**2. Endothelial dysfunction as the first step in the development of vascular disease**

Endothelial dysfunction is an earlier pathophysiologic stage in CVD development. Ross in 1976 published his theory of response to damage, where he hypothesized that the initial event in atherogenesis is the endothelial injury, followed by the proliferation of smooth muscle cells [13]. Over the past years, this theory has been consolidated as endothelial damage is shown to be decisive in the promotion of vascular diseases. Indeed, diverse pharmacologic and dietetic interventions are intended to prevent the imbalance of the endothelial function, trying to interfere with the development of atherosclerosis and its clinical consequences [14, 15].

Cellular senescence is an irreversible biological phenomenon triggered by potentially harmful stimuli which can damage the cell genome. During this process, the cell interrupts the division process, entering a state of cell cycle arrest and becoming quiescent. Senescence is a protective mechanism which affects the major part of the cells within the organism, including the vascular cells [3, 4]. It is considered indispensable to prevent tumor development, although turns to be pathologic when senescent cells extensively accumulate in tissues as a consequence of aging.

Cell senescence can be triggered prematurely due to aging-associated pathologies such as CVD or chronic kidney disease (CKD). In fact, several studies confirm that CKD patients manifest premature aging in several tissues, including those in the cardiovascular system [5]. This is partly explained because CKD patients show "classic" cardiovascular risk factors (age, lifestyle, left ventricular hypertrophy, dyslipidemia, hypertension and diabetes mellitus). Kidney failure leads to the accumulation of circulating uremic toxins in the blood of those patients, causing stress and damage to the endothelium and activating endothelial cells senescence. Furthermore, CKD patients often show subclinical chronic inflammation associated with an immunosenescence process, which seems to be induced by the uremic toxins and other factors [6]. The renal replacement therapies may have a significant role in

Taken together, these concepts show that blood circulating toxins cause endothelial cells to become senescent leading to the appearance of several CVD. For example, some studies have proved that, at least in atherosclerotic processes, the pathogenic basis by which the CVD is developed is endothelial senescence [8, 9]. When endothelial cells become senescent, their imbalanced functionality may lead to the loss of the vascular structure. Moreover, the senescent endothelium cannot regulate correctly the repairing and regenerative activity of endothelial progenitor cells (EPCs), which increases the harmful effect in the vascular bed [10]. It is easy to understand in this context that endothelial senescence acts as the first element in the

Recently, microvesicles (MVs) have been proposed as endothelial response elements that can take part both in damaging and repairing processes in the endothelium [10–12]. There is certain knowledge, yet scarce, about the mechanisms underlying the participation of MVs in endothelial homeostasis, although the implication of those MVs in endothelial senescence

Therefore, to understand and characterize the mechanisms by which the senescent endothelial cells show an imbalanced functionality, it is necessary to identify early biomarkers and to

Endothelial dysfunction is an earlier pathophysiologic stage in CVD development. Ross in 1976 published his theory of response to damage, where he hypothesized that the initial event in atherogenesis is the endothelial injury, followed by the proliferation of smooth muscle

**2. Endothelial dysfunction as the first step in the development of** 

this process, as they induce the activation of immunocompetent cells [7].

development of CVD.

**vascular disease**

remains an unresolved question.

50 Endothelial Dysfunction - Old Concepts and New Challenges

design therapeutic targets for CVD.

The endothelium is a thin monocellular layer that covers the inner surface of blood vessels, separating the circulating blood from the interstitial fluid [16]. The endothelium is not an inert organ, as it can respond to physical or chemical stimuli by liberating the adequate regulatory substance to keep the correct vasomotor equilibrium and homeostasis [17]. The endothelium acts as an autocrine, paracrine and endocrine gland. Endothelial cells produce vasodilating, antiproliferative, antithrombotic and antiadherent mediators, like nitric oxide (NO), prostacyclin, the endothelium-derived hyperpolarizing factor (EDHF) and the natriuretic peptide, type C (CNP). The actions of those molecules are compensated by the release of substances with the opposing effect, as endothelin 1, thromboxane A2, prostaglandin H2 and the superoxide anion. Thus, endothelium regulates the tone of the smooth muscle cells of the vessel wall, causing its relaxation or contraction and conditioning the vasodilation or vasoconstriction processes. Also, it regulates hemostasis by controlling the production of prothrombotic or antithrombotic molecules, as well as fibrinolytic and antifibrinolytic substances. Endothelium takes part in inflammatory and immune processes by regulating proliferation and cell migration, as well as adherence and leukocytes activation. It is capable of producing cytokines and adhesion molecules that regulate the inflammatory process, contributing to the defensive function of the organism by the activation of neutrophils and macrophages [18].

Cardiovascular risk factors provoke an oxidative stress which alter the function of the endothelial cells and provoke endothelial dysfunction by reducing the ability of the endothelium to maintain the homeostasis and concluding with the development of vascular diseases [19]. The term "endothelial dysfunction" has been used to define diverse syndromes which include a change of the endothelial phenotype from a "basal" to an active state. It is a complex disorder which includes alterations in the vasomotor and antithrombotic responses, in the vascular permeability, the leukocytes recruitment and the proliferation of endothelial cells [20, 21]. In the progress of endothelial dysfunction, the presence of pathologic conditions can contribute accelerating CVD development [22, 23].

Among the cardiovascular risk factors, the age arises as a critical factor. It is associated with damage and endothelial dysfunction, as well with atherosclerosis development which will lead to vascular pathologies [24]. Epidemiologic studies have demonstrated that aging is the most important risk factor for the development of CVD, mainly atherosclerotic [23]. During the gradual aging, the incidence and prevalence of atherothrombotic and coronary diseases and cerebrovascular accidents increases. For that reason, there must be a causal relationship between the age-associated changes and vascular damage. It has been demonstrated that, during aging, the vasculature of healthy subjects suffers several changes, as endothelial dysfunction [21], the arterial wall thickening and remodeling [25], angiogenesis alterations, incorrect vascular repair [26] and increased atherosclerosis prevalence [27]. The relationship between the development of these disorders and the aging process remain poorly understood, but it is possible that throughout the physiologic aging of the organisms some similar changes occur, comparable to those in the vascular diseases and sharing common cellular mechanisms.
