Endothelium-derived reactive oxygen species: their relationship to endothelium-dependent hyperpolarization and vascular tone

Opinions on the role of reactive oxygen species (ROS) in the vasculature have shifted in recent years, such that they are no longer merely regarded as indicators of cellular damage or byproducts of metabolism--they may also be putative mediators of physiological functions. Hydrogen peroxide (H2O2), in particular, can initiate vascular myocyte proliferation (and, incongruously, apoptosis), hyperplasia, cell adhesion, migration, and the regulation of smooth muscle tone. Endothelial cells express enzymes that produce ROS in response to various stimuli, and H2O2 is a potent relaxant of vascular smooth muscle. H2O2 itself can mediate endothelium-dependent relaxations in some vascular beds. Although nitric oxide (NO) is well recognized as an endothelium-derived dilator, it is also well established, particularly in the microvasculature, that another factor, endothelium-derived hyperpolarizing factor (EDHF), is a significant determinant of vasodilatory tone. This review primarily focuses on the hypothesis that H2O2 is an EDHF in resistance arteries. Putative endothelial sources of H2O2 and the effects of H2O2 on potassium channels, calcium homeostasis, and vascular smooth muscle tone are discussed. Furthermore, given the perception that ROS can more likely elicit cytotoxic effects than perform signalling functions, the arguments for and against H2O2 being an endogenous vasodilator are assessed.     

Regulation of Vascular Smooth Muscle Relaxation by the Endothelium in Health and in Diabetes (with a Focus on Endothelium-Derived Hyperpolarizing Factor)

Amongst its wide repertoire of functions, the vascular endothelium plays a pivotal role in the regulation of vascular smooth muscle tone and ultimately tissue perfusion. In healthy vessels, the endothelium exerts a vasodilator influence on the underlying smooth muscle cells. In diabetes mellitus, endothelium-dependent vasodilation is impaired in various vascular beds and may contribute to the increased vascular tone and reduced tissue perfusion, which are features of this disease. There are regional variations in the extent of endothelial vasodilator dysfunction in diabetes, and the basis for this variation has yet to be resolved. The complement of vasodilators involved in endothelium-dependent relaxation varies in different vascular beds. In larger arteries and conduit vessels, the role of nitric oxide (NO) has been the focus of human and animal studies on diabetes. Small arteries and arterioles are important in the local regulation of tissue perfusion, and in many of these, another endothelial vasodilator, endothelium-derived hyperpolarizing factor (EDHF), plays an increasingly prominent role in overall endothelium-dependent relaxation. Surprisingly few studies have explored the influence of diabetes on EDHF; however, there is emerging evidence from a diverse range of vascular beds that the actions of EDHF are seriously compromised in diabetes. Vascular disease remains the leading cause of morbidity and mortality associated with diabetes mellitus. A better understanding of the regional differences and mechanisms involved in endothelial function and dysfunction in small arteries may reveal new strategies to aid in the prevention and/or therapeutic management of the vascular complications of diabetes mellitus.     

Modulation of endothelial SK3 channel activity by Ca²⁺-dependent caveolar trafficking

Small- and intermediate-conductance Ca(2+)-activated K(+) channels (SK3/Kcnn3 and IK1/Kcnn4) are expressed in vascular endothelium. Their activities play important roles in regulating vascular tone through their modulation of intracellular concentration ([Ca(2+)](i)) required for the production of endothelium-derived vasoactive agents. Activation of endothelial IK1 or SK3 channels hyperpolarizes endothelial cell membrane potential, increases Ca(2+) influx, and leads to the release of vasoactive factors, thereby impacting blood pressure. To examine the distinct roles of IK1 and SK3 channels, we used electrophysiological recordings to investigate IK1 and SK3 channel trafficking in acutely dissociated endothelial cells from mouse aorta. The results show that SK3 channels undergo Ca(2+)-dependent cycling between the plasma membrane and intracellular organelles; disrupting Ca(2+)-dependent endothelial caveolae cycling abolishes SK3 channel trafficking. Moreover, transmitter-induced changes in SK3 channel activity and surface expression modulate endothelial membrane potential. In contrast, IK1 channels do not undergo rapid trafficking and their activity remains unchanged when either exo- or endocytosis is block. Thus modulation of SK3 surface expression may play an important role in regulating endothelial membrane potential in a Ca(2+)-dependent manner.     

