Wall shear stress rather than shear rate regulates cytoplasmic Ca++ responses to flow in vascular endothelial cells

Fabricating a multi-component microfluidic system for exercise-induced endothelial cell mechanobiology guided by hemodynamic similarity

Generating precise in vivo arterial endothelial hemodynamic microenvironments using microfluidics is essential for exploring endothelial mechanobiology. However, a hemodynamic principle guiding the fabrication of microfluidic systems is still lacking. We propose a hemodynamic similarity principle for quickly obtaining the input impedance of the microfluidic system in vitro derived from that of the arterial system in vivo to precisely generate the desired endothelial hemodynamic microenvironments. First, based on the equivalent of blood pressure (BP) and wall shear stress (WSS) waveforms, we establish a hemodynamic similarity principle to efficiently map the input impedance in vivo to that in vitro, after which the multi-component microfluidic system is designed and fabricated using a lumped parameter hemodynamic model. Second, numerical simulation and experimental studies are carried out to validate the performance of the designed microfluidic system. Finally, the intracellular Ca²⁺ responses after exposure to different intensities of exercise-induced BP and WSS waveforms are measured to improve the reliability of EC mechanobiological studies using the designed microfluidic system. Overall, the proposed hemodynamic similarity principle can guide the fabrication of a multi-component microfluidic system for endothelial cell mechanobiology.

Abnormal Flow Dynamics Result in Low Wall Shear Stress and High Oscillatory Shear Index in Abdominal Aortic Dilatation: Initial in vivo Assessment with 4D-flow MRI

PURPOSE

To characterize the non-laminar flow dynamics and resultant decreased wall shear stress (WSS) and high oscillatory shear index (OSI) of the infrarenal abdominal aortic dilatation, cardiac phase-resolved 3D phase-contrast MRI (4D-flow MRI) was performed.

METHODS

The prospective single-arm study was approved by the Institutional Review Board and included 18 subjects (median 67.5 years) with the dilated infrarenal aorta (median diameter 35 mm). 4D-flow MRI was conducted on a 1.5T MRI system. On 3D streamline images, laminar and non-laminar (i.e., vortex or helical) flow patterns were visually assessed both for the dilated aorta and for the undilated upstream aorta. Cardiac phase-resolved flow velocities, WSS and OSI, were also measured for the dilated aorta and the upstream undilated aorta.

RESULTS

Non-laminar flow represented by vortex or helical flow was more frequent and overt in the dilated aorta than in the undilated upstream aorta (P < 0.0156) with a very good interobserver agreement (weighted kappa: 0.82-1.0). The WSS was lower, and the OSI was higher on the dilated aortic wall compared with the proximal undilated segments. In mid-systole, mean spatially-averaged WSS was 0.20 ± 0.016 Pa for the dilated aorta vs. 0.68 ± 0.071 Pa for undilated upstream aorta (P < 0.0001), and OSI on the dilated aortic wall was 0.093 ± 0.010 vs. 0.041 ± 0.0089 (P = 0.013). The maximum values and the amplitudes of the WSS at the dilated aorta were inversely proportional to the ratio of dilated/undilated aortic diameter (r = -0.694, P = 0.0014).

CONCLUSION

4D-flow can characterize abnormal non-laminar flow dynamics within the dilated aorta in vivo. The wall of the infrarenal aortic dilatation is continuously and increasingly affected by atherogenic stimuli due to the flow disturbances represented by vortex or helical flow, which is reflected by lower WSS and higher OSI.

Microfluidic devices for studying coagulation biology

The ability to study the behavior of cells, proteins, and cell-cell or cell-protein interactions under dynamic forces such as shear stress under fluid flow, provides a more accurate understanding of the physiopathology of hemostasis. This review touches upon the traditional methods for studying blood coagulation and platelet aggregation and provides an overview on cellular and protein response to shear stress. We also elaborate on the biological aspects of how cells recognize mechanical forces and convert them into biochemical signals that can drive various signaling pathways. We give a detailed description of the various types of microfluidic devices that are employed to study the complex processes of platelet aggregation and blood coagulation under flow conditions as well as to investigate endothelial shear-response. We also highlight works mimicking artificial vessels as platforms to study the mechanisms of coagulation, and finish our review by describing anticipated clinical uses of microfluidics devices and their standardization.

A biomechanically derived minimum work model of the fish gill lamellar system exhibits its exquisite morphological arrangement and perfusate regulation for oxygen uptake from water

To evaluate the efficiency of oxygen (O₂) uptake from water through the fish gill lamellar system, a cost function (CF) representing mechanical power expenditure for water ventilation and blood circulation through the gill was formulated, by applying steady-state fluid mechanics to a homogeneous lamellar-channel model. This approach allowed us to express CF as the function of inter-lamellar water channel width (w) and to derive an analytical solution of the width (w_min) at the minimum CF. Morphometric and physiological data for rainbow trout in the literature were referred to calculate CF(w) curves and their w_min values at five intensity stages of swimming exercise. Obtained w_min values were evenly distributed around the standard measure of the width (w_s = 24 μm) in this fish. Individual levels of CF(w_min) were also fairly close to the corresponding CF(w_s) values within a 10% deviation, suggesting the reliability of approximating [CF(w_min) = CF(w_s)]. The cost-performance of O₂ uptake through the gill (η_g) was then assessed from reported data of total O₂ uptake/CF(w_s) at each intensity stage. The η_g levels at any swimming stage exceeded 95% of the theoretical maximum value, implying that O₂ uptake is nearly optimally performed in the lamellar-channel system at all swimming speeds. Further analyses of O₂ transport in this fresh water fish revealed that the water ventilation by the buccal/opercular pumping evokes a critical limit of swimming velocity, due to confined O₂ supply to the peripheral skeletal muscles, which is avoided in ram ventilators such as tuna.

Piezo1 Channels in Vascular Development and the Sensing of Shear Stress

A critical point in mammalian development occurs before mid-embryogenesis when the heart starts to beat, pushing blood into the nascent endothelial lattice. This pushing force is a signal, detected by endothelial cells as a frictional force (shear stress) to trigger cellular changes that underlie the essential processes of vascular remodeling and expansion required for embryonic growth. The processes are complex and multifactorial and Piezo1 became a recognized player only 2years ago, 4years after Piezo1's initial discovery as a functional membrane protein. Piezo1 is now known to be critical in murine embryonic development just at the time when the pushing force is first detected by endothelial cells. Murine Piezo1 gene disruption in endothelial cells is embryonic lethal and mutations in human PIEZO1 associate with severe disease phenotype due to abnormal lymphatic vascular development. Piezo1 proteins coassemble to form calcium-permeable nonselective cationic channels, most likely as trimers. They are large proteins with little if any resemblance to other proteins or ion channel subunits. The channels appear to sense mechanical force directly, including the force imposed on endothelial cells by physiological shear stress. Here, we review current knowledge of Piezo1 in the vascular setting and discuss hypotheses about how it might serve its vascular functions and integrate with other mechanisms. Piezo1 is a new important player for investigators in this field and promises much as a basis for better understanding of vascular physiology and pathophysiology and perhaps also discovery of new therapies.

