The relation between atherosclerosis and thrombosis has been recognized for over a century. Until very recently, however, this pathophysiological association has been largely based on observation with little information available to dissect molecular and cellular mechanism.

Atherosclerosis is a slowly progressive disease characterized by the accumulation of cholesterol within the vessel wall. Plasma lipoproteins are particles of complex lipid and protein composition that transport lipids in blood. Low density lipoproteins (LDL) and high density lipoproteins (HDL) are the major cholesterol-carrier lipoproteins. LDL seem to be responsible for the delivery of lipids (cholesterol) from the liver to the tissues. Compelling evidence supports the concept that the lipids deposited in the arteriosclerotic lesions are derived primarily from plasma LDL. The term "reverse cholesterol transport" describes the transport of cholesterol from extrahepatic tissues to the liver, where it may be metabolized. Reverse cholesterol transport seems to be the major route for removal of the exchangeable cholesterol deposited in the extrahepatic tissue. It has been postulated that a major role of the plasma HDL particle is to act as a scavenger of tissue cholesterol. The hypothesis that high levels of plasma HDL are protective against coronary artery disease (CAD) was initially proposed by Barr et al in the early 1950s, but it was overlooked until confirmed by the Tromso and Framingham studies in 1977, which showed an inverse relation between HDL plasma levels and incidence of CAD. Similar observations have been made in 15 major experimental studies including eight countries.(ABSTRACT TRUNCATED AT 250 WORDS)

Angiography in patients with unstable angina or myocardial infarction with subtotal coronary occlusion often reveals eccentric stenoses with irregular borders, suggesting ruptured atherosclerotic plaques and thrombosis, as documented by angioscopy and at autopsy. We have simulated and studied these processes in an ex vivo perfusion chamber and in an in vivo swine model. Our results suggest that specific local factors at the time of plaque disruption influence the degree of thrombogenicity, the stability of the growing thrombus, and, therefore, possibly also the various clinical syndromes. These factors can be divided into two groups: local vessel wall-related factors and systemic factors with local action at the area of risk. These factors include the following. 1) Exposed substrate-related effects: Plaque rupture produces a rough surface and stimulates the development of occlusive thrombus in proportion to the degree of damage. 2) Fluid dynamics-related factors: The more severe the stenotic lesion after plaque rupture, the higher the local shear rate, resulting in enhanced platelet deposition and thrombus formation. 3) Vasoconstrictive effects: Vasospasm is an important contributor to the pathogenesis of ischemic heart disease. 4) Systemic factors: There is clinical and experimental evidence to suggest that various systemic factors at the time of plaque rupture may enhance thrombogenicity (i.e., levels of epinephrine, levels of serum cholesterol, impaired fibrinolysis). We have investigated the role of residual thrombus on the process of rethrombosis and found that a residual thrombus is a very thrombogenic surface that may significantly contribute to reocclusion even in heparinized blood. Using recombinant hirudin as a pharmacological tool in our flow studies, we observed that rethrombosis is partially caused by thrombin bound to fibrin in the original thrombus, because the effect is abolished by the specific thrombin inhibitor.

Arterial injury induces vascular smooth muscle cells (VSMC) to modulate from a quiescent to a proliferative state characterized by cell division, migration, and secretion of matrix. These changes have been implicated in the development of intimal hyperplasia after balloon angioplasty. The transition of VSMC to a proliferative state is preceded by the accumulation of platelets and leukocytes, which may release growth factors and cytokines at the site of injury. Platelet-derived growth factor (PDGF) is secreted by platelets and a variety of cellular elements associated with the vessel wall and, as a VSMC mitogen and chemoattractant, has been implicated in the pathogenesis of intimal hyperplasia. We have found that, in addition to its effects on VSMC growth and migration, PDGF induces the expression in VSMC of the JE and KC genes, which encode monocyte and neutrophil chemoattractants, respectively. The induction of JE and KC by PDGF in VSMC culture involves several distinct transmembrane signaling pathways. In addition to their induction in VSMC culture, JE and KC messenger RNA accumulates rapidly and transiently in adult rabbit aorta after balloon dilatation, suggesting a role for these chemoattractants in the early vascular response to injury in vivo. The VSMC may therefore play a dual role in vessel injury, both as a mediator of the inflammatory response through chemoattractant release and as an effector of the hyperplastic response through proliferation.

