Dimiter S. Dimitrov

Dynamic interactions between approaching surfaces of biological interest

Abstract The dynamic interactions arising when two interfaces approach each other are reviewed. Experimental methods are summarized and briefly discussed. The basic physical laws describing the dynamic behavior of interfaces and the liquid film between them are presented and the thin film approximation is used in order to simplify them. It was shown that interface deformability decreases the approach rate (increases interaction) while surface tangential mobility and membrane permeability increase it. Formulae are also presented for the dynamic interactions between solid particles of different shapes. The influence of the rheological properties of the liquid film between approaching surfaces is considered using viscoelastic, liquid-crystalline and non-linear models. The interactions between surface shape perturbations which are important for liquid film stability are considered with special attention to the case of membranes. A hydrodynamic theory of bilayer membrane formation and some similar phenomena is described which coincides semi-quantitatively with available experimental data. It is pointed out that viscous interactions can significantly decrease the rate of aggregation and fusion in multiparticle systems. The influence of external fields is also briefly discussed. The general conclusion is that in any case of very close approach of two interfaces the dynamic interactions can significantly increase leading to a decrease in the rate of adhesion and fusion.

Mechanical aspects of the lungs as cancer cell-killing organs during hematogenous metastasis*

A substantial proportion of many different types of circulating cancer cells appear to be killed during their interactions with the pulmonary microcirculation. Different tensions exist during respiration within alveolar units, and hence the pulmonary capillaries. We have calculated the effects of these tensions on the entry and subsequent fate of circulating cancer cells. Our calculations indicate that during expiration, when tension in the capillary walls is low, cancer cells can enter and travel along the capillaries without damage, because the vessels are deformed by the cells and the hydrodynamic field surrounding them. During normal inspiration when the alveoli are stretched, the increased tension within the capillary walls serves to compress the contained cancer cells. This compression, together with previously calculated blood pressure differentials between the ends of the cells, is thought in some cases, to increase their membrane tensions above the critical level for rupture, resulting in cytolysis, in accord with experimental observations. In deep inspiration, when a very substantial increase in capillary wall tension occurs, cancer cells already within the capillaries, entering them and in transit along them are expected to develop membrane tensions greatly exceeding the critical values for rupture. It is suggested that these respiration-induced effects may act as an important rate-regulating step in the metastatic process, where the development of pulmonary metastases plays a central role. Furthermore, induced deep inspiration may conceivably be utilized in the inhibition of pulmonary metastasis.

Correlation between physical parameters in electrofusion and electroporation of protoplasts

Abstract Electrofusion requires the destabilization of two membranes at close approach; electroporation requires the destabilization of one single membrane. Therefore, the mechanisms of both processes can be similar. A difference can arise from membrane interactions. One of the possible mechanisms for membrane destabilization is the growth of fluctuation waves. The fluctuation wave mechanism predicts that the amplitude and duration of the pulse to form pores and fuse membranes are interrelated. This paper presents experimental evidence that the parameters of electrical costraints which induce electrofusion correlate with those for electroporation. Pea protoplasts were placed in a homogeneous electric field. Fusion was detected microscopically. Pore formation was indicated by the increased conductivity of the protoplast suspension. The pulse amplitude and duration and the temperature of the medium were varied. The basic results are (1) the voltage of membrane rupture decreases with increasing pulse duration and temperature, and (2) the pulse amplitude to induce fusion correlates with that for the formation of pores or other defects in membranes. These results are in qualitative agreement with previous observations for other systems. They support the fluctuation wave mechanism of membrane electrofusion and electroporation but do not disprove other mechanisms.

Dynamics of viscoelastic spherical membranes—the balloon model of the alveolus

Abstract Double-valued pressure-volume relationships in dynamic conditions for spherical membranes, modelling the lung alveoli, were obtained at small deformations. This hysteretic behavior was considered to be produced by at least three independent mechanisms: (1) the lung parenchyma exhibits viscoelastic properties; (2) the lung surface film, independent of the tissue, exhibits viscoelastic properties and (3) the pressure acting on the inner membrane surface depends on the rate of the alveolus volume change, due to the air viscous resistance in the bronchial tree. In each case, the maximum volume change, the hysteresis loop area, the tilt angle of the hysteresis loop and the relaxation time of the system were calculated. The results show pronounced hysteresis at normal breathing due to the air viscous resistance and smaller one due to the tissue and surface viscoelastic properties. In quasistatic conditions the values of the surface viscoelasticity and the tissue viscoelasticity effects are comparable or different, depending on the concrete external conditions. Comparison with the available experimental data is discussed in detail.

