effect on the protein and in part to the interaction of the drugs with the bilayer. The drugs had small inhibitory effects on the lubrol-solubilized enzyme.The glucagon-stimulated enzyme was initially inhibited by the anionic drugs at low concentrations, then activated, and finally inhibited with increasing drug concentration. The reasons for such changes are complex, but there was no evidence from electron spin resonance studies to suggest that the elevations in activity were due to increases in bilayer fluidity.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jss.400140104
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    Paper (German National Licenses)
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  • 4
    Electronic Resource
    Electronic Resource
    Spin label studies on rat liver and heart plasma membranes: Effects of temperature, calcium, and lanthanum on membrane fluidity (1978)
    New York, N.Y. : Wiley-Blackwell
    Journal of Supramolecular Structure 9 (1978), S. 299-326 
    Details
    ISSN: 0091-7419
    Keywords: lanthanum ; calcium ; lipid phase separation ; lipid clusters ; spin label method ; membrane fluidity ; temperature ; Life Sciences ; Molecular Cell Biology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Biology , Chemistry and Pharmacology , Medicine
    Notes: The structures of rat liver and heart plasma membranes were studied with the 5-nitroxide stearic acid spin probe, I(1 2,3). The polarity-corrected order parameters (S) of liver and heart plasma membranes were independent of probe concentration only if experimentally determined low I(1 2,3)/lipid ratios were employed. At higher probe/lipid ratios, the order parameters of both membrane systems decreased with increasing probe concentration, and these effects were attributed to enhanced nitroxide radical interactions. Examination of the temperature dependence of approximate and polarity-corrected order parameters indicated that lipid phase separations occur in liver (between 19° and 28°C) and heart (between 21° and 32°C) plasma membranes. The possibility that a wide variety of membrane-associated functions may be influenced by these thermotropic phase separations is considered.Addition of 3.9 mM CaCl2 to I(1 2,3)-labeled liver plasma membrane decreased the fluidity as indicated by a 5% increase in S at 37°C. Similarly, titrating I(1 2,3)-labeled heart plasma membranes with either CaCl2 or LaCl3 decreased the lipid fluidity at 37°C, although the magnitude of the La3+ effect was larger and occurred at lower concentrations than that induced by Ca2+; addition of 0.2 mM La3+ or 3.2 mM Ca2+ increased S by approximately 7% and 5%, respectively. The above cation effects reflected only alterations in the membrane fluidity and were not due to changes in probe-probe interactions. Ca2+ and La3+ at these concentrations decrease the activities of such plasma membrane enzymes as Na+, K+-ATPase and adenylyl cyclase, and it is suggested that the inhibition of these enzymes may be due in part to cation-mediated decreases in the lipid fluidity.
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jss.400090303
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    Paper (German National Licenses)
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