Secretory function of the endothelium as a factor of vascular tone regulation in the norm and in cardiovascular pathology

Endothelin-1 and nitric oxide are the most potent factors of the endothelium-derived substances. The factors play opposite roles in regulation of cardiovascular system, and their interaction underlies the balance of vasoconstrictor and vasodilator influences on vascular tone under normal conditions. In our experiments, changes in endothelin-1 blood concentration were associated with affected production of endogenous nitric oxide. The altered interrelationships between the endothelium-derived vasoactive substances may precede pathological shifts in the cardiovascular system.     

Endothelial control of vascular tone by nitric oxide and gap junctions: a haemodynamic perspective

Local haemodynamic forces acting on the endothelium modulate vascular tone through mechanisms that normalize intimal shear stress. This flow-dependent diameter response contributes to the optimization of circulatory function and is mediated via shear stress-induced release of NO, vasodilator prostanoids and a putative endothelium-derived hyperpolarizing factor or EDHF. There is growing evidence that NO/prostanoid independent relaxations involve direct heterocellular signalling between endothelial and smooth muscle cells via gap junctions.     

Endothelium-derived relaxing and contracting factors

Key discoveries in the past decade revealed that the endothelium can modulate the tone of underlying vascular smooth muscle by the synthesis/release of potent vasorelaxant (endothelium-derived relaxing factors; EDRF) and vasoconstrictor substances (endothelium-derived contracting factors; EDCF). It has become evident that the synthesis and release of these substances contribute to the multitude of physiological functions the vascular endothelium performs. Accumulating evidence suggests that at least one of the EDRFs is identical with nitric oxide (NO) or a labile nitroso compound, which is produced from L-arginine by an NADPH- and Ca(2+)-dependent enzyme, arginine oxidase. The existence of more than one chemically distinct EDRF has been proposed, including an endothelium-derived hyperpolarizing factor (EDHF). The target of EDRF (NO) is soluble guanylate cyclase (increase in cyclic GMP) while EDHF appears to activate a K(+)-channel in vascular smooth muscle. Recent data suggest that muscarinic receptor subtypes selectively mediate the release of EDRF(NO) (M2) and EDHF (M1). EDRF(NO) affects not only the underlying vascular smooth muscle, but also platelets, inhibiting their aggregation and adhesion to the endothelium. The antiaggregatory effect of EDRF is synergistic with prostacyclin, so their combined release may represent a physiological mechanism aimed at preventing thrombus formation. An additional proposed biological function of EDRF(NO) is cytoprotection by virtue of scavenging superoxide radicals. The endothelium can also mediate vasoconstriction by the release of a variety of endothelium-derived contracting factors (EDCF). Other than the unique peptide endothelin, the nature of EDCFs has not yet been firmly established. Autoregulation of cerebral and renal blood flow and hypoxic pulmonary vasoconstriction may represent the physiological role of endothelium-dependent vasoconstriction. Growing evidence indicates that the endothelium can serve as a unique mechanoreceptor, sensing and transducing physical stimuli (e.g., shear forces, pressure) into changes in vascular tone by the release of EDRFs or EDCFs. In physiological states, a delicate balance exists between endothelium-derived vasodilators and vasoconstrictors. Alterations in this balance can result in local (vasospasm) and generalized (hypertension) increase in vascular tone and also in facilitated thrombus formation. Endothelial dysfunction may also contribute to the pathophysiology of angiopathies associated with hypercholesterolemia and atherosclerosis.     

