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R M Hochmuth

Publications and source records attributed to R M Hochmuth.

At least 19 recordsLinked to original sources
PubMed1992

Analysis of lateral diffusion from a spherical cell surface to a tubular projection.

Cell surfaces are often heterogeneous with respect to the lateral distribution and mobility of membrane components. Because lateral mobility is related to membrane structure, measurement of a particular component's local diffusion coefficient within a distinct surface region provides useful information about the formation and maintenance of that region. Many structurally interesting cell surface features can be described as narrow tubular projections from the body of the cell. In a companion paper, we consider the thin "tethers" that can be mechanically drawn from the red blood cell membrane, and we measure the transport of fluorescent integral proteins from the surface of the cell body onto the tether. In this paper we present an analysis to describe the surface diffusion of membrane particles from a spherical shell onto a thin cylindrical process. Provision is made for different rates of diffusion within the two morphologically distinct regions. The relative role of each region in controlling the diffusive flux between regions is determined primarily by a single dimensionless parameter. This parameter incorporates the ratio of the two diffusion coefficients as well as the dimensions of each region. The analysis can be applied to a fluorescence photobleaching experiment in which the extended process is bleached. If the dimensions of the spherical cell body and the cylindrical extension are known, then the diffusion coefficients of both regions can be determined from the experimental fluorescence recovery curve.

Cell Membrane↗
PubMed1992

Lateral mobility of integral proteins in red blood cell tethers.

The red blood cell membrane is a complex material that exhibits both solid- and liquidlike behavior. It is distinguished from a simple lipid bilayer capsule by its mechanical properties, particularly its shear viscoelastic behavior and by the long-range mobility of integral proteins on the membrane surface. Subject to sufficiently large extension, the membrane loses its shear rigidity and flows as a two-dimensional fluid. These experiments examine the change in integral protein mobility that accompanies the mechanical phenomenon of extensional failure and liquidlike flow. A flow channel apparatus is used to create red cell tethers, hollow cylinders of greatly deformed membrane, up to 36-microns long. The diffusion of proteins within the surface of the membrane is measured by the technique of fluorescence redistribution after photobleaching (FRAP). Integral membrane proteins are labeled directly with a fluorescein dye (DTAF). Mobility in normal membrane is measured by photobleaching half of the cell and measuring the rate of fluorescence recovery. Protein mobility in tether membrane is calculated from the fluorescence recovery rate after the entire tether has been bleached. Fluorescence recovery rates for normal membrane indicate that more than half the labeled proteins are mobile with a diffusion coefficient of approximately 4 x 10(-11) cm2/s, in agreement with results from other studies. The diffusion coefficient for proteins in tether membrane is greater than 1.5 x 10(-9) cm2/s. This dramatic increase in diffusion coefficient indicates that extensional failure involves the uncoupling of the lipid bilayer from the membrane skeleton.

Erythrocyte Membrane↗
PubMed1992

A sensitive measure of surface stress in the resting neutrophil.

The simplest parameterized model of the "passive" or "resting receptive" neutrophil views the cell as being composed of an outer cortex surrounding an essentially liquid-like highly viscous cytoplasm. This cortex has been measured to maintain a small persistent tension of approximately 0.035 dyn/cm (Evans and Yeung. 1989. Biophys. J. 56:151-160) and is responsible for recovering the spherical shape of the cell after large deformation. The origin of the cortical tension is at present unknown, but speculations are that it may be an active process related to the sensitivity of a given cell to external stimulation and the "passive-active" transition. In order to characterize further this feature of the neutrophil we have used a new micropipet manipulation method to give a sensitive measure of the surface stress as a function of the surface area dilation of the highly ruffled cellular membrane. In the experiment, a single cell is driven down a tapered pipet in a series equilibrium deformation positions. Each equilibrium position represents a balance between the stress in the membrane and the pressure drop across the cell. For most cells that seemed to be "passive," as judged by their spherical appearance and lack of pseudopod activity, area dilations of approximately 30% were accompanied by only a small increase in the membrane tension, indicative of a very small apparent elastic area expansion modulus (approximately 0.04 dyn/cm). Extrapolations back to zero area dilation gave a value for the tension in the resting membrane of 0.024 +/- 0.003 dyn/cm, in close agreement with earlier measures. A few cells showed virtually no change in cortical tension and fit the persistent cortical tension model of Evans and Yeung (1989. Biophys. J. 56:151-160). However, other cells that also appeared "passive," as judged by their spherical appearance, had membrane tensions that increased as the apparent surface area was increased. Thus, the postulated,persistent "cortical tension" does not appear to be a unique and constant parameter for all cells as the membrane area is dilated.This measurement of membrane tension could represent a sensitive indication of the first stages of cell activation and the"passive-active" transition.

