Endothelium-dependent contractions

The endothelial cells help to control the tone of the underlying vascular smooth muscle by releasing vasoactive factors. In physiological circumstances, the release of relaxing factors (nitric oxide and endothelium-derived hyperpolarizing factor) appears to predominate. However, in certain blood vessels (peripheral veins and large cerebral arteries), the normal endothelium has the propensity to release vasoconstrictor substances, among which are superoxide anion and thromboxane A2; the release of these endothelium-derived vasoconstrictors may contribute to the autoregulatory processes. In most blood vessels, anoxic conditions initiate the release of an unidentified endothelium-dependent contracting factor. Cultured endothelial cells, and blood vessels maintained under culture conditions for prolonged periods of time, release the vasoconstrictor peptide endothelin. A characteristic of vascular diseases is that the ability of the endothelial cells to release relaxing factor(s) is reduced, while the generation of contracting factor is maintained or enhanced.     

Imbalance of endothelium-derived relaxing and contracting factors. A new concept in hypertension?

The endothelium has a strategical anatomical position between the circulating blood and vascular smooth muscle cells. It has recently been recognized that endothelial cells play an important regulatory role in the circulation. The cells metabolize or activate vasoactive hormones (ie, norepinephrine, serotonin, bradykinin, angiotensin II), produce substances involved in coagulation and can release endothelium-derived relaxing factors and contracting factors. Nitric oxide and prostacyclin are vasodilators and inhibitors of platelet function. Endothelin is the most potent vasoconstrictor substance known. Thus, the endothelium can profoundly affect platelet adhesion and aggregation, vascular smooth muscle tone and possibly also vascular smooth muscle growth. Under physiological conditions, endothelium-derived relaxing factors appear to dominate. In contrast, in hypertensive and atherosclerotic arteries the release of endothelium-derived relaxing factors and/or the responsiveness of vascular smooth muscle cells to the relaxing factors is reduced, while that of endothelium-derived contracting factors is augmented. This imbalance of endothelium-derived relaxing and contracting factors may be important in the pathogenesis of hypertension and its cardiovascular complications.     

Endothelial control of vascular tone in large and small coronary arteries

The endothelium modulates coronary vascular tone by the release of endothelium-derived relaxing or contracting substances. The endothelium-derived relaxing factor has been identified as nitric oxide synthesized in endothelial cells from L-arginine. The endothelium can release other relaxing substances such as prostacyclin and a hyperpolarizing factor. Endothelin-1 is a potent vasoconstrictor peptide formed by endothelial cells, and is likely to be the physiologic antagonist of endothelium-derived relaxing factor. Other putative contracting factors include superoxide anions and products of arachidonic acid metabolism. Endothelium-derived relaxing factor is released spontaneously and in response to flow, platelet-derived products (that is, serotonin, thrombin and adenosine diphosphate) and certain autacoids (that is, acetylcholine, bradykinin, histamine, substance P, vasopressin, alpha-adrenergic agonists). A considerable heterogeneity of responses exists among vessels of different size from different anatomic origin and different species. Hypercholesterolemia, atherosclerosis, hypertension and myocardial ischemia or reperfusion, or both, impair endothelium-dependent relaxation. Under normal conditions, endothelium-derived relaxing factor appears to dominate the control of vascular tone of large and small coronary vessels, whereas in disease states, endothelium-derived contracting factors are released. Impairments of endothelial function may be important in the development of various forms of cardiovascular disease.     

Endothelium-derived hyperpolarizing factor

Although nitric oxide appears to be the major endothelium-derived relaxing factor (EDRF), it cannot explain all endothelium-dependent responses of isolated arteries. Thus, acetylcholine causes an endothelium-dependent, transient hyperpolarization, which is due to the release from the endothelial cells of a diffusible substance (endothelium-derived hyperpolarizing factor, EDHF) other than nitric oxide. The muscarinic receptors on the endothelium that trigger the release of EDHF belong to the M1-muscarinic subtype, while those activating the liberation of EDRF are M2-muscarinic in nature. The importance of endothelium-dependent hyperpolarization varies among different blood vessels. The hyperpolarization, and the resulting relaxation caused by EDHF can be attributed to an increase in K+ conductance in the vascular smooth muscle. Although the nature of EDHF remains elusive, it may be a labile metabolic of arachidonic acid.     

