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Biomedical subjects

P Mazur

Publications and source records attributed to P Mazur.

At least 55 records · Page 3Linked to original sources
PubMed1991

When and why is the random force in Brownian motion a Gaussian process.

It is shown that the assumptions of causality and time-reversal invariance severely restrict the possibility to describe the fluctuations of a variable in a non-linear Markovian system using a Langevin equation. In fact a theorem is proven which implies that with the aforementioned assumptions a Langevin force which is independent of the state of the system is necessarily Gaussian and white. The theorem furthermore implies that such a description is only possible if the socalled "systematic force" is proportional to the derivative of the logarithm of the equilibrium distribution of the variable. Our analysis is given for a system with one variable which may be either even or odd under time reversal.

Journal Article↗
PubMed1991

All-trans-retinal is the chromophore bound to the photoreceptor of the alga Chlamydomonas reinhardtii.

Rhodopsin is the general name for a family of visual pigments that receive light and transmit this signal to the rest of an organism. Chlamydomonas reinhardtii is a unicellular eukaryote whose light-tracking system consists of a single eye. Through spectral studies of Chlamydomonas' reaction to light of different wavelengths (action spectroscopy), it has been shown in vivo that the photoreceptor of Chlamydomonas is functionally similar to vertebrate rhodopsin. We seek to characterize the photoreceptor further by identifying the molecule that is incorporated into the rhodopsin of Chlamydomonas forming the chromophore. High performance liquid chromatography analysis of organic extracts of retinaloximes from membrane fractions enriched in eye-spots and in cells virtually free of interfering carotenoids identified syn-all-trans as the existing retinaloxime isomer. We conclude that all-trans-retinal is the native molecule that is available to be incorporated into the rhodopsin of Chlamydomonas and therefore forms the functioning chromophore on binding.

Animals↗
PubMed1990

Color regulation in the archaebacterial phototaxis receptor phoborhodopsin (sensory rhodopsin II).

Phoborhodopsin, a repellent phototaxis receptor in Halobacterium halobium, exhibits vibrational fine structure, a feature that has not been identified for any other rhodopsin pigment at physiological temperatures. This conclusion follows form analysis of the absorption properties of the pigment in H. halobium membranes containing native retinal and an array of retinal analogues. The absorption spectrum of the native pigment has a maximum at 487 nm with a pronounced shoulder at 460 nm; however, the bandwidth is that expected for a single retinylidene species. Gaussian band-shape simulation with a spacing corresponding to the vibrational frequencies of polyene stretching modes reproduces the structured absorption spectra of native pigment as well as of analogue phoborhodopsin. Absorption shifts produced by a series of dihydroretinal and other retinal analogues strongly indicate that the dominant factor regulating the color of the pigment is planarization of the retinal ring with respect to the polyene chain.

Archaeal Proteins↗
PubMed1990

Separation of carotenes on cyclodextrin-bonded phases.

The separation of carotenoids and retinoids on a beta-cyclodextrin-bonded stationary phase with conventional mobile phases is reported. Compounds studied include beta-carotene (all-trans), 15,15'-cis-beta-carotene, 7,8,7',8'-dihydro-beta-carotene, alpha-carotene, lycopene, lutein, zeaxanthin, retinal, retinol, retinol palmitate and retinol acetate. The best resolution of carotenes was obtained with low concentrations (less than or equal to 1%) of polar solvents (e.g., 2-propanol or ethyl acetate) in hexane or cyclohexane. Xanthophylls required much higher concentrations of polar solvents. The best solvent for the resolution of lutein and zeaxanthin was found to be dichloromethane. The resolution of cis/trans-isomers and the tentative identification of other isomers present in newly synthesized carotenoid standards is also reported. All-trans-isomers were found to be eluted before cis-isomers.

Carotenoids↗
PubMed1990

Equilibrium, quasi-equilibrium, and nonequilibrium freezing of mammalian embryos.