Acetylcholine released by endothelial cells facilitates flow-mediated dilatation

KEY POINTS

The endothelium plays a pivotal role in the vascular response to chemical and mechanical stimuli. The endothelium is exquisitely sensitive to ACh, although the physiological significance of ACh-induced activation of the endothelium is unknown. In the present study, we investigated the mechanisms of flow-mediated endothelial calcium signalling. Our data establish that flow-mediated endothelial calcium responses arise from the autocrine action of non-neuronal ACh released by the endothelium.

ABSTRACT

Circulating blood generates frictional forces (shear stress) on the walls of blood vessels. These frictional forces critically regulate vascular function. The endothelium senses these frictional forces and, in response, releases various vasodilators that relax smooth muscle cells in a process termed flow-mediated dilatation. Although some elements of the signalling mechanisms have been identified, precisely how flow is sensed and transduced to cause the release of relaxing factors is poorly understood. By imaging signalling in large areas of the endothelium of intact arteries, we show that the endothelium responds to flow by releasing ACh. Once liberated, ACh acts to trigger calcium release from the internal store in endothelial cells, nitric oxide production and artery relaxation. Flow-activated release of ACh from the endothelium is non-vesicular and occurs via organic cation transporters. ACh is generated following mitochondrial production of acetylCoA. Thus, we show ACh is an autocrine signalling molecule released from endothelial cells, and identify a new role for the classical neurotransmitter in endothelial mechanotransduction.

Endothelial Mechanosignaling: Does One Sensor Fit All?

SIGNIFICANCE

Forces are important in the cardiovascular system, acting as regulators of vascular physiology and pathology. Residing at the blood vessel interface, cells (endothelial cell, EC) are constantly exposed to vascular forces, including shear stress. Shear stress is the frictional force exerted by blood flow, and its patterns differ based on vessel geometry and type. These patterns range from uniform laminar flow to nonuniform disturbed flow. Although ECs sense and differentially respond to flow patterns unique to their microenvironment, the mechanisms underlying endothelial mechanosensing remain incompletely understood.

RECENT ADVANCES

A large body of work suggests that ECs possess many mechanosensors that decorate their apical, junctional, and basal surfaces. These potential mechanosensors sense blood flow, translating physical force into biochemical signaling events.

CRITICAL ISSUES

Understanding the mechanisms by which proposed mechanosensors sense and respond to shear stress requires an integrative approach. It is also critical to understand the role of these mechanosensors not only during embryonic development but also in the different vascular beds in the adult. Possible cross talk and integration of mechanosensing via the various mechanosensors remain a challenge.

FUTURE DIRECTIONS

Determination of the hierarchy of endothelial mechanosensors is critical for future work, as is determination of the extent to which mechanosensors work together to achieve force-dependent signaling. The role and primary sensors of shear stress during development also remain an open question. Finally, integrative approaches must be used to determine absolute mechanosensory function of potential mechanosensors. Antioxid. Redox Signal. 25, 373-388.

Ion Channels in Endothelial Responses to Fluid Shear Stress

Fluid shear stress is an important environmental cue that governs vascular physiology and pathology, but the molecular mechanisms that mediate endothelial responses to flow are only partially understood. Gating of ion channels by flow is one mechanism that may underlie many of the known responses. Here, we review the literature on endothelial ion channels whose activity is modulated by flow with an eye toward identifying important questions for future research.

Shear stress-induced redistribution of vascular endothelial-protein-tyrosine phosphatase (VE-PTP) in endothelial cells and its role in cell elongation

Vascular endothelial cells (ECs) are continuously exposed to shear stress (SS) generated by blood flow. Such stress plays a key role in regulation of various aspects of EC function including cell proliferation and motility as well as changes in cell morphology. Vascular endothelial-protein-tyrosine phosphatase (VE-PTP) is an R3-subtype PTP that possesses multiple fibronectin type III-like domains in its extracellular region and is expressed specifically in ECs. The role of VE-PTP in EC responses to SS has remained unknown, however. Here we show that VE-PTP is diffusely localized in ECs maintained under static culture conditions, whereas it undergoes rapid accumulation at the downstream edge of the cells relative to the direction of flow in response to SS. This redistribution of VE-PTP triggered by SS was found to require its extracellular and transmembrane regions and was promoted by integrin engagement of extracellular matrix ligands. Inhibition of actin polymerization or of Cdc42, Rab5, or Arf6 activities attenuated the SS-induced redistribution of VE-PTP. VE-PTP also underwent endocytosis in the static and SS conditions. SS induced the polarized distribution of internalized VE-PTP. Such an effect was promoted by integrin engagement of fibronectin but prevented by inhibition of Cdc42 activity or of actin polymerization. In addition, depletion of VE-PTP by RNA interference in human umbilical vein ECs blocked cell elongation in the direction of flow induced by SS. Our results suggest that the polarized redistribution of VE-PTP in response to SS plays an important role in the regulation of EC function by blood flow.

CABG MODELS FLOW SIMULATION STUDY ON THE EFFECTS OF VALVE REMNANTS IN THE VENOUS GRAFT

Venous valves and sinuses are frequently observed in vein grafts in the coronary artery bypass grafts (CABG). However, from the biomedical engineering viewpoint, vein grafts are always assumed as smooth tubes in the existing simulations, and no effort has been made to investigate the effects of jaggedness of the graft inner wall due to the valve cusps remnants and valve sinus (in case of valve-stripped saphenous vein (SV) grafts) on the blood flow patterns and hemodynamic parameters (HPs). In this paper, the effects of the inner surface irregularities of a vein graft on the blood flow is investigated in the graft as well as in the distal anastomotic region, with a more realistic geometry of valve-stripped SV, by means of numerical simulation of pulsatile, Newtonian blood flow. The simulation results demonstrate that the valve remnants and sinuses cause disturbances in the flow field within the graft (due to vortices formation within the valve sinuses) and undesirable distribution of HPs, which can result in early atherosclerotic lesion development in the graft.

Mechanotransduction of shear in the endothelium: basic studies and clinical implications

The endothelium plays an integral role in the development and progression of atherosclerosis. Hemodynamic forces, particularly shear stress, have a powerful influence on endothelial phenotype and function; however, there is no clear consensus on how endothelial cells sense shear. Nevertheless, multiple endothelial cell signal transduction pathways are activated when exposed to shear stress in vitro. The type of shear, laminar or oscillatory, impacts which signal transduction pathways are initiated as well as which subsequent genes are up- or down-regulated, thereby influencing endothelial phenotype and function. Recently, human studies have examined the impact of shear stress and different shear patterns at rest and during exercise on endothelial function. Current evidence supports the theory that augmented exercise-induced shear stress contributes to improved endothelial function following acute exercise and exercise training, whereas retrograde shear initiates vascular dysfunction. The purpose of this review is to examine the current theories on how endothelial cells sense shear stress, to provide an overview on shear stress-induced signal transduction pathways and subsequent gene expression, and to review the current literature pertaining to shear stress and shear patterns at rest as well as during exercise in humans and the related effects on endothelial function.