Restenosis presently limits the long-term success of percutaneous transluminal coronary angioplasty (PTCA) and allied treatments of atheroma. Current views of the pathophysiology of restenosis fail to explain certain important clinical features: Thrombosis, often invoked as a cause of smooth muscle proliferation, wanes before intimal thickening peaks. Prolonged antithrombotic therapy does not eliminate restenosis after PTCA. Furthermore, only a minority of angioplastied lesions develop clinically significant restenosis.

This review discusses the role of growth factors on the proliferation of smooth muscle cells (SMCs) in injured arteries. We have used a variety of procedures to injure rat carotid arteries and noted that removal of endothelium followed by platelet adherence does not always initiate SMC replication. Furthermore, thrombocytopenia did not reduce the early SMC replication induced by balloon catheter injury. Platelet-derived growth factor (PDGF) has been found to exert little effect on SMC replication but markedly influences the ability of SMCs to migrate to the intima. Basic fibroblast growth factor (bFGF) is a potent mitogen for SMCs in denuded arteries while having no effect on cells in control uninjured arteries. We have hypothesized that arterial injury leads to release of bFGF from injured SMCs and so stimulates cell replication. Rats were treated with antibodies to bFGF immediately before balloon injury, and this significantly reduced the SMC replication. These findings suggest that in vivo bFGF is an important mitogen for initiating SMC replication and that PDGF is important as a chemotactic for SMCs.

Rupture of the plaque surface, often with thrombosis superimposed, occurs frequently during the evolution of coronary atherosclerotic lesions. It is probably the most important mechanism underlying the sudden, rapid plaque progression responsible for acute coronary syndromes. The risk of plaque rupture depends on plaque type (composition) rather than plaque size (volume), because only plaques rich in soft extracellular lipids are vulnerable (rupture-prone). Most ruptures are tiny, occurring at the periphery of the fibrous cap that covers the lipid-rich core--points where the cap is usually thinnest and most heavily infiltrated by macrophage foam cells. Compared with intact caps, ruptured ones usually have less tensile strength and are more extensible, containing less collagen and glycosaminoglycans, more extracellular lipid, fewer smooth muscle cells, and more macrophages. Progressive extracellular lipid accumulation (lipid core formation) and cap weakening (macrophage related?) predispose the plaque to rupture and determine the actual vulnerability, which may change with time. Luckily, the plaque components responsible for vulnerability (soft lipid and probably macrophages) are apparently most likely to regress with treatment. The dynamic interplay between the actual plaque vulnerability and external stresses ("triggers") probably determines the particular moment and point of rupture, if this occurs. Vulnerability probably plays a more important role in rupture than triggers, because exercise stress testing of patients with advanced coronary artery disease rarely triggers a rupture/thrombus-related acute heart attack. A prerequisite is the presence of a vulnerable plaque.

The adhesive protein von Willebrand factor is essential for the formation of platelet thrombi under flow conditions characterized by high shear stress. This function requires the interaction with two distinct platelet receptors, the glycoprotein complexes Ib-IX-V and IIb-IIIa. Interaction with the former results in platelet activation, a necessary step for binding to the latter and supporting stable aggregation. The inhibition of von Willebrand factor binding to glycoprotein Ib can be achieved with small recombinant fragments containing the specific functional domain of the molecule that interacts with this platelet receptor. Such fragments may provide a new selective approach to antithrombotic therapy.

This article briefly addresses the role of the monocyte-macrophage in atherogenesis and the potential mechanisms participating in intimal blood monocyte recruitment. The obligatory components of the recruitment process, i.e., contact with the vascular endothelium, attachment, and migration into the subendothelial space, are addressed with reference to the influence of blood flow, plasma proteins (particularly low density lipoprotein), and inflammatory mediators and cytokines. The potentially important role of monocyte chemotactic protein-1 in regulating monocyte recruitment in plaque pathogenesis is discussed.

