Nitric oxide production in arterial vessels of cirrhotic rats

Cirrhotic portal hypertension: From pathophysiology to novel therapeutics

Portal hypertension and bleeding from gastroesophageal varices is the major cause of morbidity and mortality in patients with cirrhosis. Portal hypertension is initiated by increased intrahepatic vascular resistance and a hyperdynamic circulatory state. The latter is characterized by a high cardiac output, increased total blood volume and splanchnic vasodilatation, resulting in increased mesenteric blood flow. Pharmacological manipulation of cirrhotic portal hypertension targets both the splanchnic and hepatic vascular beds. Drugs such as angiotensin converting enzyme inhibitors and angiotensin II type receptor 1 blockers, which target the components of the classical renin angiotensin system (RAS), are expected to reduce intrahepatic vascular tone by reducing extracellular matrix deposition and vasoactivity of contractile cells and thereby improve portal hypertension. However, these drugs have been shown to produce significant off-target effects such as systemic hypotension and renal failure. Therefore, the current pharmacological mainstay in clinical practice to prevent variceal bleeding and improving patient survival by reducing portal pressure is non-selective -blockers (NSBBs). These NSBBs work by reducing cardiac output and splanchnic vasodilatation but most patients do not achieve an optimal therapeutic response and a significant proportion of patients are unable to tolerate these drugs. Although statins, used alone or in combination with NSBBs, have been shown to improve portal pressure and overall mortality in cirrhotic patients, further randomized clinical trials are warranted involving larger patient populations with clear clinical end points. On the other hand, recent findings from studies that have investigated the potential use of the blockers of the components of the alternate RAS provided compelling evidence that could lead to the development of drugs targeting the splanchnic vascular bed to inhibit splanchnic vasodilatation in portal hypertension. This review outlines the mechanisms related to the pathogenesis of portal hypertension and attempts to provide an update on currently available therapeutic approaches in the management of portal hypertension with special emphasis on how the alternate RAS could be manipulated in our search for development of safe, specific and effective novel therapies to treat portal hypertension in cirrhosis.

Chronic administration of aminoguanidine reduces vascular nitric oxide production and attenuates liver damage in bile duct-ligated rats

BACKGROUND

Nitric oxide (NO) has been implicated in the pathogenesis of liver cirrhosis. This study investigated the activity of nitric oxide synthase (NOS) in cirrhosis induced by bile duct-ligation (BDL) with NOS inhibitors.

METHOD

Three days after operation, rats were randomized to receive aminoguanidine (AG, 25 mg/kg/day) or L-N(G)-nitro-L-arginine methyl ester (L-NAME, 10 mg/kg/day) for 21 days.

RESULTS

Vascular NO production, which was increased in BDL cirrhotic rats, was reduced by 75% with AG but not L-NAME chronic administration. AG treatment attenuated liver damage, while L-NAME aggravated it. AG significantly suppressed inducible NOS (iNOS) expression in aorta of BDL rats at both mRNA and protein level, but much less efficient in reducing it in liver. In contrast, endothelial NOS (eNOS) expression was not markedly affected. Calcium-independent NOS activity, which was dramatically increased in aorta of BDL rats, was abolished by AG treatment. In liver, however, both calcium-dependent and -independent NOS activity were increased by AG treatment.

CONCLUSION

Chronic administration of AG could reduce systemic NO levels as well as suppress iNOS expression and activity in aorta of BDL rats. It also improved liver function, possibly because of its ability to increase hepatic NOS activity, and to correct the systemic hemodynamic disorders by decreasing vascular NO production.

The role of nitric oxide in the pathogenesis of systemic and splanchnic vasodilation in cirrhotic rats before and after the onset of ascites

BACKGROUND

The role of nitric oxide (NO) in the pathogenesis of splanchnic arterial vasodilation in cirrhosis has been recently debated by some experimental studies.

AIMS

We investigated the role of NO in the pathogenesis of the splanchnic arterial vasodilation along the course of CCl(4)-induced experimental cirrhosis.

