4.1. NO and Superoxide Anion
Both NO and O2- are significant mediators of inflammation. Activated macrophages are known to release both NO and O2-. It is generally believed that the interaction of these radicals produces the even more cytotoxic peroxynitrite [
222,
223]. However, there is evidence suggesting that co-incubation of cells with NO and O2- results in a cross-protective effect, whereas separately both radicals cause apoptosis or necrosis [
224,
225]. It is thought that in this case, NO acts as a scavenger of O2-, neutralizing its negative effects. Probably, the protective effect requires a balanced presence of NO and O2- and a certain redox state of the cell, as it is necessary to neutralize the formation ONOO-, which is very likely in this situation [
226]. Under normal physiological conditions, a balance between superoxide and nitric oxide exists in vivo. NO and superoxide react together at a diffusion-controlled rate to form peroxynitrite (ONOO-), which causes cellular damage by oxidizing many biological molecules. Additionally, ONOO- is involved in the inactivation of Mn and Fe superoxide dismutase [
227,
228]. NO can protect cells from cytotoxicity, ROS-mediated by removing superoxide anions, which are involved in toxicity through the formation of hydrogen peroxide or hydroxyl radicals [
229]. Nitric oxide has been shown to inhibit the formation of superoxide anions. The mechanism of this inhibition is thought to be due to the inactivation of nicotinamide adenine dinucleotide phosphate oxidase because of the scavenging action of NO on superoxide [
230]. Inhibition of caspases. Since cysteine is present in the active center of caspases, and reactive nitrogen species can nitrosylate SH-groups, the initial explanation for the suppression of the caspase cascade by nitric oxide was through such nitrosylation of functionally important Cys showed not only suppression of active caspases by nitric oxide but also interruption of caspase activation itself. The proteolytic activation of caspases 3 and 8 was found to be effectively inhibited by both endogenous and exogenous NO, and part of this inhibition was unrelated to S-nitrosylation [
231,
232].
4.2. NO and Arterial Hypertension
In studies on rats with spontaneous hypertension, it has been found that the central component of NO-ergic regulation of blood pressure involves neurons located in various regions of the brain, including the hypothalamus (such as the paraventricular and supraoptic nuclei, as well as the median eminence) and the medulla oblongata (including the nucleus of the solitary tract, dorsal nucleus, and ambiguous nucleus). Additionally, the peripheral component comprises NO-producing vascular endothelial cells and neurons in the adrenal medulla.
The development of arterial hypertension is accompanied by specific changes in the activity of NO-ergic neurons in the brain involved in blood pressure regulation. These changes include a decrease in the number of neurons positive for neuronal nitric oxide synthase (NOS) in the small cell zone of the paraventricular nucleus, fibers of the median eminence of the hypothalamus, and neurons of the nucleus of the solitary tract. Conversely, there is an increase in the number and activity of NOS-positive neurons in the endocrine nuclei of the hypothalamus, as well as the dorsal and ambiguous nuclei of the medulla oblongata. The systemic increase in blood pressure in spontaneous hypertension leads to the inhibition of NO-producing function in the endothelium of both muscular and elastic vessels. Additionally, the change in NO-ergic activity in adrenal medullary neurons exhibits a dynamic character. Our studies also revealed a depression of NO formation alongside a decrease in total nitric oxide synthase (NOS) activity, both in mitochondria and in the cytosol of the myocardium in all groups of SHR [
116].
