Liver sinusoidal endothelial cells are responsible for nitric oxide modulation of resistance in the hepatic sinusoids.

The mechanisms that regulate vascular resistance in the liver are an area of active investigation. Previously, we have shown that nitric oxide (NO) modulates hepatic vascular tone in the normal rat liver. In this study, the production of NO is examined in further detail by isolating sinusoidal endothelial cells (SEC) from the rat liver. Endothelial NO synthase (eNOS) was present in SEC based on Western blotting and confocal immunofluorescence microscopy. Exposure of SEC to flow increased the release of NO. To investigate the relevance of these in vitro findings to the intact liver, we modified an in situ perfusion system to allow for direct measurement of NO release from the hepatic vasculature. NO was released from the hepatic vasculature in a time-dependent manner, and administration of N-monomethyl-L-arginine reduced NO release and increased portal pressure. Immunostaining of intact liver demonstrated eNOS localization to endothelial cells lining the hepatic sinusoids. These findings demonstrate that SEC in vitro and in vivo express eNOS and produce NO basally, and increase their production in response to flow. Additionally, an increase in portal pressure concomitant with the blockade of NO release directly demonstrates that endogenous endothelial-derived NO modulates portal pressure.

Bacterial translocation in cirrhotic rats stimulates eNOS-derived NO production and impairs mesenteric vascular contractility.

Nitric oxide (NO) has been implicated in the arterial vasodilation and associated vascular hyporesponsiveness to vasoconstrictors observed in liver cirrhosis. Bacteria, potent activators of NO and TNF-alpha synthesis, are found in the mesenteric lymph nodes (MLNs) of ascitic cirrhotic rats. Here, we investigated the impact of bacterial translocation (BT) to MLNs on TNF-alpha production, vascular NO release, and contractility in the mesenteric vasculature of ascitic cirrhotic rats. Vascular response to the alpha-adrenoagonist methoxamine, which is diminished in the superior mesenteric arterial beds of cirrhotic rats, is further blunted in the presence of BT. BT promoted vascular NO release in cirrhotic rats, an effect that depended on pressure-induced shear stress and was blocked by the NO inhibitor N(omega)-nitro-L-arginine. Removing the endothelium had the same effect. Endothelial NO synthase (eNOS), but not the inducible isoform (iNOS), was present in mesenteric vasculature of cirrhotic rats with and without BT, and its expression was enhanced compared with controls. TNF-alpha was induced in MLNs by BT and accumulated in parallel in the serum. This TNF-alpha production was associated with elevated levels of tetrahydrobiopterin (BH(4)), a TNF-alpha-stimulated cofactor and enhancer of eNOS-derived NO biosynthesis and NOS activity in mesenteric vasculature. These findings establish a link between BT to MLNs and increased TNF-alpha production and elevated BH(4) levels enhancing eNOS-derived NO overproduction, further impairing contractility in the cirrhotic mesenteric vasculature.

Propranolol ameliorates the development of portal-systemic shunting in a chronic murine schistosomiasis model of portal hypertension.

We investigated the role of early portal hypotensive pharmacotherapy in preventing the development of portal-systemic shunting in a portal hypertensive model of chronic murine schistosomiasis induced by infecting C3H mice with 60 cercariae of Schistosoma mansoni. Propranolol was administered in drinking water to 20 animals for a period of 6 wk at a dose of 10 mg.kg-1d-1, starting at 5 wk of schistosomal infection. 32 age-matched mice with chronic schistosomal infection served as controls. All animals were studied 11 wk after the infection. Compared with controls the portal pressure (10.8 +/- 0.40 mmHg) was significantly lower (P less than 0.001) in the propranolol-treated animals (7.9 +/- 0.80 mmHg). Portal-systemic shunting was decreased by 79%, from 12.2 +/- 3.34% in controls to 2.5 +/- 0.99% in the propranolol group (P less than 0.05). Portal venous inflow was reduced by 38% in the propranolol treated animals (2.50 +/- 0.73 ml/min; n = 6) compared with controls (4.00 +/- 0.34 ml/min; n = 8; P less than 0.05). The worm burden, the granulomatous reaction, the collagen content of the liver, and the serum bile acid levels were not significantly different between the two groups of animals. These results demonstrate that in chronic liver disease induced by schistosomiasis, the development of portal-systemic shunting can be decreased or prevented by the reduction of flow and pressure in the portal system.

