The role of nitric oxide synthase isoforms in extrahepatic portal hypertension: studies in gene-knockout mice

Hepatic Encephalopathy-Associated Cerebral Vasculopathy in Acute-on-Chronic Liver Failure: Alterations on Endothelial Factor Release and Influence on Cerebrovascular Function

The acute-on-chronic liver failure (ACLF) is a syndrome characterized by liver decompensation, hepatic encephalopathy (HE) and high mortality. We aimed to determine the mechanisms implicated in the development of HE-associated cerebral vasculopathy in a microsurgical liver cholestasis (MHC) model of ACLF. Microsurgical liver cholestasis was induced by ligating and extracting the common bile duct and four bile ducts. Sham-operated and MHC rats were maintained for eight postoperative weeks Bradykinin-induced vasodilation was greater in middle cerebral arteries from MHC rats. Both Nω-Nitro-L-arginine methyl ester and indomethacin diminished bradykinin-induced vasodilation largely in arteries from MHC rats. Nitrite and prostaglandin (PG) F_1α releases were increased, whereas thromboxane (TX) B₂ was not modified in arteries from MHC. Expressions of endothelial nitric oxide synthase (eNOS), inducible NOS, and cyclooxygenase (COX) 2 were augmented, and neuronal NOS (nNOS), COX-1, PGI₂ synthase, and TXA₂S were unmodified. Phosphorylation was augmented for eNOS and unmodified for nNOS. Altogether, these endothelial alterations might collaborate to increase brain blood flow in HE.

Metabolomic Characterization of Human Model of Liver Rejection Identifies Aberrancies Linked to Cyclooxygenase (COX) and Nitric Oxide Synthase (NOS)

BACKGROUND Acute liver rejection (ALR), a significant complication of liver transplantation, burdens patients, healthcare payers, and the healthcare providers due to an increase in morbidity, cost, and resources. Despite clinical resolution, ALR is associated with an increased risk of graft loss. A unique protocol of delayed immunosuppression used in our institute provided a model to characterize metabolomic profiles in human ALR. MATERIAL AND METHODS Twenty liver allograft biopsies obtained 48 hours after liver transplantation in the absence of immunosuppression were studied. Hepatic metabolites were quantitated in these biopsies by liquid chromatography and mass spectroscopy (LC/MS). Metabolite profiles were compared among: 1) biopsies with reperfusion injury but no histological evidence of rejection (n=7), 2) biopsies with histological evidence of moderate or severe rejection (n=5), and 3) biopsies with histological evidence of mild rejection (n=8). RESULTS There were 133 metabolites consistently detected by LC/MS and these were prioritized using variable importance to projection (VIP) analysis, comparing moderate or severe rejection vs. no rejection or mild rejection using partial least squares discriminant statistical analysis (PLS-DA). Twenty metabolites were identified as progressively different. Further PLS-DA using these metabolites identified 3 metabolites (linoleic acid, γ-linolenic acid, and citrulline) which are associated with either cyclooxygenase or nitric oxide synthase functionality. CONCLUSIONS Hepatic metabolic aberrancies associated with cyclooxygenase and nitric oxide synthase function occur contemporaneous with ALR. Additional studies are required to better characterize the role of these metabolic pathways to enhance utility of the metabolomics approach in diagnosis and outcomes of ALR.

Mechanisms of decompensation and organ failure in cirrhosis: From peripheral arterial vasodilation to systemic inflammation hypothesis

The peripheral arterial vasodilation hypothesis has been most influential in the field of cirrhosis and its complications. It has given rise to hundreds of pathophysiological studies in experimental and human cirrhosis and is the theoretical basis of life-saving treatments. It is undisputed that splanchnic arterial vasodilation contributes to portal hypertension and is the basis for manifestations such as ascites and hepatorenal syndrome, but the body of research generated by the hypothesis has revealed gaps in the original pathophysiological interpretation of these complications. The expansion of our knowledge on the mechanisms regulating vascular tone, inflammation and the host-microbiota interaction require a broader approach to advanced cirrhosis encompassing the whole spectrum of its manifestations. Indeed, multiorgan dysfunction and failure likely result from a complex interplay where the systemic spread of bacterial products represents the primary event. The consequent activation of the host innate immune response triggers endothelial molecular mechanisms responsible for arterial vasodilation, and also jeopardizes organ integrity with a storm of pro-inflammatory cytokines and reactive oxygen and nitrogen species. Thus, the picture of advanced cirrhosis could be seen as the result of an inflammatory syndrome in contradiction with a simple hemodynamic disturbance.

Attenuated portal hypertension in germ-free mice: Function of bacterial flora on the development of mesenteric lymphatic and blood vessels

Intestinal bacterial flora may induce splanchnic hemodynamic and histological alterations that are associated with portal hypertension (PH). We hypothesized that experimental PH would be attenuated in the complete absence of intestinal bacteria. We induced prehepatic PH by partial portal vein ligation (PPVL) in germ-free (GF) or mice colonized with altered Schaedler's flora (ASF). After 2 or 7 days, we performed hemodynamic measurements, including portal pressure (PP) and portosystemic shunts (PSS), and collected tissues for histomorphology, microbiology, and gene expression studies. Mice colonized with intestinal microbiota presented significantly higher PP levels after PPVL, compared to GF, mice. Presence of bacterial flora was also associated with significantly increased PSS and spleen weight. However, there were no hemodynamic differences between sham-operated mice in the presence or absence of intestinal flora. Bacterial translocation to the spleen was demonstrated 2 days, but not 7 days, after PPVL. Intestinal lymphatic and blood vessels were more abundant in colonized and in portal hypertensive mice, as compared to GF and sham-operated mice. Expression of the intestinal antimicrobial peptide, angiogenin-4, was suppressed in GF mice, but increased significantly after PPVL, whereas other angiogenic factors remained unchanged. Moreover, colonization of GF mice with ASF 2 days after PPVL led to a significant increase in intestinal blood vessels, compared to controls. The relative increase in PP after PPVL in ASF and specific pathogen-free mice was not significantly different.