Nitric oxide--the endothelium-derived relaxing factor and its role in endothelial functions

Vascular endothelium plays a key role in the local regulation of vascular tone and vascular architecture by release of vasodilator and vasoconstrictor substances, as well as factors with pro-coagulant, anticoagulant, fibrinolytic, antibacterial properties, growth factors, chemokines, free radicals, etc. Release of endothelium-derived relaxing factors such as nitric oxide (NO), prostaglandins and endothelium-derived hyperpolarizing factor, as well as vasoconstricting factors such as endothelin, superoxide and thromboxanes play an influential role in the maintenance and regulation of vascular tone and the corresponding peripheral vascular resistance. Under physiological conditions, the release of anticoagulant and smooth muscle relaxing factors exceeds the release of other substances. The first part of this review presents the functions of the endothelium itself, the nature of the endothelium-derived relaxing factor, its production by NO synthases, mechanisms of its action via activation of soluble guanylyl cyclase and production of cyclic 3'-5'-guanosine monophosphate. The resulting biological effects include vasodilatation, regulation of vessel wall structure, increased regional blood perfusion, lowering of systemic blood pressure, antithrombosis and antiatherosclerosis effects, which counteract the vascular actions of endogenous vasoconstrictor substances. Impaired endothelial function, either as a consequence of reduced production/release or increased inactivation of endothelium-derived vasodilators, as well as interactions of NO with angiotensin, reactive oxygen species and oxidized lipoproteins, has detrimental functional consequences and is one of the most important cardiovascular risk factors. Therefore the second part of this review assesses the pathophysiologic impact of the endothelium in examples of cardiovascular pathologies, e.g. endotheliopathies caused by increased angiotensin production, lipid peroxidation, ischemia/reperfusion or diabetes.     

Role of 5,6-epoxyeicosatrienoic acid in the regulation of newborn piglet pulmonary vascular tone

We examined the responses of newborn piglet pulmonary resistance arteries (PRAs) to 5,6-epoxyeicosatrienoic acid (5,6-EET), a cytochrome P-450 metabolite of arachidonic acid. In PRAs preconstricted with a thromboxane A(2) mimetic, 5,6-EET caused a concentration-dependent dilation. This dilation was partially inhibited by the combination of charybdotoxin (CTX) and apamin, inhibitors of large and small conductance calcium-dependent potassium (K(Ca)) channels, and was abolished by depolarization of vascular smooth muscle with KCl. Disruption of the endothelium significantly attenuated the dilation, suggesting involvement of one or more endothelium-derived vasodilator pathways in this response. The dilation was partially inhibited by nitro-L-arginine (L-NA), an inhibitor of nitric oxide synthase (NOS), but was unaffected by indomethacin, a cyclooxygenase (COX) inhibitor. The combined inhibition of NOS and K(Ca) channels with L-NA, CTX, and apamin abolished 5,6-EET-mediated dilation. Similarly, combined inhibition of NOS and COX abolished the response. We conclude that 5,6-EET is a potent vasodilator in newborn piglet PRAs. This dilation is mediated by redundant pathways that include release of nitric oxide (NO) and COX metabolites and activation of K(Ca) channels. The endothelium dependence of this response suggests that 5,6-EET is not itself an endothelium-derived hyperpolarizing factor (EDHF) but may induce the release of one or more endothelium-derived relaxing factors, such as NO and/or EDHF.     

Modulation of pulmonary vasomotor tone in the fetus and neonate

The high pulmonary vascular resistance (PVR) of atelectatic, hypoxic, fetal lungs limits intrauterine pulmonary blood flow (PBF) to less than 10% of combined right and left ventricular output. At birth, PVR decreases precipitously to accommodate the entire cardiac output. The present review focuses on the role of endothelium-derived nitric oxide (NO), prostacyclin, and vascular smooth muscle potassium channels in mediating the decrease in PVR that occurs at birth, and in maintaining reduced pulmonary vasomotor tone during the neonatal period. The contribution of vasodilator and vasoconstrictor modulator activity to the pathophysiology of neonatal pulmonary hypertension is also addressed.     