Hypersensitivity of mesenteric veins to 5-hydroxytryptamine- and ketanserin-induced reduction of portal pressure in portal hypertensive rats.

Isolated superior mesenteric veins from portal hypertensive rats were 3 to 10 times more sensitive to 5-hydroxytryptamine (5-HT) and 3 times less sensitive to (-)-noradrenaline than veins from sham-operated rats. The sensitivity to vasopressin did not differ in the 2 groups. Ketanserin competitively antagonized the effects of 5-HT in superior mesenteric veins and portal veins with high affinity (KB values 0.1-0.3 nM), as expected for 5-HT2-receptors. The affinity of ketanserin for 5-HT2-receptors was similar in veins from normal, sham-operated or portal-hypertensive rats. Intraportal injections of low doses of 5-HT caused increases in portal pressure which were more pronounced in portal hypertensive rats than in sham-operated rats and were blocked by 0.3 mg kg-1 ketanserin in both groups. Ketanserin 0.3 mg kg-1 did not block the portal pressor response to (-)-noradrenaline in either group of rats. In portal hypertensive rats but not in sham-operated rats, 0.3 mg kg-1 ketanserin caused decreases in portal pressure, portal flow and cardiac output, as estimated by radioactive microspheres. The reduction in portal pressure caused by ketanserin was due mainly to a decrease in portal venous inflow secondary to a decreased cardiac output. The reduction in cardiac output, which was observed only in the portal hypertensive rats but not in sham-operated rats, is consistent with venous dilatation and pooling of blood in the portal venous system. The venous pooling could be secondary to the blockade of 5-HT2-receptors in the portal venous system. It is proposed that ketanserin should be explored for the treatment of patients with portal hypertension.

Pathophysiology of portal hypertension.

Portal hypertension is a common clinical syndrome associated with chronic liver diseases, and is characterized by a pathological increase in portal pressure that leads to the formation of portosystemic collaterals resulting in shunting of portal blood into the systemic circulation. The increase in portal pressure is due to an increase in vascular resistance and an elevated portal blood flow. The site of increased resistance is variable, and dependent upon the disease process. The site of relative obstruction may be prehepatic, hepatic, or posthepatic. There are several intrahepatic lesions that lead to increased resistance. Some of these lesions are irreversible, like fibrosis, regenerating nodules, and capillarization of the space of Disse; however, there is a functional component, increased vascular tone, which contributes to increased intrahepatic resistance and is potentially reversible. Another important factor contributing to the increased portal pressure is elevated portal blood flow. Peripheral vasodilatation initiates the classical profile of decreased systemic resistance, expanded plasma volume, elevated splanchnic blood flow, and elevated cardiac index, which characterize the hyperdynamic circulatory state. This hyperdynamic circulation is responsible for various complications of portal hypertension.

Pharmacological vs. mechanical reduction in portal pressure: a comparative study.