CONCLUSION

In the complete absence of gut microbial flora PP is normal, but experimental PH is significantly attenuated. Intestinal mucosal lymphatic and blood vessels induced by bacterial colonization may contribute to development of PH.

Endothelial Gab1 deficiency aggravates splenomegaly in portal hypertension independent of angiogenesis

Certain pathological changes, including angiogenesis, actively contribute to the pathogenesis of splenomegaly in portal hypertension (PH), although the detailed molecular and cellular mechanisms remain elusive. In this study, we demonstrated that endothelial Grb-2-associated binder 1 (Gab1) plays a negative role in PH-associated splenomegaly independent of angiogenesis. PH, which was induced by partial portal vein ligation, significantly enhanced Gab1 expression in endothelial cells in a time-dependent manner. Compared with controls, endothelium-specific Gab1 knockout (EGKO) mice exhibited a significant increase in spleen size while their PH levels remained similar. Pathological analysis indicated that EGKO mice developed more severe hyperactive white pulp and fibrosis in the enlarged spleen but less angiogenesis in both the spleen and mesenteric tissues. Mechanistic studies showed that the phosphorylation of endothelial nitric oxide synthase (eNOS) in EGKO mice was significantly lower than in controls. In addition, the dysregulation of fibrosis and inflammation-related transcription factors [e.g., Krüppel-like factor (KLF) 2 and KLF5] and the upregulation of cytokine genes (e.g., TNF-α and IL-6) were observed in EGKO mice. We thus propose that endothelial Gab1 mediates multiple pathways in inhibition of the pathogenesis of splenomegaly in PH via prevention of endothelial dysfunction and overproduction of proinflammatory/profibrotic cytokines.

Pathophysiology of Portal Hypertension

The bases of our current knowledge on the physiology of the hepatic portal system are largely owed to the work of three pioneering vascular researchers from the sixteenth and the seventeenth centuries: A. Vesalius, W. Harvey, and F. Glisson. Vesalius is referred to as the founder of modern human anatomy, and in his influential book, De humani corporis fabrica libri septem, he elaborated the first anatomical atlas of the hepatic portal venous system (Vesalius 2013). Sir William Harvey laid the foundations of modern cardiovascular research with his Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus (Harvey 1931) in which he established the nature of blood circulation. Finally, F. Glisson characterized the gastrointestinal-hepatic vascular system (Child 1955). These physiological descriptions were later complemented with clinical observations. In the eighteenth and nineteenth centuries, Morgagni, Puckelt, Cruveilhier, and Osler were the first to make the connection between common hepatic complications – ascites, splenomegaly, and gastrointestinal bleeding – and obstruction of the portal system (Sandblom 1993). These were the foundations that allowed Gilbert, Villaret, and Thompson to establish an early definition of portal hypertension at the beginning of the twentieth century. In this period, Thompson performed the first direct measurement of portal pressure by laparotomy in some patients (Gilbert and Villaret 1906; Thompson et al. 1937). Considering all these milestones, and paraphrasing Sir Isaac Newton, if hepatologists have seen further, it is by standing on the shoulders of giants.

Nowadays, our understanding of the pathogenesis of portal hypertension has largely improved thanks to the progress in preclinical and clinical research. However, this field is ever-changing and hepatologists are continually identifying novel pathological mechanisms and developing new therapeutic strategies for this clinical condition. Hence, the aim of this chapter is to summarize the current knowledge about this clinical condition.

Inflammation and portal hypertension - the undiscovered country

Portal hypertension has traditionally been viewed as a progressive process, involving ultrastructural changes including fibrosis, nodule formation, and vascular thrombosis, leading to increased intrahepatic resistance to flow. However, it is increasingly recognized that a significant component of this vascular resistance results from a dynamic process, regulated by complex interactions between the injured hepatocyte, the sinusoidal endothelial cell, the Kupffer cell and the hepatic stellate cell, which impact on sinusoidal calibre. Recent findings suggest these haemodynamic findings are most marked in patients with superimposed inflammation. The precise mechanisms for vascular dysfunction in cirrhosis with superimposed inflammation remain to be fully elucidated but several studies over the past decade have started to generate the hypothesis that inflammation may be a key mediator of the pathogenesis and severity of portal hypertension in this context. This review provides a comprehensive overview of the biological mechanisms for inflammation playing a key role in the severity of portal hypertension, and illustrates potential novel therapies that act by modifying these processes.

Splanchnic-aortic inflammatory axis in experimental portal hypertension

Splanchnic and systemic low-grade inflammation has been proposed to be a consequence of long-term prehepatic portal hypertension. This experimental model causes minimal alternations in the liver, thus making a more selective study possible for the pathological changes characteristic of prehepatic portal hypertension. Low-grade splanchnic inflammation after long-term triple partial portal vein ligation could be associated with liver steatosis and portal hypertensive intestinal vasculopathy. In fact, we have previously shown that prehepatic portal hypertension in the rat induces liver steatosis and changes in lipid and carbohydrate metabolism similar to those produced in chronic inflammatory conditions described in metabolic syndrome in humans. Dysbiosis and bacterial translocation in this experimental model suggest the existence of a portal hypertensive intestinal microbiome implicated in both the splanchnic and systemic alterations related to prehepatic portal hypertension. Among the systemic impairments, aortopathy characterized by oxidative stress, increased levels of proinflammatory cytokines and profibrogenic mediators stand out. In this experimental model of long-term triple portal vein ligated-rats, the abdominal aortic proinflammatory response could be attributed to oxidative stress. Thus, the increased aortic reduced-nicotinamide-adenine dinucleotide phosphate [NAD(P)H] oxidase activity could be associated with reactive oxygen species production and promote aortic inflammation. Also, oxidative stress mediated by NAD(P)H oxidase has been associated with risk factors for inflammation and atherosclerosis. The splanchnic and systemic pathology that is produced in the long term after triple partial portal vein ligation in the rat reinforces the validity of this experimental model to study the chronic low-grade inflammatory response induced by prehepatic portal hypertension.

