Neurophysiology, Vol. 43, No. 1, July, 2011 (Original Vol. 43, No. 1, January-February, 2011)
REVIEWS
Hydrogen Sulfide as a Regulator of Systemic Functions in Vertebrates A. A. Varaksin1 and E. V. Puschina1 Neirofiziologiya/Neurophysiology, Vol. 43, No. 1, pp. 73-84, January-February, 2011. Received January 23, 2011. This review deals with new published data and also with our own findings related to the physiological and pathological effects of hydrogen sulfide (H2S), a gasotransmitter, which has recently attracted intense attention. In the organism, hydrogen sulfide is synthesized from cysteine with the involvement of pyridoxal5′-phosphate-dependent enzymes, cystathionine-β-synthase (CBS) or cystathionine-γ-lyase (CSE). Under the action of H 2S, ATP-dependent potassium channels (K АТР) are activated in vascular smooth muscle cells, neurons, cardiomyocytes, and pancreatic β-cells; this explains the important and extensive role of this agent in the control of vascular tone and contractile activity of cardiomyocytes, in the processes of neurotransmission, and in secretion of insulin. Possible roles of Н 2S-producing systems in the pathogenesis of arterial and pulmonary hypertension, Alzheimer’s disease, and hepatocirrhosis are discussed. A description of some aspects of the action of H2S “beyond the nervous system” should, probably, also attract the interest of readers, allowing them to form a clearer view of the role of this gasotransmitter in the control of systemic functions.
Keywords: hydrogen sulfide, gasotransmitters, visceral systems.
INTRODUCTION
and endocrine interactions, examination of cell-tocell communications that are provided by gaseous mediators was begun relatively recently. Nitric oxide was the first gasotransmitter identified in pilot studies on isolated blood vessels [2]. Hydrogen sulfide was firstly described as a factor involved in modulation of neuronal activity [3]; later on, its vasorelaxing properties were demonstrated [4]. Despite the fact that investigation of the physiology of gasotransmitters began comparatively recently, these compounds are at present implicitly recognized as biologically highly significant and clinically important messengers [5].
More and more data emerge at present in favor of the fact that pathways of signal transmission in the control of homeostasis in higher animals (vertebrates) are numerous and diverse. From this aspect, nitric monoxide (oxide, NO), carbon monoxide (СО), and hydrogen sulfide (H 2S) play important roles. The gaseous (when they are out of solution) mediators are qualified as gasotransmitters [1], and, at the same time, they may be highly toxic agents. A number of criteria for the corresponding gasotransmitters have been formulated. In the free state, gasotransmitters exist as gases, their molecules freely penetrate biological membranes, the above compounds are produced endogenously, their synthesis is controlled by enzymes, they (in physiological concentrations) fulfill definite functions, and they have specific cellular and molecular targets [1]. As compared with traditional studies of the processes of signalization during neuro-
METABOLISM OF H 2S IN THE ORGANISM A sulfur-containing amino acid, L-cysteine, obtained from food or synthesized from L-methionine by means of the so-called transsulfurization with formation of homocysteine as an intermediate product, is the substrate for synthesis of endogenous H 2S. There are two main pathways of catabolism of cysteine [6]. The first pathway is oxidation of SH groups by cysteine dioxygenase into cysteine sulfinate, which can be decarboxylated and transformed into hypotaurine or
Zhirmunskii Institute of Marine Biology, Far East Branch of the Russian Academy of Sciences, Vladivostok, Russia. Correspondence should be addressed to A. A. Varaksin (e-mail:
[email protected]) or E. V. Puschina (e-mail:
[email protected]). 1
62 0090-2977/11/4301-0062 © 2011 Springer Science+Business Media, Inc.