Adjustment of Forest Management Strategies to Changing Climate

Research work on the influence of global warming on forests predicts a rise in air temperature and changes in precipitation for a large part of Europe. Climate change has been forecast to increase runoff and nutrient leaching from the boreal catchments. Windiness, cloudiness and more frequent extreme-weather events are expected in the temperate region. The Mediterranean region is expected to suffer considerable impacts because of increased drought conditions. The need to understand and control the hydrological role of forests is rising, especially in upper watershed areas (headwaters), where forest expansion and stocking may result in water scarcity downstream. However, observations and studies of natural processes related to the water cycle suggest that forests and water are interrelated in complex ways. Therefore, forests should not be defined and treated as mere consumers of water significantly influencing runoff, but also as natural filters and reservoirs. Forest management systems and operations will have to be adapted to changes in forest composition and distribution within new water and disturbance regimes. Water protection measures like buffer zones and sedimentation ponds will gain in importance, and in general, a new water-yield or water-saving silviculture will play an important role in future forest management.

Electrical breakdown of protoplast membranes under different osmotic pressures

Abstract The electrical breakdown of membranes depends on their tension. The cell membrane tension increases with decreasing osmolarity of the solution. We measured the breakdown voltage of pea protoplasts in hypo-, iso- and hypertonic mannitol solutions. In hypotonic solutions, near the isotonic point, the breakdown voltage decreased strongly with increasing osmotic pressure difference. For lower osmolarities, the breakdown voltage did not change so significantly. In hypertonic solutions, the protoplasts broke at the same voltage as in isotonic ones. We developed a theory, based on the fluctuation waves mechanism of membrane rupture, which describes the experimental results approximately. It does not, however, fit the data for solutions of low osmolarity. Possible mechanisms, not taken into account by the theory, and applications to biotechnology are discussed.

Iodide quenching of tryptophan fluorescence in phosphorylase kinase

Abstract 1. 1. The quenching of tryptophan fluorescence by iodide in rabbit muscle phosphorylase kinase was investigated. 2. 2. It has been shown that the action of Mg 2+ -ATP brought a change in the Stern-Volmer constant. 3. 3. The pH influence of tryptophan fluorescence have been demonstrated.

Red blood cell dielectrophoresis in axisymmetric fields

Dielectrophoretic velocities of human red blood cells in an axisymmetric field were measured as a function of the applied voltage and the distance from the axis of symmetry. The voltage of the alternating electric field (frequency 2 MHz), applied between two concentric cylindrical metal electrodes (outer and inner radii 0.24 and 1 mm, respectively), was varied up to 19 V. Two kinds of mediums were used: (a) 90% of 2.1% glycine solution and 10% of 5.5% glucose solution and (b) 5.4% sorbitol solution. The results have shown that in both mediums the cell velocities are proportional to the square of the applied voltage and inversely proportional to the cube of the distance from the axis of symmetry, as predicted by the theory. The coefficient of proportionality (dielectrophoretic coefficient) is on the order of 10−25 A2s4kg−1. It depends on the donor of red blood cells and might be used for diagnostic purposes. These results will be used in future investigations of membrane adhesion, stability and fusion.

Protoplast Dielectrophoresis in Axisymmetric Fields

Abstract Pea protoplasts dielectrophoretic coefficients were measured in alternating electric fields of frequency 1 M Hz and voltage 5 V applied between two concentric cylindrical metal electrodes of outer and inner radii 0.24 mm and 1 mm, respectively. They do not vary significantly with solution osmolarity, but show a clear expressed maximum in isotonic conditions; the values in 0.4 ᴍ, 0.5 ᴍ, 0.6 ᴍ and 0.7 ᴍ manitol solutions are (6.5, 10.2, 8.8 and 5.6) × 10−24 A2s4/Kg, pectively. The average cell radii in those conditions are 14.2 µm, 14.6 µm, 13.8 µm and 13.6 µm. The radii of cell-to-electrode contacts follow the same dependence on the osmolarity as the dielectrophoretic coefficients; they do not depend on the applied voltages up to 18V. The times of cell approach near to the electrode were too short to be explained by the action only of the dielectrophoretic force; consequently an attractive force appeared at very close approach. These results may be of use in future studies of membrane adhesion and fusion.

The influence of Ca2+ ions on the diffrence absorption spectra of phosphorylase kinase

A low iodide concentration causes a positive difference band in the 250–260 mm UV range. The effect of Ca2+ changes the direction of difference spectra. The concentration dependence is explained by multistage interaction. The addition of EDTA the solution restores the positive band.