The bile acid membrane receptor TGR5: a novel pharmacological target in metabolic, inflammatory and neoplastic disorders

TGR5 is the G-protein-coupled bile acid-activated receptor, found in many human and animal tissues. Considering different endocrine and paracrine functions of bile acids, the current review focuses on the role of TGR5 as a novel pharmacological target in the metabolic syndrome and related disorders, such as diabetes, obesity, atherosclerosis, liver diseases and cancer. TGR5 ligands improve insulin sensitivity and glucose homeostasis through the secretion of incretins. The bile acid/TGR5/cAMP signaling pathway increases energy expenditure in brown adipose tissue and skeletal muscle. Activation of TGR5 in macrophages inhibits production of proinflammatory cytokines and attenuates the development of atherosclerosis. This receptor has been detected in many cell types of the liver where it has anti-inflammatory effects, thus reducing liver steatosis and damage. TGR5 also modulates hepatic microcirculation and fluid secretion in the biliary tree. In cell culture models TGR5 has been linked to signaling pathways involved in metabolism, cell survival, proliferation and apoptosis, which suggest a possible role of TGR5 in cancer development. Despite the fact that TGR5 ligands may represent novel drugs for prevention and treatment of different aspects of the metabolic syndrome, clinical studies are awaited with the perspective that they will complete TGR5 biology and identify efficient and safe TGR5 agonists.

Current concepts on the role of nitric oxide in portal hypertension

Portal hypertension (PHT) is defined as a pathological increase in portal venous pressure and frequently accompanies cirrhosis. Portal pressure can be increased by a rise in portal blood flow, an increase in vascular resistance, or the combination. In cirrhosis, the primary factor leading to PHT is an increase in intra-hepatic resistance to blood flow. Although much of this increase is a mechanical consequence of architectural disturbances, there is a dynamic and reversible component that represents up to a third of the increased vascular resistance in cirrhosis. Many vasoactive substances contribute to the development of PHT. Among these, nitric oxide (NO) is the key mediator that paradoxically regulates the sinusoidal (intra-hepatic) and systemic/splanchnic circulations. NO deficiency in the liver leads to increased intra-hepatic resistance while increased NO in the circulation contributes to the hyperdynamic systemic/splanchnic circulation. NO mediated-angiogenesis also plays a role in splanchnic vasodilation and collateral circulation formation. NO donors reduce PHT in animals models but the key clinical challenge is the development of an NO donor or drug delivery system that selectively targets the liver.

Chronological changes in renal vascular reactivity in portal hypertensive rats

BACKGROUND

Circulatory dysfunction in portal hypertension is characterized by increased cardiac output, decreased systemic vascular resistance, a fall in mean arterial pressure secondary to splanchnic and systemic vasodilation and hence renal hypoperfusion. Previous studies have disclosed that renal vasculatures of portal hypertensive rats had lower perfusion pressure and hyporesponsiveness to endogenous vasoconstrictors. However, the sequences of altered renal haemodynamics have never been described. This study aimed to explore the evolution of renal vascular hyporeactivity and associated mechanisms during portal hypertension.

MATERIALS AND METHODS

All rats were randomized into partial portal vein ligation (PVL) or shamed surgery. Isolated kidney perfusion was performed at postoperative day 1, 4, 7 and 14, respectively, to evaluate chronologically renal vascular response to endothelin-1. Renal arteries and kidneys were harvested for further analysis.

RESULTS

Impaired renal vascular reactivity to endothelin-1 developed 1 week following PVL. There were extensive up-regulations of vasodilative nitric oxide synthase (NOS) and cyclooxygenase-2 in renal arteries of PVL rats. Among them, the changes in endothelial NOS paralleled with the evolution of renal vascular hyporesponsiveness. Preincubation of NOS inhibitor attenuated the renal vascular hyporeactivity in PVL rats. Up-regulated NOS and down-regulated cyclooxygenase-2 in kidneys of PVL rats might play a critical role to maintain renal circulation and body fluid homoeostasis in response to systemic hypotension.

CONCLUSIONS

This investigation highlights the versatile nature of renal vasculatures in portal hypertension, which is replete with compensatory mechanisms. It may help to unveil potential mechanisms of severe renal dysfunction in advanced liver disease.

Perspective: TGR5 (Gpbar-1) in liver physiology and disease

Bile acids are signaling molecules with diverse endocrine functions. Bile acid effects are mediated through the nuclear receptor farnesoid X receptor (FXR), the G-protein coupled receptor TGR5 (Gpbar-1) and various other bile acid sensing molecules. TGR5 is almost ubiquitously expressed and has been detected in different non-parenchymal cells of human and rodent liver. Here, TGR5 has anti-inflammatory, anti-apoptotic and choleretic functions. Mice with targeted deletion of TGR5 are protected from the development of cholesterol gallstones. Administration of specific TGR5 agonists lowers serum and liver triglyceride levels thereby reducing liver steatosis. Furthermore, activation of TGR5 promotes intestinal glucagon-like peptide-1 (GLP-1) release, thereby modulating glucose homeostasis and energy expenditure in brown adipose tissue and skeletal muscle. Additionally, TGR5 exerts anti-inflammatory actions resulting in decreased liver injury in animal models of sepsis. These beneficial effects make TGR5 an attractive therapeutic target for metabolic diseases, such as diabetes, obesity, atherosclerosis and steatohepatitis.