METHODS

We analyzed the effect on mean arterial pressure (MAP), cardiac output (CO), total peripheral resistance (TPR), and resistance in the superior mesenteric artery (RSMA), before and after the administration of a unspecific NO synthase (NOS) inhibitor (Nomega-nitro-L-arginine-methyl-ester, L-NAME) and a specific NOS2 inhibitor (L-N-(1-iminoethyl)-lysine, L-NIL) to cirrhotic rats with and without ascites, and to control rats. NOS2 and NOS3 protein expression was also assessed in systemic and splanchnic arteries of these animals.

RESULTS

L-NAME in cirrhotic rats markedly improved MAP, and TPR and decreased CO regardless of whether they had ascites or not. L-NIL did not produce any significant effect on systemic haemodynamics in control and cirrhotic rats. NOS3 overexpression in the aorta of cirrhotic animals paralleled the progression of the liver disease. L-NAME increased RSMA in cirrhotic rats, but this effect was much less intense in rats with ascites. L-NIL had an effect only on RSMA in rats with ascites, which was of a similar extent to that produced by L-NAME. Western blot experiment showed a faint overexpression of NOS3 in the mesenteric artery of cirrhotic rats with and without ascites and a clear induction of NOS2 only in the mesenteric artery of rats with ascites.

CONCLUSIONS

These results indicate that NO contributes significantly to the pathogenesis of arterial splanchnic circulation in the early stages of experimental cirrhosis but has only a minor role in its maintenance after the development of ascites. Furthermore, the expression of the different NOS isoforms varies along the course of the liver disease.

Nitric oxide synthase 1 is partly compensating for nitric oxide synthase 3 deficiency in nitric oxide synthase 3 knock-out mice and is elevated in murine and human cirrhosis

BACKGROUND

The role of endothelial nitric oxide synthase 3 (NOS-3) in the hyperdynamic circulation associated with cirrhosis is established but not that of the neuronal (NOS-1) isoform. We therefore investigated aortic NOS-1 levels in NOS-3 knock-out (KO) and wildtype (WT) mice and in hepatic arteries of patients.

METHODS

Mice rendered cirrhotic by bile duct ligation (BDL) were compared with sham-operated controls. Hepatic arteries of cirrhotic patients were collected during liver transplantation; donor vessels served as controls. mRNA levels were quantified by real-time PCR, protein levels by Western blotting and NO production by Nomega-nitro-L-arginine methyl ester inhibitable arginine-citrulline assay.

RESULTS

Aortae of NOS-3 KO mice exhibited higher NOS-1mRNA (5.6-fold, P < 0.004) and protein levels (8.8-fold) compared with WT. NO production in aortae of NOS-3 KO mice was 52% compared with WT (P = 0.002). BDL increased NOS-1 mRNA (2.4-fold, P = 0.01) and protein (7.1-fold) levels in aortae of WT, but no further in the NOS-3 KO mice. Hepatic artery NOS-1 mRNA levels in cirrhotic patients were markedly increased compared with controls (24.5-fold, P = 0.0007).

CONCLUSIONS

Increased NOS-1 mRNA and protein levels and partially maintained in vitro NO-production in aortae of NOS-3 KO mice suggest that NOS-1 may partially compensate for NOS-3 deficiency. BDL-induced increase in aortic NOS-1 mRNA and protein levels hint that not only NOS-3, but also NOS-1 may be involved in the regulation of systemic hyperdynamic circulation and portal hypertension. Upregulation of NOS-1 mRNA levels in hepatic arteries of portal hypertensive patients suggests possible clinical significance for these experimental findings.