The endothelium: influencing vascular smooth muscle in many ways

The endothelium, although only a single layer of cells lining the vascular and lymphatic systems, contributes in multiple ways to vascular homeostasis. Subsequent to the 1980 report by Robert Furchgott and John Zawadzki, there has been a phenomenal increase in our knowledge concerning the signalling molecules and pathways that regulate endothelial - vascular smooth muscle communication. It is now recognised that the endothelium is not only an important source of nitric oxide (NO), but also numerous other signalling molecules, including the putative endothelium-derived hyperpolarizing factor (EDHF), prostacyclin (PGI(2)), and hydrogen peroxide (H(2)O(2)), which have both vasodilator and vasoconstrictor properties. In addition, the endothelium, either via transferred chemical mediators, such as NO and PGI(2), and (or) low-resistance electrical coupling through myoendothelial gap junctions, modulates flow-mediated vasodilatation as well as influencing mitogenic activity, platelet aggregation, and neutrophil adhesion. Disruption of endothelial function is an early indicator of the development of vascular disease, and thus an important area for further research and identification of potentially new therapeutic targets. This review focuses on the signalling pathways that regulate endothelial - vascular smooth muscle communication and the mechanisms that initiate endothelial dysfunction, particularly with respect to diabetic vascular disease.     

Mechanisms of endothelial dysfunction in resistance arteries from patients with end-stage renal disease

The study focuses on the mechanisms of endothelial dysfunction in the uremic milieu. Subcutaneous resistance arteries from 35 end-stage renal disease (ESRD) patients and 28 matched controls were studied ex-vivo. Basal and receptor-dependent effects of endothelium-derived factors, expression of endothelial NO synthase (eNOS), prerequisites for myoendothelial gap junctions (MEGJ), and associations between endothelium-dependent responses and plasma levels of endothelial dysfunction markers were assessed. The contribution of endothelium-derived hyperpolarizing factor (EDHF) to endothelium-dependent relaxation was impaired in uremic arteries after stimulation with bradykinin, but not acetylcholine, reflecting the agonist-specific differences. Diminished vasodilator influences of the endothelium on basal tone and enhanced plasma levels of asymmetrical dimethyl L-arginine (ADMA) suggest impairment in NO-mediated regulation of uremic arteries. eNOS expression and contribution of MEGJs to EDHF type responses were unaltered. Plasma levels of ADMA were negatively associated with endothelium-dependent responses in uremic arteries. Preserved responses of smooth muscle to pinacidil and NO-donor indicate alterations within the endothelium and tolerance of vasodilator mechanisms to the uremic retention products at the level of smooth muscle. We conclude that both EDHF and NO pathways that control resistance artery tone are impaired in the uremic milieu. For the first time, we validate the alterations in EDHF type responses linked to kinin receptors in ESRD patients. The association between plasma ADMA concentrations and endothelial function in uremic resistance vasculature may have diagnostic and future therapeutic implications.     

Signaling across myoendothelial gap junctions--fact or fiction?