Calcium and endothelium-mediated vasodilator signaling

Vascular tone refers to the balance between arterial constrictor and dilator activity. The mechanisms that underlie tone are critical for the control of haemodynamics and matching circulatory needs with metabolism, and thus alterations in tone are a primary factor for vascular disease etiology. The dynamic spatiotemporal control of intracellular Ca(2+) levels in arterial endothelial and smooth muscle cells facilitates the modulation of multiple vascular signaling pathways. Thus, control of Ca(2+) levels in these cells is integral for the maintenance of tone and blood flow, and intimately associated with both physiological and pathophysiological states. Hence, understanding the mechanisms that underlie the modulation of vascular Ca(2+) activity is critical for both fundamental knowledge of artery function, and for the development of targeted therapies. This brief review highlights the role of Ca(2+) signaling in vascular endothelial function, with a focus on contact-mediated vasodilator mechanisms associated with endothelium-derived hyperpolarization and the longitudinal conduction of responses over distance.

A novel method for measuring tension generated in stress fibers by applying external forces

The distribution of contractile forces generated in cytoskeletal stress fibers (SFs) contributes to cellular dynamic functions such as migration and mechanotransduction. Here we describe a novel (to our knowledge) method for measuring local tensions in SFs based on the following procedure: 1), known forces of different magnitudes are applied to an SF in the direction perpendicular to its longitudinal axis; 2), force balance equations are used to calculate the resulting tensions in the SF from changes in the SF angle; and 3), the relationship between tension and applied force thus established is extrapolated to an applied force of zero to determine the preexisting tension in the SF. In this study, we measured tensions in SFs by attaching magnetic particles to them and applying known forces with an electromagnetic needle. Fluorescence microscopy was used to capture images of SFs fluorescently labeled with myosin II antibodies, and analysis of these images allowed the tension in the SFs to be measured. The average tension measured in this study was comparable to previous reports, which indicates that this method may become a powerful tool for elucidating the mechanisms by which cytoskeletal tensions affect cellular functions.

Rheology of embryonic avian blood

Shear stress, a mechanical force created by blood flow, is known to affect the developing cardiovascular system. Shear stress is a function of both shear rate and viscosity. While established techniques for measuring shear rate in embryos have been developed, the viscosity of embryonic blood has never been known but always assumed to be like adult blood. Blood is a non-Newtonian fluid, where the relationship between shear rate and shear stress is nonlinear. In this work, we analyzed the non-Newtonian behavior of embryonic chicken blood using a microviscometer and present the apparent viscosity at different hematocrits, different shear rates, and at different stages during development from 4 days (Hamburger-Hamilton stage 22) to 8 days (about Hamburger-Hamilton stage 34) of incubation. We chose the chicken embryo since it has become a common animal model for studying hemodynamics in the developing cardiovascular system. We found that the hematocrit increases with the stage of development. The viscosity of embryonic avian blood in all developmental stages studied was shear rate dependent and behaved in a non-Newtonian manner similar to that of adult blood. The range of shear rates and hematocrits at which non-Newtonian behavior was observed is, however, outside the physiological range for the larger vessels of the embryo. Under low shear stress conditions, the spherical nucleated blood cells that make up embryonic blood formed into small aggregates of cells. We found that the apparent blood viscosity decreases at a given hematocrit during embryonic development, not due to changes in protein composition of the plasma but possibly due to the changes in cellular composition of embryonic blood. This decrease in apparent viscosity was only visible at high hematocrit. At physiological values of hematocrit, embryonic blood viscosity did not change significantly with the stage of development.

Shear stress-dependent effects of lysophosphatidic acid on agonist-induced vasomotor responses in rat mesenteric artery

We have previously shown that lysophosphatidic acid (LPA), a bioactive plasma lysophospholipid, markedly accelerates shear stress-induced Ca2+ responses in cultured vascular endothelial cells (ECs). This study aimed to demonstrate the impact of LPA and luminal shear stress on vasomotor regulation in the isolated rat mesenteric artery (MA) using a videomicroscopic technique. Although the addition of LPA to the perfusate in a concentration range of 0.03-0.3 μM had no significant effect on the basal MA tone, LPA in a similar concentration range led to increased phenylephrine-induced MA contraction and reduced acetylcholine-induced MA relaxation under physiological shear conditions. These vasomodulatory actions of LPA, which vanished upon removal of ECs, were positively dependent on luminal shear stress levels and were markedly inhibited by the LPA receptor antagonist Ki16425, the cyclooxygenase inhibitor indomethacin, and the thromboxane A2 receptor antagonist SQ29548. These data thus suggest that LPA can modify the agonist-induced vasomotor responses in MAs in a shear stress-dependent manner. This effect of LPA was mediated through ECs, the LPA receptor, and cyclooxygenase/thromboxane A2 signaling.

Effects of shear stress and stretch on endothelial function

Vascular endothelial cells (ECs) play a central role in the control of blood vessel function and circulatory system homeostasis. It is well known that that EC functions are regulated by chemical mediators, including hormones, cytokines, and neurotransmitters, but it has recently become apparent that EC functions are also controlled by hemodynamic forces such as shear stress and stretch (cyclic strain). ECs recognize shear stress and cyclic strain as mechanical stimuli, and transmit the signal into the interior of the cells, thereby triggering a variety of cellular responses that involve alterations in cell morphology, cell function, and gene expression. Impaired EC responses to shear stress and cyclic strain lead to vascular diseases, including hypertension, thrombosis, and atherosclerosis. A great deal of research has already been conducted on the mechanotransduction of shear stress and cyclic strain, and its molecular mechanisms are gradually coming to be understood. However, much remains unclear, and further studies of mechanotransduction should increase our understanding of the molecular basis of the hemodynamic-force-mediated control of vascular functions.

New molecular mechanisms for cardiovascular disease:blood flow sensing mechanism in vascular endothelial cells

Endothelial cells (ECs) lining blood vessels have a variety of functions and play a critical role in the homeostasis of the circulatory system. It has become clear that biomechanical forces generated by blood flow regulate EC functions. ECs are in direct contact with blood flow and exposed to shear stress, a frictional force generated by flowing blood. A number of recent studies have revealed that ECs recognize changes in shear stress and transmit signals to the interior of the cell, which leads to cell responses that involve changes in cell morphology, cell function, and gene expression. These EC responses to shear stress are thought to play important roles in blood flow-dependent phenomena such as vascular tone control, angiogenesis, vascular remodeling, and atherogenesis. Much research has been done on shear stress sensing and signal transduction, and their molecular mechanisms are gradually becoming understood. However, much remains uncertain, and many candidates have been proposed for shear stress sensors. More extensive studies of vascular mechanobiology should increase our understanding of the molecular basis of the blood flow-mediated control of vascular functions.

