2.1. NO effects on vasodilation and endothelial dysfunction

F2a and thromboxane A2) and/or relaxing (prostaglandin I2 and nitric oxide, NO) factors [2, 3] can modulate blood vessel tone. These substances, known as endothelium-derived contracting factors (EDCF) or endothelium-derived relaxing factor (EDRF), can modify the vascular smooth muscle tone directly, acting on smooth muscle cells, or indirectly, by altering sympathetic transmission [4]. Nevertheless, when endothelium integrity and/or function is compromised, such regulation can be impaired. Indeed, evidence suggests that endothelial dysfunction (present an altered NO production and oxidative stress) may contribute to the pathogenesis of hypertension. As a consequence, an increase in peripheral vascular resistance occurs in conditions where endothelium is somehow injured. For example, endothelium dysfunction leads to the enhancement of contractile responses to vasoconstrictor agents [2, 5–8]. Nevertheless, in the literature, there are also innumerous other factors that can also influence endothelium function and, therefore, vascular responsiveness, such as tetrahydrobiopterin (BH4), sex hormones and gender, angiotensin, insulin, vascular endothelial growth factor, vitamin D, adiponectin, uric acid, lipids, oxygen-derived free radicals, aldosterone and epithe-

In this chapter, the impact of endothelial dysfunction on vascular neurotransmission is debated

The vascular wall is composed of layers that can be identified by their respective morphology and by the different functions exhibited by respective cells which, ultimately, are responsible for the vascular tone, influencing blood pressure. Arteries and veins have a similar structure presenting three layers: intima or endothelium, media or smooth muscle and adventitia.

The tunica intima is the inner and thinnest layer and surrounds the lumen. It is made up of endothelial cells lining the entire vasculature and includes circular elastic bands, the internal elastic lamina. The tunica media, also called muscle layer, is composed of vascular smooth muscle, which helps regulate the size of the lumen and externally present circular elastic bands, the external elastic lamina. This tunica differs between arteries and veins: arteries contain more smooth muscle than the tunica media of their counterpart, the veins, and this allows arteries to constrict and dilate to adjust the volume of blood needed by the tissues that they support. Additionally, the structure of arteries differs between large arteries and resistant arteries: in the first type, arteries present a media with large amount of elastic fibers disposed between smooth muscle cells and the thickness of the vascular wall is thinner than that exhibited by resistant arteries that often have multiple strands of smooth muscle layers. The external layer, adventitia layer is composed of connective tissue allowing the blood vessel to withstand forces acting on the vessel wall and of collagen fibers that anchor the vessel to

The endothelium can evoke effects, dilation or contraction of the underlying vascular smooth muscle, by releasing endothelium-derived relaxing factors (EDRF) such as NO or endothelium-

derived contracting factors (EDCF) such as endothelin or prostanoids.

with particular focus on adenosinergic and nitroxidergic system dynamics.

lial sodium channels.

surrounding tissues.

2. Endothelium and vasodilation

328 Endothelial Dysfunction - Old Concepts and New Challenges

NO is a well-known EDRF that induces vasodilation through the activation of soluble guanylyl cyclase in the vascular smooth muscle cells producing cyclic guanosine monophosphate (therefore, through the signaling pathway that can be represented as NO-cGMP/cGMP-dependent kinases).

It is well accepted that the benefits of NO released from endothelium are compromised in vascular diseases and aging since there is a reduced amount of NO. However, evidence also show that the production of NO can be upregulated, for example, by estrogens, exercise and dietary factors and downregulated by oxidative stress, smoking, pollution and oxidized lowdensity lipoproteins.

Moreover, when endothelium is dysfunctional, the vasodilation induced by endothelial mediators is impaired and it can even lead to vascular smooth muscle cells contraction. For instance, in aged subjects and in vascular diseases (essential hypertension and diabetes) when the production of NO is compromised, endothelium-dependent contractions are intensified.