Endothelium-derived vasoactive factors and their role in the coronary circulation

The endothelium-due to its strategic anatomic position between the circulating blood and vascular smooth muscle-plays an important functional role in the coronary circulation. Endothelial cells release factors interfering with coagulation, platelet function, vascular tone, and growth. Endothelium-derived nitric oxide (NO) is the endogenous nitrovasodilator that is a potent inhibitor of platelet function and vasodilator. Together with prostacyclin, NO plays an important protective role in preventing platelet adhesion, aggregation, and coronary vasospasm. Endothelial cells also are a source of contracting factors such as endothelin-1, thromboxane A(2), and endoperoxides. Cardiovascular risk factors (hyperlipidemia, hypertension, and diabetes) inhibit the formation of endothelium-derived relaxing factors and promote that of contracting factors. In coronary arteries with advanced atherosclerosis, endothelial function is severely impaired. The reduced release of endothelium-derived NO is associated with an increased platelet-vessel-wall interaction and, in turn, platelet activation and vasospasm, both known events in coronary artery disease.     

Different activation of the endothelial L-arginine and cyclooxygenase pathway in the human internal mammary artery and saphenous vein.

The endothelium releases substances controlling vascular tone and platelet function. We investigated mediators of endothelium-dependent responses in human internal mammary arteries and saphenous veins. The inhibitor of nitric oxide formation, NG-monomethyl L-arginine, enhanced the sensitivity to norepinephrine (fivefold) and evoked more pronounced endothelium-dependent contractions in internal mammary arteries (19 +/- 6% of 100 mM KCl) than in saphenous veins (2 +/- 1%; p less than 0.005). In internal mammary arteries, NG-monomethyl L-arginine, but not indomethacin, markedly reduced endothelium-dependent relaxations to acetylcholine (from 95 +/- 2% to 39 +/- 7%; p less than 0.005) and prevented those to histamine (78 +/- 6% to 4 +/- 3%; p less than 0.005). In saphenous veins, endothelium-dependent relaxations to acetylcholine were weak (24 +/- 11%), while nitric oxide caused comparable relaxations (85 +/- 3%) as in internal mammary arteries (80 +/- 5%; NS). NG-Monomethyl L-arginine prevented the relaxations to acetylcholine and unmasked endothelium-dependent contractions (30 +/- 10%). Indomethacin and the thromboxane synthetase inhibitor CGS-13080 augmented relaxations of saphenous veins to acetylcholine from 24 +/- 11% to 46 +/- 9% (p less than 0.05). Histamine-evoked contractions were converted to endothelium-dependent relaxations by indomethacin and the thromboxane A2/endoperoxide receptor antagonist SQ-30741 (38 +/- 3% and 40 +/- 6%; p less than 0.05) but not CGS-13080. Thus, 1) nitric oxide mediates endothelium-dependent relaxations in human arteries and veins; 2) internal mammary arteries release more nitric oxide than do saphenous veins, and 3) in saphenous veins, the effects of nitric oxide are reduced by endothelium-derived contracting factors originating from the cyclooxygenase pathway.     

Endothelium-derived relaxing factors. A perspective from in vivo data

We review below published studies of endothelium-dependent vasodilation in vivo. Endothelium-dependent vasodilation has been demonstrated in conduit arteries in vivo and in the cerebral, coronary, mesenteric, and femoral vascular beds as well as in the microcirculation of the brain and the microcirculation of cremaster muscle. The available evidence, although not complete, strongly suggests that the endothelium-derived relaxing factor generated by acetylcholine in the cerebral microcirculation is a nitrosothiol. The endothelium-derived relaxing factor generated by bradykinin in this vascular bed is an oxygen radical generated in association with enhanced arachidonate metabolism via cyclooxygenase. In the microcirculation of skeletal muscle, on the other hand, the vasodilation from bradykinin is mediated partly by prostacyclin and partly by an endothelium-derived relaxing factor similar to that generated by acetylcholine. Basal secretion of endothelium-derived relaxing factor is controversial in vivo but is usually present in vitro. On the other hand, it appears that endothelium-derived relaxing factor mediates flow-dependent vasodilation in both large vessels and in the microcirculation in vivo. The generation and release of endothelium-derived relaxing factor from endothelium may be abnormal in a variety of conditions including acute and chronic hypertension, atherosclerosis, and ischemia followed by reperfusion. Several mechanisms for these abnormalities have been identified. These include inability to generate endothelium-derived relaxing factor or destruction of endothelium-derived relaxing factor by oxidants after its release in the extracellular space. These abnormalities in endothelium-dependent relaxation may contribute to the vascular abnormalities in these conditions.     