Lysophosphatidic acid induces shear stress-dependent Ca2+ influx in mouse aortic endothelial cells in situ

Using real-time two-photon laser scanning microscopy, we have demonstrated that lysophosphatidic acid (LPA), a bioactive lipid mediator, causes shear stress-dependent oscillatory local increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) in fluo-4-loaded endothelial cells of isolated mouse aortic strips in situ. The increase in [Ca(2+)](i) occurred independently in the individual endothelial cells in a stepwise manner or repetitively during constant flow. The percentage of cells that responded and the averaged level of increase in [Ca(2+)](i) were dependent on both the concentration of LPA (0.3-10 μm) and the shear stress (10-80 dyn cm(-2)). The response was inhibited by removing extracellular Ca(2+), but not by thapsigargin, an inhibitor of the endoplasmic reticulum Ca(2+)-ATPase. The spatiotemporal properties of the [Ca(2+)](i) response were completely different from those of a Ca(2+) wave induced by ATP, a Ca(2+)-mobilizing agonist. These results were almost the same as those in the previous investigation using cultured bovine aortic endothelial cells, and suggest that LPA enhanced the shear stress-induced oscillatory Ca(2+) influx, termed 'Ca(2+) spot', in endothelial cells via activation of elementary Ca(2+) influx. In conclusion, the present study demonstrates, for the first time, that LPA functions as an endogenous sensitizer for mechanotransduction in endothelial cells in shear conditions in aortic strips in situ as well as in cultured cells. This indicates an important role for LPA as an endogenous factor in fluid flow-induced endothelial function.

Hemodynamic forces in endothelial dysfunction and vascular aging

Aging is a key risk factor associated with the onset of cardiovascular disease. Notably, vascular aging and cardiovascular disease are both associated with endothelial dysfunction, or a marked decrease in production and bioavailability the vasodilator of nitric oxide (NO). As a result of decreased nitric oxide availability, aging vessels often exhibit endothelial cell senescence and increased oxidative stress. One of the most potent activators of NO production is fluid shear stress produced by blood flow. Interestingly, age-related decrease in NO production partially results from endothelial insensitivity to shear stress. While the endothelial cell response to fluid shear stress has been well characterized in recent years, the exact mechanisms of how the mechanical force of fluid shear stress is converted into intracellular biochemical signals are relatively unknown. Therefore, gaining a better knowledge of mechanosignaling events in endothelial cells may prove to be beneficial for developing potential therapies for cardiovascular diseases.

Design optimization of a helical endothelial cell culture device

The specific roles of mass transfer and fluid dynamic stresses on endothelial function, important in atherogenesis, are not known. Further, the effects of mass transfer and fluid dynamic stresses are difficult to separate because areas of "abnormal" mass transfer and "abnormal" wall shear stress tend to co-localize (where abnormal is defined as any deviation from the mass transfer rate or wall shear stress present in a long straight artery with the same flow rate and diameter). Our goal was to design a cell culture device which gives maximum separation between areas of abnormal shear stress and areas of abnormal mass transfer. We used design optimization principles to design a helical cell culture device. Using shear stress and mass transfer fields predicted by solving the governing equations, the area of the device which was exposed to low rates of mass transfer and normal levels of wall shear stress was determined. The design optimization method then maximized this area by varying the design variables, resulting in the optimum design. The optimum design had Reynolds number = 50, helical radius = 3.23 and helical pitch = 3.82. The area of the device which was exposed to low rates of mass transfer and regular levels of wall shear stress was about 4.5 times the inlet cross-sectional area of the device or about 5% of the device total internal surface area. An optimum design was successfully determined and the methodology used was shown to be robust. The area of the device which was exposed to low rates of mass transfer and regular levels of wall shear stress occurred in a defined region which should aid further experimental work.

Vascular mechanobiology: endothelial cell responses to fluid shear stress

Endothelial cells (ECs) lining blood vessel walls respond to shear stress, a fluid mechanical force generated by flowing blood, and the EC responses play an important role in the homeostasis of the circulatory system. Abnormal EC responses to shear stress impair various vascular functions and lead to vascular diseases, including hypertension, thrombosis, and atherosclerosis. Bioengineering approaches in which cultured ECs are subjected to shear stress in fluid-dynamically designed flow-loading devices have been widely used to analyze EC responses at the cellular and molecular levels. Remarkable progress has been made, and the results have shown that ECs alter their morphology, function, and gene expression in response to shear stress. Shear stress affects immature cells, as well as mature ECs, and promotes differentiation of bone-marrow-derived endothelial progenitor cells and embryonic stem cells into ECs. Much research has been done on shear stress sensing and signal transduction, and their molecular mechanisms are gradually coming to be understood. However, much remains uncertain, and many candidates have been proposed for shear stress sensors. More extensive studies of vascular mechanobiology should increase our understanding of the molecular basis of the blood-flow-mediated control of vascular functions.

A computational model of nitric oxide production and transport in a parallel plate flow chamber

We developed a mathematical model to investigate the production and transport of nitric oxide (NO) generated by a monolayer of cultured endothelial cells exposed to flow in a parallel plate flow chamber. The objectives were to provide a theoretical framework for interpreting experimental observations and to suggest a quantitative relationship between shear stress and NO production rate. NO production was described as a combination of a basal production rate term and a shear-dependent term. Our results show that the shear stress-dependence of the production of NO by the endothelium influences the nature of mass transport within the boundary layer. We found that the steady state NO concentration near the endothelial surface exhibits a biphasic dependence on shear stress, in which at low flow, NO concentration decreases owing to the enhanced removal by convective transport while only at higher shear stresses does the increased production cause an increase in NO concentration. The unsteady response to step changes in flow exhibits transient fluctuations in NO that can be explained by time-dependent changes in the diffusive and convective mass transport as the concentration profile evolves. Our results indicate that this model can be used to determine the relationship between shear stress and NO production rate from measurements of NO concentration.

Fluid shear stress induces arterial differentiation of endothelial progenitor cells

Endothelial progenitor cells (EPCs) are mobilized from bone marrow to peripheral blood and contribute to angiogenesis in tissues. In the process, EPCs are exposed to the shear stress generated by blood flow and tissue fluid flow. Our previous study showed that shear stress promotes differentiation of EPCs into mature endothelial cells. In this study, we investigated whether EPCs differentiate into arterial or venous endothelial cells in response to shear stress. When cultured EPCs derived from human peripheral blood were exposed to controlled levels of shear stress in a flow-loading device, the mRNA levels of the arterial endothelial cell markers ephrinB2, Notch1/3, Hey1/2, and activin receptor-like kinase 1 increased, but the mRNA levels of the venous endothelial cell markers EphB4 and neuropilin-2 decreased. Both the ephrinB2 increase and the EphB4 decrease were shear stress dependent rather than shear rate dependent. EphrinB2 protein was increased in shear-stressed EPCs, and the increase in ephrinB2 expression was due to activated transcription and not mRNA stabilization. Deletion analysis of the ephrinB2 promoter indicated that the cis-element (shear stress response element) is present within 106 bp 5' upstream from the transcription initiation site. This region contains the Sp1 consensus sequence, and a mutation in its sequence decreased the basal level of transcription and abolished shear stress-induced ephrinB2 transcription. Electrophoretic mobility shift assays and chromatin immunoprecipitation assays showed that shear stress markedly increased binding of Sp1 to its consensus sequence. These results indicate that shear stress induces differentiation of EPCs into arterial endothelial cells by increasing ephrinB2 expression in EPCs through Sp1 activation.