Gap junctions interconnect vascular cells homocellularly, thereby allowing the spread of signals along the vessel wall, which serve to coordinate vessel behavior. In addition, gap junctions provide heterocellular coupling between endothelial and vascular smooth muscle cells, creating so-called myoendothelial gap junctions (MEGJs). Endothelial cells control vascular tone by the release of factors that relax vascular smooth muscle. Endothelial factors include nitric oxide, prostaglandins, and an additional dilator principle, which acts by smooth muscle hyperpolarization and is therefore named endothelium-derived hyperpolarizing factor (EDHF). Whether this principle indeed relies on a factor or on intact MEGJs, which allow direct current transfer from endothelial to smooth muscle cells, has recently been questioned. Careful studies revealed the presence of vascular cell projections that make contact through the internal elastic lamina, exhibit the typical GJ morphology, and express connexins in many vessels. The functional study of the physiological role of MEGJs is confined by the difficulty of selectively blocking these channels. However, in different vessels studied in vitro, the dilation related to EDHF was sensitive to experimental interventions that block MEGJs more or less specifically. Additionally, bidirectional electrical coupling between endothelial and smooth muscle cells was demonstrated in isolated small vessels. In marked contrast, similar approaches used in conjunction with intravital microscopy, which allows examination of vascular behavior in the intact animal, did not verify electrical or dye-coupling in different models investigated. The discrepancy between in vitro and in vivo investigations may be due to size and origin of the vessels studied using these distinct experimental approaches. Additionally, MEGJ coupling is possibly tightly controlled in vivo by yet unknown mechanisms that prevent unrestricted direct signaling between endothelial and smooth muscle cells.     

EDHF-mediated vasodilation involves different mechanisms in normotensive and hypertensive rat lungs

The role of endothelium-derived hyperpolarizing factor (EDHF) in regulating the pulmonary circulation and the participation of cytochrome P-450 (CYP450) activity and gap junction intercellular communication in EDHF-mediated pulmonary vasodilation are unclear. We tested whether tonic EDHF activity regulated pulmonary vascular tone and examined the mechanism of EDHF-mediated pulmonary vasodilation induced by thapsigargin in salt solution-perfused normotensive and hypoxia-induced hypertensive rat lungs. After blockade of both cyclooxygenase and nitric oxide synthase, inhibition of EDHF with charybdotoxin plus apamin did not affect either normotensive or hypertensive vascular tone or acute hypoxic vasoconstriction but abolished thapsigargin vasodilation in both groups of lungs. The CYP450 inhibitors 7-ethoxyresorufin and sulfaphenazole and the gap junction inhibitor palmitoleic acid, but not 18alpha-glycyrrhetinic acid, inhibited thapsigargin vasodilation in normotensive lungs. None of these agents inhibited the vasodilation in hypertensive lungs. Thus tonic EDHF activity does not regulate either normotensive or hypertensive pulmonary vascular tone or acute hypoxic vasoconstriction. Whereas thapsigargin-induced EDHF-mediated vasodilation in normotensive rat lungs involves CYP450 activity and might act through gap junctions, the mechanism of vasodilation is apparently different in hypertensive lungs.     

Ovariectomy-Induced Reductions in Endothelial SK3 Channel Activity and Endothelium-Dependent Vasorelaxation in Murine Mesenteric Arteries

Mesenteric artery endothelium expresses both small (SK3)- and intermediate (IK1)-conductance Ca²⁺-activated K⁺ (K_Ca) channels whose activity modulates vascular tone via endothelium-dependent hyperpolarization (EDH). Two other major endothelium-dependent vasodilation pathways utilize nitric oxide (NO) and prostacyclin (PGI₂). To examine how ovariectomy (ovx) affects the basal activity and acetylcholine (ACh)-induced activity of each of these three pathways to vasorelaxation, we used wire myograph and electrophysiological recordings. The results from functional studies using isolated murine mesenteric arteries show that ovx reduces ACh-induced endothelium-dependent vasodilation due to decreased EDH and NO contributions, although the contribution of PGI₂ is upregulated. Both endothelial SK3 and IK1 channels are functionally coupled to TRPV4 (transient receptor potential, vanilloid type 4) channels: the activation of TRPV4 channels activates SK3 and IK1 channels, leading to EDH-mediated vascular relaxation. The decreased EDH-mediated vasorelaxation in ovx vessels is due to reduced SK3 channel contribution to the pathway. Further, whole-cell recordings using dispersed endothelial cells also show reduced SK3 current density in ovx endothelial cells. Consequently, activation of TRPV4 channels induces smaller changes in whole-cell current density. Thus, ovariectomy leads to a reduction in endothelial SK3 channel activity thereby reducing the SK3 contribution to EDH vasorelaxation.     