NO is produced by three isoforms of NO synthase, presenting a more general distribution in the human body than that initially predicted: neuronal NOS (nNOS), inducible NOS (iNOS) and endothelial NOS (eNOS). nNOS is constitutively expressed in central and peripheral nervous system contributing to regulation of blood pressure, smooth muscle dilation and vasodilation via peripheral nitrergic nerves. iNOS is expressed in several cell types and generates large amounts of NO, which is involved in the pathophysiology of inflammatory diseases, as regulatory effector molecule of the innate immune response and septic shock. eNOS is expressed mainly in endothelial cells and has several vasoprotective and anti-atherosclerotic effects as well as an important role in vascular tone and thus blood pressure regulation.

Many cardiovascular risk factors lead to oxidative stress, eNOS uncoupling and endothelial dysfunction in the vasculature. eNOS generates NO which results from the activity of two domains, the oxygenase domain that convert L-arginine to L-citrulline plus NO and the reductase domain that convert nitrites to NO [9].

As mentioned above, NO production from endothelium can be upregulated or downregulated by a number of factors of which vascular endothelial growth factor (VEGF) can upregulate eNOS. Interestingly, a chronic side effect of VEGF inhibitors is the occurrence of hypertension, suggesting a physiological role for VEGF in maintaining endothelial control of vasomotor tone [10–12]. In humans, in hypertension, VEGF inhibitors may cause increased production of endothelin-1 [13, 14] and reduced vascular response to acetylcholine [15, 16].

Acute and chronic increases in flow as well as the resulting augmentation in shear stress of the blood on the endothelial cells can be altered through Ca2+-dependent and Ca2+-independent pathways. It has been described that Ca2+-independent pathway can increase both the expression and activity of eNOS and thus the release of NO [17]. The role played by the endothelial cells to protect against thrombin and other platelet products by increasing the activity of eNOS has been demonstrated both in vitro [18–26] and in vivo [27]. Serotonin and adenosine diphosphate are mediators released by aggregating platelets, which may activate eNOS and increase NO production. When endothelium is absent/dysfunctional, vasodilation is no longer observed, and aggregating platelets induce contractions, because they release vasoconstrictors (thromboxane A2 and serotonin). When platelet aggregation occurs in a healthy artery (i.e, with an intact and physiologically active endothelium), serotonin (and ADP) release by the platelets as well as production of thrombin will increase NO release from endothelial cells. Thus, NO will be increased in the vicinity of smooth muscle cells inducing dilation, and consequently, increasing blood flow.

enveloped in Schwann cells: most nerve fibers travel through individual channels in the Schwann cell, but small fibers are sometimes bundled together within a single channel [42]. The SNS signals to dilate or constrict the vessel, changing the lumen size, i.e., regulating

Vascular Sympathetic Neurotransmission and Endothelial Dysfunction

http://dx.doi.org/10.5772/intechopen.72442

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Nowadays, it is well established that SNS contributes to the modulation of vascular function and that this relationship is a key factor in the development of cardiovascular diseases. Several factors, such as the renin-angiotensin system, NO, ROS and endothelin, influence this modulation at central and peripheral level [43–45]. Moreover, endothelial function also seems to be regulated by SNS, mainly in the control of vascular tone. Additionally, endothelial dysfunction as well as increase in sympathetic activity has been associated to cardiovascular risk factors and disease. For example, in studies carried out in healthy subjects, an increase in sympathetic activity was associated with a decrease in endothelial function [46] Moreover, in humans, stiffness of large artery was also associated with an increased activity of SNS [47]. On the other hand, large artery stiffness can interfere with autonomic regulation by impairing carotid baroreflex sensitivity [48].

The influence of endothelium in noradrenaline release has also been previously demonstrated [49, 50]. This conclusion was obtained not only in arteries without endothelium but also in a model of endothelial dysfunction (i.e, essential hypertensive arteries), which is shown in Table 1. This type of information can be obtained from experiments where synapse events are mimicked allowing the evaluation of putative players able to alter neurotransmitter release from the nerves. Indeed, in such experiments, the use of selective pharmacological tools, such as agonists/antagonists of receptors or of activators/inhibitors of proteins or enzymes, can reveal their respective role in the neurotransmission dynamic. For instance, in experiments where rat vascular tissues,