A wound-like inflammatory aortic response in chronic portal hypertensive rats

Long-term prehepatic portal hypertension in the rat produces a low-grade splanchnic inflammation with liver steatosis and dyslipidemia. It has been suggested that in this experimental model these inflammatory alterations could represent a risk factor of vascular disease. Therefore, our aim was to investigate whether long-term prehepatic portal hypertension (PH) induces vascular pathology, fundamentally inflammatory aortopathy. Male Wistar sham-operated (SO) rats and rats with triple partial portal vein ligation in the very long-term (22 months) of postoperative evolution were used. Serum lipid profile, pro- and anti- inflammatory cytokines and ACTH and corticosterone were assayed by spectrophotometric and ELISA techniques. Aorta mRNA expression of oxidative and nitrosative stress enzymes, NFκB e IκB, immune-related cytokine production and vascular fibrosis parameters, were evaluated by real time RT-PCR. In addition, aortic p22phox subunit immunostaining, morphometry and vascular fibrosis in aorta were analyzed. PH rats have increased serum cholesterol, triglyceride, low-density lipoproteins (LDL) and very low-density lipoproteins (VLDL), while high-density lipoproteins (HDL) were lower than in SO rats. Serum ACTH and corticosterone decreased in PH rats. Also, serum TNF-α, IL-1β and IL-6 were significantly higher in PH-rats. Portal hypertensive-rats showed aortic oxidative stress with increased mRNA expressions of NAD(P)H oxidase p22phox, XDh, SOD and eNOS; higher aortic levels of pro-inflammatory cytokines, including TNF-α, IL-1β and IL-6; remodeling markers, like collagen I, CTGF and MMP-9; and finally, higher protein production of p22phox and collagen and extracellular matrix density were significantly higher in rats with PH. The results from the current study suggest that very long-term prehepatic portal hypertension in rats induces an abdominal aortic inflammatory and fibrotic response. Therefore, it could be considered that portal hypertension aggravates aortic inflammaging and one of its more severe complications, which is remodeling by a wound healing reaction.

iNOS expression in vascular resident macrophages contributes to circulatory dysfunction of splanchnic vascular smooth muscle contractions in portal hypertensive rats

Portal hypertension, a major complication of cirrhosis, is caused by both increased portal blood flow due to arterial vasodilation and augmented intrahepatic vascular resistance due to sinusoidal constriction. In this study, we examined the possible involvement of resident macrophages in the tone regulation of splanchnic blood vessels using bile duct ligated (BDL) portal hypertensive rats and an in vitro organ culture method. In BDL cirrhosis, the number of ED2-positive resident macrophages increased by two- to fourfold in the vascular walls of the mesenteric artery and extrahepatic portal vein compared with those in sham-operated rats. Many ED1-positive monocytes were also recruited into this area. The expression of inducible nitric oxide (NO) synthase (iNOS) mRNA was increased in the vascular tissues isolated from BDL rats, and accordingly, nitrate/nitrite production was increased. Immunohistochemistry revealed that iNOS was largely expressed in ED1-positive and ED2-positive cells. We further analyzed the effect of iNOS expression on vascular smooth muscle contraction using an in vitro organ culture system. iNOS mRNA expression and nitrate production significantly increased in vascular tissues (without endothelium) incubated with 1 μg/ml lipopolysaccharide (LPS) for 6 h. Immunohistochemistry indicated that iNOS was largely expressed in ED2-positive resident macrophages. α-Adrenergic-stimulated contractility of the mesenteric artery was greatly suppressed by LPS treatment and was restored by N(G)-nitro-L-arginine methyl ester (NO synthase inhibitor); in contrast, portal vein contractility was largely unaffected by LPS. Sodium nitroprusside (NO donor) and 8-bromo-cGMP showed greater contractile inhibition in the mesenteric artery than in the portal vein with decreasing myosin light chain phosphorylation. In the presence of an α-adrenergic agonist, the mesenteric artery cytosolic Ca(2+) level was greatly reduced by sodium nitroprusside; however, the portal vein Ca(2+) level was largely unaffected. These results suggest that the induction of iNOS in monocytes/macrophages contributes to a hypercirculatory state in the cirrhosis model rat in which the imbalance of the responsiveness of visceral vascular walls to NO (mesenteric artery >> portal vein) may account for the increased portal venous flow in portal hypertension.

Role of endothelial nitric oxide synthase in the development of portal hypertension in the carbon tetrachloride-induced liver fibrosis model

Portal hypertension (PHT) is a complication of liver cirrhosis and directly increases mortality and morbidity by increasing the propensity of venous hemorrhage. There are two main underlying causations for PHT, increased hepatic resistance and systemic hyperdynamic circulation. Both are related to localized aberrations in endothelial nitric oxide synthase (eNOS) function and NO biosynthesis. This study investigates the importance of eNOS and systemic hyperdynamic-associated hyperemia to better understand the pathophysiology of PHT. Wild-type and eNOS(-/-) mice were given the hepatotoxin CCl(4) for 4-12 wk. Hepatic fibrosis was determined histologically following collagen staining. Portal venous pressure, hepatic resistance, and hyperemia were determined by measuring splenic pulp pressure (SPP), hepatic portal-venous perfusion pressure (HPVPP), abdominal aortic flow (Qao), and portal venous flow (Qpv). Hepatic fibrosis developed equally in wild-type and eNOS(-/-) CCl(4)-exposed mice. SPP, Qao, and Qpv increased rapidly in wild-type CCl(4)-exposed mice, but HPVPP did not. In eNOS(-/-) CCl(4) mice, Qao was not increased, SPP was partially increased, and HPVPP and Qpv were increased nonsignificantly. We concluded that the systemic hyperemia component of hyperdynamic circulation is eNOS dependent and precedes increased changes in hepatic resistance. Alternative mechanisms, possibly involving cyclooxygenase, may contribute. eNOS maintains normal hepatic resistance following CCl(4)-induced fibrosis. Consequently, increased portal pressure following chronic CCl(4) exposure is linked to hyperdynamic circulation in wild-type mice and increased hepatic resistance in eNOS(-/-) mice.