The third pathway: endothelium-dependent hyperpolarization.

In response to various neurohumoral substances endothelial cells release nitric oxide (NO), prostacyclin and produce hyperpolarization of the underlying vascular smooth muscle cells, possibly by releasing another factor termed endothelium-derived hyperpolarizing factor (EDHF). EDHF-mediated responses are sensitive to the combination of two toxins, charybdotoxin plus apamin, but do not involve ATP-sensitive or large conductance calcium-activated potassium channels. As hyperpolarization of the endothelial cells is required in order to observe endothelium-dependent hyperpolarization, and electrical coupling through myo-endothelial gap junctions may explain the phenomenon. An alternative explanation is that the hyperpolarization of the endothelial cells causes an efflux of potassium that in turn activates the inwardly rectifying potassium conductance and the Na+/K+ pump of the smooth muscle cells. Endothelial cells produce metabolites of the cytochrome P450-monooxygenase that activate BKCa, and induce hyperpolarization of coronary arterial smooth muscle cells. The elucidation of the mechanism underlying endothelium-dependent hyperpolarization and the discovery of specific inhibitors of the phenomenon are prerequisite for the understanding of the physiological role of this alternative endothelial pathway involved in the control of vascular tone in health and disease.     

Reduced expression of SKCa and IKCa channel proteins in rat small mesenteric arteries during angiotensin II-induced hypertension

Ca(2+)-activated K(+) channels (K(Ca)), in particular, the small and intermediate K(Ca) (SK(Ca) and IK(Ca), respectively) channels, are key players in endothelium-derived hyperpolarizing factor (EDHF)-mediated relaxation in small arteries. Hypertension is characterized by an endothelial dysfunction, possibly via reduced EDHF release and/or function. We hypothesize that during angiotensin II (14 days)-induced hypertension (ANG II-14d), the contribution of SK(Ca) and IK(Ca) channels in ACh-induced relaxations is reduced due to decreased expression of SK(Ca) and IK(Ca) channel proteins in rat small mesenteric arteries (MAs). Nitric oxide- and prostacyclin-independent vasorelaxation to ACh was similar in small MAs of sham-operated and ANG II-14d rats. Catalase had no inhibitory effects on these relaxations. The highly selective SK(Ca) channel blocker UCL-1684 almost completely blocked these responses in MAs of sham-operated rats but partially in MAs of ANG II-14d rats. These changes were pressure dependent since UCL-1684 caused a greater inhibition in MAs of 1-day ANG II-treated normotensive rats compared with ANG II-14d rats. Expression levels of both mRNA and protein SK3 were significantly reduced in MAs of ANG II-14d rats. The IK(Ca) channel blocker 1-[(2-chlorophenyl)diphenylmethyl]-1H-pyrazole (TRAM-34) resulted in comparable reductions in the relaxation responses to ACh in MAs of sham-operated and ANG II-14d rats. Relative mRNA expression levels of IK1 were significantly reduced in MAs of ANG II-14d rats, whereas protein levels of IK1 were not but tended to be lower in MAs of ANG II-14d rats. The findings demonstrate that EDHF-like responses are not compromised in a situation of reduced functional activity and expression of SK3 channels in small MAs of ANG II-induced hypertensive rats. The role of IK1 channels is less clear but might compensate for reduced SK3 activity.     