The first successful freezing of early embryos to -196 degrees C in 1972 required that they be cooled slowly at approximately 1 degree C/min to about -70 degrees C. Subsequent observations and physical/chemical analyses indicate that embryos cooled at that rate dehydrate sufficiently to maintain the chemical potential of their intracellular water close to that of the water in the partly frozen extracellular solution. Consequently, such slow freezing is referred to as equilibrium freezing. In 1972 and since, a number of investigators have studied the responses of embryos to departures from equilibrium freezing. When disequilibrium is achieved by the use of higher constant cooling rates to -70 degrees C, the results is usually intracellular ice formation and embryo death. That result is quantitatively in accord with the predictions of the physical/chemical analysis of the kinetics of water loss as a function of cooling rate. However, other procedures involving rapid nonequilibrium cooling do not result in high mortality. One common element in these other nonequilibrium procedures is that, before the temperature has dropped to a level that permits intracellular ice formation, the embryo water content is reduced to the point at which the subsequent rapid nonequilibrium cooling results in either the formation of small innocuous intracellular ice crystals or the conversion of the intracellular solution into a glass. In both cases, high survival requires that subsequent warming be rapid, to prevent recrystallization or devitrification. The physical/chemical analysis developed for initially nondehydrated cells appears generally applicable to these other nonequilibrium procedures as well.

Animals↗
PubMed1989

Roles of unfrozen fraction, salt concentration, and changes in cell volume in the survival of frozen human erythrocytes.

The cause of slow freezing injury and the basis of the protection by solutes like glycerol are subjects of debate. During slow freezing, cells are sequestered in unfrozen channels between ice crystals that grow by removing pure water from the channels. As a consequence, the solute concentration in the channels rises and the volume of liquid in the channels progressively decreases. The rise in solute concentration, in turn, causes the cells to progressively shrink osmotically. Until recently cryobiologists have ascribed slow freezing injury to either the rise in solute (electrolyte) concentrations in the channels or to the consequent cell shrinkage, rather than to the decrease in the of the channels. Although ordinarily reciprocally coupled, it is possible to separate the composition of the channels from their size, or more precisely from the magnitude of the unfrozen fraction, by suspending cells in NaCl/cryoprotectant solutions in which the mole ratio of the two is held constant, but the molality of the NaCl is allowed to vary below and above isotonic. When human red cells are frozen in such solutions to temperatures that produce given NaCl concentrations (ms), but varying unfrozen fractions (U), survival at low U is found to be strongly dependent on U but independent of ms. At higher values of U, survival becomes inversely dependent on both ms and U. Although cell volume during freezing is independent of the NaCl tonicity in the solution, the cells in the several solutions differ in volume both prior to the onset of freezing and after the completion of thawing. We have now examined and compared the effect of returning the thawed cells to isotonic solutions and isotonic volume or nearly so, and find that there is little change in survival after exposure to low U, but that survival after exposure to high U values exhibits substantially increased sensitivity to ms, a sensitivity that is probably a manifestation of posthypertonic hemolysis. Low values of U were in general attained by the use of solutions with low tonicities of NaCl, and as a consequence cells frozen to low U values had larger volumes prior to freezing than cells frozen to higher U values. The significance of this confounding is discussed.

Blood Preservation↗
PubMed1987

Relative influence of unfrozen fraction and salt concentration on the survival of slowly frozen eight-cell mouse embryos.

This laboratory has previously reported that the survival of frozen-thawed human erythrocytes is determined more by the fraction of the extracellular solution that remains unfrozen than by the salt concentration in that fraction, especially when the cells are frozen at low hematocrit. To determine the extent to which these findings are applicable to nucleated mammalian cells, we have studied the survival of some 3300 mouse embryos as a function of the unfrozen fraction and the concentration of salt in that unfrozen fraction. Also varied in the study was the weight percentage ratio of glycerol to salt. The concentration of embryos in these experiments (i.e., the cytocrit) was so low that embryo-embryo contacts should have been rare during the freezing. As in the case of the red cells at low hematocrit, we find that the survival of slowly frozen eight-cell embryos is not affected by the high concentrations of salt produced by freezing, at least up to 3.3 molal NaCl, and therefore is not affected by the extent to which the cells shrink below their isotonic volume, nor in general is survival influenced by the temperature at which given salt concentrations and unfrozen fractions are attained or by the glycerol concentration at those temperatures. On the other hand, the attainment of low values of the unfrozen fraction (U) is damaging, but the damage appears in part to be due to the fact that low values of U had to be achieved by placing embryos in solutions hypotonic with respect to NaCl, which caused their volume to be greater than isotonic prior to freezing.

Animals↗
PubMed1986

Osmotic responses of preimplantation mouse and bovine embryos and their cryobiological implications.