Shear-stress dependence of dinoflagellate bioluminescence

Fluid flow stimulates bioluminescence in dinoflagellates. However, many aspects of the cellular mechanotransduction are incompletely known. The objective of our study was to formally test the hypothesis that flow-stimulated dinoflagellate bioluminescence is dependent on shear stress, signifying that organisms are responding to the applied fluid force. The dinoflagellate Lingulodinium polyedrum was exposed to steady shear using simple Couette flow in which fluid viscosity was manipulated to alter shear stress. At a constant shear rate, a higher shear stress due to increased viscosity increased both bioluminescence intensity and decay rate, supporting our hypothesis that bioluminescence is shear-stress dependent. Although the flow response of non-marine attached cells is known to be mediated through shear stress, our results indicate that suspended cells such as dinoflagellates also sense and respond to shear stress. Shear-stress dependence of flow-stimulated bioluminescence in dinoflagellates is consistent with mechanical stimulation due to direct predator handling in the context of predator-prey interactions.

Haemodynamics and wall remodelling of a growing cerebral aneurysm: A computational model

We have developed a computational simulation model for investigating an often postulated hypothesis connected with aneurysm growth. This hypothesis involves a combination of two parallel and interconnected mechanisms: according to the first mechanism, an endothelium-originating and wall shear stress-driven apoptotic behavior of smooth muscle cells, leading to loss of vascular tone is believed to be important to the aneurysm behavior. Vascular tone refers to the degree of constriction experienced by a blood vessel relative to its maximally dilated state. All resistance and capacitance vessels under basal conditions exhibit some degree of smooth muscle contraction that determines the diameter, and hence tone, of the vessel. The second mechanism is connected to the arterial wall remodeling. Remodeling of the arterial wall under constant tension is a biomechanical process of rupture, degradation and reconstruction of the medial elastin and collagen fibers. In order to investigate these two mechanisms within a computationally tractable framework, we devise mechanical analogues that involve three-dimensional haemodynamics, yielding estimates of the wall shear stress and pressure fields and a quasi-steady approach for the apoptosis and remodeling of the wall. These analogues are guided by experimental information for the connection of stimuli to responses at a cellular level, properly averaged over volumes or surfaces. The model predicts aneurysm growth and can attribute specific roles to the two mechanisms involved: the smooth muscle cell-related loss of tone is important to the initiation of aneurysm growth, but cannot account alone for the formation of fully grown sacks; the fiber-related remodeling is pivotal for the latter.

Flow-activated Chloride Channels in Vascular Endothelium

Although activation of outward rectifying Cl(-) channels is one of the fastest responses of endothelial cells (ECs) to shear stress, little is known about these channels. In this study, we used whole-cell patch clamp recordings to characterize the flow-activated Cl(-) current in bovine aortic ECs (BAECs). Application of shear stress induced rapid development of a Cl(-) current that was effectively blocked by the Cl(-) channel antagonist 5-nitro-2-(3-phenopropylamino)benzoic acid (100 microM). The current initiated at a shear stress as low as 0.3 dyne/cm(2), attained its peak within minutes of flow onset, and saturated above 3.5 dynes/cm(2) approximately 2.5-3.5-fold increase over pre-flow levels). The Cl(-) current desensitized slowly in response to sustained flow, and step increases in shear stress elicited increased current only if the shear stress levels were below the 3.5 dynes/cm(2) saturation level. Oscillatory flow with a physiological oscillation frequency of 1 Hz, as occurs in disturbed flow zones prone to atherosclerosis, failed to elicit the Cl(-) current, whereas lower oscillation frequencies led to partial recovery of the current. Nonreversing pulsatile flow, generally considered protective of atherosclerosis, was as effective in eliciting the current as steady flow. Measurements using fluids of different viscosities indicated that the Cl(-) current is responsive to shear stress rather than shear rate. Blocking the flow-activated Cl(-) current abolished flow-induced Akt phosphorylation in BAECs, whereas blocking flow-sensitive K(+) currents had no effect, suggesting that flow-activated Cl(-) channels play an important role in regulating EC flow signaling.

Hemodynamic shear stresses in mouse aortas: implications for atherogenesis

OBJECTIVE

The hemodynamic environment is a determinant of susceptibility to atherosclerosis in the vasculature. Although mouse models are commonly used in atherosclerosis studies, little is known about local variations in wall shear stress (WSS) in the mouse and whether the levels of WSS are comparable to those in humans. The objective of this study was to determine WSS values in the mouse aorta and to relate these to expression of gene products associated with atherosclerosis.

METHODS AND RESULTS

Using micro-CT and ultrasound methodologies we developed a computational fluid dynamics model of the mouse aorta and found values of WSS to be much larger than those for humans. We also used a quantum dot-based approach to study vascular cell adhesion molecule-1 and intercellular adhesion molecule-1 expression on the aortic intima and demonstrated that increased expression for these molecules occurs where WSS was relatively low for the mouse.

CONCLUSIONS

Despite large differences in WSS in the two species, the spatial distributions of atherogenic molecules in the mouse aorta are similar to atherosclerotic plaque localization found in human aortas. These results suggest that relative differences in WSS or in the direction of WSS, as opposed to the absolute magnitude, may be relevant determinants of flow-mediated inflammatory responses.

Are intramural suction-squeezing effects generated by the variations in radial wall stress during each heart beat the motor of atherosclerosis? A new concept

In the early sixties, the existence of predilection sites for atherosclerotic lesions inside the arterial circulation led to the concept that low wall shear stress (WSS) was responsible, together with systemic factors like high blood pressure, hypercholesterolemia, or diabetes, for the genesis and progression of atherosclerosis. It was found later that oscillating WSS and high WSS gradients could also be incriminated. Yet, this concept, which is broadly accepted today, fails to explains several facts, for instance that some arteries (e.g. epicardial coronary arteries) are more prone to become atherosclerotic than other ones exposed to the same systemic factors (e.g. hepatic and brain arteries). In this paper, we present a quite different concept. It is based on the fact that the increase in intravascular pressure and flow that occur in the arteries during systole generates, at the predilection sites of atherosclerotic lesions (bends, bifurcations, and branchings), an increase of radial wall stress in the outer layers of the arterial wall so that this stress becomes momentarily tensile. These cyclic stress increases have a suction effect that is likely to facilitate the diffusion of atherogenic cells and substances inside the wall. Furthermore, since arteries are not primarily structured to resist inversions of radial stress, they may also create damages (e.g. disruptions of cell membranes and elastic lamellae) followed by inflammations and micro-haemorrhages in the wall. This new concept may provide a complementary (or possibly alternative) explanation of atherosclerosis.

The State of the Art of Nanobioscience in Japan

This paper reviews a part of the state of the art of nanobioscience in Japan. The importance of combination and integration of interdisciplinary principles is emphasized for the development of nanobioscience. Biomagnetics, biomechanics, nanomachining, self-replicating cell model, neuronal network, drug delivery system, and tissue engineering are discussed.