Interdependence of pharmacologically-induced and endothelium-mediated coronary vasodilation in antianginal therapy

Summary Recent advances in the understanding of vascular physiology have furnished new aspects in the treatment of angina pectoris by various vasodilators. Upon stimulation by various factors (viscous drag from increased flow, pulsatile stretch, ADP/ATP, norepinephrine, serotonin), the coronary endothelium releases a vasodilator called endothelium-derived relaxant factor (EDRF). This factor has recently been shown to probably be nitric oxide (NO), which is identical to the active compound of nitroglycerin. EDRF (NO) dilates both large epicardial arteries and also coronary resistance vessels. It also has a strong platelet antiaggregant effect. The predominant effect of Ca²⁺ antagonists is on resistance vessels, increasing myocardial perfusion and viscous drag acting upon the endothelial lining. This, in turn, stimulates EDRF (NO) release in epicardial arteries and dilation. This additional nitrate-like effect augments the direct vasodilator effect of Ca²⁺ antagonists. Lack of normal endothelial function results in diminished capacity to dilate, and sometimes even in a shift from dilator to constrictor effects, paralleled by an increased tendency for platelet adhesion, activation, and thrombosis, which is still enhanced when plasma low density lipoprotein (LDL) is augmented. EDRF release, vasodilator capacity, and antiaggregant effects are reduced when LDL is high. Nitrates have a direct, endothelium-independent dilator effect, particularly on large coronary arteries, which seems even more pronounced when the endothelium is absent, but only when the vessel segment is still compliant. Therefore nitrates may particularly be effective in vessels with deficient EDRF release.     

Endothelium-derived relaxing and contracting factors: potential role in coronary artery disease

Endothelial cells can release substances which profoundly affectvascular tone and platelet function. The inhibitory substancesinclude endothelium-derived relaxing factor (EDRF or nitricoxide), prostacyclin and probably an endothelium-derived hyperpolarizingfactor. Endothelin is a potent vasoconstrictor peptide releasedfrom endothelial cells. Under certain conditions, the endotheliumcan also produce angiotensin II, thromboxane A₂ and a cyclooxygenase-dependentendothelium-derived contracting factor. In normal arteries,the effects of EDRF appear to dominate. In diseased arteries,the release and action of EDRF is impaired and that of endothelium-derivedcontracting factors is increased. Hyperlipidaemia, atherosclerosisand hypertension reduce endothelium-dependent relaxations. Hypoxiainhibits the release of EDRF and prolonged ischaemia severelyimpairs the response. Regenerated endothelium at sites of mechanicalinjury exhibits selective defects in response to aggregatingplatelets. The more effective release of EDRF in arterial comparedwith venous bypass grafts further suggests an involvement ofthe factor in preventing vascular occlusion. Therapeutic interventionswith specific drugs and diets can augment the impaired endothelium-dependentrelaxation of diseased arteries. Thus, functional changes ofthe endothelium in coronary artery disease may be an importantfactor in the development of vasospasm, ischaemia and thrombosis.     

Heterogeneity of endothelium-dependent responses to acetylcholine in canine femoral arteries and veins. Separation of the role played by endothelial and smooth muscle cells

The purpose of this study was to determine whether heterogeneity in endothelium-dependent responses to acetylcholine between canine blood vessels of different anatomical origin reflects variations in endothelial function or in responsiveness of vascular smooth muscle cells. Experiments were conducted in a bioassay system, where segments of femoral artery or vein with endothelium were perfused intraluminally and the perfusate used to superfuse rings of femoral arteries or veins without endothelium. Indomethacin was present in all experiments to prevent the synthesis of prostanoids. The blood vessels were contracted by phenylephrine. Measurement of wall tension in both the perfused segment and bioassay ring allowed simultaneous detection of endothelium-derived relaxing factor(s) released abluminally (segment) and intraluminally (ring). Intraluminal infusion of acetylcholine (ACh) induced relaxations in the perfused artery but not in vein segments. During arterial superfusion ACh induced relaxation in femoral arterial rings but contraction in venous rings. After treatment with atropine the arterial perfusate evoked relaxations in venous rings. Infusion of ACh through the femoral vein evoked only moderate relaxations in arterial rings. These data demonstrate that depressed endothelium-dependent relaxation to ACh in femoral veins compared to femoral arteries is due to a masking effect of the direct stimulating action of ACh and decreased release of the same mediator or the release of a different relaxing factor from venous endothelium.     