Cell Membrane↗
PubMed1991

Relationship of clinical severity to packed cell rheology in sickle cell anemia.

There is wide variation in the clinical manifestations of sickle cell disease (SCD) from one affected individual to another. Many investigators have sought to discern parameters that would explain this variability. In the present studies we have attempted to correlate the frequency of painful events and the extent of end organ failure in SCD with rheologic properties of packed suspensions of sickle cells, using a magneto-acoustic ball microrheometer developed in our laboratory. Using this device we have measured the steady-state viscosity, and the viscous and elastic moduli of cell suspensions in 16 individuals with hemoglobin SS disease who were untransfused and in their steady state. The rheologic parameters were then correlated with clinical parameters. The clinical parameters measured were emergency department visits, hospitalizations, hemoglobin, reticulocyte count, age, and end organ failure (nephropathy, avascular necrosis of bone, stroke, retinopathy, resting hypoxemia after acute chest syndrome(s), leg ulcer, and priapism with impotence). The P value for the correlation between the steady state viscosity and end organ failure was .001 with a correlation coefficient (R value) of .73. The P value for the correlation between the viscous modulus of viscosity and end organ failure was .00006 with an R value of .83. The P value for the correlation between the elastic modulus of viscosity and end organ failure was .0006 with an R value of .76. However, there was no significant correlation between any component of packed cell rheology and emergency department visits or hospitalizations for pain.

Adult↗
PubMed1991

Time-dependent recovery of passive neutrophils after large deformation.

Experiments are performed in which a passive human neutrophil is deformed into an elongated "sausage" shape by aspirating it into a small glass pipette. When expelled from the pipette the neutrophil recovers its natural spherical shape in approximately 1 minute. This recovery process is analyzed according to a Newtonian, liquid-drop model in which a variational method is used to simultaneously solve the hydrodynamic equations for low Reynolds-number flow and the equations for membrane equilibrium with a constant membrane tension. The theoretical model gives a good fit to the experimental data for a ratio of membrane cortical tension to cytoplasmic viscosity of approximately 1.7 x 10(-5) cm/s (0.17 micron/s). However, when the cell is held in the pipette for only a short time period of 5 s or less, and then expelled, the cell undergoes an initial, rapid elastic rebound suggesting that the cell behaves in this instance as a Maxwell viscoelastic liquid rather than a Newtonian liquid with constant cortical tension.

Elasticity↗
PubMed1990

The influence of oxygen tension, temperature, and hemoglobin concentration on the rheologic properties of sickle erythrocytes.

With the use of micromanipulation techniques, the shear modulus or "rigidity" mu, the recovery time tc, and the unfolding time tf for individual sickle cells have been measured at different oxygen tensions, temperatures, and cell densities. In these experiments, the partial pressure of oxygen was varied from 156 to 40 mm Hg and the temperature was controlled at 25 degrees C or 37 degrees C. Three mean cellular hemoglobin concentrations were studied: 29 g/dL, 33 g/dL, and 46 g/dL. The lighter cells (29 and 33 g/dL) exhibited at most a threefold increase in rigidity as the pO2 was decreased from 156 to 40 mm Hg. At 25 degrees C, the densest cells (46 g/dL) also exhibited a threefold increase. However, at 37 degrees C, the rigidity of these cells increased eightfold between 156 to 40 mm Hg. Compared with normal cells, this gives a rigidity that is 18 times larger. In contrast to the values for mu, the values for tc and tf remained essentially unchanged (within the accuracy of the experiments) for the lighter cells and could not be measured for the densest cells.

Adolescent↗
PubMed1990

Cell biomechanics: a brief overview.

In this issue of the Journal of Biomechanical Engineering, there are eleven papers and one technical brief in the general area of "Cell Biomechanics." In general, the work in these papers focuses on measuring and characterizing the mechanical and adhesive properties of cells and membranes. Included are studies of lipid membranes, erythrocytes, endothelial cells, and neutrophils. Characterizing and measuring the properties and behavior of cells in both passive and active states present a major challenge to investigators in this field. In this paper in the comments to follow, a simple overview of the field is presented and the principles and techniques used in the studies of cell biomechanics are discussed.

Biomechanical Phenomena↗
PubMed1990

Rapid flow of passive neutrophils into a 4 microns pipet and measurement of cytoplasmic viscosity.

