Oxidative stress is an imbalance between the production of reactive oxygen species (ROS) and the antioxidant system reducing its capability to detoxify ROS or repair the resulting damage, i.e., ROS overwhelms antioxidants. Oxidative stress is a key pathogenic factor in chronic liver injury of various etiologies, such as alcoholic liver disease ^[1], nonalcoholic fatty liver diseases (NAFLD) ^[2], chronic viral hepatitis, and cholestatic diseases. This damage will ultimately lead to cirrhosis (defined as hepatic architectural damage characterized by nodular regeneration and diffuse fibrosis) ^[3]. Furthermore, oxidative stress also plays an important pathogenic role in portal hypertension (defined as a portal venous pressure of greater than 12 mm Hg) ^[4], cirrhotic cardiomyopathy (CCM) ^[5], hepatorenal syndrome (HRS) ^[6], cirrhosis-related pulmonary complications ^[7], and hepatic encephalopathy ^[8].

2. Pathogenic Mechanisms of Oxidative Stress

Oxidative stress plays a crucial role in the progression of chronic liver diseases to cirrhosis and the development of associated complications. The liver plays a central role in the detoxification of endogenous and exogenous toxins, and harbors a high antioxidant function ^[3]. In cirrhosis, liver function is significantly impaired and therefore antioxidant function is also jeopardized. This is coupled with excess oxidative stress resulting from portal hypertension driven by gastrointestinal congestion and bacterial translocation ^[9]. The overactivated ROS can damage the intestine structurally and functionally, as increased lipid peroxidation and protein oxidation have been found in the intestinal mucosa of cirrhotic rats ^[10] and decompensated cirrhotic patients ^[11]. Using a carbon tetrachloride (CCl₄) model of cirrhosis, Ramachandran and coworkers ^[10] evaluated oxidative stress status in the gastrointestinal tract. In comparison with controls, xanthine oxidase (XO) activity (an index of oxidative stress) was significantly increased, and xanthine dehydrogenase activity (a parameter of antioxidant status) was significantly decreased in the intestine of cirrhotic rats. This alteration of oxidative stress was associated with a significant reduction in villus fraction, increased enterocyte necrosis, loss of tight junctions, and abnormal intestinal brush border. The damaged intestinal mucosa is thought to enhance intestinal permeability and bacterial translocation that in combination with collateral circulation and an impaired liver results in endotoxemia. Endotoxemia can then further increase oxidative stress, intestinal barrier dysfunction, and organ injury, leading to a ‘vicious cycle’: gut barrier dysfunction → endotoxemia → organ injury. Endotoxemia in cirrhotic subjects plays critical role in cirrhotic complications such as CCM ^[11], and AKI/HRS ^[12].

3. Overview of Cardiovascular Abnormalities of Cirrhosis and Portal Hypertension

The cardiovascular system in cirrhotic patients is abnormal, and is characterized by portal hypertension, systemic hyperdynamic circulation (increased cardiac output and decreased peripheral vascular resistance), and arterial pressure ^[13]. The decrease in peripheral vascular resistance is thought to be due to an imbalance between vasoconstrictive and vasodilatory factors. The former is mainly driven by the sympathetic system ^[14], renin–angiotensin system (RAS) ^[15], endothelin-1 ^[16], and thromboxane ^[17]; the latter by glucagon ^[18], prostaglandins ^[19], bile acids, nitric oxide (NO) ^[20], carbon monoxide (CO) ^[21], and hydrogen sulfate (H₂S) ^[22]. Although the vasoconstrictors such as the sympathetic system [23,24]^[23][24] and RAS [25,26]^[25][26] are increased in subjects with cirrhosis, the response of cirrhotic subjects to vasoconstrictors as a whole is impaired ^[27]. Vasodilators play a dominant role in cirrhotic patients and therefore the cardiovascular system in patients with cirrhosis is characterized by peripheral vascular dilation, decreased systemic vascular resistance (SVR), and mean arterial pressure (MAP). The cardiac output is increased at rest with cirrhotic patients having been described to have hyperdynamic circulation ^[22]. Under stress, the cardiac functional reserve is insufficient in cirrhosis, with decreased left ventricular responsiveness having been described ^[28]. Moreover, cardiac diastolic dysfunction has also been reported and can lead to significantly smaller increases in stroke volume when challenged ^[28]. This collection of cardiac functional abnormalities in patients with cirrhosis is called cirrhotic cardiomyopathy. The diagnosis of cirrhotic cardiomyopathy is based on advanced imaging examination at rest ^[29]. The pathophysiology of cardiovascular changes in cirrhosis is multifaceted, with inflammation (evidenced by increases of proinflammatory cytokines) ^[30] and oxidative stress as significant contributors to these changes ^[5]. There is evidence that oxidative stress also plays an essential role in non-cirrhotic cardiovascular diseases ^[31]. However, the role of oxidative stress in the pathophysiology of cardiovascular changes in cirrhosis remains incompletely clarified.