Cells subjected to the events occurring before, during, and after freezing and thawing are exposed to major changes in the osmotic pressure of the surrounding medium; i.e., the osmolalities can exceed 30. An important question in understanding the mechanisms of injury is whether cells respond as ideal osmometers to these strongly anisotonic solutions. Mouse and bovine embryos from eight-cell to blastocyst stage were used to investigate the question. They were found to behave as ideal osmometers at room temperature over a wide range of tonicities; i.e., from four times isotonic to almost 1/3 times isotonic, ideality being defined by a Boyle-van't Hoff equation. Embryo volumes increased from 40 to 200% of isotonic over this range and survivals of mouse embryos were unaffected. However, outside this range the membrane apparently becomes leaky and the survival of mouse embryos drops sharply. Osmolalities rise to high values during freezing and the paper develops the thermodynamic equations to show how computed cell volumes as a function of subzero temperature can be translated into the Boyle-van't Hoff format of cell volume as a function of the reciprocal of osmolality.

Animals↗
PubMed1985

Influence of cell concentration on the contribution of unfrozen fraction and salt concentration to the survival of slowly frozen human erythrocytes.

We have shown previously that the survival of human red blood cells during slow freezing at 2% hematocrit is dependent more on the magnitude of the unfrozen fraction than on the salt concentration in that unfrozen fraction. In parallel, first Nei and more recently Pegg and colleagues have shown that survival is affected by the hematocrit of the suspension. Freezing at hematocrits above 30% becomes increasingly damaging. The present studies were designed to see whether there is a link between the two phenomena. Cells were suspended at nominal hematocrits of 0.4, 2, 8, 40, or 60% in five test solutions of glycerol-NaCl. The test solutions were of such composition that when frozen to a specified temperature, the magnitude of the unfrozen fraction differed but the NaCl concentration (ms) remained constant. At low hematocrits (0.4 to 8%), red cell survival was dependent predominantly on the unfrozen fraction and was relatively independent of the salt concentration in that fraction. This we term the "rheological" effect because injury appears to be related to interaction with the ice walls and perhaps is due to shearing forces or cell deformation. But at high hematocrits (40 or 60%), cell survival became dependent on both the unfrozen fraction and the salt concentration in that fraction. When freezing occurs at high hematocrits, increasing numbers of cells are presumably brought into contact with their neighbors. Furthermore, they are increasingly shrunken cells, for the progressive removal of liquid water, which is responsible for the crowding, also causes a rise in ms and the consequent osmotic shrinkage of cells. Our data suggest that at unfrozen fractions above those producing injurious rheological forces, the tight packing of less shrunken cells (i.e., high hematocrit, low ms) and the extensive shrinking of loosely packed cells (high ms, low hematocrit) are both quite innocuous. Injury becomes substantial only when extensively shrunken cells are brought into close contact (i.e., high ms, high hematocrit). At high hematocrit the cells occupy a substantial fraction of the unfrozen space, and the water that they lose during slow freezing adds substantially to the volume of extracellular ice. Accordingly, we defined other measures of unfrozen fraction that include these perturbations. However, we found that the conclusions on the relation between survival, unfrozen fraction, and hematocrit were not affected by the method of expressing the unfrozen fraction. Freezing at high hematocrit to high ms and low values of unfrozen fraction is one way to produce contact between shrunken cells at low temperatures.(ABSTRACT TRUNCATED AT 400 WORDS)

Blood Preservation↗
PubMed1984

Kinetics of water loss and the likelihood of intracellular freezing in mouse ova. Influence of the method of calculating the temperature dependence of water permeability.

To avoid intracellular freezing and its usually lethal consequences, cells must lose their freezable water before reaching their ice-nucleation temperature. One major factor determining the rate of water loss is the temperature dependence of the water permeability, Lp (hydraulic conductivity). Because of the paucity of water permeability measurements at subzero temperatures, that temperature dependence has usually been extrapolated from above-zero measurements. The extrapolation has often been based on an exponential dependence of Lp on temperature. This paper compares the kinetics of water loss based on that extrapolation with that based on an Arrhenius relation between Lp and temperature, and finds substantial differences below -20 to -25 degrees C. Since the ice-nucleation temperature of mouse ova in the cryoprotectants DMSO and glycerol is usually below -30 degrees C, the Arrhenius form of the water-loss equation was used to compute the extent of supercooling in ova cooled at rates between 1 and 8 degrees C/min and the consequent likelihood of intracellular freezing. The predicted likelihood agrees well with that previously observed. The water-loss equation was also used to compute the volumes of ova as a function of cooling rate and temperature. The computed cell volumes agree qualitatively with previously observed volumes, but differ quantitatively.

Animals↗