Dual nitric oxide mechanisms of cholestasis-induced bradycardia in the rat

1. Cholestatic liver disease is associated with nitric oxide (NO) overproduction and bradycardia. Nitric oxide has a dual effect on sinoatrial node and its effects depend on its concentration. Nitric oxide can increase heart rate by activating hyperpolarization-activated pacemaker current (If) but, at high concentrations, it can potentially decrease heart rate by inhibition of L-type calcium current. In the present study, the responsiveness of isolated atria to CsCl (an inhibitor of the If current) and acetylcholine (ACh; which decreases L-type calcium current through a NO-dependent pathway) were evaluated in bile duct-ligated and sham-operated control rats. 2. Bile duct ligation induced a significant decrease in the negative chronotropic effect of CsCl (0.2-5 mmol/L), but increased the responsiveness of isolated atria to ACh (10-8 to 10-3 mol/L). These effects were restored after incubation of the atria in the presence of the NO synthase inhibitor NG-nitro-l-arginine methyl ester (0.1 mmol/L). 3. Anaesthetized bile duct-ligated rats showed bradycardia and the plasma levels of NO2-/NO3- were significantly higher in bile duct-ligated rats compared with sham-operated animals. 4. Different and opposite responses of atria of cholestatic rats to CsCl and ACh can be explained by NO overproduction in bile duct-ligated animals. A dual role of NO in the regulation of the sinoatrial node may be responsible for this opposite effect and may have a role in the pathophysiology of cholestasis-induced bradycardia.

Accelerated reversal of carbon tetrachloride-induced cirrhosis in rats by the endothelin receptor antagonist TAK-044

BACKGROUND

A multifunctional mediator, endothelin (ET)-1 is implicated in the pathophysiology of liver cirrhosis. Carbon tetrachloride (CCl4)-induced cirrhosis in rats resolves upon termination of CCl4 treatment. We determined the hepatic ET-1 system during such reversal and assessed whether ET-1 receptor antagonism enhances this process.

METHODS

Cirrhosis was induced in rats by CCl4 treatment for 8 weeks. Treatment with an ETA/ETB antagonist TAK-044 (10 mg/kg per day) was then started and determinations were made at 1, 2 and 4 weeks.

RESULTS

After termination of CCl4 treatment, accelerated normalization of liver architecture and portal hypertension occurred in TAK-044-treated rats compared with saline-treated rats. The increased hepatic hydroxyproline concentration and collagen I mRNA expression also declined to greater extents in the TAK-044-treated group. Higher collagenase activity in cirrhosis decreased in saline-treated rats, but did not reach basal values. In TAK-044-treated rats, collagenase activity tended to increase at weeks 2 and 4. Increased ET-1 concentration and ETA receptor density declined to normal values in both groups. In contrast, increased ETB receptor density did not change in saline-treated rats, but decreased to control values in TAK-044-treated rats.

CONCLUSIONS

Our results emphasize the role of ET-1 in chronic liver disease and strongly indicate the potential for ET-1 receptor antagonists in its treatment.

Relationship among gastric motility, autonomic activity, and portal hemodynamics in patients with liver cirrhosis

BACKGROUND AND AIMS

We examined the effects of the autonomic nervous function and the volume of portal blood flow to clarify the mechanism of the abnormal gastric motility in patients with liver cirrhosis.

METHODS

Heart rate variability, electrogastrogram (EGG), and volume of portal blood flow were measured before and after a meal in 27 patients with liver cirrhosis (LC group) and in 20 normal subjects (N group). Autonomic nervous function was evaluated by using spectral analysis of heart rate variability. We used the cine phase-contrast (PC) method, using magnetic resonance imaging (MRI) to measure the portal flow, while the peak frequency and spectral power of the EGG were measured at pre- and postprandial change.

RESULTS

The ratio of low frequency power to high frequency power (LF/HF) was significantly higher, and the HF power was significantly lower in the LC group than in the N group both before and after a meal. In both groups, the electrogastrographic peak power ratio before and after a meal showed a positive correlation with the HF ratio, and an inverse correlation with the LF/HF ratio. In addition, portal blood flow volume was significantly decreased in the LC group than in the N group. However, the increased rate of portal blood flow after a meal correlated positively with the increased rate of electrogastrographic peak power. Moreover, gastric motility was positively correlated with esophageal varices and coma scale with the use of multivariate analysis.