Previous studies have shown that decreasing blood flow in the superior mesenteric artery (SMA) by the infusion of intra-arterial vasopressin into or partial mechanical obstruction of the SMA by a balloon catheter (partial balloon obstruction) causes similar alterations in splanchnic hemodynamics, but divergent changes in systemic hemodynamics. The effects of these two methods of reducing SMA blood flow were compared in each of six anesthetized normal dogs. Vasopressin and partial balloon obstruction induce similar reductions in portal pressure (-54 +/- 12% vs. -46 +/- 11%), wedge hepatic vein pressure (-54 +/- 13% vs. -53 +/- 18%), and portal venous flow (-34 +/- 7% vs. -37 +/- 7%). Significantly different effects between intra-arterial vasopressin and partial balloon obstruction were observed, however, in cardiac output (a decrease of -24 +/- 5% vs. an increase of +12 +/- 4%) (P less than 0.001), heart rate (-8 +/- 3% vs. 0) (P less than 0.05), and systemic vascular resistance (+36 +/- 8% vs. -2 +/- 2%) (P less than 0.005), respectively. These results indicate that the two procedures are equally effective in reducing portal venous pressure and blood flow. Partial balloon obstruction, however, does not induce the potentially deleterious systemic hemodynamic effects seen with vasopressin infusion. In fact, some of the changes observed with partial balloon obstruction, especially the increase in cardiac output, are considered to be beneficial. In an additional five dogs, partial balloon obstruction was maintained for 5 hours. Throughout, the reduction in portal venous pressure (hepatic venous wedge minus hepatic venous free pressure) was maintained at less than half of the baseline levels (4.75 +/- 0.43 vs. 2.25 +/- 0.32 mm Hg), and the mean arterial pressure at baseline values. All of the dogs survived and were well at 1 week after the prolonged partial obstruction. No abnormalities were observed in the anatomical or histological studies of the small intestine. This study suggests that partial balloon obstruction of the SMA has theoretical therapeutic advantages over intra-arterial vasopressin for reducing portal venous pressure.

Resistance to reinfection in experimental murine schistosomiasis: role of porto-hepatic hemodynamics.

Experiments were conducted to evaluate critically, and independently of the immune system, the possible role of hemodynamic mechanisms in resistance to schistosomal reinfection. The effects of a challenge schistosomal infection were compared in groups of mice which were either previously infected with schistosomiasis, vaccinated with irradiated cercariae, or underwent partial portal vein ligation for the induction of portal hypertension and porto-systemic shunting. Following infection with 60 cercariae, the appearance of portal hypertension preceded by approximately 2 weeks the development of porto-systemic shunting, which reached maximal values 11 weeks postinfection. Such a primary infection conferred on C3H mice an estimated 90% protection to a 2nd infection, measured by the reduction of worm burden. Worm burdens were also reduced in vaccinated and ligated animals as compared to normal controls. The protection amounted to 30% and 56%, respectively, in the C3H strain and 63% and 75-85%, respectively, in the C57Bl/6 strain. Reduction in worm burden in the ligated animals is believed to be due to the extrahepatic porto-systemic vascular shunts. Hemodynamic as well as immunological factors may account for the resistance to reinfection observed in chronic murine schistosomiasis.

Pathophysiology of portal hypertension and variceal bleeding.

Portal hypertension results from an interaction of abnormal intrahepatic resistance and increases in portal blood flow. Intrahepatic resistance is probably multifactorial in nature and may include compression of hepatic veins by regenerating nodules, collagen deposition in sinusoids and venules, hepatocyte enlargement, and constriction of sinusoids by contractile myofibroblasts. The increase in splanchnic blood flow observed is incompletely understood, but it may involve circulating vasodilators and alteration in volume and sodium balance. The end result of these interactions is the development of increased portal pressure and portosystemic collaterals, the most important of which are esophageal varices. The rupture of esophageal varices is a devastating complication of portal hypertension. Increased portal pressure is necessary for the development and rupture of varices but apparently not sufficient, because many patients with elevated portal pressures never bleed. Presumably, local factors must be involved. Variceal wall tension is probably the best single descriptor of risk from variceal hemorrhage. The wall-tension formula unites the contributions of portal pressure, varix size, and wall thickness to variceal rupture. Lowering portal pressure, reducing varix size, and supporting varices in scar tissue may all lower the risk of hemorrhage.

Randomised clinical trial: the safety and efficacy of long-acting octreotide in patients with portal hypertension.