4. Oxidative Stress in Pathogenesis of Hyperdynamic Circulation

Cirrhosis and portal hypertension cause hyperdynamic circulation. There are two theories explaining hyperdynamic circulation in subjects with cirrhosis and portal hypertension: the humoral theory and central neural dysregulation ^[9].

5. Oxidative Stress in the Humoral Theory

The humoral hypothesis suggests that in cirrhosis, mesenteric congestion causes endotoxemia. Endotoxin stimulates the generation of vasodilators such as glucagon ^[18], prostaglandins ^[19], NO ^[32], CO ^[33], and H₂S ^[34]. All these vasodilators additively/synergically dilate peripheral vasculature which results in hyperdynamic circulation. Oxidative stress also plays an important role in hyperdynamic circulation. In bile duct ligation (BDL)-induced cirrhosis in rats, Lee et al. ^[35] found that mesenteric markers of oxidative stress, such as thiobarbituric acid reactive substances (TBARS, an index of lipoperoxidation), and malondialdehyde (MDA), are significantly increased. Congruent with these data, ouresearchers' group has also shown that mesenteric myeloperoxidase (MPO) is significantly increased in portal hypertensive rats ^[9]. Furthermore, the levels of oxidative stress in the mesentery are closely related to circulating proinflammatory cytokines, such as TNF-α, IL-1β, and IL-6. Lee’s study suggests that oxidative stress may have an additive/synergic effect on systemic inflammation on hyperdynamic circulation in cirrhotic animal models ^[35]. Now it is clear that cellular enzymes called nicotinamide adenine dinucleotide phosphate (NADPH) oxidases produce a considerable amount of ROS in humans ^[36] and a rat model of partial portal vein ligation (PPVL) ^[37]. Deng and coworkers ^[37] demonstrated that H₂O₂ is significantly increased in mesenteric tissues and in parallel, phosphorylated eNOS (p-eNOS) is elevated. NADPH oxidase inhibitor, GKT137831, significantly reduces mesenteric H₂O₂ and p-eNOS, linking oxidative stress and the vasodilator, NO ^[37]. Interestingly, GKT137831 also reduces cardiac index, portal vein pressure, portal vein blood flow, and portal–systemic shunting in PPVL rats. GKT137831 reverses the decreased mesenteric artery contractile response to norepinephrine in PPVL rats. The final effect of NADPH oxidase inhibition is ameliorating hyperdynamic circulation ^[37]. In 1998, the Moore lab ^[38] demonstrated that PPVL rats develop hyperdynamic circulation; this was reversed when these rats were treated with N-acetylcysteine (NAC). Licks et al. ^[39] later found that in PPVL rats, in parallel with hyperdynamic circulation, the levels of gastric TBARS, nitrates, and nitrites are increased. In their four groups of rats (Sham, Sham + NAC, PPVL, and PPVL + NAC), the levels of oxidative stress markers paralleled that of nitrates and nitrites. The antioxidant parameters, such as superoxide dismutase (SOD) and glutathione peroxidase (GPx), were significantly decreased in PPVL rats. Histology showed that the vessels in the gastric mucosa in the PPVL group were dilated. NAC treatment significantly reversed these changes and resulted in a circulation that resembled what was observed in sham operated control rats. With these results, they concluded that oxidative stress contributes to portal hypertension and hyperdynamic circulation via the regulation of nitrates and nitrites, and antioxidant reverses these changes in the rat PPVL model. Interestingly, Iwakiri et al. ^[40] used a double knockout (iNOS, eNOS) mice in a PPVL model and observed that these mice still develop hyperdynamic circulation. This suggests that other variables in addition to the humoral theory may be at play in this complex condition.