We observed a significant increase in the expression of inducible nitric oxide synthase (iNOS) in myocardial mitochondria of SHR rats compared to normotensive animals. The discoordination between the activity of total NOS in mitochondria and the formation of stable NO metabolites in the myocardium under conditions of experimental atherosclerosis alongside arterial hypertension, in our opinion, is associated with a surge of “parasitic” reactions. These reactions occur when NOS produces not only NO but also its cytotoxic derivatives, such as peroxynitrite and the nitrosonium ion, etc. Such reactions may occur in conditions of L-arginine deficiency, antioxidant deficiency, mitochondrial dysfunction, increased iNOS expression, and under the influence of proinflammatory factors [
233]. Our assumption is confirmed by the detection of increased content of the nitrosative stress marker nitrotyrosine against the background of increased iNOS expression in the mitochondrial fraction of SHR heart homogenate. Additionally, in the myocardial cytosol of SHR rats, we observed a low level of stable NO metabolites (2.4-1.6 times lower) compared to normotensive rats, alongside inhibition of endothelial nitric oxide synthase (eNOS) activity. Analyzing the obtained results of studies on the NO system parameters and reduced intermediates of the thiol-disulfide system, we can conclude that in SHR rats with the most pronounced shifts of the myocardial thiol-disulfide system (including deficit of reduced equivalents, increased oxidation of intermediates, and deprivation of glutathione reductase activity), there were significant changes in the neurochemical profile of NO. It transitioned from a molecular messenger to an agent of nitrosative stress [
138]. We have demonstrated that arterial hypertension is accompanied by the inhibition of NOS activity and NO deficiency. This deficiency, combined with the corresponding redox status of mitochondria, leads to protective effects that increase the cell’s resistance to adverse effects. In this context, the expression of inducible NOS increases in mitochondria, particularly when arterial hypertension is combined with diabetes and atherosclerosis. This increased expression has a compensatory value aimed at reducing blood pressure. However, under conditions of deficiency of reduced equivalents in the thiol-disulfide system of cardiac mitochondria, inducible NOS appears as an initiator of nitrosative stress. In this regard, it is important to determine the factor that determines whether NO exhibits cytoprotective or cytotoxic properties at a certain stage of the molecular-biochemical cascade. The thiol-disulfide system seems to play a special role in the development of mechanisms underlying NO cytotoxicity and target organ damage. Intermediates of the thiol-disulfide system possess transport properties with respect to NO, thereby increasing its bioavailability. Moreover, many thiols, such as glutathione, cysteine, and methionine, can significantly limit the cytotoxicity of NO and its derivatives, thus reducing the degree of damage to the target organ [
138,
183,
234].
4.3. NO and the Thiol-Disulfide System
The addition of CDNB (80 μmol), a selective inhibitor of glutathione-S-transferase and a glutathione conjugate, to the incubation medium of neurons resulted in the depletion of the glutathione linkage of thiol-disulfide system (TDS), as evidenced by the deficiency of reduced forms of glutathione due to the inhibition of glutathione reductase (GR) and glutathione-S-transferase (G-S-T) activity. This depletion leads to uncontrolled production of reactive oxygen species, nitrogen and nitrosative stress, as indicated by the observed increase in the level of nitrotyrosine in the neuronal suspension [
183,
235].
Thus, the increase of nitrotyrosine in neurons treated with CDNB was found to be more than 2.2-fold. Concurrently, there was a shift of the TDS towards oxidized thiols, as evidenced by a decrease in the level of reduced glutathione by 6.6-fold and an increase in its oxidized form by 3-fold. Accumulation of glutathione disulfide proceeded against the background of decreased activity of key enzymes of TDS: glutathione-S-transferase (G-S-T) decreased by 2.7-fold and glutathione reductase (GR) decreased by 2.3-fold compared to intact neurons at 60 minutes of incubation. It is important to note that the described pathophysiological changes led to an increase in cellular damage in the neuron suspension, as evidenced by a statistically significant (p≤0.05) increase in the number of degenerately changed neurons in the test with Hoechst 33342. A possible mechanism of cell damage in neurons incubated with CDNB, in our opinion, may involve disruption of the TDS and the formation of mitochondrial dysfunction. It has been established that the deficit of glutathione not only occurs in conditions of accumulation of active derivatives of NO but also the decrease of its reduced form can be a triggering factor for the development of nitrosative stress. Restored thiols are intracellular NO scavengers. Nitric oxide interacts with cysteine to form S-nitrosocysteine and with glutathione to form S-nitroglutathione. S-nitroglutathione serves as the main transport molecule for NO transfer [