[Hyperdynamic circulation in patients with liver cirrhosis and portal hypertension]

Hyperdynamic circulation in patients with liver cirrhosis is characterized by increased cardiac output and heart rate, and decreased systemic vascular resistance with low arterial blood pressure and currently focused on understanding the pathogenesis because of possibility of developing novel treatment modality. Basically, these hemodynamic alternations arise from portal hypertension. Portosystemic collaterals develop to counterbalance the increased intrahepatic vascular resistance to portal blood flow and induce an increase in venous return to heart. Increased shear stress in vascular endothelial cell related high blood flow by portosystemic shunting contributes to this upregulation of eNOS resulting in NO overproduction. Additionally, bypassing through portosystemic collaterals and escaping degradation of over-produced circulating vasodilators in the diseased liver can promote the peripheral arterial vasodilation. Vasodilation of the systemic and splanchnic circulations lead to a reduced systemic vascular resistance, and increased cardiac output and splanchnic blood flow. Furthermore, neurohumoral vasoconstrictive systems including systemic nervous system, rennin angiotensin aldosterone system, and vasopressin are intensively activated secondary to vasodilation. However, hyperdynamic circulation would be more aggravated by the activated vasoconstrictive systems. With the progression of the cirrhotic process, hyperdynamic alternations can be more profound due to hyporesponsiveness to vasoconstrictors and increased shunt formation in conjunction with autonomic neuropathy. Eventually, splanchnic arterial vasodilation results in an increase portal venous inflow, maintaining the elevated portal venous pressure. Hyperdynamic circulation is intimately involved in portal hypertension with liver cirrhosis, therefore it is reasonable to have an interest in complete understanding of the pathogenesis of hyperdynamic circulation to develop novel treatment modality.

Role of cyclooxygenase isoforms in prostacyclin biosynthesis and murine prehepatic portal hypertension

Portal hypertension (PHT) is a common complication of liver cirrhosis and significantly increases morbidity and mortality. Abrogation of PHT using NSAIDs has demonstrated that prostacyclin (PGI(2)), a direct downstream metabolic product of cyclooxygenase (COX) activity, is an important mediator in the development of experimental and clinical PHT. However, the role of COX isoforms in PGI(2) biosynthesis and PHT is not fully understood. Prehepatic PHT was induced by portal vein ligation (PVL) in wild-type, COX-1(-/-), and COX-2(-/-) mice treated with and without COX-2 (NS398) or COX-1 (SC560) inhibitors. Hemodynamic measurements and PGI(2) biosynthesis were determined 1-7 days after PVL or sham surgery. Gene deletion or pharmacological inhibition of COX-1 or COX-2 attenuated but did not ameliorate PGI(2) biosynthesis after PVL or prevent PHT. In contrast, treatment of COX-1(-/-) mice with NS398 or COX-2(-/-) mice with SC560 restricted PGI(2) biosynthesis and abrogated the development of PHT following PVL. In conclusion, either COX-1 or COX-2 can mediate elevated PGI(2) biosynthesis and the development of experimental prehepatic PHT. Consequently, PGI(2) rather then COX-selective drugs are indicated in the treatment of PHT. Identification of additional target sites downstream of COX may benefit the >27,000 patients whom die annually from cirrhosis in the United States alone.

Splanchnic and systemic vasodilation: the experimental models

Experimental models are a sine qua non condition for unraveling the specific components and mechanisms contributing to vascular dysfunction and arterial vasodilation in portal hypertension. Moreover, a careful selection of the type of animal model, vascular bed, and methodology is crucial for any investigation of this issue. In this review, some critical aspects related to experimental models in portal hypertension and the techniques applied are highlighted. In addition, a detailed summary of the mechanisms of arterial vasodilation in portal hypertension is presented. First, humoral and endothelial vasodilators, predominantly nitric oxide but also carbon monoxide and endothelium-derived hyperpolarizing factor, and others are discussed. Second, time course and potential stimuli triggering and/or perpetuating splanchnic vasodilation are delineated. Finally, a brief general overview of vascular smooth muscle signaling sets the stage for a discussion on cotransmission, receptor desensitization, and the observed impairment in vasoconstrictor-induced smooth muscle contraction in the splanchnic and systemic circulation during portal hypertension.

Portal branch ligation induces a hepatic arterial buffer response, microvascular remodeling, normoxygenation, and cell proliferation in portal blood-deprived liver tissue

Portal branch ligation (PBL) may prevent liver failure after extended hepatic resection. However, clinical studies indicate that tumors within the ligated lobe develop accelerated growth. Although it is well known that tumor growth depends on the host's microvascularization, there is no information about how PBL affects the hepatic microcirculation. Our aims were to determine hepatic artery response, liver microcirculation, tissue oxygenation, and cell proliferation after PBL. Therefore, we used intravital multifluorescence microscopy, laser-Doppler flowmetry, immunohistochemistry, and biochemical techniques to examine microcirculatory responses, microvascular remodeling, and cellular consequences after left lateral PBL in BALB/c mice. During the first 7 days, PBL induced a reduction of left hilar blood flow by approximately 50%. This resulted in 80% sinusoidal perfusion failure, significant parenchymal hypoxia, and liver atrophy. After 14 days, however, left hilar blood flow was found to be restored. However, remodeling of the microvasculature included a rarefaction of the sinusoidal network, however, without substantial perfusion failure, compensated by a hepatic arterial buffer response and significant sinusoidal dilatation. This resulted in normalization of tissue oxygenation, indicating arterialization of the ligated lobe. Interestingly, late microvascular remodeling was associated with increased endothelial nitric oxide synthase expression, significant hepatocellular proliferation, and weight gain of the ligated lobe. Thus PBL induces only an initial microcirculatory failure with liver atrophy, followed by a hepatic arterial buffer response, microvascular remodeling, normoxygenation, and hepatocellular proliferation. This may explain the accelerated tumor progression occasionally observed in patients after PBL.