6. Oxidative Stress in Central Neural Dysregulation

Hyperdynamic circulation due to the dysregulation of the central neural system ^[41] has also been described in subjects with cirrhosis and portal hypertension. This theory proposes that there is a reflex arc which regulates the cardiovascular system. The reflex arc includes the receptors in mesentery, afferent nerves, cardiovascular nuclei, and efferent nerves. The signals originating from receptors in mesentery are relayed via afferent nerves to the central neural system which then dispatches signals to the cardiovascular system via efferent nerves. The integrity of the reflex arc is essential for the regulation of the cardiovascular system [9,41,42]^[9][41][42]. Portal hypertension causes congestion in the mesentery which activates chemoreceptors and/or baroreceptors in the splanchnic area. OuResearcher studys' research found that 10 days after PPVL, the MAP and SVR are significantly decreased and cardiac output significantly increased, indicating a hyperdynamic circulation ^[42]. WThe researchers examined different interventions to test the role of this reflex arc in hyperdynamic circulation. WeThe researchers first tested the role of capsaicin-sensitive nerves in hyperdynamic circulation. Capsaicin was used to denervate the afferent nerves in rat pups. These rats were subjected to PPVL or BDL-induced cirrhosis when they reached adulthood ^[43]. WThe researchers showed that capsaicin-treated PPVL or cirrhotic rats had similar cardiac output and systemic vascular resistance compared to the sham-operated group. In comparison, the PPVL or cirrhotic rats treated with vehicle (DMSO + ethanol) demonstrated hyperdynamic circulation. These data confirmed that capsaicin-treated rats have no capacity to develop hyperdynamic circulation when subjected to PPVL or cirrhosis. Capsaicin had no hemodynamic effect on sham-operated rats. These results showed that capsaicin treatment blocks the development of vasodilation in cirrhotic and portal hypertensive rats. Thus, primary afferent innervation is important in the pathogenesis of hyperdynamic circulation in portal hypertension and cirrhosis. WThe researchers then tested the central neural system in the regulation of hyperdynamic circulation in PPVL rats ^[42]. c-fos is an immediate-early gene and has important roles in cellular signal transduction. The c-fos protein product Fos is significantly increased in central cardiovascular nuclei such as the nucleus tractus solitarius (NTS) and paraventricular nucleus (PVN). Fos is an activation marker in the central neural system. WThe researchers found that Fos expression is the prerequisite of hyperdynamic circulation in PPVL rats. Fos is detectable at day 1 after PPVL with immunohistochemistry and persistently increased when examined daily in the PPVL rats. However, the hyperdynamic circulation developed on day 3 and remained thereafter. When Fos expression in the NTS was blocked by local microinjection of c-fos antisense oligonucleotides, the increased cardiac output decreased SVR and MAP were reversed in PPVL rats. c-fos antisense oligonucleotides had no effect on circulation in sham-control rats. This experiment indicated that cardiovascular nuclei are crucial to the regulation of hyperdynamic circulation in PPVL rats ^[42]. WThe researchers then tried to clarify the afferent nervous pathway. An inflatable cuff around the portal vein was used to acutely increase the portal pressure, then the vagal nerve electrical activity in the cervical area was recorded ^[41]. The acute increase in portal pressure immediately increased the vagal nerve electrical activity. In another experiment, the cervical vagus nerve was ablated by capsaicin. Three weeks after vagal ablation, the rats were subjected to PPVL. OurThe results showed that vagal nerve ablation significantly decreased Fos expression in the PVN of PPVL rats. Furthermore, vagal nerve blocked the development of hyperdynamic circulation in PPVL rats. This implied that an intact vagal nerve is a sine qua non in the pathogenesis of hyperdynamic circulation in PPVL rats ^[41]. The initial signal that triggers the hyperdynamic circulation in PPVL rats is thought to be mesenteric congestion. The congestion impacts two types of receptors, baroreceptors and chemoreceptors, as congestion not only increases the mesenteric venous pressure, but also causes ischemia. Chen et al. ^[44] induced cirrhosis in rats by CCl₄, and reported that H₂O₂ content in the mesenteric arterial wall was significantly increased in comparison to control animals. Several studies demonstrated that an antioxidant such as NAC significantly reverses hyperdynamic circulation in portal hypertensive animal models. One of these studies from the Moore Lab demonstrated that NAC alleviates oxidative stress and prevents hyperdynamic circulation in the PPVL rats. They showed that the portal pressure remains significantly higher in the PPVL + NAC animals compared with sham + NAC rats ^[38]. However, that study did not further investigate the mechanism of NAC on the attenuation of hyperdynamic circulation. WThe researchers therefore performed a studyresearch to investigate the role of oxidative stress in the pathogenesis of hyperdynamic circulation, specifically aiming to evaluate whether oxidative stress is the initiating signal in the gut. We aThe researchers also used the PPVL model in rats because prehepatic portal hypertension creates mesenteric congestion and ischemia without significant liver parenchymal injury. Our studyResearchers' research also reconfirmed the effect of NAC on hyperdynamic circulation in PPVL rats ^[10]. Furthermore, wthe researchers found that jejunal MPO, an index of intestinal oxidative stress, is significantly increased in the PPVL model in rats; NAC treatment significantly decreased the activity of jejunal MPO. Interestingly, NAC also significantly decreased cardiac output, increased MAP and SVR, and reversed the hyperdynamic circulation in PPVL rats. To confirm that it is the oxidative stress that inaugurates hyperdynamic circulation, H₂O₂ was applied directly to the mesenteric area. The Weresearchers confirmed that H₂O₂ stimulates Fos expression in PVN, and decreased MAP, a direct index of hyperdynamic circulation. All these data indicate that it is the oxidative stress that triggers the hyperdynamic circulation in portal hypertension ^[9].