138]. The deficiency of sulfhydryl (SH) groups inside the cell leads to a decrease in NO bioactivity and accumulation of reactive oxygen species (ROS). Additionally, uncontrolled growth of ROS leads to oxidation of the alkyl groups of the mitochondrial respiratory chain and inactivation of mitochondrial superoxide dismutase (SOD), further depleting the antioxidant system of the neuron. When modeling acute cerebral ischemia in Wistar rats, we observed marked differences in the concentrations of glutathione (GSH) and nitrotyrosine among the groups of animals with mild, moderate, and severe neurological disorders, as reported by P. McGraw. After conducting statistical analysis using Pearson’s coefficient, a negative correlation of -0.8289 was observed between neurological symptoms and reduced glutathione, while a positive correlation of 0.8272 was found with nitrotyrosine levels. The strong correlations suggest a clear dependence between the studied parameters. Consequently, it appears feasible to compute the ratio of nitrotyrosine level to reduced glutathione and utilize it for diagnosing neurological disorders. The calculated coefficients indicate that, under normal conditions, the ratio of nitrotyrosine to glutathione (Kn/GSH) is approximately 1.3.A mild degree of neurological deficit is characterized by a Kn/GSH close to 5.0; in severe neurological disorders, the Kn/GSH ratio increases substantially to about 138.5. Thus, the interaction within the “NO - reduced thiols” system plays a crucial role in the mechanisms of neurodegradation and endogenous neuroprotection, with its ratio determining the fate of neurons under conditions of central ischemia. The key factor of equilibrium in this system is the maintenance of the pool of reduced thiols and, especially, glutathione at a certain level. The reduced glutathione equivalents not only ensure the bioavailability of NO but also safeguard the proper functioning of the NO system within neurons, thereby preventing the formation of its neurotoxic derivatives [
183]. A statistically significant linear correlation between the severity of neurological deficit and the functionality of the conjugated “NO - reduced thiols” system was identified. These findings provide experimental support for utilizing the nitrotyrosine/reduced glutathione coefficient as a diagnostic parameter for assessing the severity of cerebral stroke in clinical biochemistry. Testing its effectiveness in treating patients with cerebral blood flow disorders appears promising.
4.4. NO and Cerebral Ischemia
Numerous studies have demonstrated the direct involvement of nitric oxide (NO) in the neuronal destruction process during ischemia. This has been observed when selective inhibitors of neuronal and inducible NOS isoforms are administered to animals with acute cerebral circulatory disorders (ACBD), as well as in experiments involving animals with a deficiency in the gene encoding iNOS. Data also indicate an elevation in NO concentration in the brains of animals experiencing both focal and global ischemia [
138]. The concentration of NO begins to rise within the first minutes of ischemia, peaking on the 1st to 3rd day. Measurement of NOS activity revealed a significant increase in enzyme activity both within the ischemic core and in the penumbra. However, this assessment did not differentiate between the various NOS isoforms. The involvement of NO in neuronal damage and death exhibits specificity determined by NOS isoforms and the type and stage of stroke development. In the initial phase of ischemia, expression of constitutive calcium-dependent NOS, triggered by transmitter autocoidosis. NO production during this phase is not directly responsible for neuronal death but contributes to indirect mechanisms such as the activation of phospholipases, augmentation of hydroxyl radical formation, and modulation of NMDA receptor activity. Subsequently, from 7-14 days in global ischemia and from 1-3 days in focal ischemia, during the delayed post-ischemic period, there is a surge in NO production involving inducible NOS activated within glia, macrophages, and neutrophils [
138]. The delayed induction of inducible NOS expression correlates with the subsequent activation of astro- and microglia as well as inflammatory cells. In focal ischemia, these cells, known as NO producers, are localized within the penumbra, while in global ischemia, they are primarily found in structures most vulnerable to oxygen deficiency. Apart from NO synthases, nitrate/nitrite reductases in warm-blooded organisms serve as sources of NO, capable of reducing nitrate and nitrite. Gliocytes and thymocytes exhibit nitroreductase activity. Although xanthine oxidase has demonstrated the ability to convert nitrate and nitrite into NO, its role in neurodegeneration remains understudied. Currently, there is active research into the targets of nitric oxide and efforts to elucidate whether NO itself is sufficiently cytotoxic or if its derivatives are more active [
236].
