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Y Tardy

Publications and source records attributed to Y Tardy.

At least 19 recordsLinked to original sources
PubMed1998

Measuring actin dynamics in endothelial cells.

Cytoplasmic actin distributes between monomeric and filamentous phases in cells. As cells crawl, actin polymerizes near the plasma membrane of expanding peripheral cytoplasm and depolymerizes elsewhere. Thus, the finite actin filament lifetime, the diffusivity of actin monomer, and the distribution of actin between the polymer and monomer phases are key parameters in cell motility. The dynamics of cellular actin can be determined by following the evolution of fluorescence in the techniques of photoactivated fluorescence (PAF) or fluorescence recovery after photobleaching (FRAP) of microinjected actin derivatives. A mathematical model is discussed that measures monomer diffusion coefficients, filament turnover rates, and the fraction of actin polymerized from measurements of the evolution of fluorescence from a photoactivated band [Tardy et al. (1995) Biophys. J., 69:1674-1682; McGrath et al. (1998) Biophys. J., in press]. Applying this model to subconfluent endothelial cells shows that approximately 40% of the actin is polymer and that these filaments turn over on average every 6 minutes. This report discusses how PAF and FRAP can be combined with more traditional biochemistry to probe actin cytoskeleton remodeling in endothelial cells.

Actins↗
PubMed1998

Modeling actin filament reorganization in endothelial cells subjected to cyclic stretch.

Hemodynamic forces affect endothelial cell morphology and function. In particular, circumferential cyclic stretch of blood vessels, due to pressure changes during the cardiac cycle, is known to affect the endothelial cell shape, mediating the alignment of the cells in the direction perpendicular to stretch. This change in cell shape proceeds a drastic reorganization at the internal level. The cellular scaffolding, mainly composed of actin filaments, reorganize in the direction which later becomes the cell's long axis. How this external mechanical stimulus is 'sensed' and transduced into the cell is still unknown. Here, we develop a mathematical model depicting the dynamics of actin filaments, and the influence of the cyclic stretch of the substratum based on the experimental evidence that external stimuli may be transduced inside the cell via transmembrane proteins which are coupled with actin filaments on the cytoplasmic side. Based on this view, we investigate two approaches describing the formulation of the transduction mechanisms involving the coupling between filaments and the membrane proteins. As a result, we find that the mechanical stimulus could cause the experimentally observed reorganization of the entire cytoskeleton simply by altering the dynamics of the filaments connected with the integral membrane proteins, as described in our model. Comparison of our results with previous studies of cytoskeletal dynamics reveals that the cytoskeleton, which, in the absence of the effect of stretch would maintain its isotropic distribution, slowly aligns with the precise direction set by the external stimulus. It is found that even a feeble stimulus, coupled with a strong internal dynamics, is sufficient to align actin filaments perpendicular to the direction of stretch.

Actins↗
PubMed1998

Simultaneous measurements of actin filament turnover, filament fraction, and monomer diffusion in endothelial cells.

The analogous techniques of photoactivation of fluorescence (PAF) and fluorescence recovery after photobleaching (FRAP) have been applied previously to the study of actin dynamics in living cells. Traditionally, separate experiments estimate the mobility of actin monomer or the lifetime of actin filaments. A mathematical description of the dynamics of the actin cytoskeleton, however, predicts that the evolution of fluorescence in PAF and FRAP experiments depends simultaneously on the diffusion coefficient of actin monomer, D, the fraction of actin in filaments, FF, and the lifetime of actin filaments, tau (, Biophys. J. 69:1674-1682). Here we report the application of this mathematical model to the interpretation of PAF and FRAP experiments in subconfluent bovine aortic endothelial cells (BAECs). The following parameters apply for actin in the bulk cytoskeleton of subconfluent BAECs. PAF: D = 3.1 +/- 0.4 x 10(-8) cm2/s, FF = 0.36 +/- 0.04, tau = 7.5 +/- 2.0 min. FRAP: D = 5.8 +/- 1.2 x 10(-8) cm2/s, FF = 0.5 +/- 0.04, tau = 4.8 +/- 0.97 min. Differences in the parameters are attributed to differences in the actin derivatives employed in the two studies and not to inherent differences in the PAF and FRAP techniques. Control experiments confirm the modeling assumption that the evolution of fluorescence is dominated by the diffusion of actin monomer, and the cyclic turnover of actin filaments, but not by filament diffusion. The work establishes the dynamic state of actin in subconfluent endothelial cells and provides an improved framework for future applications of PAF and FRAP.