H 2S as a Regulator of Systemic Functions converted into pyruvate and sulfite. Then, the latter is oxidized by sulfite oxidase to sulfate. The second pathway is related to elimination of the sulfur atom from the cysteine molecule without oxidation of this atom, with its subsequent involvement in the synthesis of H 2S. These processes are catalyzed by cystathionine-β-synthase (CBS, EC 4.2.1.22) and cystathionine-γ-lyase (CSE, EC 4.4.1.1). Both enzymes depend on pyridoxal 5′-phosphate (vitamin В 6), but the mechanisms underlying H 2S synthesis with their participation differ from each other. CSE catalyzes transformation of cysteine into thiocysteine, pyruvate, and ammonium; then, thiocysteine is, in a non-enzymatic way, decomposed into cysteine and H 2S [7]. The main mechanism of formation of H 2S with the involvement of CBS is related to condensation of homocysteine and cysteine; this event results in production of cystathionine and during this reaction, H 2S is released. CBS and CSE are extensively distributed in tissues; however, CBS is the main source of H 2S in the CNS, while CSE is the main H 2S-producing enzyme in the cardiovascular system (CVS). In some organs, such as the liver and kidneys, both these enzymes are involved in the synthesis of H 2S. In the brain of vertebrates, the activity of CBS is rapidly intensified upon the action of electrical stimulation and glutamate [8]. S-Adenosylmethionine, an intermediate product of metabolism of methionine and a basic donor of methyl groups, is an allosteric activator of CBS. It seems probable that sex steroid hormones are involved in the control of H 2S production in the brain. The activity of CBS and the level of H 2S are higher in males than those in females; castration of mice leads to a drop in H 2S production [8, 9]. Catabolism of H 2S remains little studied, and most data in this aspect were obtained with the use of exogenous H 2S. Hydrogen sulfide is rapidly oxidized in the mitochondria to thiosulfate; then, the latter is converted into sulfite and sulfate. Oxidation of H 2S to thiosulfate is, probably, related to the transport of electrons in the process of mitochondrial respiration [10]. Transformation of thiosulfate into sulfite is catalyzed by rhodanese (EC 2.8.1.1), which carries sulfur from thiosulfate to cyanide or other acceptors. Sulfite, which is produced during this reaction, is rapidly oxidized to sulfate by sulfite oxidase. Therefore, sulfate is the main final product of metabolism of H 2S in the organism. Thiosulfate in the urine is considered a specific marker of H 2S, since it is known that a greater portion of sulfate in urine is produced during oxidation by cysteine dioxygenase
63 [11]. The other metabolic pathway of H 2S is based on its methylation by thiol-S-methyltransferase with production of methyl mercaptan and dimethyl sulfide [12]. This reaction is realized mostly in the cytosol. In addition, H 2S can be bound with methemoglobin with the formation of sulfhemoglobin.
H 2S IN THE NERVOUS SYSTEM It was found that homogenates of the brain are capable of producing H 2S under in vitro conditions [3]. In the hippocampus and cerebellum of rats and fishes, high levels of CBS expression were found. In our own studies, we found that, in the masu (sima) salmon (Oncorhynchus masou), CBS labels neurons of the reticular formation (Fig. 1A), vessels (B), neurons of the ventral spinal column (D), and climbing fibers in the cerebellum (F). It has been demonstrated that endogenous H 2S increases the sensitivity of NMDA receptors to glutamate [13]. Such activation of these receptors promotes long-term potentiation (LTP) in the hippocampus. The mechanisms by which H 2S stimulates the function of NMDA receptors remain unknown. It is hypothesized that H 2S modulates the redox potential of thiol groups in extracellular domains of NMDA receptors and activates them at the expense of its own reducing properties. Intracellularly, H 2S modulates the response mediated by NMDA receptors via intensification of the production of cAMP [14]. Exogenous H 2S increases the production of cAMP in the primary culture of cerebral neurons, in the cerebellum, and also in glial cells [13]. In the course of development of LTP, cAMP activates cAMP-dependent protein kinase [15]. It has been shown that H 2S is capable of controlling the activity of GABA В receptors localized pre- and postsynaptically [16]. Stimulation of postsynaptic GABA receptors induces long-term depression of postsynaptic transmission. This is related to a rise in the level of K + and is an important factor in fine tuning of inhibitory neurotransmission. In neurons of the dorsal raphe nucleus and hippocampal CA1 area, H 2S participates in induction of hyperpolarization by increasing the influx of K + via ATP-dependent potassium channels (K ATP). H 2S is also involved in the control of blood pressure by its action on K ATP channels in hypothalamic neurons [17]. In presynaptic sites, GABA В receptors regulate the release of GABA and L-glutamate via the blockade of voltage-dependent calcium channels. Thus, H 2S that activates GABA B receptors is involved, to a
A. A. Varaksin and E. V. Puschina
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Fig. 1. Immunohistochemical detection of cystathionine-β-synthase (CBS) in the CNS in cerebral vessels of the masu salmon (Oncorhynchus masou) and carp (Cyprinus carpio). A) CBS-immunopositive cells (indicated by filled arrows) and capillary (indicated by open arrows) in the magnocellular reticular formation of the medulla of the masu salmon; B) CBS-immunopositive sites in the cerebral vessels of the masu salmon (indicated by open arrows); C) CBS-immunopositive gliocytes in the periventricular region of the carp brain (cells are indicated by small arrows); D) CBS-immunopositive gliocytes of the carp at a greater magnification; E) CBS-immunopositive neurons of the masu salmon spinal cord (indicated by filled arrows); and F) CBS-immunopositive climbing fibers in the cortex of the salmon cerebellum (indicated by small arrows). PCs) Purkinje cells; GLC) granular layer of the cerebellum. Results of our own studies are presented in this figure.