[Shear-stress sensing via P2 purinoceptors in vascular endothelial cells]

The mechanisms by which shear stress elevates intracellular Ca(2+) in endothelial cells (EC) are not fully understood. Here we report that endogenously released ATP contributes to shear stress-induced Ca(2+) responses. Application of a flow of Hank's balanced solution to human pulmonary artery EC (HPAEC) elicited shear stress-dependent increases in Ca(2+) concentration. Chelation of extracellular Ca(2+) with EGTA completely abolished the Ca(2+) responses, whereas the phospholipase C inhibitor U-73122 and the Ca(2+)-ATPase inhibitor thapsigargin had no effect, indicating that the response was due to the influx of extracellular Ca(2+). The Ca(2+) influx was significantly suppressed by apyrase, which degrades ATP, and by antisense oligonucleotide targeted to P2X4 receptors. A luciferase luminometric assay showed that shear stress induced dose-dependent release of ATP. When the ATP release was inhibited by the ATP synthase inhibitors angiostatin or oligomycin, the Ca(2+) influx was markedly suppressed but was restored by removal of these inhibitors or addition of extracellular ATP. These results suggest that shear stress stimulates HPAEC to release ATP, which activates Ca(2+) influx via P2X4 receptors.

Function of caveolae in Ca2+ entry and Ca2+-dependent signal transduction

The correct spatial and temporal control of Ca2+ signaling is essential for such cellular activities as fertilization, secretion, motility, and cell division. There has been a long-standing interest in the role of caveolae in regulating intracellular Ca2+ concentration. In this review we provide an updated view of how caveolae may regulate both Ca2+ entry into cells and Ca2+-dependent signal transduction

Endogenously released ATP mediates shear stress-induced Ca2+ influx into pulmonary artery endothelial cells

The mechanisms by which flow-imposed shear stress elevates intracellular Ca2+ in cultured endothelial cells (ECs) are not fully understood. Here we report finding that endogenously released ATP contributes to shear stress-induced Ca2+ responses. Application of flow of Hanks' balanced solution to human pulmonary artery ECs (HPAECs) elicited shear stress-dependent increases in Ca2+ concentrations. Chelation of extracellular Ca2+ with EGTA completely abolished the Ca2+ responses, whereas the phospholipase C inhibitor U-73122 or the Ca2+-ATPase inhibitor thapsigargin had no effect, which thereby indicates that the response was due to the influx of extracellular Ca2+. The Ca2+ influx was significantly suppressed by apyrase, which degrades ATP, or antisense oligonucleotide targeted to P2X4 purinoceptors. A luciferase luminometric assay showed that shear stress induced dose-dependent release of ATP. When the ATP release was inhibited by the ATP synthase inhibitors angiostatin or oligomycin, the Ca2+ influx was markedly suppressed but was restored by removal of these inhibitors or addition of extracellular ATP. These results suggest that shear stress stimulates HPAECs to release ATP, which activates Ca2+ influx via P2X4 receptors.

Spatial microstimuli in endothelial mechanosignaling

Descriptive and quantitative analyses of microstimuli in living endothelial cells strongly support an integrated mechanism of mechanotransduction regulated by the spatial organization of multiple structural and signaling networks. Endothelial responses to blood flow are regulated at multiple levels of organization extending over scales from vascular beds to single cells, subcellular structures, and individual molecules. Microstimuli at the cellular and subcellular levels exhibit temporal and spatial complexities that are increasingly accessible to measurement. We address the cell and subcellular physical interface between flow-related forces and biomechanical responses of the endothelial cell. Live cell imaging and computational analyses of structural dynamics, two important approaches to microstimulation at this scale, are briefly reviewed.

Sites of Ca2+ wave initiation move with caveolae to the trailing edge of migrating cells

The caveola is a membrane domain that compartmentalizes signal transduction at the cell surface. Normally in endothelial cells, groups of caveolae are found clustered along stress fibers or at the lateral margins in all regions of the cell. Subsets of these clusters appear to contain the signaling machinery for initiating Ca2+ wave formation. Here we report that induction of cell migration, either by wounding a cell monolayer or by exposing cells to laminar shear stress, causes caveolae to move to the trailing edge of the cell. Concomitant with the relocation of the caveolae,sites of Ca2+ wave initiation move to the same location. In as much as the relocated caveolae contain elements of the signaling machinery required for ATP-stimulated release of Ca2+ from the ER, these results suggest that caveolae function as containers that carry this machinery to different cellular locations.

Intimal hyperplasia and wall shear in arterial bypass graft distal anastomoses: an in vivo model study

The observation of intimal hyperplasia at bypass graft anastomoses has suggested a potential interaction between local hemodynamics and vascular wall response. Wall shear has been particularly implicated because of its known effects upon the endothelium of normal vessels and, thus, was examined as to its possible role in the development of intimal hyperplasia in arterial bypass graft distal anastomoses. Tapered (4-7 mm I.D.) e-PTFE synthetic grafts 6 cm long were placed as bilateral carotid artery bypasses in six adult, mongrel dogs weighing between 25 and 30 kg with distal anastomotic graft-to-artery diameter ratios (DR) of either 1.0 or 1.5. Immediately following implantation, simultaneous axial velocity measurements were made in the toe and artery floor regions in the plane of the anastomosis at radial increments of 0.35 mm, 0.70 mm, and 1.05 mm using a specially designed 20 MHz triple crystal ultrasonic wall shear rate transducer Mean, peak, and pulse amplitude wall shear rates (WSRs), their absolute values, the spatial and temporal wall shear stress gradients (WSSG), and the oscillatory shear index (OSI) were computed from these velocity measurements. All grafts were harvested after 12 weeks implantation and measurements of the degree of intimal hyperplasia (IH) were made along the toe region and the artery floor of the host artery in 1 mm increments. While some IH occurred along the toe region (8.35+/-23.1 microm) and was significantly different between DR groups (p<0.003), the greatest amount occurred along the artery floor (81.6+/-106.5 microm, mean +/- S.D.) (p < 0.001) although no significant differences were found between DR groups. Linear regressions were performed on the paired IH and mean, peak, and pulse amplitude WSR data as well as the absolute mean, peak, and pulse amplitude WSR data from all grafts. The mean and absolute mean WSRs showed a modest correlation with IH (r = -0.406 and -0.370, respectively) with further improvements seen (r = -0.482 and -0.445, respectively) when using an exponential relationship. The overall best correlation was seen against an exponential function of the OSI (r = 0.600). Although these correlation coefficients were not high, they were found to be statistically significant as evidenced by the large F-statistic obtained. Finally, it was observed that over 75 percent of the IH occurred at or below a mean WSR value of 100 s(-1) while approximately 92 percent of the IH occurred at or below a mean WSR equal to one-half that of the native artery. Therefore, while not being the only factor involved, wall shear (and in particular, oscillators wall shear) appears to provide a stimulus for the development of anastomotic intimal hyperplasia.