Actins↗
PubMed1997

Monocyte adhesion to activated aortic endothelium: role of L-selectin and heparan sulfate proteoglycans.

This study examines the role of L-selectin in monocyte adhesion to arterial endothelium, a key pathogenic event of atherosclerosis. Using a nonstatic (rotation) adhesion assay, we observed that monocyte binding to bovine aortic endothelium at 4 degrees C increased four to nine times upon endothelium activation with tumor necrosis factor (TNF)-alpha. mAb-blocking experiments demonstrated that L-selectin mediates a major part (64 +/- 18%) of monocyte attachment. Videomicroscopy experiments performed under flow indicated that monocytes abruptly halted on 8-h TNF-alpha-activated aortic endothelium, approximately 80% of monocyte attachment being mediated by L-selectin. Flow cytometric studies with a L-selectin/IgM heavy chain chimeric protein showed calcium-dependent L-selectin binding to cytokine-activated and, unexpectedly, unactivated aortic cells. Soluble L-selectin binding was completely inhibited by anti-L-selectin mAb or by aortic cell exposure to trypsin. Experiments with cycloheximide, chlorate, or neuraminidase showed that protein synthesis and sulfate groups, but not sialic acid residues, were essential for L-selectin counterreceptor function. Moreover, heparin lyases partially inhibited soluble L-selectin binding to cytokine-activated aortic cells, whereas a stronger inhibition was seen with unstimulated endothelial cells, suggesting that cytokine activation could induce the expression of additional ligand(s) for L-selectin, distinct from heparan sulfate proteoglycans. Under flow, endothelial cell treatment with heparinase inhibited by approximately 80% monocyte attachment to TNF-alpha-activated aortic endothelium, indicating a major role for heparan sulfate proteoglycans in monocyte-endothelial interactions. Thus, L-selectin mediates monocyte attachment to activated aortic endothelium, and heparan sulfate proteoglycans serve as arterial ligands for monocyte L-selectin.

Animals↗
PubMed1997

Model for the alignment of actin filaments in endothelial cells subjected to fluid shear stress.

Cultured vascular endothelial cells undergo significant morphological changes when subjected to sustained fluid shear stress. The cells elongate and align in the direction of applied flow. Accompanying this shape change is a reorganization at the intracellular level. The cytoskeletal actin filaments reorient in the direction of the cells' long axis. How this external stimulus is transmitted to the endothelial cytoskeleton still remains unclear. In this article, we present a theoretical model accounting for the cytoskeletal reorganization under the influence of fluid shear stress. We develop a system of integro-partial-differential equations describing the dynamics of actin filaments, the actin-binding proteins, and the drift of transmembrane proteins due to the fluid shear forces applied on the plasma membrane. Numerical simulations of the equations show that under certain conditions, initially randomly oriented cytoskeletal actin filaments reorient in structures parallel to the externally applied fluid shear forces. Thus, the model suggests a mechanism by which shear forces acting on the cell membrane can be transmitted to the entire cytoskeleton via molecular interactions alone.

Actins↗
PubMed1997

Shear stress gradients remodel endothelial monolayers in vitro via a cell proliferation-migration-loss cycle.

Wall shear stress has been implicated in the genesis of atherosclerosis because a strong correlation exists between the location of developing arterial lesions and regions where particular gradients in stress occur. Studying the behavior of endothelial cells in such regions may contribute to our understanding of the disease etiology. We report the detailed migratory history of endothelial cells subjected to large shear stress gradients caused by a surface protuberance in an in vitro model system. The history of cell migration, cell division, and cell loss from the surface was continuously monitored in confluent human umbilical vein endothelial cell monolayers for 48 hours after the onset of flow. Individual cells were tracked using time-lapse video microscopy. In contrast to a uniform laminar flow field in which cells were observed to continually rearrange their relative position with no net migration, in a disturbed flow field there was a net migration directed away from the region of high shear gradient. This organized migration pattern under disturbed flow conditions was accompanied by more than a twofold increase in cell motility. In addition, cell division increased in the vicinity of the flow separation (maximum shear stress gradient of 34 dyne/cm2 per mm) whereas cell loss was increased upstream and downstream in the regions where the shear gradient diminishes. These data suggest a steady cell proliferation-migration-loss cycle and indicate that local shear stress gradient may play a key role in the morphological remodeling of the vascular endothelium in vivo.

Arteriosclerosis↗
PubMed1995

Interpreting photoactivated fluorescence microscopy measurements of steady-state actin dynamics.