Normal and pathophysiologic considerations of endothelial regulation of vascular tone and their relevance to nitrate therapy.

During the past decade, it has become clear that the vascular endothelium critically influences vascular permeability, controls vessel growth, modulates hemostasis, and regulates vasomotion. This latter role of the endothelium is mediated by the liberation of a number of potent vasoactive compounds, including endothelium-derived relaxing factors, one of which is either nitric oxide or a compound that releases nitric oxide, vasoactive prostaglandins, hyperpolarizing factors, and a number of constricting factors. This role of the endothelium is dramatically altered by several diseases, including atherosclerosis, hypertension, and diabetes. Abnormalities of endothelial regulation of vascular tone may contribute to a number of clinical syndromes, including variant angina, unstable angina, syndrome X, and perhaps many others. In this review, several aspects of the endothelium-derived relaxing factor will be considered, including recent concepts regarding its synthesis, its chemical identity, and alterations in atherosclerosis. Finally, its action in the coronary microcirculation as contrasted to that of nitroglycerin will be considered.     

Vascular biology of coronary bypass grafts

Clinical studies on the natural history of coronary bypass grafts strongly suggest that biologic properties of the vessels used importantly affect their function and patency. The endothelium is a source of substances regulating vascular tone, platelet function, and vascular growth; it produces nitric acid from L-arginine, which is a potent vasodilator and inhibitor of platelet function and has antiproliferative properties. Nitric oxide and prostacyclin, which have a similar profile of action, are released in greater amounts in arterial than venous coronary bypass vessels. Vascular smooth muscle cells are primary regulators of local blood flow and contribute to proliferative responses. The right gastroepiploic artery exhibits more pronounced contractions than the mammary artery but has a similar sensitivity to vasoconstrictors. Proliferative responses in coronary bypass vessels appear to be induced by changes in transmural pressure (particularly in veins), endothelial damage, and in turn, local release of platelet-derived growth factors, low-density lipoproteins, and intrinsic characteristics of the blood vessels. Thus, biologic properties of the endothelium and vascular smooth muscle significantly contribute to the function and patency of coronary bypass grafts. While the mammary artery has near-ideal characteristics, the right gastroepiploic artery exhibits marked contractile responses and the saphenous vein has unsatisfactory antithrombotic properties and more pronounced proliferative responses.     

Endothelium-derived contracting factors.

The endothelium not only mediates relaxation but is a source of contracting factors. Endothelium-dependent contractions are elicited by physical and chemical stimuli (i.e., hypoxia, pressure, and stretch) and autacoids, local and circulating hormones. The mechanism of endothelium-dependent contractions to hypoxia involves withdrawal of nitric oxide. The endothelial cyclooxygenase pathway can produce thromboxane A2, prostaglandin H2, and superoxide anions. The peptide endothelin is a potent contracting factor; its production is stimulated by vasopressor hormones, platelet-derived factors, coagulation products, and cytokines, whereas endothelium-derived nitric oxide, prostacyclin, and a smooth muscle cell-derived inhibitory factor reduce endothelin production. In hypertension, the release of cyclooxygenase-dependent endothelium-derived contracting factors to stretch, acetylcholine, and platelet-derived products is augmented. Vascular endothelin production in hypertension remains controversial but appears mostly normal; it is augmented in the presence of vascular disease or renal insufficiency. The endothelium-dependent inhibition of endothelin-induced contractions is reduced in hypertension while the reactivity of vascular smooth muscle may be normal, increased, or reduced. The potentiating effects of low concentrations of endothelin on contractions to norepinephrine are augmented with aging and hypertension. In atherosclerosis, the production of the cyclooxygenase-dependent endothelium-derived contracting factors and endothelin is enhanced. Thus, endothelium-derived contracting factors can profoundly affect vascular tone and counteract relaxing factors produced within the endothelium. In hypertension and atherosclerosis, the role of contracting factors appears to become more dominant, leading to an imbalance of endothelium-dependent vascular regulation.     