Lysophosphatidic acid positively regulates the fluid flow-induced local Ca(2+) influx in bovine aortic endothelial cells

Using real-time confocal microscopy, we have demonstrated that lysophosphatidic acid (LPA), a bioactive phospholipid existing in plasma, positively regulates fluid flow-induced [Ca(2+)](i) response in fluo 4-loaded, cultured, bovine aortic endothelial cells. The initial increase in [Ca(2+)](i) was localized to a circular area with a diameter of <4 microm and spread concentrically, resulting in a mean global increase in [Ca(2+)](i). The local increase often occurred in a stepwise manner or repetitively during constant flow. The percentage of cells that responded and the averaged level of increase in [Ca(2+)](i) were dependent on both the concentration of LPA (0.1 to 10 micromol/L) and the flow rate (25 to 250 mm/s). The response was inhibited by removing extracellular Ca(2+) or by the application of Gd(3+), an inhibitor of mechanosensitive (MS) channels, but not by thapsigargin, an inhibitor of the endoplasmic reticular Ca(2+)-ATPASE: It was also inhibited by 8-bromo-cGMP, and the inhibition was completely reversed by KT5823, an inhibitor of protein kinase G (PKG). These results suggest that the [Ca(2+)](i) response arises from Ca(2+) influx through Gd(3+)-sensitive MS channels, which are negatively regulated by the activation of PKG. The spatiotemporal properties of the [Ca(2+)](i) response were completely different from those of a Ca(2+) wave induced by ATP, a Ca(2+)-mobilizing agonist. Therefore, we called the phenomenon Ca(2+) spots. We conclude that LPA positively regulates fluid flow-induced local and oscillatory [Ca(2+)](i) increase, ie, the Ca(2+) spots, in endothelial cells via the activation of elementary Ca(2+) influx through PKG-regulating MS channels. This indicates an important role for LPA as an endogenous factor in fluid flow-induced endothelial function.

Sp1-mediated downregulation of P2X4 receptor gene transcription in endothelial cells exposed to shear stress

Endothelial purinoceptors play an important role in vascular responses to extracellular adenine nucleotides and hemodynamic forces. Here we report that P2X4 purinoceptor expression in human umbilical vein endothelial cells is transcriptionally downregulated by fluid shear stress. When human umbilical vein endothelial cells were subjected to a laminar shear stress of 15 dyn/cm(2), P2X4 mRNA levels began to decrease within 1 h and further decreased with time, reaching 60% at 24 h. Functional analysis of the 1.9-kb P2X4 5'-promoter indicated that a 131-bp segment (-112 to +19 bp relative to the transcription start site) containing a consensus binding site for the Sp1 transcription factor was critical for the shear stress responsiveness. Mutations of the Sp1 site decreased the basal level of transcription and abolished the response of the P2X4 promoter to shear stress. Electrophoretic mobility shift assays showed a marked decrease in binding of Sp1 to the Sp1 consensus element in shear-stressed cells, suggesting that Sp1 mediates the shear stress-induced downregulation of P2X4 gene transcription.

MRNA expression on shape-engineered endothelial cells: adhesion molecules ICAM-1 and VCAM-1

This study was designed to assess the effect of cell shape on mRNA expression of two adhesion molecules, intracellular adhesion molecule-1 and vascular adhesion molecule-1, on endothelial cells. Photo-microprocessing using photoreactive poly(ethylene glycol) produced two different patterned-cell adhesive regions on tissue culture dishes: one is a striped region on which adhered cells are highly elongated and aligned along the long axis of the striped pattern, and the other is a circular region on which cells are less spread out and rounded. mRNA expressions, measured by the reverse transcription-polymerase chain reaction technique, revealed higher mRNA expression for intracellular adhesion molecule-1 and lower mRNA expression for vascular adhesion molecule-1 on elongated cells than those on round cells. This indicates that surface-induced cell shape induces changes in the mRNA expression of these molecules. The significance of cell-shape-induced mRNA expression is discussed in conjunction with the experimental results of flow-induced expression at molecular and mRNA levels.

Fluid shear stress activates Ca(2+) influx into human endothelial cells via P2X4 purinoceptors

Ca(2+) signaling plays an important role in endothelial cell (EC) responses to shear stress generated by blood flow. Our previous studies demonstrated that bovine fetal aortic ECs showed a shear stress-dependent Ca(2+) influx when exposed to flow in the presence of extracellular ATP. However, the molecular mechanisms of this process, including the ion channels responsible for the Ca(2+) response, have not been clarified. Here, we demonstrate that P2X4 purinoceptors, a subtype of ATP-operated cation channels, are involved in the shear stress-mediated Ca(2+) influx. Human umbilical vein ECs loaded with the Ca(2+) indicator Indo-1/AM were exposed to laminar flow of Hanks' balanced salt solution at various concentrations of ATP, and changes in [Ca(2+)](i) were monitored with confocal laser scanning microscopy. A stepwise increase in shear stress elicited a corresponding stepwise increase in [Ca(2+)](i) at 250 nmol/L ATP. The shear stress-dependent increase in [Ca(2+)](i) was not affected by phospholipase C inhibitor (U-73122) but disappeared after the chelation of extracellular Ca(2+) with EGTA, indicating that the Ca(2+) increase was due to Ca(2+) influx. Antisense oligonucleotides designed to knockout P2X4 expression abolished the shear stress-dependent Ca(2+) influx seen at 250 nmol/L ATP in human umbilical vein ECs. Human embryonic kidney 293 cells showed no Ca(2+) response to flow at 2 micromol/L ATP, but when transfected with P2X4 cDNA, they began to express P2X4 purinoceptors and to show shear stress-dependent Ca(2+) influx. P2X4 purinoceptors may have a "shear-transducer" property through which shear stress is perceived directly or indirectly and transmitted into the cell interior via Ca(2+) signaling.

P2X(4) receptors mediate ATP-induced calcium influx in human vascular endothelial cells

ATP induces Ca(2+) influx across the cell membrane and activates release from intracellular Ca(2+) pools in vascular endothelial cells (ECs). Ca(2+) signaling leads to the modification of a variety of EC functions, including the production of vasoactive substances such as nitric oxide and prostacyclin. However, the molecular mechanisms for ATP-induced Ca(2+) influx in ECs have not been thoroughly clarified. Here we demonstrate evidence that a P2X(4) receptor for an ATP-gated cation channel is predominantly expressed in human ECs and is involved in the ATP-induced Ca(2+) influx. Northern blot analysis distinctly showed the expression of P2X(4) mRNA in human ECs cultured from the umbilical vein, aorta, pulmonary artery, and skin microvessels. Competitive PCR revealed that P2X(4) mRNA expression was much higher in ECs than was the expression of other subtypes, including P2X(1), P2X(3), P2X(5), and P2X(7). Treatment of ECs with antisense oligonucleotides designed to target the P2X(4) receptor decreased the P2X(4) mRNA and protein levels to approximately 25% of control levels and markedly prevented the ATP-induced Ca(2+) influx.

Gradients in cytoplasmic calcium concentration ([Ca2+]i) in migrating human umbilical vein endothelial cells (HUVECs) stimulated by shear-stress

Using a parallel-plate flow-chamber and confocal laser scanning microscopy (CLSM), we studied the distribution and temporal changes in intracellular Ca2+ concentration ([Ca2+]i) in migrating HUVECs stimulated by shear-stress. In the presence or absence of ATP, shear-stress (10 dyne/cm2) caused morphological change and migration of individual HUVECs in the random direction. After 120 minute exposure to shear-stress, 70% of the cells migrated in the direction of flow, whereas, as many as 30% of the cells migrated to the upstream against flow. A nonspecific plasma membrane Ca2+ channel blocker, Ni2+, abolished such responses markedly, suggesting that Ca2+ influx may be essential for shear-stress dependent morphological change and migration of HUVECs. Analysis of [Ca2+]i distribution revealed the appearance of localized [Ca2+]i elevation inside lamellipodium formed in the direction of cell migration. The localized rise in [Ca2+]i might be closely related with morphological change to regulate the direction of cell migration induced by shear-stress.

