Number 22, 2004
Endothelial Dysfunction

Endothelial function and dysfunction

Back to the Summary

Paul M. Vanhoutte
Department of Pharmacology, University of Hong Kong
Correspondence: P.M. Vanhoutte, Department of Pharmacology, Faculty of Medicine, 21 Sassoon Road, Hong Kong. e-mail: vanhoutte.hku@hku.hk

Introduction
In 1980, Furchgott and Zawadzki [1] demonstrated that endothelial cells have an obligatory role in the relaxation of isolated arteries in response to acetylcholine. This pivotal observation has revolutionized thinking about the local control of vasomotor function. The endothelium-dependent responses are caused by the release of several diffusible substances (endothelium-derived relaxing [EDRF] and contracting factors) from the endothelial cells. This review briefly summarizes the observations, obtained mainly in the author's laboratory, that have examined how the production of relaxing factors by endothelial cells underlies moment-to-moment changes in the tone of the surrounding vascular smooth muscle cells, and how a lack of this function by endothelial cells eventually initiates atherosclerosis and thus vascular disease. It updates similar, more exhaustive overviews [213].

Endothelium-derived relaxing factors

Endothelium-derived nitric oxide
The short-lived diffusible factor that underlies endothelium-dependent relaxation in response to acetylcholine [1] has been identified as nitric oxide. Endothelial nitric oxide is formed from the guanidine-nitrogen terminal of L-arginine by the action of endothelial constitutive nitric oxide synthase (nitric oxide synthase III, eNOS). The activation of eNOS depends on the intracellular concentration of calcium ions in the endothelial cells, and is Ca²⁺-calmodulin-dependent (Figure 1). The activity of the enzyme requires cofactors: in particular, reduced nicotinamide adenine dinucleotide phosphate, and 5,6,7,8-tetrahydrobiopterin. eNOS can be inhibited competitively by synthetic L-arginine analogs such as N^G-monomethyl-L-arginine or N^G-nitro-L-arginine, or by the endogenous inhibitor, asymmetric dimethyl arginine. Nitric oxide diffuses to the underlying smooth muscle cells and, in them, stimulates cytosolic soluble guanylate cylase, which accelerates the formation of cyclic 325-guanosine monophosphate (cyclic GMP). The cyclic nucleotide in turn inhibits the contractile process. Nitric oxide is the major contributor to endothelium-dependent relaxation in large arteries [115]. In the intact organism, both animal and human, the inhibitors of nitric oxide synthase cause vasoconstriction in most vascular beds and an increase in systemic arterial pressure, not only because they prevent the direct inhibitory action of nitric oxide on the vascular smooth muscle, but also because nitric oxide inhibits the production of renin and of endothelin 1 [16].

Nitric oxide is also released in the lumen of the blood vessel. Because it is scavenged by the oxyhemoglobin of the blood, it does not fulfil a hormonal role. However, at the interface between the blood and the blood vessel wall, it inhibits the adhesion of platelets and white cells to the endothelium. It acts (in strong synergy with prostacyclin) to inhibit platelet aggregation [3,4,9,15]. It also inhibits the growth of the vascular smooth muscle cells and prevents the production of adhesion molecules [17] (Figure 2).

The activity of eNOS can be upregulated acutely. For example, the shear forces exerted by the flowing blood on the endothelial cells are one of the main regulators of the local release of nitric oxide, a mechanism that explains flow dependent vasodilation. Several substances, whether circulating in the blood or produced by the blood vessel wall, can increase the release of nitric oxide through activation of specific receptors on the endothelial cell membrane (Figure 3). They include hormones (eg, estrogen, catecholamines, vasopressin), neurotransmitters (eg, substance P), autacoids (bradykinin, histamine), and products formed during platelet aggregation (serotonin, adenosine diphosphate [ADP] or blood coagulation (thrombin). The cell membrane receptors for these substances are coupled to the activation of eNOS by two different families of G proteins (Figure 2). Thus, in coronary arteries, ₂-adrenergic receptors, serotonin receptors, and thrombin receptors are coupled to pertussis toxin-sensitive Gi proteins, whereas, in contrast, the receptors for ADP or bradykinin are not coupled to the production of nitric oxide by pertussis-toxin sensitive G proteins [18]. The activation of eNOS by bradykinin involves low molecular weight G proteins of the Rho family [19]. In coronary and cerebral arteries, aggregating platelets induce endothelium-dependent relaxation, and the presence of a healthy endothelium inhibits the constriction induced by the platelet products (thromboxane A₂ and serotonin). Serotonin, acting on 5-HT1D serotonin receptors, plays the major part in this response, whereas ADP, activating P2y-purinoceptors, contributes little (Figure 2). The release of nitric oxide, both toward the underlying smooth muscle and at the interface with the blood, in response to thrombin and platelet-derived serotonin is pivotal for the protective role played by the healthy endothelium against the platelet attack (Figure 4) [3,8,13].