H 2S as a Regulator of Systemic Functions considerable extent, in the maintenance of equilibrium between the processes of excitation and inhibition in the brain. Some authors believe that H 2S is the main modulator of calcium homeostasis in neurons since it activates calcium influx into the cytosol through L-type calcium channels [18]. In addition to participation in the processes of neuromodulation, H 2S is involved in protection of neurons from oxidative stress. It is known that reduced glutathione plays a role of one of the main antioxidant protectors in the brain. This protective function is realized via the intense uptake of free radicals and other reactive groups and also via the removal of hydrogen peroxide and lipid peroxides; these processes prevent oxidation of other biomolecules [19]. Under in vitro conditions, H 2S manifests neuroprotective properties typical of those of glutamate. Under experimental conditions, introduction of a donor of H 2S, NaHS, leads to an increase in the amount of glutathione, which results from an increase in the activity of γ-glutamyl cysteine synthase and the rate of cysteine transport. It was demonstrated that a rise in the amount of glutathione protects neurons from programmed cellular death (apoptosis) under conditions of oxidative stress triggered by an abnormal increase in the L-glutamate concentration. H 2S plays an important neuromodulatory role in glial cells. Astrocytes are necessary for the maintenance of normal physiological properties of neurons, since these glial elements are capable of controlling acid-base homeostasis and of absorbing different neurotransmitters, including L-glutamate [20]. As compared with neurons that transmit signals mostly via generation of action potentials (APs), astrocytes and other glial cells “communicate” with each other by means of calcium signalization. Modulation of the states of neurons and vessels is, to a considerable extent, based on the above phenomenon [20, 21]. It was demonstrated that exogenous H 2S induces calcium waves in the primary culture of astrocytes and in hippocampal slices. Microglial cells, as is known, can be activated by numerous external influences [22]. It is hypothesized that changes in the state of microglia are significant pathogenetic factors in the development of Alzheimer’s disease [23] and Parkinson’s disease [24]. Exogenous H 2S reversibly increases the amount of intracellular calcium ([Ca 2+] i) in microglial cells at the expense of calcium release from intracellular stores and Ca 2+ entry into the cell via the plasma membrane [25]. Since H 2S, similarly to other gasotransmitters, is capable of rapid diffusing, it is supposed that it can play an
65 important role in activation of vast populations of microgliocytes, increasing the intracellular calcium level in neighboring cells. It is known that inflammatory responses mediated by activation of immune, neuronal, and glial cells are formed in many neuropathological states. Activated glial cells that release various pro- and anti-inflammatory chemokines are responsible for initiation of the activity of immune cells, their infiltration, and coordination of their activity in the brain [26]. Activated microglial cells produce and release proinflammatory factors, such as NO, tumor necrosis factor (TNF), and interleukin-1β, which are, later on, capable of increasing the level of damage of the tissues and inducing the cell death [23].