It is well-established that NO within target cells forms active derivatives such as nitrosonium (NO+), nitroxyl (NO-), and peroxynitrite (ONOO-). Recent studies have further emphasized the role of NO and its transformation products, including peroxynitrite (ONOO-), nitrosonium ion (NO+), nitroxyl (NO-), and diazotrioxide (N2O3), as primary factors in inducing nitrosative stress [
183,
237]. This stress arises from both the direct interaction of NO with metals, such as heme iron in hemoglobin and myoglobin, iron-containing enzymes, and non-heme iron in iron-sulfur proteins and DNA, as well as copper and zinc in enzyme active centers. Additionally, the indirect interaction of NO+ through S-, N-, and O-nitrosation with thiol, phenolic, hydroxyl, and amino groups of proteins and DNA further contributes to nitrosative stress. Such interactions lead to receptor desensitization, inhibition of mitochondrial enzyme activity, and nucleic acid fragmentation. Consequently, nitric oxide (NO), which reversibly binds to the Fe3+ active center of catalase, significantly inhibits its function both during the initial period of ischemia and in the post-ischemic phase of focal cerebral ischemia. Excessive NO levels depress heme enzymes within the mitochondrial electron transport chain. In the post-ischemic period, elevated NO concentrations can interact with heme iron and paired thiol groups to form dinitrosyl iron complex (DNIC) [
183]. Unlike NO, DNIC serves as a potent nitrosylating agent, interacting with protein thiols, histidine, aspartate, glutamine, methionine, cysteine, and glutathione, forming N- and S-nitrosothiols. Under ischemic conditions, DNIC undergoes irreversible nitrosylation of iron-sulfur clusters in mitochondrial proteins (such as NADH-ubiquinone oxidoreductase, succinate-ubiquinone oxidoreductase, and aconitase), thereby contributing to mitochondrial dysfunction [
183]. Our research has demonstrated that DNIC significantly inhibits the activity of superoxide dismutase (SOD), as well as enzymes involved in regulating thiol-disulfide equilibrium within cells, including glutathione reductase, glutathione-S-transferase, and glutathione peroxidase in neuronal suspensions.
Under ischemic conditions, the inhibition of these enzymes leads to oxidative modification of low-molecular-weight thiols, resulting in the formation of homocysteine and subsequent impairment of NO transport. This impairment leads to the generation of cytotoxic derivatives of NO, which further exacerbate thiol oxidation. Neurons equipped with a sufficiently active thiol antioxidant system capable of regulating NO transport exhibit resistance to nitrosative stress, which represents the earliest neurodegradative mechanism under ischemic conditions. It is well-documented that within the initial minutes of brain ischemia, NO (whether macrophage-derived or exogenous) inhibits oxidative phosphorylation in the mitochondria of target cells through reversible binding to mitochondrial cytochrome-C oxidase. Suppression of electron transport in mitochondria leads to the generation of superoxide, resulting in the formation of ОNOО–. Subsequently, peroxynitrite synthesis occurs in cells with high activity of NO synthase and enzymes producing ROS (such as xanthine oxidase, NADH- oxidoreductase, cyclooxygenase, lipoxygenase, and electron transport chain enzymes). Recent studies have revealed that during the initial stages of ischemia, peroxynitrite levels can be mitigated by mitochondrial nitroreductase, which reconverts it back to NO using NADPH and NADH as cofactors. The targets of oxidative and nitrosative attacks by peroxynitrite encompass thiols, CO2, metalloproteins, nucleic acids, transmitters, and lipids [
183].
Peroxynitrite, being a relatively stable compound, undergoes rapid protonation to form its primary product, the nitrate anion, along with hydroxyl radicals and nitrogen dioxide, thereby determining its oxidative properties. Hence, during the initial stages of ischemia, peroxynitrite interacts with thiols via nitrosylation, leading to the formation of nitrosothiols. As the process progresses and lactate acidosis ensues, this interaction shifts towards oxidation, resulting in the formation of more persistent disulfides. These reactions significantly contribute to the mechanisms of neurodegradation by shifting the thiol disulfide system towards oxidized thiol compounds, thereby reducing the cell’s reductive potential. This oxidation process also disrupts gene expression by irreversibly oxidizing cysteine residues within redox-dependent domains and causing dissociation of the MAP kinase cascade. Moreover, peroxynitrite inhibits the activity of metabolic cycles involving methionine and cysteine, thereby impeding key enzymes regulating cysteine levels and promoting homocysteine formation. Additionally, peroxynitrite reacts with the metabolitotropic transmitter CO2 to form a potent nitrosylating agent, nitrosoperoxycarbonate. An essential mechanism of peroxynitrite’s neurotoxic action is its reaction with thiosin to form nitrotyrosine. Peroxynitrite significantly inhibits the activity of Cu-Zn-SOD and Mn-SOD by nitration of its 34th tyrosine residue and by binding to copper, altering its valence. Moreover, peroxynitrite serves as a specific agent that irreversibly depresses mitochondrial respiration during ischemia (
Figure 2). Direct interaction with the iron of active centers of key enzymes and nitrosylation of thiol, phenol, hydroxyl, and amino groups of the protein component of these enzymes by S-, N-, O-elements, results in their irreversible oxidation under heightened nitrosative stress. Suppression of mitochondrial respiration leads to a decline in mitochondrial charge, which can trigger the apoptotic process and, in the absence of glucose, necrosis [
238,
239,
240]. Evidence also suggests direct activation of the giant pore opening by nitric oxide, leading to the release of cytochrome C and triggering of the caspase cascade. These findings were obtained when mitochondria were exposed to cytotoxic derivatives of NO such as peroxynitrite and nitrosonium ion, whose mechanism is based on the modification of thiol proteins in the mitochondrial pore.