Reviewed are various aspects of atherosclerotic plaque stabilization and regression in humans and experimental animals. Plaque regression is a function of the dynamic balance among initiation, progression, stabilization, and removal of plaque constituents. Pseudoregression, the result of the triad thrombolysis, age- or lesion-dependent arterial dilatation, and relaxation of vasospasm, may readily give rise to angiographic misinterpretation. Although lowering of plasma cholesterol and low density lipoprotein-cholesterol has demonstrated significant clinical benefits in a number of clinical trials, the magnitude of angiographic regressive changes is relatively small despite aggressive lipid-lowering regimens. The emerging need for alternative or complementary therapeutic interventions has been emphasized. In particular, they should be targeted to pivotal cellular or molecular mechanisms in initiation, progression, or stabilization. Potentially important therapeutic targets include the use of antioxidants or free radical scavengers such as Probucol or its analogues, butylated hydroxytoluene, tocopherols, and possibly the tocotrienols. Other therapeutic targets include intimal monocyte-macrophage recruitment, macrophage cholesterol acyltransferase inhibition, stimulation of the high density lipoprotein-mediated reverse cholesterol transport system, smooth muscle cell migration to and proliferation in the arterial intima, and intimal connective tissue synthesis. Whether the isoprenylated proteins associated with the cholesterol biosynthetic pathway will give rise to compounds regulating smooth muscle cell growth has yet to be determined. Because of the importance of thrombosis in the pathogenesis and progression of lesions, the need to develop interventional strategies targeted at endothelial cell thromboresistance and thromboregulation must assume a high priority in future research and development. Other areas of therapeutic promise include the calcium channel blockers and angiotensin converting enzyme inhibitors.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium channel blockers have been shown to retard the development of atherosclerosis in rabbits fed cholesterol-rich diets. The mechanism accounting for this effect is controversial but may be by stimulation of cholesteryl ester hydrolase activity in smooth muscle cells, by amelioration of hypercholesterolemia-induced endothelial dysfunction, or by inhibition of smooth muscle cell proliferation and migration. The effect of calcium channel blockers on the evolution of coronary atherosclerosis in humans has been assessed in two clinical trials. In the Montreal Heart Institute trial, nicardipine did not influence the overall rate of progression and regression; however, nicardipine-treated patients experienced significantly less progression of minimal lesions, defined as stenoses of < or = 20% severity. In the International Nifedipine Trial on Antiatherosclerotic Therapy, nifedipine had no effect on overall progression and regression but reduced the rate of appearance of new coronary lesions. These studies constitute a potentially important new approach to the management of coronary atherosclerosis.

Coronary artery disease develops and progresses over decades with lipid incorporation and coalescence into the arterial wall, leading to recurrent plaque disruption, thrombosis, and organization into fibromuscular lesions. The role of thrombin in arterial thrombosis is documented by observations that specific thrombin inhibition can completely prevent intraluminal mural thrombus in vivo, even though other factors that activate platelets are not inhibited. Platelet-rich thrombi associated with deep injury have a high thrombin content and require blood levels of specific thrombin inhibition with hirudin that are eight to 10 times those required to prevent thrombi associated with lesser injury or stasis. Total prevention of mural thrombus in deeply injured arteries is impossible with aspirin or conventional antithrombotic therapy, which can reduce occlusive thrombus. Thus, conventional antithrombotic therapies can reduce clinical events but are not effective in reducing progression of coronary artery disease. Whether newer antithrombins that can totally prevent mural thrombus after deep injury can be developed and administered orally and chronically without high risk of bleeding and will be able to reduce progression of coronary artery disease is unknown. Lesion growth, vascular injury, acute coronary syndromes, and principles of thrombus formation on ruptured plaque are discussed elsewhere in this issue. This article extends these findings and discusses the prevention and treatment of mural thrombosis, the pivotal role of thrombin in arterial thrombosis, and antithrombotic therapy for the prevention and the progression of coronary artery disease.

The initiation of atherosclerosis may result from blood flow oscillatory shear stress in certain vascular sites (bending points, bifurcations, etc.) producing chronic minimal injury resulting in functional alteration of the arterial endothelium type I injury; experimentally, this is potentiated by atherogenic risk factors such as hypercholesterolemia, hypertension, immunocomplexes, viral infections, and tobacco smoke. Such minimal injury leads to accumulation of lipid and monocytes (macrophages), and subsequently, toxic products released by the macrophages produce damage of the intimal surface with denuding endothelium type II injury or damage, which attracts platelets; all of these cells release growth factors, prompting migration and proliferation of smooth muscle cells and producing a "fibro-intimal lesion" or the outside of the capsule of a predominant "lipid lesion." The lipid lesions surrounded by a thin capsule tend to be small and rupture easily, causing type III injury or damage; that is, they are soft and weak, contain large numbers of macrophages, which may release collagenase and elastase to form abscesses, and by their location, are under the effect of flow shear forces. After plaque disruption there is thrombus formation; when thrombi are small, they can become organized and contribute to the growth of the atherosclerotic plaque; when thrombi are large and occlusive, they lead to the acute coronary syndromes. New data suggest that, at the time of plaque disruption, certain "thrombogenic" risk factors modulate the degree of thrombogenicity and, thereby, the growth of the plaque versus the various acute coronary syndromes. Aside from the need for better understanding of the basic biology of atherogenesis, emphasis on identifying and modifying the primary atherogenic and thrombogenic risk factors should continue for primary prevention. Also, new approaches should focus on the identification, stabilization, and regression of the small "lipid plaques" prone to rupture (these are not necessarily angiographically apparent), as well as on the use of better and safer antithrombotic agents for prevention of progression.