INVOLVEMENT OF H 2S IN POSTNATAL MORPHOGENESIS OF THE CNS In the periventricular region of the medulla and ventral and lateral zones of the cerebellum of the carp (Cyprinus carpio), we found cells devoid of processes (obviously, glial units), which were highly immunoreactive with respect to the most important enzyme involved in the synthesis of H 2S, CВS (Fig. 1C, D). CBS-immunoreactive cells in the CNS of the carp are localized in the periventricular zone corresponding to the region of primary proliferation. The sizes of these cells, their localization in the brain, and interaction with H 2S-producing neurons indicate that H 2S-producing glia is present in the periventricular cerebral zone [27]. Since at present it has been found that some neurotransmitters localized in precursor cells of the periventricular cerebral region can fulfill the function of regulators of the process of nonembryonic neurogenesis (adult neurogenesis) [28], the assumption that H 2S, similarly to NO, can also play the role of a regulator of postnatal neurogenesis is rather appropriate [27]. Recent studies showed that neurons, shortly after their formation from precursor cells and long before the formation of neuron-to-neuron connections and initiation of synaptogenesis, begin to secrete specific signal molecules [29]. Such molecules can be neuropeptides, enzymes of the synthesis of “classic” neurotransmitters and NO, and transmembrane and vesicular transporters. A great part of signal molecules participate in autocrine and paracrine regulation of differentiation of target neurons playing the role of morphogenetic and transcription factors [29]. It was shown that NO plays the role of
66 a signal agent controlling the processes of directed growth of the axons and dendrites and also migration of differentiating neurons [30]. In mammals, the duration of action of signal molecules is limited by certain periods of ontogenesis, during which longterm morphogenetic effects on the differentiation of target neurons and expression of a specific phenotype are realized [31]. In fishes, the processes of postnatal neurogenesis and gliogenesis in the periventricular region are realized also in the adult state [32]. Recent studies demonstrated the presence of NADPH-positive and NOS-immunoreactive cells in the periventricular cerebral regions of the Pacific masu salmon (Oncorhynchus masou). In Cyprinoidei, the activity of NADPH/NOS is absent in the periventricular region; apparently, H 2S can be such a signal molecule in this cerebral region of the carp [33].
H 2S AND REPRODUCTION Using immunohistochemical techniques, it was found that the distributions of CBS and CSE in rat testicles are dissimilar. СВS was revealed mostly in Leydig cells and also in Sertoli cells and gametes within different stages of development, while CSE was observed in Sertoli cells and spermatogonia [34]. In murine ovaries, CBS is expressed in follicular cells at all stages of the development of follicles. In late antrum-containing follicles, CBS-immunoreactive protein is observed in granulosa cells adjacent to the antrum and also in the cumulus cells localized around the oocyte where the expression of CBS was not observed [35]. It is hypothesized that a high level of CBS expression in cumulus cells intensifies the synthesis of glutathione that protects these cellular elements from the action of free radicals. A decrease in CBS expression during in vitro culturing of oocytes that have finished growth causes a delay of maturation of such cells [36]. In the course of the process of fertilization, glutathione participates in decondensation of the nuclear material of spermatozoids, which is a necessary condition for the formation of the male pronucleus [37]. Similarly to NO, H 2S is involved in the control of erection. Intracavernous injection of NaHS results in a considerable increase in the size of the rat penis and also in a rise in the pressure inside the cavernous bodies, while propargyl glycine to a significant extent suppresses these processes [38]. Later on, it was shown that smooth muscles of the corpus cavernosum are capable of endogenously synthesizing H 2S, which
A. A. Varaksin and E. V. Puschina demonstrates its proerectal pharmacological effect. Inhibitors of CBS intensify, to a significant extent, noradrenaline-induced contraction of smooth muscles of the corpus cavernosum and did not influence appreciably NO-ergic relaxation in preparations of these muscles whose contraction was preliminarily caused by noradrenaline. It is supposed that the action of H 2S on the erectile function in mammals is realized with the involvement of cAMP [39]. The data obtained in studies of smooth muscles of the uterus of pregnant rats indicate that spontaneous contractions of the myometrium are weakened by the action of a donor of Н 2S, sodium hydrosulfide (NaHS).