The G-protein coupled bile salt receptor TGR5 is expressed in liver sinusoidal endothelial cells

Sinusoidal endothelial cells (SEC) constitute a permeable barrier between hepatocytes and blood. SEC are exposed to high concentrations of bile salts from the enterohepatic circulation. Whether SEC are responsive to bile salts is unknown. TGR5, a G-protein-coupled bile acid receptor, which triggers cAMP formation, has been discovered recently in macrophages. In this study, rat TGR5 was cloned and antibodies directed against the C-terminus of rat TGR5 were developed, which detected TGR5 as a glycoprotein in transfected HepG2-cells. Apart from Kupffer cells, TGR5 was detected in SEC of rat liver. SEC expressed TGR5 over the entire acinus, whereas endothelial cells of the portal or central veins were not immunoreactive toward TGR5 antibodies. In isolated SEC, TGR5 mRNA and protein were detected by reverse transcription (RT) PCR, immunofluorescence microscopy, and Western blot analysis. Bile salts increased cAMP in isolated SEC and induced mRNA expression of endothelial NO synthase (eNOS), a known cAMP-dependent gene. In addition, bile acids activated eNOS by phosphorylation of eNOS at amino acid position 1177. In line with eNOS activation, bile acids induced NO production in liver slices. This is the first report on the expression of TGR5 in SEC.

CONCLUSION

The data suggest that SEC are directly responsive toward specific bile salts. Regulation of eNOS in SEC by TGR5 connects bile salts with hepatic hemodynamics. This is of particular importance in cholestatic livers when bile salt concentrations are increased.

Animal models of portal hypertension

Animal models have allowed detailed study of hemodynamic alterations typical of portal hypertension and the molecular mechanisms involved in abnormalities in splanchnic and systemic circulation associated with this syndrome. Models of prehepatic portal hypertension can be used to study alterations in the splanchnic circulation and the pathophysiology of the hyperdynamic circulation. Models of cirrhosis allow study of the alterations in intrahepatic microcirculation that lead to increased resistance to portal flow. This review summarizes the currently available literature on animal models of portal hypertension and analyzes their relative utility. The criteria for choosing a particular model, depending on the specific objectives of the study, are also discussed.

The future treatment of portal hypertension

Increased understanding of the hyperdynamic circulation syndrome has resulted in novel therapeutic approaches, some of which have already reached clinical practice. Central to the hyperdynamic circulation syndrome is an imbalance between the increase in different vasodilators (foremost among which is nitric oxide) and the compensatory increase in vasoconstrictors--usually accompanied by a blunted response. This chapter discusses the role of endothelin in the pathogenesis of the syndrome and in future treatment approaches. A relatively new area of research in this field is the role of infection and inflammation in the initiation and maintenance of the hyperdynamic circulation syndrome. The use of antibiotics in the setting of acute variceal bleeding is standard practice. Studies have suggested that chronic manipulation of the intestinal flora could have beneficial effects in the treatment of portal hypertension. The bile salts are another novel and interesting target. Although their vasoactive properties have been known for some time, recent data demonstrate that their effects could be central in the pathogenesis of the hyperdynamic circulation syndrome, and that manipulation of the composition of the bile acid pool could be a therapeutic approach to portal hypertension. Finally, hypoxia and angiogenesis play a role in the development of portal hypertension and the formation of collaterals. This role needs to be further defined but it appears likely that this phenomenon is yet another target for therapeutic intervention.

Animal models of portal hypertension

Animal models have allowed detailed study of hemodynamic alterations typical of portal hypertension and the molecular mechanisms involved in abnormalities in splanchnic and systemic circulation associated with this syndrome. Models of prehepatic portal hypertension can be used to study alterations in the splanchnic circulation and the pathophysiology of the hyperdynamic circulation. Models of cirrhosis allow study of the alterations in intrahepatic microcirculation that lead to increased resistance to portal flow. This review summarizes the currently available literature on animal models of portal hypertension and analyzes their relative utility. The criteria for choosing a particular model, depending on the specific objectives of the study, are also discussed.

Increased angiogenesis and permeability in the mesenteric microvasculature of rats with cirrhosis and portal hypertension: an in vivo study

BACKGROUND

In vivo evidence for angiogenesis in the splanchnic vasodilation in portal hypertension (PHT) and cirrhosis is lacking. Vascular endothelial growth factor (VEGF) and endothelial nitric oxide synthase (eNOS) are mediators of angiogenesis. The present study visualises in vivo structural changes (angiogenesis and vascular hyperpermeability) and examines the presence of VEGF and eNOS in the mesenteric microvasculature of animal models of PHT with and without cirrhosis.

METHODS

Portal hypertension was induced by partial portal vein ligation (PPVL) and cirrhosis was induced by common bile duct ligation (CBDL) in rats. The mesenteric microcirculation was examined by intravital microscopy. Expression of VEGF, eNOS and CD31 in mesenteric tissue were studied by immunohistochemistry.

RESULTS

An increased mesenteric angiogenesis was observed in PPVL and CBDL rats compared with Sham-operated and control rats, as shown by intravital microscopy and CD 31 staining. VEGF and eNOS expression was higher in CBDL and PPVL rats compared with control groups and correlated positively with vascular density. Macromolecular leakage was increased in cirrhotic rats compared with control and PPVL rats.