Neutrophils from five different individuals are isolated with a density separation technique. A total of 151 unactivated (passive) cells are rapidly aspirated at constant suction pressure and at room temperature into a pipet with a diameter of 4 microns. The suction pressures in excess of an initial yield threshold are 0.5, 1 and 2 kPa and are comparable to those encountered in the microcirculation. These pressures are well in excess of the small suction pressure of approximately 20 Pa that is required to form a static hemispherical bump on the cell. At a given aspiration pressure, the leading edge of an individual cell is "tracked" as it flows into the pipet. A theory based on the flow of a Newtonian liquid from either a hemisphere or a spherical segment into a cylinder is used to model the entry process. Both theory and experiment show that during most of the entry process the leading edge of the cell moves at a nearly constant velocity with a rapid acceleration at the end. For cells from five different individuals at the three different excess aspiration pressures, Newtonian theory gives a cytoplasmic viscosity of 135 +/- 54 Pa.s and overall entry times of 3.3s (0.5 kPa), 1.6s (1 kPa) and 0.82s (2 kPa). These results and those of Evans and Yeung at lower aspiration pressures indicate that the complex cytoplasm inside unactivated neutrophils behaves as a nearly Newtonian fluid with a viscosity on the order of 10(2) Pa.s over almost a two order of magnitude range in aspiration pressure and, thus, rate of deformation.

Cytoplasm↗
PubMed1990

The viscosity of neutrophils and their transit times through small pores.

Passive neutrophils from five different individuals are rapidly aspirated at constant suction pressure and at room temperature into a pipet with a diameter of 4 microns. The excess suction pressures (i.e., the pressures in excess of the small threshold pressure required to produce continuous flow into the pipet) are 5000, 10,000 and 20,000 dyn/cm2 (0.5, 1 and 2 kPa) and are comparable to those encountered in the microcirculation. The rate of entry into the pipet is modeled with a linearized version of a theory by Yeung and Evans for the newtonian flow of a neutrophil into a pipet or pore. From this theory and measurements of the cell size and its rate of entry into the pipet, we can calculate a value for the cytoplasmic viscosity. A linear (newtonian) fit of the theory to the experimental data gives a value for the viscosity of 1050 poise. A non-linear fit predicts a decrease in the "apparent viscosity" from about 1500 poise at zero excess pressure to 1000 poise at an excess aspiration pressure of 20,000 dyn/cm2. Our experiments and analysis also allow us to calculate a value for the transit time through short pores over a wide range of excess aspiration pressures and pore diameters. For example, for a pore diameter of 3 microns and an aspiration pressure of 1250 dyn/cm2, we predict a transit time of about 70 s. At 6 microns and 20,000 dyn/cm2, the predicted transit time is only about 0.04 s.

Blood Flow Velocity↗
PubMed1989

Electro-mechanical permeabilization of lipid vesicles. Role of membrane tension and compressibility.

A simple micropipet technique was used to determine the critical electric field strength for membrane breakdown as a function of the applied membrane tension for three different reconstituted membranes: stearoyloleoylphosphatidylcholine (SOPC), red blood cell (RBC) lipid extract, and SOPC cholesterol (CHOL), 1:1. For these membranes the elastic area expansivity modulus increases from approximately 200 to 600 dyn/cm, and the tension at lysis increases from 5.7 to 13.2 dyn/cm, i.e., the membranes become more cohesive with increasing cholesterol content. The critical membrane voltage, Vc, required for breakdown was also found to increase with increasing cholesterol from 1.1 to 1.8 V at zero membrane tension. We have modeled the behavior in terms of the bilayer expansivity. Membrane area can be increased by either tensile or electrocompressive stresses. Both can store elastic energy in the membrane and eventually cause breakdown at a critical area dilation or critical energy. The model predicts a relation between tension and voltage at breakdown and this relation is verified experimentally for the three reconstituted membrane systems studied here.

Cell Membrane↗
PubMed1989

A rheological study of packed red blood cell suspensions with an oscillating ball microrheometer.

Rheological properties of concentrated red blood cell suspensions are studied with a magneto acoustic microrheometer in which a ball is suspended in a vertically oriented cylindrical tube. The rheometer uses a conventional falling ball technique to measure steady state viscosity and a vertically oscillating, magnetically driven ball for viscoelastic measurements. The motion of the ball is tracked by ultrasound echo location in which sound waves are transmitted and received by an ultrasound transducer mounted at the base of the tube. The compact size of the rheometer allows rheological studies to be made with microliter quantities of opaque suspensions and permits sudden and accurate changes in temperature. Also, values for the adiabatic compressibility are evaluated from measurements of the speed of sound.