A continuum model describing the steady-state actin dynamics of the cytoskeleton of living cells has been developed to aid in the interpretation of photoactivated fluorescence experiments. In a simplified cell geometry, the model assumes uniform concentrations of cytosolic and cytoskeletal actin throughout the cell and no net growth of either pool. The spatiotemporal evolution of the fluorescent actin population is described by a system of two coupled linear partial-differential equations. An analytical solution is found using a Fourier-Laplace transform and important limiting cases relevant to the design of experiments are discussed. The results demonstrate that, despite being a complex function of the parameters, the fluorescence decay in photoactivated fluorescence experiments has a biphasic behavior featuring a short-term decay controlled by monomer diffusion and a long-term decay governed by the monomer exchange rate between the polymerized and unpolymerized actin pools. This biphasic behavior suggests a convenient mechanism for extracting the parameters governing the fluorescence decay from data records. These parameters include the actin monomer diffusion coefficient, filament turnover rate, and ratio of polymerized to unpolymerized actin.

Actins↗
PubMed1993

Nonlinear separation of forward and backward running waves in elastic conduits.

A new method for the separation of forward and backward running waves in elastic conduits, with possible extension to the arterial system, has been developed. The mathematical model is based on the one-dimensional flow equations which allow the treatment of non-periodic or transient pressure and flow pulses. The method is fully nonlinear, i.e. no linearizing assumptions are made. The method includes the effects of convective acceleration and pressure-dependent vessel compliance. A first approximation for the fluid friction at the wall is also included. The application of the method requires the knowledge of the elastic properties, the instantaneous pressure and flow, as well as the instantaneous spatial derivatives of pressure and flow. Analysis of simulated data shows good results and suggests that the proposed method, unlike previous quasi-nonlinear and frequency domain methods, can be applied to strongly nonlinear and/or nonperiodic flows. The method predicts that if a linear analysis is applied to a nonlinear system errors arise.

Arteries↗
PubMed1993

Spontaneous diameter oscillations of the radial artery in humans.

In this study we investigated whether there exists a periodic contractile activity of the radial artery, and we evaluated the impact of such an oscillatory behavior on the mechanical properties of this medium-sized muscular vessel. The internal diameter of the right radial artery was measured noninvasively in six healthy male volunteers aged 18-42 yr using a high-precision ultrasonic echo-tracking device. Blood pressure was simultaneously recorded on the same side at the middle finger by photoplethysmography. The electrical activity of the heart was monitored during the entire experiment by electrocardiography. The frequency components of the arterial diameter, blood pressure, and heart rate were obtained using spectral analysis. Under resting conditions, the radial arterial diameter exhibited a spontaneous oscillation with a period ranging from 45 to 70 s and an amplitude of 80 +/- 14 microns (+/- SE). No very low-frequency mode (< or = 0.02 Hz) was identified in either heart rate or blood pressure. These diameter oscillations affected the distensibility-pressure curves acquired simultaneously. Thus the 3-4% oscillatory variation in arterial diameter was paralleled by a 1.5- to 2-fold change in distensibility. These low-frequency oscillations of large arteries seem to be mediated by an intrinsic vascular mechanism.

Adolescent↗
PubMed1992

Conduit artery compliance and distensibility are not necessarily reduced in hypertension.

The goal of this study was to investigate whether the elastic behavior of conduit arteries of humans or rats is altered as a result of concomitant hypertension. Forearm arterial cross-sectional compliance-pressure curves were determined noninvasively by means of a high precision ultrasonic echo-tracking device coupled to a photoplethysmograph (Finapres system) allowing simultaneous arterial diameter and finger blood pressure monitoring. Seventeen newly diagnosed hypertensive patients with a humeral blood pressure of 163/103 +/- 4.4/2.2 mm Hg (mean +/- SEM) and 17 age- and sex-matched normotensive controls with a humeral blood pressure of 121/77 +/- 3.2/1.9 mm Hg were included in the study. Compliance-pressure curves were also established at the carotid artery of 16-week-old anesthetized spontaneously hypertensive rats (n = 14) as well as Wistar-Kyoto normotensive animals (n = 15) using the same echo-tracking device. In these animals, intra-arterial pressure was monitored in the contralateral carotid artery. Mean blood pressures averaged 197 +/- 4 and 140 +/- 3 mm Hg in the hypertensive and normotensive rats, respectively. Despite the considerable differences in blood pressure, the diameter-pressure and cross-sectional compliance-pressure and distensibility-pressure curves were not different when hypertensive patients or animals were compared with their respective controls. These results suggest that the elastic behavior of a medium size muscular artery (radial) in humans and of an elastic artery (carotid) in rats is not necessarily altered by an increase in blood pressure.