H 2S AND REGULATION OF THE FUNCTIONS OF THE CVS CSE is the main H 2S-producing enzyme within the CVS. The data obtained in immunohistochemical studies and the use of the reverse polymerase chain reaction demonstrated that CSE is expressed in smooth muscle cells of the vessels and is not observed in endothelial cells [40]. A blocker of K ATP channels, glibenclamide, weakens the hypotensive action of H 2S in vivo and the vasodilating action in vitro [40]. According to the data of electrophysiological studies with the use of the patch-clamp technique, H 2S increases the K ATPmediated current and induces hyperpolarization in the isolated smooth muscle cells of the vessels [41, 42]. Inhibitors of CSE decrease the conductivity of K ATP channels, which serves as proof of the involvement of endogenous H 2S in the maintenance of functioning of potassium channels at the basic level. Therefore, H 2S is responsible for relaxation of the walls of blood vessels resulting from opening of ATP-dependent potassium channels in smooth muscle cells of these vessels. In contrast to the direct action on smooth-muscle cells, H 2S-induced vasorelaxation, which is dependent on the endothelium, is not related to the effects on K ATP channels [41]. Nitric oxide also activates the K ATP channels, but this effect is mediated by cyclic guanosine monophosphate (cGMP) [43]. Therefore, it is obvious that H 2S is responsible for the unique mechanism underlying the action on the vessels, which is not typical of other well-known gasotransmitters (NO and CO). The formation of endogenous H 2S from cysteine is intensified under the action of testosterone. Under in vitro conditions, this hormone induces
H 2S as a Regulator of Systemic Functions dose-dependent vasodilation of the rat aorta. It is hypothesized that this action is related to modulation of the enzymatic activity of CSE [44]. It is interesting that the synthesis of H 2S in smooth muscle cells of the vessels and vasorelaxing properties of this agent are typical of all vertebrates examined at present, namely fishes, amphibia, reptiles, birds, and mammals. It is obvious that H 2S is an evolutionary older signal agent than NO; the vasorelaxing properties of the latter are firstly manifested only at the stage of evolution corresponding to amphibia [45, 46]. As is hypothesized, H 2S even in low doses can induce vasoconstriction, which is related to change in the level of endothelial NO. Under conditions of parallel action of donors of H 2S and NO, it was demonstrated that in this case the vasodilating effects of NO in vitro and its hypothensive action in vivo are suppressed. A donor of H 2S (NaHS) used in low concentrations modulates the NO-dependent effects of acetylcholine and histamine and does not influence the vasodilating action of isoprenaline [47]. Thus, H 2S within the CVS in general counteracts the effects of NO. It is supposed that H 2S participates in the control of hemodynamics influencing the baroreceptor reflexes. In the myocardium, CBS mRNA is expressed; thus, H 2S can be endogenously produced in the heart. Sodium hydrosulfide suppresses the contractile abilities of the cardiac muscle and decreases the heart rate under both in vitro and in vivo conditions. These effects are reduced but are not eliminated completely under the action of the inhibitor of K ATP channels glibenclamide [48]. Experimental modeling of myocardial infarction in rats showed that CBS-immunoreactive protein is revealed in the ischemic zone. Under hypoxic conditions, the number of dying smooth muscle cells in the ventricles of the heart increases in a progradient manner. Preliminary treatment of the myocardium with a solution of NaHS enhanced the viability of such cells, while propargyl glycine exerted an opposite effect. As Zhu et al. believe [49], endogenous H 2S possesses a clear cardioprotective effect in rats with experimental myocardial infarction.
H 2S IN THE DIGESTIVE SYSTEM CBS and CSE are expressed in the stomachic mucosa; endogenous H 2S is, obviously, a protective factor in the case of damage to this mucosa. Acetylsalicylic acid and nonsteroidal anti-inflammatory preparations decrease the expression of the CSE gene and suppression of production of H 2S in the stomachic mucosa.