CONCLUSIONS

Parasympathetic hypofunction, sympathetic hyperfunction and portal hemodynamics were closely related with gastric motility in cirrhotic patients. In addition, gastric motility was decreased, at least in part, by the ingestion of food in cirrhotic patients because of abnormalities in autonomic functions and portal blood flow following a meal.

Enhancement of endothelial nitric oxide production by chenodeoxycholic acids in patients with hepatobiliary diseases

The purpose of this study was to clarify whether physiological concentrations of bile acids could affect endothelial nitric oxide production. We investigated the relationships between clinical concentrations of individual bile acids observed in patients with hepatobiliary diseases and endothelial nitric oxide production induced by each bile acid. Fifteen serum bile acids were measured using high-performance liquid chromatography combined with enzymatic fluorometry in 8 patients with liver cirrhosis, obstructive jaundice, and 8 healthy subjects. The effects of individual bile acids on nitric oxide production were examined in human umbilical endothelial cells by measuring the concentration of NO2- in the cultured medium. NO release in the blood was also determined by measuring the NO2-/NO3- concentration in these patients. In patients with hepatobiliary diseases, the plasma concentrations of chenodeoxycholic acid, ursodeoxycholic acid and cholic acid (free acid, taurine and glycine conjugates) were markedly elevated. Incubation of cells with chenodeoxycholic acid and deoxycholic acid (free acid, taurine and glycine conjugates) enhanced NO2- production in a concentration-dependent manner, while cholic acid (free and its conjugates) did not. The effects of individual bile acids on nitric oxide production were additive. Patients with liver cirrhosis and obstructive jaundice had higher plasma levels of NO2-/NO3- levels than the control subjects. These results suggest that increased plasma concentrations of chenodeoxycholic acid (free, taurine and glycine conjugates) in patients with hepatobiliary diseases may induce endothelial nitric oxide production. Thus, nitric oxide production induced by bile acids may be involved in the pathogenesis of circulatory abnormalities in patients with liver diseases.

Levels of circulating endothelin-1 and nitrates/nitrites in patients with virus-related hepatocellular carcinoma

A balance between endothelins (ET) and nitric oxide (NO) might interfere with liver haemodynamics and disease progression in various liver diseases. Increased levels of endothelin 1 (ET-1) and nitrites and nitrates (NOx, the end products of NO metabolism) have been reported in hepatocellular carcinoma (HCC), but the balance has not been studied. The purpose of this study was to assess the ratio of NOx to ET-1 in patients with virus-related hepatocellular carcinoma and to investigate its correlation with the extent of the disease. Eighteen patients with virus-related HCC (six Okuda stage I, six Okuda stage II and six Okuda stage III) were included in the study and were compared with 22 patients with viral cirrhosis (14 decompensated, eight compensated) and seven normal controls. ET-1 was measured with an ELISA assay and NOx with a modification of the Griess reaction. Patients with virus-related HCC had the highest levels of circulating ET-1 and NOx (13.24 +/- 0.82 pg/ml and 112.28 +/- 18.56 micromol/l) compared to compensated cirrhosis (9.47 +/- 0.50 pg/ml, P < 0.004 and 54.47 +/- 2.36 micromol/l, P < 0.01), decompensated cirrhosis (9.57 +/- 0.32 pg/ml, P < 0.001 and 90.20 +/- 11.23 micromol/l, NS) and normal controls (8.84 +/- 0.61 pg/ml, P < 0.001 and 51.17 +/- 6.18 micromol/l, P < 0.01). There was a significant increase of ET-1 and NOx at HCC stage III compared to HCC stages I and II, cirhotics and controls. HCC stage III patients also had a NOx/ET-1 ratio that was higher than HCC stages I and II patients, normal controls and patients with compensated cirrhosis. Virus-related HCC patients have high levels of circulating ET-1, compared to compensated or decompensated cirrhosis. Highest levels of ET-1 are produced in Okuda III tumours. NOx are also increased but only in Okuda stage III tumours. The NOx/ET-1 ratio is increased in virus-related HCC and DC. This increase may account for the known increase in tumour blood flow.