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B T Stokke

Publications and source records attributed to B T Stokke.

28 records · Page 2Linked to original sources
PubMed1986

The human erythrocyte membrane skeleton may be an ionic gel. II. Numerical analyses of cell shapes and shape transformations.

In the first paper in this series (Stokke et al. Eur Biophys J 1986, 13:203-218) we developed the general theory of the mechanochemical properties and the elastic free energy of the protein gel--lipid bilayer membrane model. Here we report on an extensive numerical analysis of the human erythrocyte shapes and shape transformations predicted by this new cell membrane model. We have calculated the total elastic free energy of deformation of four different cell shape classes: disc-shaped cells, cup-shaped cells, crenated cells, and cells with membrane invaginations. We find that which of these shape classes is favoured depends strongly on the spectrin gel osmotic tension, IIGu, and the surface tensions, IIEu and IIPu, of the extracellular and protoplasmic halves of the membrane lipid bilayer, respectively. For constant ratio IIEu/IIPu greater than O large negative or positive values of IIGu favour respectively the crenated and invaginated cell shape classes. For small absolute values of IIGu, IIEu, and IIPu, biconcave or cup-shaped cells are the stable ones. Our numerical analysis shows that the higher the membrane skeleton compressibility is, the smaller are the values of IIGu needed to induce cell shape transformation. We find that the stable and metastable shapes of discocytes and stomatocytes generally depend both on the shape of the stressfree membrane skeleton and the membrane skeleton compressibility.

Elasticity↗
PubMed1986

Spectrin, human erythrocyte shapes, and mechanochemical properties.

Physical studies of human erythrocyte spectrin indicate that isolated spectrin dimers and tetramers in solution are worm-like coils with a persistence length of approximately 20 nm. This finding, the known polyelectrolytic nature of spectrin, and other structural information about spectrin and the membrane skeleton molecular organization have lead us to the hypothesis that the human erythrocyte membrane skeleton constitutes a two-dimensional ionic gel (swollen ionic elastomer). This concept is incorporated in what we refer to as the protein gel-lipid bilayer membrane model. The model accounts quantitatively for red elastic shear modulus and the maximum elastic extension ratio reported for the human erythrocytes membrane. Gel theory further predicts that depending on the environmental conditions, the membrane skeleton modulus of area compression may be small or large relative to the membrane elastic shear modulus. Our analyses show that the ratio between these two parameters affects both the geometry and the stability of the favored cell shapes and that the higher the membrane skeleton compressibility the smaller the values of the gel tension needed to induce cell shape transformations. The main virtue of the protein gel-lipid bilayer membrane model is that it offers a novel theoretical and molecular basis for the various mechanical properties of the membrane skeleton such as the membrane skeleton modulus of area compression and osmotic tension, and the effects of these properties on local membrane skeleton density, cell shape, and shape transformations.

Erythrocyte Membrane↗
PubMed1985

An electrophoretic device concentrating charged macromolecules to a predetermined final solution volume.

An apparatus for electrophoretic concentration of charged macromolecules to a predetermined final solution volume has been developed. The concentration process has a yield of near 100%, which implies that it is possible to predetermine the final macromolecule concentration as well. Both the final macromolecule solution volume and concentration are nearly independent of the electrophoresis time when it exceeds a certain minimum value. The electric field strength across the boundary containing the concentrated macromolecule solution is very low. This considerably reduces macromolecule aggregation, adsorption, and denaturation at this boundary compared to conventional electrophoretic concentrator designs. Both one-stage and two-stage versions of the apparatus have been developed. The one-stage version easily yields a 10-fold and the two-stage version a 50-fold concentration of the macromolecules. Typical macromolecule solution start volumes are 20-50 ml.

Electrochemistry↗
PubMed1985

Human erythrocyte spectrin dimer intrinsic viscosity: temperature dependence and implications for the molecular basis of the erythrocyte membrane free energy.

We have determined experimentally the temperature dependence of human erythrocyte spectrin dimer intrinsic viscosity at shear rates 8-12 s-1 using a Cartesian diver viscometer. We find that the intrinsic viscosity decreases from 43 +/- 3 ml/g at 4 degrees C to 34 +/- 3 ml/g when the temperature is increased to 38 degrees C. Our results show that spectrin dimers are flexible worm-like macromolecules with persistence length about 20 nm and that the mean square end-to-end distance for this worm-like macromolecules decreases when the temperature is increased. This implies that the spectrin dimer internal energy decreases when the end-to-end distance is increased and that the free energy increase associated with making the end-to-end distance longer than the equilibrium value for the free molecules is of entropic origin. The temperature dependence of the erythrocyte membrane shear modulus reported previously in the literature therefore appears mainly to be due to temperature dependent alterations in the membrane skeleton topology.

Erythrocyte Membrane↗
PubMed1985

Some viscoelastic properties of human erythrocyte spectrin networks end-linked in vitro.