CONCLUSION

Our study provides in vivo evidence of an increased angiogenesis in the mesenteric microvasculature of animal models of PHT and cirrhosis. Increased VEGF and eNOS expression in the mesentery of PPVL and CBDL rats may suggest their contribution. Microvascular permeability in the mesenteric vessels was only increased in cirrhotic rats.

Molecular and structural basis of portal hypertension

Portal hypertension is a complication of diseases that obstruct portal blood flow, such as cirrhosis or portal vein thrombosis. In these diseases, increased vascular resistance to portal blood flow is the primary mechanism that increases portal pressure. In cirrhosis, increased intrahepatic vascular resistance is a result of both intrahepatic vasoconstriction and surrounding mechanical factors including collagen deposition and regenerative nodules. This article summarizes recent progress in the understanding of molecular mechanisms underlying the portal hypertension-associated arterial alterations in splanchnic systemic territories and those involved in the development of portal-systemic collateral circulation.

Effect of verapamil on nitric oxide synthase in a portal vein-ligated rat model: Role of prostaglandin

AIM: To investigate the effects of verapamil on nitric oxide (NO) synthesis in a portal vein-ligated rat model.

METHODS: Systemic and splanchnic hemodynamics were measured by radiolabeled microspheres in portal hypertensive rats after acute administration of verapamil (2 mg/kg) on chronic treatment with N^w–nitro-L-arginine (NNA)(80 mg/kg) and/or indomethacin (2 mg/kg) .

RESULTS: Verapamil (2 mg/kg) caused a marked fall in both arterial pressure and cardiac output accompanied by an insignificant change in the portal pressure and no change in portal venous inflow. This result suggested that verapamil did not cause a reduction in portal vascular resistance of portal hypertensive rats, which was similar between N^w- nitro–L-arginine-treated and indomethacin-treated groups.

CONCLUSION: In portal hypertensive rats pretreated with NNA and/or indomethacin, acute verapamil administration can not reduce the portal pressure, suggesting that NO and prostaglandin play an important role in the pathogenesis of splanchnic arterial vasodilation in portal hypertension.

Modelos animales en el estudio de la hipertensión portal

Animal models allow detailed study of the hemodynamic alterations in portal hypertension syndrome and of the molecular mechanisms involved in the abnormalities in splenic and systemic circulation associated with this syndrome. Models of prehepatic portal hypertension can be used to study alterations in the splenic circulation and the physiopathology of hyperdynamic circulation. Moreover, models of cirrhosis allow the alterations in intrahepatic microcirculation that lead to increased resistance to portal flow to be studied. The present review summarizes currently available animal models of portal hypertension and analyzes their relative utility in investigating the distinct disorders associated with this entity. The criteria for the choice of a particular model, depending on the specific objectives of the study, are also discussed.

Endothelial nitric oxide synthase is not essential for the development of fibrosis and portal hypertension in bile duct ligated mice

BACKGROUND/AIMS

It is postulated that nitric oxide (NO) is responsible for the hyperdynamic circulation of portal hypertension. Therefore, we investigated induction of fibrosis and hyperdynamic circulation in endothelial NO synthase knock-out (KO) mice.

METHODS

Fibrosis was induced by bile duct ligation. Hemodynamic studies were performed after portal vein ligation. All studies were performed in wild-type (WT) and KO mice.

RESULTS

Three to 4 weeks after bile duct ligation (BDL), both WT and KO groups had similar degrees of portal hypertension, 12 (9-14) and 11(8-15) mmHg, median (range), and liver function. Fibrosis increased from 0.0% in sham operated to 1.0 and 1.1% in WT and KO mice, respectively. Cardiac output was similar after portal vein ligation (20 and 17 ml/min in WT and KO mice, respectively). There was no difference in liver of mRNA for endothelin 1, inducible NO synthase (iNOS) and hem-oxygenase 1 (HO1); proteins of iNOS, HO1 and HO2; nor in endothelin A and B (EtA and EtB) receptor density between WT and KO mice after BDL.

CONCLUSIONS

These results suggest that endothelial NO synthase is neither essential for the development of fibrosis and portal hypertension in bile duct ligated mice, nor for the hyperdynamic circulation associated with portal hypertension in the portal vein ligated mice.

Involvement of constitutive nitric oxide synthase in the portal-systemic collaterals of portal hypertensive rats

BACKGROUND

Recent studies have shown that endothelial nitric oxide (NO) is involved in modulating the vascular response to vasoconstrictors in portal-systemic collaterals of portal hypertensive rats. This study investigated which isoform of NO synthase is involved in the collateral circulation of portal hypertensive rats.

METHODS

The relaxation response to acetylcholine (10(-8) M, 10(-7) M and 10(-6) M) in norepinephrine (NE)-preconstricted portal-systemic collaterals was investigated after incubation with vehicle (Krebs solution), a preferential inducible NO synthase inhibitor (aminoguanidine [AG]), or a non-selective NO synthase inhibitor (Nomega-nitro-L-arginine [NNA]), in rats with partial portal vein ligation. Mean arterial pressure was measured before the perfusion experiments.

RESULTS

Bodyweight and mean arterial pressure before the perfusion studies were similar in the vehicle, AG and NNA groups. Preincubation with NNA, but not AG, produced a significant increase in baseline perfusion pressure compared with the vehicle group (p < 0.05). The increase in perfusion pressure in response to NE was enhanced in the presence of NNA (p < 0.05), but not AG. In addition, preincubation with NNA, but not AG, significantly suppressed acetylcholine-induced relaxation in the portal-systemic collaterals (p < 0.05).

CONCLUSION

These results suggest that constitutive, rather than inducible, NO synthase is involved in the vascular response to vasoconstrictors in the portal-systemic collaterals of portal hypertensive rats.