Grape-derived polyphenols improve aging-related endothelial dysfunction in rat mesenteric artery: role of oxidative stress and the angiotensin system

Aging is characterized by the development of an endothelial dysfunction, which affects both the nitric oxide (NO)- and the endothelium-derived hyperpolarizing factor (EDHF)-mediated relaxations, associated with vascular oxidative stress and the activation of the angiotensin system. This study investigated whether red wine polyphenols (RWPs), antioxidants and potent stimulators of NO- and EDHF-mediated relaxations improve aging-related endothelial dysfunction, and, if so, examined the underlying mechanism. Mesenteric artery reactivity was determined in organ chambers, vascular oxidative stress by dihydroethidine and MitoSOX staining, and expression of target proteins by immunohistochemical staining. Control young rats (16 weeks) received solvent (ethanol, 3% v/v), and middle-aged rats (46 weeks) either solvent or RWPs (100 mg/kg/day) in the drinking water. The acetylcholine-induced endothelium-dependent NO component was slightly reduced whereas the EDHF component was markedly blunted in rings of middle-aged rats compared to young rats. The endothelial dysfunction was associated with oxidative stress, an upregulation of angiotensin II and AT1 receptors and a down-regulation of SK(Ca), IK(Ca), and angiotensin converting enzyme. Intake of RWPs for either one or two weeks improved the NO and the EDHF components of the relaxation, and normalized oxidative stress, the expression of SK(Ca), IK(Ca) and the components of the angiotensin system. The protective effect of the 2-week RWPs treatment persisted for one and two weeks following stopping intake of RWPs. Thus, intake of RWPs caused a persistent improvement of the endothelial function, particularly the EDHF component, in middle-aged rats and this effect seems to involve the normalization of the expression of SK(Ca), IK(Ca) and the angiotensin system.     

Endothelial function and coronary artery disease

The endothelium produces a number of vasodilator and vasoconstrictor substances that not only regulate vasomotor tone, but also the recruitment and activity of inflammatory cells and the propensity towards thrombosis. Endothelial vasomotor function is a convenient way to assess these other functions, and is related to the long-term risk of cardiovascular disease. Lipids (particularly low density lipoprotein cholesterol) and oxidant stress play a major role in impairing these functions, by reducing the bioavailability of nitric oxide and activating pro-inflammatory signalling pathways such as nuclear factor kappa B. Biomechanical forces on the endothelium, including low shear stress from disturbed blood flow, also activate the endothelium increasing vasomotor dysfunction and promoting inflammation by upregulating pro-atherogenic genes. In contrast, normal laminar shear stress promotes the expression of genes that may protect against atherosclerosis. The sub-cellular structure of endothelial cells includes caveolae that play an integral part in regulating the activity of endothelial nitric oxide synthase. Low density lipoprotein cholesterol and oxidant stress impair caveolae structure and function and adversely affect endothelial function. Lipid-independent pathways of endothelial cell activation are increasingly recognized, and may provide new therapeutic targets. Endothelial vasoconstrictors, such as endothelin, antagonize endothelium-derived vasodilators and contribute to endothelial dysfunction. Some but not all studies have linked certain genetic polymorphisms of the nitric oxide synthase enzyme to vascular disease and impaired endothelial function. Such genetic heterogeneity may nonetheless offer new insights into the variability of endothelial function.     

Communication between endothelial and smooth muscle cells

Vascular endothelial cells play a fundamental role in the control of vascular tone, and therefore in the control of local blood flow, by releasing various contracting (endothelin, prostaglandins) and relaxing (prostacycline, NO) factors. An additional mechanism involving the hyperpolarization of the vascular smooth muscle cells is observed mainly in the coronary vascular bed and in the periphery. This phenomenon was attributed to an elusive endothelial factor called endothelium-derived hyperpolarizing factor (EDHF). This mechanism is now better understood. It involves first an increase in the endothelial intracellular concentration of calcium, the activation of endothelial potassium channels and the resulting hyperpolarization of the endothelial cells. The hyperpolarization of the endothelial cells is transmitted to the smooth muscle cells by different pathways. This hyperpolarization propagates along the vessels not only via the smooth muscle cells but also via the endothelial cells. Therefore, the endothelial layer can also be considered as a conducting tissue. The discovery of specific inhibitors of the endothelial cell hyperpolarization allows the assessment of the contribution of EDHF-mediated responses in the control of vascular tone.