by liquid scintillation spectrometry. In addition, by altering the receptors or proteins activated (with pharmacological tools), it is possible to evaluate the activity/role of a specific player in neurotransmitter release (please see previous articles from our group where the methodology is described in detail [49, 51, 52]). For example, in Table 1, data refer to tissues that were stimulated twice at 30-min interval: outflow (bn) refers to the 5-min period immediately before each stimulation period. The electrically evoked tritium overflow (Sn) was calculated by subtracting the estimated basal outflow from total outflow observed during and in the 25-min period subsequent

animal models have been used: spontaneously hypertensive rats (SHR), a well-established model of essential hypertension [53, 54], and the respective controls, the Wistar Kyoto (WKY) rats. Moreover, in WKY animals, some arteries were endothelium denuded. The influence of these

The results in this table show that the outflow observed in the endothelium-denuded vascular tissue is lower than that obtained in intact tissue. Also, the S2 values obtained in the endothelium-denuded arteries are altered, with values higher than those observed in intact tissues. These data reveal the importance of a healthy endothelium to the sympathetic neurotransmission homeostasis, once it seems to present a transsynaptic influence mediated by endothelium. In pathological conditions, this influence can be impaired augmenting the

conditions on the release of S1 was evaluated, and the results are presented in Table 1.

H]-noradrenaline, are electrically stimulated (5 Hz, 100 pulses, 1 ms, 50 mA),

H content at the onset of stimulation. Two

H is induced (which mimics a physiological depolarization) and can be measured

vascular tone and, therefore, affecting blood pressure.

to S1 and expressed as a percentage of the tissue <sup>3</sup>

amount of noradrenaline release and causing vasoconstriction.

preincubated with [3

the release of <sup>3</sup>

Another important factor influencing NO production relies on the presence of reactive oxygen/ nitrogen species (ROS/RNS). Indeed, several enzymes from endothelium can produce superoxide anions such as nicotinamide adenine dinucleotide phosphate oxidase (NOX), xanthine oxidase (XO), cyclooxygenases (COX) and also eNOS but only when there is a deficient supply of substrate or of the cofactor BH4. Under pathophysiological conditions, superoxide anions scavenge NO resulting in the formation of peroxynitrite, reducing considerably the bioavailability of NO. Moreover, ROS can inactivate eNOS through S-glutathionylation. Taken together, these may explain why oxidative stress is often associated with endothelial dysfunction.

Moreover, intake of a number of natural products, such as flavonoids and other polyphenols, favors endothelium-dependent dilations and protects endothelium from dysfunction through increased production of NO. The protective effects of polyphenols against endothelial dysfunction involve increased production of NO in response to endothelium-derived vasodilators resulting from: facilitation of the effects of NO on the vascular smooth muscle cells, increased levels of BH4, calcium-independent phosphorylation of eNOS, antioxidant properties preventing the uncoupling of eNOS, activation of estrogen receptors and upregulation of AMP-activated protein kinase (AMPK) and of NAD(+)-dependent deacetylase (SIRT1) [28–31].

#### 2.2. Influence of NO on another EDRF

Besides its direct role as a vasodilator, NO also modulates the release of other endotheliumderived mediators. Thus, in a number of larger arteries, endothelium-derived hyperpolarization (EDH)-mediated dilations become prominent only when the synthesis of NO is inhibited [32, 33]. Hence, EDH is able to take over, at least temporarily, in the case of 'classical' endothelial dysfunction associated with a loss of NO synthesis, demonstrating strong compensatory efficiency of EDH-mediated responses. Intriguingly, exogenous NO attenuates EDH-mediated responses in coronary arteries in vitro [34] and in coronary circulation in vivo [35, 36]. Moreover, NO has been shown to exert a negative feedback effect on endothelium-dependent dilation through cGMP-mediated desensitization in isolated coronary arteries [32]. Indeed, clinical studies show that chronic therapy with nitrate, used as a NO donor, in patients with ischemic heart disease does not yield a benefit on mortality [37, 38], confirming the importance of the physiological balance between NO and EDH. Moreover, the amount of NO formed in the endothelial cells controls the release of vasoconstrictor prostanoids [39, 40].