Endothelial dysfunction in the pulmonary vascular bed

The pulmonary endothelium modulates vascular tone by the release of endothelium-derived constricting (EDCF) and relaxing (EDRF) factors, among them endothelin-1, nitric oxide, prostacyclin, and putative endothelium-derived hyperpolarizing factors. Abnormalities in EDCF and EDRF generation have been demonstrated in a number of cardiopulmonary disease states, such as primary and secondary pulmonary hypertension, chronic obstructive lung disease, cardiopulmonary bypass, and congestive heart failure. An imbalance between EDCF and EDRF, termed "pulmonary endothelial dysfunction," may contribute to the alteration in vascular tone characteristic of pulmonary disease. The following review summarizes the present knowledge of the role of EDCF and EDRF in such processes with major focus on pulmonary endothelial dysfunction in hypoxia-induced pulmonary hypertension.     

THE ENDOGENOUS NITROVASODILATOR PRODUCED BY THE VASCULAR ENDOTHELIUM

The calibre of blood vessels and the tone of the vascular smooth muscle are determined not only by innervation, circulating hormones and metabolic factors, but also by local mediators generated in the vascular wall. Prostacyclin is produced mainly by the endothelium; it was the first endothelium-derived mediator that aroused widespread attention, for it is a powerful vasodilator and inhibitor of platelet aggregation.¹ More recently it was discovered that the vascular endothelium produced another potent humoral agent which is responsible for the vasodilator action of many endogenous substances. This substance is endothelium-derived relaxing factor (EDRF).     

Endothelial functions in cardiovascular diseases

In recent years it has become apparent that endothelial cells have important implications in the regulation of vascular smooth muscle tone, vascular permeability and platelet reactivity. One important physiological feature of these cells is the formation of the endothelium-derived relaxing factor (EDRF). This short-lived compound is a potent vascular smooth muscle relaxant and it also inhibits platelet aggregation and adhesion to the vessel wall. Its active principle seems to be nitric oxide (NO), and consequently it can be regarded as the "endogenous nitrovasodilator". In addition to EDRF, endothelial cells synthesize prostacyclin which also has platelet antiaggregatory and vasodilator properties. A reduced effectiveness of the EDRF mechanism is implicated, especially in those events of the vascular pathophysiology associated with an increased vascular tone or vasospasm. For example, a reduced production and/or action of EDRF has been found in animal models of atherosclerosis as well as in atherosclerotic human coronary arteries. In addition, prostacyclin production is reduced under these conditions. In different animal models of hypertension and diabetes mellitus, endothelium-mediated relaxation is also found to be reduced. In addition, under certain pathophysiological conditions, the endothelium seems to produce vasoconstrictor material.     

Endothelial dysfunction: from physiology to therapy

The endothelium controls the tone of the underlying vascular smooth muscle mainly through the production of vasodilator mediators. In some cases, this function is hampered by the release of constrictor substances. The endothelial mediators are also involved in the regulation by the endothelium of vascular architecture and the blood cell-vascular wall interactions. The endothelium-derived factors comprise nitric oxide (NO), prostacyclin, and a still unknown endothelium-derived hyperpolarizing factor(s) (EDHF). In most vascular diseases, the vasodilator function of the endothelium is attenuated. In advanced atherosclerotic lesions, endothelium-dependent vasodilatation may even be abolished. Various degrees and forms of endothelial dysfunction exist, including (1) the impairment of Galphai proteins, (2) less release of NO, prostacyclin and/or EDHF, (3) increased release of endoperoxides, (4) increased production of reactive oxygen species, (5) increased generation of endothelin-1, and (6) decreased sensitivity of the vascular smooth muscle to NO, prostacyclin and/or EDHF. The levels of bradykinin and angiotensin II within the vascular wall are controlled by angiotensin-converting enzyme (ACE). ACE degrades bradykinin and generates angiotensin II. Bradykinin stimulates endothelial cells to release vasodilators. The actions of the kinin are maintained despite endothelial dysfunction, except in very severe arterial lesions. Angiotensin II may be in part responsible for endothelial dysfunction because it induces resistance to the vasodilator action of NO. Thus, impairment of the generation of angiotensin II blocks the direct and indirect vasoconstrictor effect of the peptide. By potentiating bradykinin, ACE inhibitors promote the release of relaxing vasodilator mediators to restore vasodilator function, and to prevent platelet aggregation as well as the recruitment of leukocytes to the vascular wall.     