Blood Viscosity↗
PubMed1988

The influence of red cell mechanical properties on flow through single capillary-sized pores.

The resistive pulse technique was used to study the influence of specific mechanical properties of the red cell on its ability to enter and flow through single capillary-sized pores with diameters of 3.6, 5.0 and 6.3 micron and lengths of 11 micron. A two-fold increase in membrane shear elasticity resulted in a 40 percent increase in the cell's transit time through a 3.6 micron pore but produced no change in transit time through a 6.3 micron pore. A two-fold increase in membrane shear viscosity produced a 40 percent increase in transit time through the 3.6 micron pore and small but significant increases in transit times through the larger pores. Osmotically dehydrated cells showed no increase in transit time through a 6.3 micron pore, but showed increases in transit times of 50 to 70 percent through 5.0 and 3.6 micron pores. Dense red cells showed increased transit times through both 5.0 micron and 6.0 micron pores. These results indicate that for cells with normal geometric properties, the membrane's shear viscosity and elasticity only influence the cell's transit through pores of 5 micron or less in diameter. However, alterations in the cell's geometric properties can extend the influence of membrane shear properties to larger diameter pores.

Adult↗
PubMed1987

Mechanical equilibrium of thick, hollow, liquid membrane cylinders.

The mechanical equilibrium of bilayer membrane cylinders is analyzed. The analysis is motivated by the observation that mechanically formed membrane strands (tethers) can support significant axial loads and that the tether radius varies inversely with the axial force. Previously, thin shell theory has been used to analyze the tether formation process, but this approach is inadequate for describing and predicting the equilibrium state of the tether itself. In the present work the membrane is modeled as two adjacent, thick, anisotropic liquid shells. The analysis predicts an inverse relationship between axial force and tether radius, which is consistent with experimental observation. The area expansivity modulus and bending stiffness of the tether membrane are calculated using previously measured values of tether radii. These calculated values are consistent with values of membrane properties measured previously. Application of the analysis to precise measurements of the relationship between tether radius and axial force will provide a novel method for determining the mechanical properties of biomembrane.

Kinetics↗
PubMed1987

An investigation of particle flow through capillary models with the resistive pulse technique.

The use of the resistive pulse technique for the measurement of microsphere and red cell transit times through single-pore "Nuclepore" membranes (with pore diameters of 3.5 to 7.0 microns and pore length of approximately 11 microns) is described. The investigation of the fluid mechanics and electrical characteristics of the experimental system provides methods for the determination of particle and cell size, and entrance and transit times. Experimental measurement of the position dependent velocity of spherical particles through the pore shows close agreement with theoretical models. Red cell size and transit time through different sized pores at physiological shear stresses is also measured.

Blood Flow Velocity↗
PubMed1979

Red cell extensional recovery and the determination of membrane viscosity.

A theory of membrane viscoelasticity developed by Evans and Hochmuth in 1976 is used to analyze the time-dependent recovery of an elongated cell. Before release, the elongated cell is the static equilibrium where external forces are balanced by membrane elastic force resultants. Upon release, the cell recovers its initial shape with a time-dependent exponential behavior characteristic of the viscoelastic solid model. It is shown that the model describes the time-dependent recovery process very well for a time constant in the range of 0.1-0.13 s. The time constant is the ratio membrane surface viscosity eta:membrane surface elasticity mu. Measurements for the shear modulus mu of 0.006 dyne/cm give a value for the surface viscosity of red cell membrane as a viscoelastic solid material of eta = mu tc = (6-8) X 10(-4) poise . cm.

Elasticity↗
PubMed1978

Membrane mechanical properties of ATP-depleted human erythrocytes.

Although the relations between the metabolic state and the mechanical properties of human red blood cells (RBC) continue to be of current interest, literature reports in this area are not in agreement. The present investigation was designed to determine several intrinsic mechanical properties of human RBC membranes before and after metabolic depletion via incubation at 37 degrees C for 24 hr. Using micropipette and flow channel techniques, three properties were measured: (1) mu, surface shear modulus of elasticity; (2) K, elastic area compressibility modulus; (3) etap, shear viscosity in the plastic domain. Our results indicate no significant differences in these parameters between fresh and ATP-depleted human RBC membranes. These present data are thus in disagreement with other literature reports indicating large changes in membrane mechanical properties consequent to metabolic depletion. A brief discussion of the possible reasons for this disagreement is presented.

Adenosine Triphosphate↗