Nitric oxide (NO) and its derivatives can induce peroxidation of phospholipids. Consequently, cytotoxic derivatives of NO, along with hydroxyl radicals, trigger the opening of mitochondrial pores and the expression and release of proapoptotic proteins into the cytosol. This pore opening occurs due to the oxidation or nitrosylation of thiol groups within the cysteine-dependent portion of the mitochondrial inner membrane protein, specifically the ATP/ADP-antiporter, transforming it into a permeable nonspecific channel-pore. This transformation converts mitochondria from “power plants” into “furnaces” of oxidation substrates without ATP formation [
238,
239,
240]. Impaired tissue oxygenation, transmitter autocoidosis, disrupted calcium accumulation by mitochondria, and damage to the mitochondrial membrane by cytotoxic ROS and NO compounds further enhance pore opening, leading to the release of apoptogenic proteins from damaged mitochondria [
242]. The mitochondrial pore is a channel spanning both mitochondrial membranes and comprises three proteins: an adenine nucleotide translocator, a potential-dependent anion channel (porin), and a benzodiazepine receptor. When this complex binds to Ca2+, substances with small molecular weight can traverse the membrane pore. This results in a reduction in membrane potential and swelling of the matrix, ultimately compromising the integrity of the outer membrane and leading to the release of apoptotic proteins from the intermembrane space into the cytoplasm.
Nitrosylation of proteins by tyrosine residues, facilitated by ONOO-, can have significant functional consequences, as it suppresses tyrosine phosphorylation and disrupts certain signal transduction pathways within the cell [
183,
243]. The balance between NO and the thiol-disulfide system is a critical factor determining the subsequent fate of neurons under ischemic conditions, particularly the mode of cell death. During ischemic brain injury, nitrosative stress emerges early, leading to thiol nitrosation and altering the thiol-disulfide equilibrium of mitochondrial pore proteins. At this juncture, mitochondrial NOS assumes a protective role by modulating cell death, favoring a transition towards apoptosis. Subsequently, oxidative and carbonyl stress ensue, resulting in a significant shift in the thiol-disulfide equilibrium towards oxidized thiols. This leads to persistent mitochondrial dysfunction, depletion of cellular energy reserves, onset of autocoidosis, perturbation of genomic responses, and ultimately, cell death via necrosis [
244].
4.5. NO and Endothelial Dysfunction
The primary mechanism underlying endothelial dysfunction (ED) involves a reduction in the formation and bioavailability of nitric oxide (NO), accompanied by a concurrent increase in the level of superoxide ions and the production of active vasoconstrictors [
245,
246]. Consequently, ED manifests as an imbalance between mediators crucial for the optimal functioning of all endothelium-dependent processes under normal conditions [
247]. Concurrently, disruptions in the production, interaction, and breakdown of endothelial vasoactive factors are observed, alongside abnormal vascular reactivity and alterations in the structure and growth of blood vessels, which are indicative of vascular diseases [
248].
NO is synthesized from L-arginine under the influence of endothelial NO synthase (eNOS), a process involving the attachment of molecular oxygen to the terminal nitrogen atom of the guanidine group of L-arginine. The assessment of vascular wall integrity and the correction of ED in cardiovascular pathology represent one of the most promising fields of study, as they determine the likelihood of developing vascular diseases and their complications, thus contributing to the overall prognosis of the disease [
249].