Prostacyclin
Prostacyclin, formed primarily in endothelial cells, relaxes vascular smooth muscle by stimulation of adenylate cyclase, with a resulting increased production of cyclic 325-adenosine monophosphate (cyclic AMP). It acts synergistically with nitric oxide to inhibit platelet aggregation (Figure 4) [3,6,14].

Endothelium-dependent hyperpolarizing factor
In large and small arteries from different species (including the human), acetylcholine, and other endothelium-dependent vasodilators, cause endothelium-dependent hyperpolarization which can contribute to endothelium-dependent relaxation. The hyperpolarization has been attributed to a diffusible endothelium-derived hyperpolarizing factor (EDHF) different from nitric oxide and prostacyclin, although these last two can, in certain but not all blood vessels, cause hyperpolarization of vascular smooth muscle. The exact nature of EDHF remains a matter of intense debate. Among the more recent candidates to explain endothelium-dependent hyperpolarization, gap junctions, epoxyeicosatrienoic acids, potassium ions, and hydrogen peroxide appear to have major roles [2023] (Figure 1).
The contribution of hyperpolarization to endothelium-dependent relaxation varies as a function of the size of the blood vessel and thus is more pronounced in smaller than in larger arteries [23,24]. In the latter, although both mediators can contribute to endothelium-dependent relaxation, nitric oxide predominates under normal circumstances. However, in these large arteries, such as the coronaries, EDHF can maintain near normal endothelium-dependent relaxation when the synthesis of nitric oxide is dysfunctional [25]. In certain cases, nitric oxide exerts an inhibitory effect on endothelium-dependent hyperpolarization [26].

Chronic modulation
Chronic modulatory influences that can upregulate the release of relaxing factors by endothelial cells include chronic increases in blood flow, exercise training, estrogen administration, and intake of ₃-unsaturated fatty acids, red wine polyphenols, green tea, and other antioxidants [2730].

Endothelial dysfunction
In the course of aging, and in several types of vascular disease and hypertension, the endothelial cells become dysfunctional [3,4,8,1013]. This dysfunction is evident as an impairment of endothelium-dependent relaxation, mainly as the result of a reduced release of EDRFs, in particular nitric oxide, although production of endothelium-derived vasoconstrictor substances may contribute [2,3,5,31,32].

Hypercholesterolemia and atherosclerosis
Hypercholesterolemia impairs endothelium-dependent relaxation [33,34]. In contrast, endothelium-independent relaxation in response to exogenous nitric oxide remains largely normal. In the initial phase of the atherosclerotic process, endothelial dysfunction is limited to the pertussis toxin sensitive, Gi protein-dependent pathway (Figure 2). Thus the ability of regenerated endothelial cells, chronically exposed to high cholesterol concentrations, to ADP-ribosylate pertussis toxin is reduced [39]. Hence, in coronary arteries from hypercholesterolemic pigs, endothelium-dependent relaxation in response to serotonin, ₂-adrenergic agonists, aggregating platelets, or thrombin is depressed, whereas those induced by ADP and bradykinin are maintained [3339]. Oxidized low-density lipoprotein induces, in vitro, a similar selective endothelial dysfunction, whereas at higher concentrations it also inhibits endothelium-dependent relaxation in response to stimuli that are not Gi protein-dependent [40] (Figure 2).

Summary
The most important aspect of endothelial dysfunction is the reduced release or bioavailibility of nitric oxide, which probably is the fundamental, initial step of the atherosclerotic process. This hypothesis implies that aging and prolonged exposure to shear stress, coupled with risk factors such as obesity, diabetes, high blood pressure, and smoking, accelerate endothelial turnover and endothelial regeneration. Thus larger and larger sections of the endothelial lining (particularly in areas of turbulence) can no longer prevent platelet adhesion and aggregation, and become insensitive to thrombin. The negative feedback that nitric oxide, together with prostacyclin, exerts on platelet aggregation decreases steadily, whereas vasoconstrictor and growth-promoting substances (serotonin and thromboxane A₂) are released in increasing amounts, together with growth factors such as platelet-derived growth factor. This sequence of events permits the local inflammatory response and initiates the characteristic morphological changes in atherosclerosis, in particular because the local shortage of nitric oxide unleashes the growth process [3,714].

Back to the Summary

REFERENCES

Back to the Summary