Animals↗
PubMed1992

Non-invasive determination of arterial diameter and distensibility by echo-tracking techniques in hypertension.

METHODOLOGY: A new non-invasive ultrasonic device was developed to characterize the biomechanical properties of medium and large peripheral arteries. Simultaneous recordings of internal diameter and blood pressure over the whole cardiac cycle are used to establish compliance-pressure curves. Since blood pressure, which is an inherent co-determinant of arterial compliance, is taken into account, the comparison of arteries from patients with markedly different blood pressures has become possible. In a first study, the effects of three different antihypertensive drugs (20 mg lisinopril, 100 mg atenolol, 20 mg nitrendipine administered once a day) on arterial compliance and distensibility were investigated in young healthy volunteers. RESULTS: After 8 days of treatment, lisinopril induced a significant increase in arterial compliance. Subsequently, we compared the mechanical behaviour of arteries from newly diagnosed hypertensive patients (radial artery) or the carotid artery from spontaneously hypertensive rats (SHR) with that of corresponding arteries in normotensive counterparts. No decrease in arterial distensibility was found in the hypertensive groups over the measured blood pressure range. This result is not totally consistent with previous in vitro or in situ localized studies. Methodological differences, the absence of blood flow and/or denervation may partly explain these contradictory results. Finally, we tested the effects of hydralazine (5 mg/day) and captopril (25 mg/day), administered for 6 weeks in drinking water, on the behaviour of the carotid arteries of 16-week-old SHR. The two drugs effectively reduced blood pressure while shifting the distensibility-pressure curves upward in comparison to the placebo-treated animals, suggesting an improvement in arterial compliance. CONCLUSIONS: While hypertension does not itself appear to alter the elastic behaviour of large peripheral arteries, antihypertensive treatment may increase the compliance of these blood vessels.

Adult↗
PubMed1992

Dynamic non-invasive measurements of arterial diameter and wall thickness.

AIM: Non-invasive measurements of arterial diameter and wall thickness are critical in characterizing the onset and development of vascular disease. A precise dynamic method was proposed and tested for this purpose. DESIGN: A non-invasive method of measuring the variations in diameter and thickness of human arteries throughout the cardiac cycle was developed, using a high-precision ultrasonic echo-tracking system. An adaptive filtering technique was used to suppress artefacts caused by the layered tissue structure of the vessel wall. RESULTS: Based on decorrelation of microstructure noise, this technique improved the detectability of the wall interfaces, which allowed a determination of thickness and diameter. The accuracy and reproducibility of the method were tested by measurements of plastic films with known thicknesses. The discrepancies between standard micrometer and pulse-echo measurement were consistently less than 5 microns for film thicknesses ranging from 220 to 800 microns. The difference between two successive measurements was less than 2 microns. The identity of the measured vascular interfaces was checked in two ways. First, experiments on fixed bovine carotid arteries showed that the identified echogenic interfaces corresponded to the actual anatomical structure, as obtained by acoustic microscopy. Second, the radial artery thickness and diameter were extrapolated to obtain the change in wall volume over one cardiac cycle. The volume was found to be nearly constant, indicating incompressibility. CONCLUSION: This method will make it possible to obtain new information on atherogenesis and other vascular diseases.

Acoustics↗
PubMed1992

Non-invasive method for the assessment of non-linear elastic properties and stress of forearm arteries in vivo.

AIM: To propose a method for obtaining non-invasive highly accurate measurements of arterial diameter, thickness and pressure at a single location on a peripheral arm artery, and use this data to estimate the status of the arterial wall. METHODS: Diameter and thickness were measured with an A-mode ultrasonic echo-tracking device, with a precision of close to 1 micron. Pressure was measured with a photoplethysmograph at the finger level. A simple hemodynamic model was used to estimate, from the measured pressure pulse, the pressure waveform at the diameter-measuring site. RESULTS: The collected data made it possible to assess (1) the elastic response of the artery, through the compliance and distensibility, (2) the loading conditions, through the average stress-diameter curve and (3) the elastic properties of the wall material, through the incremental modulus of elasticity. CONCLUSION: This information can be used to assess the overall status of the arterial wall.

Blood Pressure↗
PubMed1992

Effect of mental stress on the tone of a medium-sized muscular artery.