Therefore, the pursuit of targeted interventions for Endothelial Dysfunction (ED) and the development of a new class of effective drugs - endothelium protectors - represent critical clinical and experimental endeavors. ED is a systemic pathology linked to compromised microstructure and secretory function of endothelium-dependent cells, resulting in reduced endothelium-dependent vasodilation, hypercoagulability, increased thrombosis, heightened vascular permeability, and lipoprotein migration into the vascular intima, as well as smooth muscle cell proliferation, and myocardial and vascular remodeling [
250,
251,
252]. The primary mechanism underlying the development of ED involves a reduction in the formation and bioavailability of NO, accompanied by the emergence of its cytotoxic forms amidst oxidative stress and a deficiency of reduced low molecular weight thiols [
98,
246]. Meanwhile, the primary causes of NO deficiency in endothelial cells may include a reduced content of its precursor, L-arginine, diminished expression or activity of endothelial nitric oxide synthase (eNOS), and a deficiency in NO synthesis cofactors, particularly tetrahydrobiopterin. Additionally, increased levels of endogenous eNOS inhibitors, such as asymmetric dimethylarginine and monomethyl-L-arginine, elevated formation of reactive oxygen species, notably superoxide anion, and the presence of low-density lipoproteins, especially their oxidized forms, contribute to NO depletion [
253,
254]. The molecular basis of vascular endothelial dysfunction remains complex and not entirely understood. However, the “eNOS - L-arginine - NO” system holds promise as a pivotal target for pharmacological correction of ED in the foreseeable future [
98,
255]. Numerous authors have highlighted the direct involvement of NO in cell death processes, including endothelial cells, under conditions such as ischemia, atherosclerosis, and alcohol intoxication. These findings were elucidated through the utilization of selective inhibitors targeting constitutive and inducible isoforms of nitric oxide synthases (NOS), alongside experiments conducted on animals with a deficiency in the gene encoding inducible NOS (iNOS). Investigations have demonstrated that NO transport occurs concomitantly with the formation of N2O3, subsequently leading to thiol nitrosylation. With the involvement of disulfide isomerase, NO is released [
235]. Additionally, there exists a mechanism for NO release from S-nitrosoglutathione, facilitated by glutamyl transpeptidase, resulting in the formation of S-nitrosocysteinylglycine, which then liberates NO. Cystine plays a crucial role in the transportation of S-nitrosoglutathione, wherein it is reduced to cysteine. The latter, upon reacting with S-nitrosoglutathione, forms S-cysteine, thereby participating in the rapid conduction of neurons and facilitating the neuron’s adaptive responses to ischemia. These reactions are regulated by glutathione reductase and glutathione transferase.
Under ischemic conditions, inhibition of these enzymes leads to oxidative modification of low molecular weight thiols, homocysteine formation, and subsequent impairment of NO transport, resulting in the generation of cytotoxic NO derivatives that exacerbate thiol oxidation [
256,
257,
258]. Given the current absence of specific drugs for correcting endothelial dysfunction (ED), insights into the effects of cardiovascular drugs from various pharmacological groups on endothelial functional characteristics hold significant value. A comprehensive approach to treating ED in conditions such as chronic cerebral ischemia, arterial hypertension, alcoholic myocardial and cerebral damage, and chronic heart failure may offer substantial practical benefits [
259]. This approach involves combining fundamental cardioprotective and neuroprotective therapies with medications that optimize energy metabolism, thereby mitigating the adverse effects of oxidative and nitrosative stress on vascular endothelium and promoting nitric oxide formation. It is conceivable that the future lies with drugs possessing not only cardioprotective or neuroprotective effects but also indirect positive impacts on endothelial function. A particularly promising avenue is the comprehensive treatment of ED in cardiovascular pathology, where reperfusion, antithrombotic, and cardio- or neuroprotective therapies are integrated with medications targeting endothelial dysfunction correction [
260].
In light of the above, it has become pertinent to investigate the endothelioprotective properties of drugs exhibiting diverse pathogenetic mechanisms of action. These drugs are known to enhance metabolism, possess antioxidant properties, serve as natural nitric oxide (NO) donors, and activate the NO synthase enzyme. They also contain “essential” phospholipids and affinity-purified antibodies to endothelial NO synthase, offering promising avenues for research in experimental models of cerebrovascular pathology [
261].