67 Deceleration of blood flow in the stomachic vessels of rats, induced by acetylsalicylic acid and nonsteroid anti-inflammatory preparations, is eliminated after introduction of NaHS [50]. NaHS decreases adhesion of leucocytes to cells of the vascular endothelium and infiltration of the mucosa by leucocytes, normalizes increased expression of TNF-α, and intensifies the synthesis of prostaglandin E 2 [50]. It was shown that H 2S suppresses spontaneous or acetylcholine-induced contractions of the wall of the ileum in different animal species; this effect is removed by glibenclamide, a blocker of K ATP channels [4, 51, 52]. With the use of immunohistochemical techniques, CBS and CSE were found in the submucous and intermuscular neural plexuses of the colon in guinea pig and humans. Applications of NaHS or L-cysteine stimulate secretion of chlorides by tissues of the colon mucous in guinea pigs and humans. This effect was blocked under the actions of tetrodotoxin and a blocker of TRPV1 receptors, capsazepine, as well as after capsaicin-induced desensitization of the afferent nerves. Thus, H 2S is generated in the enteral nervous system and acts on sensory neural terminals possessing TRPV1 receptors, which results in intensification of secretory and motor functions of the intestine [53]. In the case of experimental colorectal distention, NaHS decreased the pain sensitivity in a dosedependent manner. This influence is, obviously, not related exclusively to relaxation of the muscles of the colon and rectum; the antinociceptive effect was, probably, due to the action of H 2S on the processes of neurotransmission. It was demonstrated that experimental colitis is accompanied by increased expression of CBS and CSE in the mucosa of the colon and also by increased expression of CSE in the spinal cord. The effect of NaHS appreciably decreases hyperalgesia in animals with colitis [52]. Hydrogen sulfide decreases the intensity of noradrenalin-induced vasoconstriction in the liver of both healthy rats and animals with experimentally induced cirrhosis [54]. It was demonstrated that CSE is expressed in hepatocytes and stellate cells of the liver. H 2S influences the isolated stellate cells, inducing relaxation of the walls of hepatic microvessels. Experimental cirrhosis induced by ligation of the bile duct or the action of carbon tetrachloride is related to a decrease in the CSE expression, drop in the H 2S production in homogenates of the hepatic tissue, and decrease in the H 2S concentration in the blood plasma [54]. The relaxing action of exogenous L-cysteine on hepatic stellate cells and hepatic microvessels in animals with experimentally induced cirrhosis is
68 weakened, as compared with the analogous effect in control rats [55]. In animals and humans with cirrhosis, the resistance of the hepatic vessels to the action of H 2S is enhanced. The published data allow one to hypothesize that a deficiency of H 2S can be one of the factors in the development of portal hypertension [56]. Introduction of NaHS decreases the bile flow and excretion of bicarbonates, while blocking the production of endogenous H 2S by propargyl glycine results in the opposite effect [57]. It is known that the colonic mucus is permanently subjected to the action of H 2S produced by commensal bacteria from sulfates in food. It is hypothesized that an excess of H 2S of a bacterial nature can cause different pathologies of the colon, including ulcerative colitis and colorectal cancer [58]. It was demonstrated that H 2S stimulates proliferation of cultured intestinal epithelial cells of rats [59]. The colonic mucus is capable of metabolizing H 2S (due to the high level of the expression of rhodanese in enterocytes); it can somewhat be adapted to the excess of this agent [48]. The expression of rhodanese in the intestine upon the action of H 2S is intensified; the level of this enzyme in the intestine during the development of ulcerative colitis or colorectal cancer is lower than in healthy organisms [61]. H 2S suppresses secretion of insulin by the pancreas. It is believed that K ATP channels in the membranes of pancreatic β-cells play a crucial role in the regulation of insulin secretion [62]. An increase in the amount of glucose results in accumulation of ATP in these cells, blocking of K ATP channels, depolarization of the plasma membrane, a rise in the calcium inflow, and intensification of insulin secretion. The actions of the adenoviruses containing the CSE gene and of exogenous H 2S on the cells of rat insulinoma INS-1E suppress the process of insulin secretion induced by an increase in the level of glucose. A decrease in the concentration of endogenous H 2S upon the action of propargyl glycine exerted an opposite effect [63]. Therefore, the available data allow us to believe that H 2S suppresses insulin secretion. It is known that the cysteine concentration in blood plasma and the expression of CBS and CSE in different tissues in patients with diabetes mellitus are increased [64]. Although the concentration of H 2S in blood plasma of rats suffering from diabetes remains unchanged, its intensified synthesis is observed in homogenates of tissues obtained from the pancreas and liver of such animals. The expression of CBS in the pancreas of diabetic animals is relatively enhanced [65].