Vascular contractile response and signal transduction in endothelium-denuded aorta from cirrhotic rats

AIM: The mechanism of decreased vascular reactivity to vasoconstrictors in portal hypertension is still unclear. In addition to nitric oxide, defects in post-receptor signal transduction pathway have been suggested to play a role. However, substantial evidences observed equivocal changes of vascular reactivity following different agonists that challenged the hypothesis of the post-receptor defect. The current study was to evaluate the vascular reactivity to different agonists and the inositol trisphosphate (IP₃) changes in signal transduction cascade from cirrhotic rats with portal hypertension.

METHODS: The endothelial denuded aortic rings from cirrhotic and sham-operated rats were obtained for ex vivo tension study and measurement of the corresponding [³H] IP₃ formation following different receptor and nonreceptor-mediated agonists’ stimulation. Additionally, iNOS protein expression was measured in thoracic aorta. The contractile response curves to phenylephrine were performed in endothelial denuded aortic rings with and without preincubation with a specific iNOS inhibitor (L-N(6)-(1-iminoethyl)-lysine, L-NIL).

RESULTS: In endothelial denuded aortic rings of cirrhotic rats, the vascular responses were reduced with phenylephrine and arginine vasopressin (AVP) stimulation but were normal with U-46619, NaF/AlCl₃, and phorbol esterdibutyrate (PdBU) stimulation. Compared to the corresponding control groups, the degree of the increment of [³H] IP₃ formation from basal level was also decreased with phenylephrine and AVP stimulation, but was normal with U-46619 and NaF/AlCl₃ stimulation. The preincubation with L-NIL did not modify the hyporesponsiveness to phenylephrine. Additionally, the iNOS protein expression in thoracic aorta was not different in cirrhotic and sham-operated rats.

CONCLUSION: Without the influence of nitric oxide, vascular hyporeactivity to vasoconstrictors persisted in cirrhotic rats with portal hypertension. However, the decreased vascular reactivity is an agonist-specific phenomenon. In addition, G-protein and phospholipase C pathway associated with the IP₃ productions may be intact in cirrhotic rats with portal hypertension.

Deletion of inducible nitric oxide synthase decreases mesenteric vascular responsiveness in portal hypertensive mice

The effects of pre-hepatic portal hypertension were examined on the responsiveness of aorta and mesenteric artery from wild-type, inducible nitric oxide synthase knockout (iNOS-KO) and endothelial nitric oxide synthase knockout (eNOS-KO) mice. Mice were sham-operated or made portal hypertensive by creating a calibrated portal vein stenosis. Acetylcholine produced marked relaxations in phenylephrine (10 microM) contracted aorta and mesenteric artery from wild-type and iNOS-KO, both sham and portal hypertensive, but relaxations were abolished in vessels from eNOS-KO mice. There were no significant differences between sham and portal hypertensive animals within groups in the effects of acetylcholine. The potency of KCl was significantly increased in aorta and mesenteric artery from eNOS-KO mice. The maximum contraction to the alpha(1)-adrenoceptor agonist phenylephrine was significantly increased in aorta from eNOS-KO, as compared with wild-type mice. There were no significant differences between sham and portal hypertensive animals within each group in contractions of aorta to KCl or phenylephrine. However, in mesenteric artery, although portal hypertension did not change responsiveness in wild-type or eNOS-KO as compared to sham animals, the potency of phenylephrine was significantly reduced in portal hypertensive iNOS-KO mice as compared to shams. Hence, portal hypertension as compared to sham operation did not affect responses to vasoconstrictors in mouse aorta, but in mouse mesenteric artery portal hypertension affected vascular responses in iNOS-KO mice, suggesting that iNOS is involved in the mesenteric vascular response to portal vein ligation.

Sequential changes in redox status and nitric oxide synthases expression in the liver after bile duct ligation

Bile duct ligation (BDL) in rats induces portal fibrosis. This process has been linked to changes in the oxidative state of the hepatic cells and in the production of nitric oxide. Our objective was to find possible temporal connections between hepatic redox state, NO synthesis and liver injury. In this work we have characterized hepatic lesions 17 and 31 days after BDL and determined changes in hepatic function, oxidative state, and NO production. We have also analyzed the expression and localization of inducible NO synthase (NOS2) and constitutive NO synthase (NOS3). After 17 and 31 days from ligature, lipid peroxidation is increased and both plasma concentration and biliary excretion of nitrite+nitrate are rised. 17 days after BDL both NOS2 and NOS3 are expressed intensely and in the same regions. 31 days after BDL, the expression of NOS2 remains elevated and is localized mostly in preserved hepatocytes in portal areas and in neighborhoods of centrolobulillar vein. NOS3 is localized in vascular regions of portal spaces and centrolobulillar veins and in preserved sinusoids and although its expression is greater than in control animals (34%), it is clearly lower (50%) than 17 days after BDL. The time after BDL is crucial in the study of NO production, intrahepatic localization of NOS isoforms expression, and cell type involved, since all these parameters change with time. BDL-induced, peroxidation and fibrosis are not ligated by a cause-effect relationship, but rather they both seem to be the consequence of common inductors.

Nitric oxide in gastrointestinal health and disease

Nitric oxide is an intracellular and intercellular messenger with important functions in a number of physiologic and pathobiologic processes within gastroenterology and hepatology, including gastrointestinal tract motility, mucosal function, inflammatory responses, gastrointestinal malignancy, and blood flow regulation. Since the broad review of this topic in Gastroenterology more than 10 years ago, a number of advances have been made in the area of NO biology and its relevance to the gastrointestinal system. The aim of this review is to focus on our expanded understanding of the role NO plays in human gastrointestinal and hepatic physiology and disease processes by drawing on data from relevant in vitro and animal models as well as observational human studies.