Numerous drugs with distinct mechanisms of action exert varying degrees of influence on vascular endothelial function. For instance, nitrates replenish endogenous NO deficiency, ACE inhibitors not only reduce angiotensin-II (AT-II) synthesis but also prevent kinin degradation. Statins bolster endothelial cell barrier function against oxidized LDL, while calcium antagonists curb AT-II and endothelin activity in vascular smooth muscle, thereby amplifying NO’s vasodilatory effects. Angiotensin receptor blockers obstruct AT-II receptors, fostering NO accumulation, while endothelin-converting enzyme inhibitors and endothelin-1 receptor antagonists impede peptide activity [
262].
Of particular interest are the “specific” effects directed at enhancing NO synthesis, such as replacement therapy involving L-arginine (the substrate eNOS) and tetrahydrobiopterin (eNOS cofactor), crucial for determining the enzyme’s activity [
263].
4.7. Exogenous Nitric Oxide
Exogenous NO donors are of considerable help in studying the effect of NO on cells. These substances are widely used nowadays to create model systems in vitro, on which it is possible to study the effects of NO influence on cultures of different cells, or on separate compartments of cells (isolated mitochondria, nuclei) [
266]. These models are very popular because they greatly simplify the system of interaction between NO and cells in the body. Since nitric oxide here comes from outside, the system appears to be independent of NO synthases and their regulation, which means that the results of NO action are easier to interpret. It is evident that in this context, the effects of other signaling substances potentially accompanying biogenic NO are largely eliminated. For instance, NOS can synthesize O2- under certain conditions, and the pathways activating NOS may also trigger the production of various additional mediators [
117,
267,
268,
269]. Exogenous donors, in contrast to L-Arg, incorporate NO within the structure of the molecule, facilitating the release of this molecule in its pure form [
180].
Chemical classifications of NO donors typically include the following groups:
- -
Nitrates (such as nitroglycerin, sodium nitroprusside, nitrosorbide, commonly used in the clinic) [
270,
271,
272,
273,
274],
- -
Nitrites (amyl nitrite, NaNO2),
- -
Nitrosothiols and substances that form various complexes with NO: S-nitrosoglutathione (GSNO), S-nitroso-N-acetylpenicillamine (SNAP), diethylamine-NO (DEA-NO).
NO donors can be classified based on their mechanism of action into those that spontaneously release NO (non-enzymatic) and those requiring enzymatic interaction to release nitric oxide. Currently, there are no “ideal” NO donors for research purposes. Firstly, NO donors vary in their efficiency of NO release and their ability to affect cells to different extents. Secondly, these substances may serve as sources of side compounds, some of which can be toxic (such as cyanide released by nitroprusside) [
271,
275,
276].
Among the potential NO donors and drugs, there’s a promising original molecule called bromide 1-(β-phenylethyl)-4-amino-1,2,4-triazolium (Hypertril), a derivative of 1,2,2,4-triazole. Hypertril exhibits NO-mimetic properties, particularly when β1-receptor blockade is present. It enhances the expression and activity of endothelial NO-synthase, thus addressing NO deficiency. In the dose range of 7.5-20 mg/kg, “Hypertril” shows promising effects in mitigating disorders in the L-arginine-NO-synthase-NO system in spontaneous arterial hypertension. It achieves this by increasing NO production through enhanced expression of endothelial NOS, thereby reducing manifestations of nitrosative stress in the myocardium. Additionally, “Hypertril” reduces the expression of inducible NOS, leading to dose-dependent increases in cardiomyocyte nuclei density and cardiomyocyte area. Furthermore, it significantly increases RNA content in both nuclei and cytoplasm of cardiocytes, along with an increase in the nuclear-cytoplasmic index, indicative of decreased myocardial hypertrophy. Importantly, “Hypertril” also normalizes blood pressure [
233,
277]. Moreover, administration of “Hypertril” to animals with chronic heart failure (CHF) results in the prolongation of the depolarization phase (QRS complex) and repolarization phase of ventricles (T wave), as well as electrical diastole (TR interval). These findings suggest a crucial property of the drug in CHF therapy, specifically its ability to prevent the development of diastolic dysfunction [
278]. The obtained results of experimental studies are the basis for authorization of the first phase of clinical trials of the new drug “Hypertril” as an antianginal and antihypertensive agent.