A. A. Varaksin and E. V. Puschina ROLE OF H 2S IN PATHOLOGY OF DIFFERENT DISEASES Importance of H 2S for Pathologies of the Nervous System. It was shown that excretion of thiosulfate with urine in patients with Down’s syndrome is increased. Since thiosulfate is the final product of H 2S metabolism, it is hypothesized that the production of H 2S in patients with Down’s syndrome is intensified [66]. It seems probable that an excess of H 2S exerts a toxic effect on neurons at the expense of either suppression of the synthesis of cytochrome oxidase or intensification of activation of NMDA receptors; both these effects are among factors responsible for mental retardation in patients with trisomy 21 [67]. Influences of exogenous cysteine and NaHS lead to an increase in the volume of brain infarction induced by unilateral ligation of the medial cerebral artery, while introduction of inhibitors of CSE and CBS results in a decrease in this parameter. A high cysteine level related to the increased concentration of H 2S negatively influences the state of patients suffering from ischemic shock [68]. However, H 2S can also demonstrate protective properties with respect to neurons. In particular, H 2S protects, to a certain extent, neurons from the neurotoxic action of glutamate. Intensified production of glutamate is observed under conditions of cerebral ischemia, epileptic attacks, or traumas. The neurotoxic effect of an excess of this transmitter is usually manifested under conditions of long-term activation of the respective receptors. However, glutamate can also induce oxytosis of the neurons independently of the action on receptors of this transmitter. This mechanism of injury is based on suppression of cysteine transport in neurons by the system хс–. Extracellular glutamate blocks such type of metabolism, which results in the insufficiency of intracellular cysteine and suppression of the synthesis of glutathione; such a situation makes the cells more sensitive to oxidative stress. NaHS increases both the intracellular concentration of reduced glutathione and the concentration of cysteine and γ-glutamyl cysteine (precursor of glutathione) in neurons of the rat neocortex in vitro [69]. The mechanism used by H 2S for stimulation of transport by the хс– system remains unknown. It was demonstrated that the protective effect of H 2S is eliminated by blockers of K ATP channels [70]. Stimulation of the afferent sensory nerves can cause inflammatory processes related to the release of substance Р, neurokinin-A, and calcitonin generelated peptide (CGRP). These mediators released in
H 2S as a Regulator of Systemic Functions the cardiovascular and respiratory systems induce the development of a series of inflammatory responses, which includes vasodilation, bronchoconstriction, secretion of mucus, and release of plasma proteins leading to edema. NaHS, similarly to capsaicin, intensifies the release of substance Р and CGRP from sensory nerves in airways of the guinea pig [71]. It is interesting that i.p. injection of NaHS into healthy mice causes a significant inflammatory reaction, which is accompanied by a rise in the concentrations of substance Р, anti-inflammatory cytokines, TNF-α, and IL-1β [72]. The above effects were removed by specific antagonists of receptors of substance Р (NK1, CP-96,345). The inflammatory effect of H 2S was removed by capsazepine and was not observed in mice with deficiencies of substance Р and neurokinin-А [73]. These data indicate that H 2S can independently induce neurogenic inflammation even in the absence of other damaging factors. Role of H 2S in Pathologies of the CVS. Under conditions of experimental modeling of cardiovascular pathology, an insufficiency of H 2S leads to the development of arterial hypertension. In experiments on rats with hypertension, it was found that the expression of mRNA of CSE in the aortal wall and the H 2S concentration in the blood plasma decreased under such conditions. A deficiency of H 2S, a decreased activity of CSE, and a hypotensive effect of introduction of donors of H 2S are observed in the case where experimental hypertension was induced by inhibition of NO-synthase [74]. Under in vivo conditions, NaHS moderates negative functional and morphological changes in the vessels of rats in the case of pulmonary hypertension induced by hypoxia or chronic blocking of NOS [75, 76]. These data allow one to assume the existence of a probable action of H 2S on the walls of pulmonary vessels. H 2S inhibits aggregation of thrombocytes, suppresses proliferation of smooth muscle cells in the human aorta, induces their apoptosis, and, in such a manner, can restrict atherosclerotic damages. Another atherogenesis-associated effect of H 2S is related to the action of the above agent on vascular inflammatory reaction, which significantly influences the stability of blood plaques and the probability of their abruption [77-79]. In patients with hypertension and/or atherosclerosis, angiosteosis is usually observed. Its development results in a decrease in flexibility of the arteries, stimulates thrombosis, and makes abruption of blood plaques more probable. Angiosteosis is related to transformation of smooth muscle cells in the osteoblast-like units.