How to assess endothelial function in human blood vessels.

This brief review discusses the ways, if and when available, to examine endothelium-dependent changes diameter in human blood vessels. It stresses the problems in ensuring proper matching between arteries (and veins) from different human sources. It briefly considers the evidence in vitro supporting the role of endothelium-derived nitric oxide, hyperpolarizing factor and contracting factors (including metabolites of arachidonic acid and endothelin). It emphasizes the difficulty in extrapolating observations obtained in isolated arteries (and veins) to the intact human circulation. The overall conclusion is that the interpretations derived from animal work apply to the human vasculature.     

Endothelial dysfunction in human disease

The vascular endothelium plays a key role in the local regulation of vascular tone by the release of vasodilator substances (i.e. endothelium-derived relaxing factor (EDRF = nitric oxide, NO) and prostacyclin) and vasoconstrictor substances (i.e. thromboxane A2, free radicals, or endothelin). Using either agents like acetylcholine or changes in flow to stimulate the release of EDRF (NO), clinical studies have revealed the importance of EDRF in both basal and stimulated control of vascular tone in large epicardial coronary arteries and in the coronary microcirculation. The regulatory function of the endothelium is altered by cardiovascular risk factors or disorders such as hypercholesterolemia, chronic smoking, hypertension or chronic heart failure. Endothelial dysfunction appears to have detrimental functional consequences as well as adverse longterm effects, including vascular remodelling. Endothelial dysfunction is associated with impaired tissue perfusion particularly during stress and paradoxical vasoconstriction of large conduit vessels including the coronary arteries. These effects may cause or contribute to myocardial ischemia. Several mechanisms may be involved in the development of endothelial dysfunction, such as reduced synthesis and release of EDRF or enhanced inactivation of EDRF after its release from endothelial cells by radicals or oxidized low-density lipoprotein (LDL). Increased plasma levels of oxidized LDL have been noted in chronic smokers and are related to the extent endothelial dysfunction, raising the possibility that chronic smoking potentiates endothelial dysfunction by increasing circulating and tissue levels of oxidized LDL. In heart failure, cytokines and/or reduced flow (reflecting reduced shear stress) may be involved in the development of endothelial dysfunction and can be reversed by physical training. Other mechanisms include an activated renin-angiotensin system (i.e. postmyocardial infarction) with increased breakdown of bradykinin by enhanced angiotensin converting enzyme (ACE) activity. There is evidence that endogenous bradykinin is involved in coronary vasomotor control both in coronary conduit and resistance vessels. ACE inhibitors enhance endothelial function by a bradykinin-dependent mechanism and probably also by blunting the generation of superoxide anion. Endothelial dysfunction appears to be reversible by administering L-arginine, the precursor of nitric oxide, lowering cholesterol levels, physical training, antioxidants such as vitamin C, or ACE inhibition.     

Endothelium-derived relaxing factor

This article reviews what is known of endothelium-derived relaxing factor and its possible physiologic and pathophysiologic roles. This relaxing factor is now thought to be nitric oxide or a ready source of it. It acts as an endogenous nitrovasodilator, stimulating soluble guanylate cyclase to increase cyclic guanosine monophosphate (GMP) levels in vascular smooth muscle and platelets, with consequent relaxant and anti-aggregatory effects (predominantly when stimulated through receptor-operated channels). Its actions are thus synergistic with those of cyclic adenosine monophosphate (AMP)-mediated stimulation (for example, adenosine, prostacyclin). Endothelium-derived relaxing factor is unstable and is thought to act only very locally in vivo. Its release is continuous in the basal state and is stimulated by a number of neuropeptides and by agents released during platelet activation and thrombosis--with large differences in activity among different vessels. Endothelium-derived relaxing factor activity is also flow related, thereby coordinating vasomotor behavior in an intact vascular tree in response to changes in flow. Endothelium-derived relaxing factor activity is reduced in several pathologic states, including atherosclerosis.