BACKGROUND: It remains unclear whether a long-acting preparation of octreotide (Sandostatin LAR) can be safely used for portal hypertension in patients with compensated cirrhosis. AIM: To determine the safety and efficacy of LAR among patients with Child Pugh Class A or B cirrhosis and small oesophageal varices. METHODS: A randomised, double-blind, placebo-controlled study was conducted in 39 patients with cirrhosis and small oesophageal varices. Safety was based on frequency and severity of adverse events. Efficacy was determined by hepatic vein pressure gradient (HVPG) measured at baseline and day 84 following administration of LAR 10 mg (n = 15), 30 mg (n = 10) or saline (n = 14). Fasting and postprandial portal blood flow (PBF), superior mesenteric artery pulsatility index (SMA-PI), glucagon and octreotide levels were measured. An intention-to-treat analysis was performed. RESULTS: Four patients in the LAR 30 group (40%) withdrew from the study due to serious adverse events. No patient in the LAR 10 or control group had serious adverse events. There was no statistically significant decrease between HVPG at day 84 and baseline with LAR 30 mg (11.8 ± 2.3 mmHg vs. 14.1 ± 3.2), LAR 10 mg (15.3 ± 4.8 mmHg vs. 15.1 ± 3.8), or saline (13.3 ± 3.8 mmHg vs. 15.1 ± 4.3) (P = 0.26). Neither PBF, SMA-PI nor plasma glucagon levels were significantly decreased from baseline (P = 0.56). CONCLUSIONS: The absence of significant haemodynamic benefit, as well as the high frequency of severe adverse events associated with use of LAR, do not support the use of this agent in the treatment of portal hypertension.

Hyperdynamic state in chronic liver diseases.

Reduced splanchnic arteriolar resistance and increased portal venous outflow have been associated with chronic portal hypertension. This hyperdynamic splanchnic circulatory state is accompanied by a hyperdynamic systemic circulation characterized by a high cardiac index and low systemic vascular resistance. The systemic phenomenon occurs earlier than the regional one, possibly because the increased outflow resistance induced by portal hypertension delays the effect of circulating humoral vasodilators on the splanchnic bed. Several vasoactive substances have been advanced as the vasodilator responsible for the hyperdynamic circulation. Glucagon and bile acids have been found to be potent splanchnic vasodilators in animal studies. However, neither consistently dilates other vascular beds. For example, bile acids are splanchnic--but not systemic--vasodilators. Several studies suggest a role in chronic liver disease for nitric oxide, a potent endogenous vasodilator secreted by endothelial cells. Prostaglandins have also been proposed as the responsible vasodilator. However, mixed results have been seen with experimental suppression of prostaglandin synthesis. For a hyperdynamic state to develop and persist requires not only reduced cardiac afterload, but also an increased venous return. Recently, we found an expanded plasma volume after initial vasodilation in rats with portal hypertension. This 'refill' of the circulation may be intrinsic to development of the hyperdynamic state. Furthermore, this blood volume expansion may be the link between vasodilation and the hyperdynamic circulation.

Drug therapy of portal hypertension.

Portal hypertension is the result of increased flow and increased resistance in the portal system. Pharmacological therapy is aimed at altering these factors by the use of vasoconstrictors to reduce flow and vasodilators to decrease intrahepatic resistance. The current status of pharmacological agents to achieve these effects is reviewed.

Pharmacological treatment of portal hypertension.

The available pharmacological treatments for portal hypertension are reviewed. Vasoconstrictor treatments include vasopressin (VP), the synthetic VP analogue tGLVP, combined nitroglycerin (NTG)-VP, somatostatin (SRIF), SRIF analogues and non-selective beta-blockers. Vasodilator treatments include short- and long-acting organic nitrates. Infusions of VP > 1.0 U/min can cause severe side-effects. tGLVP can control variceal bleeding and improve survival and causes fewer complications than VP.SRIF is as effective as tGLVP in controlling bleeding and improving survival and has minimal side effects. Beta-blockers are effective in preventing the first variceal haemorrhage and are well tolerated.

The pharmacologic treatment of portal hypertension.

Surgical procedures that lower portal pressure, such as portacaval shunts, prevent variceal hemorrhage. Portal hypertension is the result of increased flow and increased resistance in the portal system. Pharmacologic therapy is aimed at altering these factors by the use of vasoconstrictors to reduce flow and vasodilators to decrease resistance. The current status of pharmacologic agents to achieve these effects is reviewed.