69 This process is accompanied by the expression of alkaline phosphatase and of the so-called proteins of bone tissue, namely osteopontin, osteocalcin, and osteonectin. Experimental angiosteosis induced by combined introduction of vitamin D and nicotine in rats is accompanied by a drop in the expression of CSE, the activity of the latter, and the level of H 2S in tissues of the aortal wall. Exogenous H 2S prevents the process of angiosteosis; this fact is confirmed by a decrease in the calcium concentration in the vessels and also by drops in the activity of acidic phosphatase and expression of osteopontin under conditions of the corresponding experiments [80]. Importance of H 2S for Inflammation Processes. Septic shock, which often accompanies sepsis, is characterized by generalized vasodilation and hypotension. The intensification of production of NO and CO mediated by inducible NO synthase and heme oxygenase-1, respectively, promotes vasodilation under such conditions [81]. In rats with experimental septic shock induced by ligation of the cecum or by introduction of lipopolysaccharides, increased production of H 2S in tissues of the vessels is observed [82]. The H 2S concentration in blood plasma negatively correlates with the blood pressure and contractility of the myocardium, and it is assumed that a certain pathogenic role is played by this agent in hemodynamic collapse. Lipopolysaccharide-induced hypertension is, obviously, related to the activation of K ATP channels, since such activation is partially removed by glibenclamide [83]. It was demonstrated that the H 2S concentration in blood plasma in rats with hemorrhagic shock is increased; inhibitors of CSE and glibenclamide increase the arterial pressure in these animals [84]. In the case of septic shock induced by ligation of the cecum or introduction of lipopolysaccharides, increased expression of CSE and its higher activity in the liver and kidneys are observed [85, 86]. Therefore, H 2S not only promotes the development of hypotension but also intensifies the inflammatory response and sepsis-related damages to the organs. The action of propargyl glycine decreases the intensity of inflammatory process in the liver and the mortality of mice with experimental sepsis. An increase in the endogenous H 2S results in intensification of the activity of myeloperoxidase in tissues and also in increases in the activity of myeloperoxidase in tissues and concentration of TNF-α in blood plasma [86]. The above-described studies showed that H 2S is, in general, a proinflammatory mediator. However, the detailed mechanisms mediating induction and
70 intensification of inflammation under the influence of H 2S remain unknown. In in vitro experiments, H 2S demonstrates both proinflammatory and antiinflammatory effects. Na 2S inhibits induced chemotaxis and degranulation of polymorphonuclear leucocytes [87]. In addition, donors of H 2S inhibit aspirin-induced adhesion of leucocytes to the endothelium in the venules of the rat mesentery, while inhibitors of production of H 2S intensify adhesion of leucocytes [88]. On the other hand, NaHS inhibits apoptosis of isolated human neutrophils with no appreciable influence on their bactericidal properties. It is interesting that NaHS did not influence the viability of eosinophils and intensified apoptosis of lymphocytes [89]. Lipopolysaccharides, similarly to proinflammatory cytokines, enhance the expression of the CSE gene, which is, obviously, the reason for the increase in the level of H 2S in different inflammatory states [90]. It was demonstrated that intensification of production of H 2S is observed under conditions of not only generalized sepsis but also of localized forms of inflammation. For example, cerulein-induced acute pancreatitis in mice is related to an increase in the expression of mRNA of CSE in the pancreas. Propargyl glycine moderates the injury of acinar cells and infiltration in the pancreas via normalization of the activity of amylase in blood plasma; this agent also suppresses inflammatory processes in the lungs [73].
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