AIM: We studied the effects of mental stress on the tone of the radial artery, a medium-sized muscular artery. METHODS: Using an A-mode echo-tracking device coupled to a photoplethysmograph, we took continuous measurements of the radial artery diameter, heart rate and finger arterial pressure in six healthy young volunteers. RESULTS: Under mental stress, the heart rate increased from 63 +/- 4 to 74 +/- 4 beats/min (mean +/- SEM, P < 0.01) and systolic pressure from 123 +/- 5 to 140 +/- 6 mmHg (P < 0.05). No consistent modification in the arterial diameter was observed at this time. CONCLUSION: The activation of sympathetic nerve activity induced by mental stress does not cause a significant contraction in the radial artery.

Adolescent↗
PubMed1991

Non-invasive estimate of the mechanical properties of peripheral arteries from ultrasonic and photoplethysmographic measurements.

The non-linear elastic response of arteries implies that their mechanical properties depend strongly on blood pressure. Thus, dynamic measurements of both the diameter and pressure curves over the whole cardiac cycle are necessary to characterise properly the elastic behaviour of an artery. We propose a novel method of estimating these mechanical properties based on the analysis of the arterial diameter against pressure curves derived from ultrasonic and photoplethysmographic measurements. An ultrasonic echo tracking device has been developed that allows continuous recording of the internal diameter of peripheral arteries. It measures the diameter 300 times per second with a resolution of 2.5 microns. This system is linked to a commercially available light-plethysmograph which continuously records the finger arterial pressure (0.25 kPa accuracy). Because of the finite pulse wave velocity, the separation between the diameter and the pressure measurement sites causes a hysteresis to appear in the recorded diameter-pressure curve. Using a model based on haemodynamic considerations, the delay between the diameter variations and the finger arterial pressure is first eliminated. As the pulse wave velocity depends on the pressure, the delay is determined for each pressure value. The relationship between pressure and diameter is then described by a non-linear mathematical expression with three parameters, which best fits the recorded data. The dynamic local behaviour of the vessel is fully characterised by these parameters. Compliance, distensibility and pulse wave velocity can then be calculated at each pressure level. Thus, the mechanical behaviour of peripheral human arteries can now be characterised non-invasively over the pressure range of the whole cardiac cycle. The results obtained in vivo on human radial and brachial arteries show that a thorough analysis of the compliance-pressure curves and their modifications (curving, shift) is needed in order to compare two different vessels in a meaningful way.

Adult↗
PubMed1991

Evaluation of arterial compliance-pressure curves. Effect of antihypertensive drugs.

A new high-precision ultrasonic device was developed to determine noninvasively arterial compliance as a function of blood pressure. Because of the nonlinear elastic properties of arterial walls, measurements of compliance can be appropriately compared only if obtained over a range of pressures. This apparatus was used to evaluate in a double-blind, parallel fashion the effect of three different antihypertensive drugs and of a placebo on radial artery compliance. Thirty-two normotensive volunteers were randomly allocated to an 8-day, once-a-day oral treatment with either a placebo, 100 mg atenolol, 20 mg nitrendipine, or 20 mg lisinopril. Blood pressure, heart rate, radial artery diameter, and arterial compliance were measured immediately before as well as 6 hours after dosing on the first and last days of the study. On the eighth day of administration, within 6 hours after dosing, lisinopril induced an acute increase in radial artery diameter, from 2.99 +/- 0.06 to 3.28 +/- 0.09 mm (mean +/- SEM, p less than 0.01). The compliance-pressure curve was shifted upward on day 1 (p less than 0.01) as well as on day 8 (p less than 0.05). None of the other drugs induced any significant modification of these parameters. Arterial compliance has a strong nonlinear dependency on intra-arterial pressure and therefore has to be defined as a function of pressure. Antihypertensive drugs acting by different mechanisms may have different effects on the mechanical properties of large arteries.

Adrenergic beta-Antagonists↗
PubMed1988

Non-invasive measurement of internal diameter of peripheral arteries during the cardiac cycle.

An ultrasonic device was developed for continuous monitoring of the internal diameter of human peripheral arteries. The diastolic internal diameter of the radial artery measured in 10 supine normotensive subjects averaged 2.36 +/- 0.12 mm (mean +/- s.e.) and the change in diameter during the pulse wave was 0.037 +/- 0.003 mm. It was surprising to find that when the transducer was used to apply pressure on the radial artery, the difference between the systolic and the diastolic diameter was significantly increased while the diastolic diameter was reduced. This method therefore allows continuous, accurate and non-invasive measurement of the internal diameter of the human radial artery.

Arteries↗