Endothelial and smooth muscle cells normally exist in a quiescent differentiated state. After injury to the vessel, these cells dedifferentiate, migrate, and proliferate as needed for repair. In culture on plastic, both endothelial and smooth muscle cells exhibit the dedifferentiated phenotype. We have found that laminin and reconstituted basement membrane proteins (Matrigel) induce a very rapid cessation of endothelial cell proliferation followed by alignment and subsequent reorganization into tubelike structures. We have also found that smooth muscle cells in culture exhibit a differentiated phenotype when exposed to Matrigel. The molecular mechanisms involved in smooth muscle differentiation resemble those of skeletal muscle, in which proliferation and differentiation appear to be mutually exclusive states controlled by both positive and negative transcriptional regulators. The dedifferentiated smooth muscle cells produce proteases and exhibit a migratory and invasive phenotype capable of destroying normal tissue architecture. These studies suggest that the modulation of endothelial and smooth muscle cells between a differentiated and dedifferentiated phenotype is regulated by extracellular matrix components as well as by cytokines. Model systems such as those described here should allow the identification of molecular events controlling the differentiation of vascular cells and facilitate the development of therapeutic agents that maintain healthy vessels.

Regulation of intracellular calcium levels is known to activate signal transduction pathways, leading to well-defined patterns of gene expression.

The original classifications of the cardiomyopathies based on anatomic criteria from radiographic and necropsy studies, as well as hemodynamic criteria from clinical and catheterization data, have been supplemented in recent years by information from noninvasive techniques. Echocardiography, radionuclide methods, and ambulatory ECG, in particular, have facilitated the ethical screening of family members and those less symptomatic than patients on whom the original classification was based. These powerful methods show a broad spectrum of anatomy and ventricular physiology along the natural history of and within the traditional categories of the cardiomyopathies. They also provide data on the effect of ventricular loading conditions affecting a range of diastolic filling patterns. This review has attempted to point out the areas of overlap among and/or controversy about the categories that have led us to a feeling of frustration when trying to neatly classify individual patients. The addition of filling patterns from Doppler echocardiography and nuclear angiography to the standard methods has been reviewed and hopefully will lend more perspective to the range of physiology seen in these conditions. The categories of cardiomyopathy should not be seen as excluding patients with the newly recognized variations in anatomy and ventricular filling patterns. Rather, the classification provides a framework on which to build and expand our understanding of these important conditions.

After the description in the past 5 years of BNP and CNP, interest in the natriuretic peptide family has dramatically increased. Molecular characterization of the receptors for this hormone family has identified a heterogeneity in the receptor subtypes not previously alluded to by pharmacological or biochemical studies. Much has been published on the physiology of ANP, but the major roles for BNP and CNP remain to be elucidated. Some experiments indicate that ANP and BNP may act synergistically, especially during cardiac stress; however, the high level of structural diversity of BNP among species and the ability of porcine BNP, but not human BNP, to activate human NPR-B suggest that an as yet unidentified receptor may exist that specifically recognizes BNP. Localization studies have implied that CNP is the most prominent neuropeptide in the natriuretic peptide family, and the restriction of its receptor, NPR-B, to the nervous system suggests that CNP and NPR-B may act in the brain to coordinate the central aspects of body fluid homeostasis. Of the three known NPRs, two, NPR-A and NPR-B, are capable of synthesizing their own second messenger, cGMP. The domain within these receptors that has high homology to protein kinases has been demonstrated to be essential for regulating this activity. No kinase activity has been measured in these proteins, but it is possible that this region is important for ATP regulation of guanylyl cyclase activity. This possibility raises interesting parallels with receptor-mediated cAMP signaling within cells. Seven transmembrane receptors, once activated by ligand, associate with G proteins to affect the activity of adenylyl cyclase.(ABSTRACT TRUNCATED AT 250 WORDS)

TEE applications for children and infants with congenital heart disease are now clinically established and TEE is being used in most major pediatric cardiovascular centers. This technology has evolved to the point that it is capable of providing diagnostic quality imaging, even for small infants. With expanding technology and increasing experience, new and more sophisticated applications will be found for pediatric TEE.