Calcium signal transduction from caveolae

Caveolae are specialized membrane microdomains that are found on the plasma membrane of most cells. Recent studies indicate that a variety of signaling molecules are highly organized in caveolae, where their interactions initiate specific signaling cascades. Molecules enriched in this membrane include G protein-coupled receptors, heterotrimeric GTP binding proteins, IP3 receptor-like protein, Ca2+ ATPase, eNOS, and several PKC isoforms. Direct measurements of calcium changes in endothelial cells suggest that caveolae may be sites that regulate intracellular Ca2+ concentration and Ca2+ dependent signal transduction. This review will focus on the role of caveolae in controlling the spatial and temporal pattern of intracellular Ca2+ signaling.

A flow-activated chloride-selective membrane current in vascular endothelial cells

Shear stress-induced activation of endothelial ion channels, one of the earliest responses to flow, is implicated in mechano-signal transduction that results in the regulation of vascular tone. The effects of laminar flow on endothelial membrane potential were studied in vitro using both fluorescent potentiometric dye measurements and whole-cell patch-clamp recordings. The application of flow stimulated membrane hyperpolarization, which was reversed to depolarization within 35 to 160 seconds. The depolarization was caused by a Cl(-)-selective membrane current activated by flow independently of the K(+) channel-mediated hyperpolarization. Thus, flow activated both K(+) and Cl(-) currents, with the net membrane potential being determined by the balance of the responses. Membrane potential sensitivity to flow was unchanged by flow preconditioning that elongated and aligned the cells.

Changes in coronary endothelial cell Ca2+ concentration during shear stress- and agonist-induced vasodilation

Increases in intraluminal shear stress are thought to cause vasodilation of coronary arterioles by activation of Ca2+-dependent endothelial nitric oxide synthase followed by release of nitric oxide. We tested the hypothesis that endothelium-dependent vasodilation of isolated coronary arterioles to shear stress and agonists is necessarily preceded by an increase in endothelial cell Ca2+ concentration ([Ca2+]i). After selective loading of endothelium in isolated rabbit coronary arterioles with fura 2, simultaneous changes in diameter and [Ca2+]i were recorded. Vasodilations recorded in response to ACh, substance P, or shear stress were accompanied by significant increases in endothelial cell [Ca2+]i. Vasodilations to shear stress were accompanied by smaller changes in endothelial cell [Ca2+]i than equivalent dilations evoked by substance P or ACh. To test the role for Ca2+ as an activator of endothelial nitric oxide synthase, the endothelium was treated with the Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N', N'-tetraacetic acid. 1,2-Bis(2-aminophenoxy)ethane-N,N,N', N'-tetraacetic acid eliminated significant changes in endothelial cell [Ca2+]i and inhibited dilations to ACh and substance P but did not significantly affect shear stress-induced vasodilation. The data indicate that endothelium-dependent vasodilation of coronary arterioles in response to agonists and shear stress is mediated in part through a rise in endothelial cell [Ca2+]i but that a substantial component of the shear stress-induced response occurs through a Ca2+-insensitive pathway.

The effect of graft caliber upon wall shear within in vivo distal vascular anastomoses

Wall shear has been widely implicated as a contributing factor in the development of intimal hyperplasia in the anastomoses of chronic arterial bypass grafts. Earlier studies have been restricted to either: (1) in vitro or computer simulation models detailing the complex hemodynamics within an anastomosis without corresponding biological responses, or (2) in vivo models that document biological effects with only approximate wall shear information. Recently, a specially designed pulse ultrasonic Doppler wall shear rate (PUDWSR) measuring device has made it possible to obtain three near-wall velocity measurements nonintrusively within 1.05 mm of the vessel luminal surface from which wall shear rates (WSRs) were derived. It was the purpose of this study to evaluate the effect of graft caliber, a surgically controllable variable, upon local hemodynamics, which, in turn, play an important role in the eventual development of anastomotic hyperplasia. Tapered (4-7 mm I.D.) 6-cm-long grafts were implanted bilaterally in an end-to-side fashion with 30 deg proximal and distal anastomoses to bypass occluded common carotid arteries of 16 canines. The bypass grafts were randomly paired in contralateral vessels and placed such that the graft-to-artery diameter ratio, DR, at the distal anastomosis was either 1.0 or 1.5. For all grafts, the average Re was 432 +/- 112 and the average Womersley parameter, alpha, was 3.59 +/- 0.39 based on artery diameter. There was a sharp skewing of flow toward the artery floor with the development of a stagnation point whose position varied with time (up to two artery diameters) and DR (generally more downstream for DR = 1.0). Mean WSRs along the artery floor for DR = 1.0 and 1.5 were found to range sharply from moderate to high retrograde values (589 s-1 and 1558 s-1, respectively) upstream to high antegrade values (2704 s-1 and 2302 s-1, respectively) immediately downstream of the stagnation point. Although there were no overall differences in mean and peak WSRs between groups, there were significant differences (p < 0.05) in oscillatory WSRs as well as in the absolute normalized mean and peak WSRs between groups. There were also significant differences (p < 0.05) in mean and peak WSRs with respect to axial position along the artery floor for both DR cases. In conclusion, WSR varies widely (1558 s-1 retrograde to 2704 s-1 antegrade) within end-to-side distal graft anastomoses, particularly along the artery floor, and may play a role in the development of intimal hyperplasia through local alteration of mass transport and mechano-signal transduction within the endothelium.

Fluid shear stress transcriptionally induces lectin-like oxidized LDL receptor-1 in vascular endothelial cells

Fluid shear stress has been shown to modulate various endothelial functions, including gene expression. In this study, we examined the effect of fluid shear stress on the expression of lectin-like oxidized LDL receptor-1 (LOX-1), a novel receptor for atherogenic oxidized LDL in cultured bovine aortic endothelial cells (BAECs). Exposure of BAECs to the physiological range of shear stress (1 to 15 dyne/cm2) upregulated LOX-1 protein and mRNA in a time-dependent fashion. LOX-1 mRNA levels peaked at 4 hours, and LOX-1 protein levels peaked at 8 hours. Inhibition of de novo RNA synthesis by actinomycin D totally abolished shear stress-induced LOX-1 mRNA expression. Furthermore, nuclear runoff assay showed that shear stress directly stimulates transcription of the LOX-1 gene. Chelation of intracellular Ca2+ with quin 2-AM completely reduced shear stress-induced LOX-1 mRNA expression; furthermore, the treatment of BAECs with ionomycin upregulated LOX-1 mRNA levels in a dose-dependent manner. Taken together, physiological levels of fluid shear stress can regulate LOX-1 expression by a mechanism dependent on intracellular Ca2+ mobilization. Inducible expression of LOX-1 by fluid mechanics may play a role in localized expression of LOX-1 and atherosclerotic lesion formation in vivo.