We have succeeded in making macroscopic networks of end-linked human erythrocyte spectrin. The network junctions were made using erythrocyte protein 4.1 irreversibly attached to 5 nm (diameter) colloidal gold particles. Rotary shadowing electron microscopy verifies that the protein 4.1-labelled colloidal gold particles bind only to the tail end of the spectrin molecules. Electron micrographs of protein 4.1-labelled colloidal gold particles incubated at 4 degrees C with spectrin dimers reveal that 1-5 spectrin dimers attach to each protein 4.1-labelled colloidal gold particle yielding a spider-like appearance of these complexes. Incubation with a low concentration of spectrin tetramers instead of dimers leads to extensive formation of spectrin microaggregates whereas use of spectrin concentrations higher than 3 mg/ml and a molar ratio between spectrin tetramers and protein 4.1/Au of 4 leads to formation of macroscopic spectrin networks. We have quantitated the viscoelastic properties of such end-linked macroscopic spectrin networks using a gravitational pendulum viscoelastometer. We find that in vitro end-linked spectrin networks can be described by linear viscoelastic theory. The dynamic storage modulus increases almost linearly with the spectrin-protein 4.1/gold particle concentration when the spectrin concentration exceeds about 3 mg/ml and the molar ratio between spectrin tetramers and protein 4.1/Au is 4. At a spectrin concentration of 6 mg/ml and the same ratio between spectrin and protein 4.1/Au, we find a dynamic storage modulus at low frequency of about 80 dyn/cm2. This is in adequate agreement with what is predicted by simple elastomer theory.

Blood Proteins↗
PubMed1985

A computerized low-shear pendulum viscoelastometer, stress-relaxation, shear creep, and dynamic elastic moduli measurements of soft biogels.

A computerized low-shear pendulum viscoelastometer that is well suited for measurements of the viscoelastic properties of soft biogels is described. The instrument has three modes of operation: (i) measurement of dynamic elastic moduli, (ii) stress relaxation, and (iii) shear creep-creep recovery. The main mechanical parts of the instrument include a specimen cuvette and a pendulum with a thin metal plate extending into the specimen cuvette. The cuvette and pendulum can undergo independent angular movements relative to a horizontal axis located about 12 cm above the specimen cuvette. The absolute angular position of the cuvette is under on-line computer control through a stepping motor driven micrometer. The angular position of the pendulum relative to the cuvette is measured using an inductive position detector. The necessary software for automatic operation, data acquisition and processing has been developed for all the three modes of operation. The instrument is well suited for measurements of dynamic elastic moduli in the range 0.02-150 dyn/cm2. Calibration data obtained using 100% ethanol are presented. The instrument suitability for measurements of stress relaxation and shear creep-creep recovery is illustrated by presenting the results of such measurements on a 5.5-mg/ml erythrocyte spectrin gel in isotonic solution.

Biophysical Phenomena↗
PubMed1985

A practical, high-resolution, microcomputer-based method for the analysis of relaxation data exhibiting multicomponent exponential decays.

We have developed and tested a practical, rapid, high-resolution, microcomputer-based method for the analysis of multicomponent exponential decays. The analysis utilizes the Fourier deconvolution technique and includes methods to reduce noise both in the input data and in the results. The developed method is particularly well suited for analysing decays consisting of a wide range of decay times. The method resolves two exponential decays differing by a factor of two when the input data are mathematically generated and without noise, and resolves two decays differing by a factor of three when 2% Gaussian noise is present in the same data. The method lends itself to routine analysis of any relaxation process consisting of exponential decays, including biomedical applications such as enzyme kinetics, circulatory transport functions, pharmacokinetics, plasma exchange therapy, and analysis of compartmental models for any process.

Birefringence↗
PubMed1984

An electro-optic study of human erythrocyte spectrin dimers. The presence of calcium ions does not alter spectrin flexibility.

In order to determine whether the presence of Ca2+ increases the stiffness of the highly elongated and flexible spectrin molecules, we have carried out a birefringence relaxation study of isolated human erythrocyte spectrin dimers. Our measurements indicate no significant change in the flexibility of spectrin in solutions containing 0-10(-3) M Ca2+. This finding indicates that decreased spectrin flexibility is not the major functional mechanism underlying the decreased erythrocyte deformability reported as result of elevated intracellular levels of Ca2+. We find that the persistence length of spectrin dimers is less than 20 nm and is not dependent on the Ca2+ concentration.

Calcium↗
PubMed1981

Human spectrin. VI. A viscometric study.

Employing viscometry, human spectrin heterodimers and heterotetramers were analyzed in aqueous solution containing different amounts of salt. (1) In aqueous 0.1 M NaCl, pH 7.5, at 4 degrees C, the intrinsic viscosity of isolated human spectrin heterodimers and heterotetramers was found to be 40 +/- 6 and 79 +/- 7 ml/g, respectively. (2) The intrinsic viscosity of isolated human spectrin heterodimers and heterotetramers increased to 78 +/- 8 and 180 +/- 10 ml/g, respectively, as the ionic strength of the solution was reduced to about 2 mM. (3) This viscometric study indicates that isolated human spectrin heterodimers and heterotetramers are flexible molecules with a contour length of at least 110 and 200 nm, respectively.

Electrophoresis, Polyacrylamide Gel↗