Endothelin-1 and endothelin-3 in cirrhosis: relations to systemic and splanchnic haemodynamics

Long-term haemodynamic effects of octreotide on postprandial splanchnic hyperemia in humans: a placebo-controlled echo-doppler study

BACKGROUND

Octreotide is a potent splanchnic hypotensive somatostatin analogue effective in the treatment of acute variceal bleeding.

AIM

To study the effects of octreotide on basal and postprandial splanchnic and systemic haemodynamics, and hormonal changes in humans.

METHODS

Twenty-four healthy volunteers were randomized to receive a liquid meal and either octreotide (OCT, 100 microg bolus) or placebo repeatedly every 4 h for 48 h. Splanchnic (Doppler ultrasound) and systemic haemodynamics (non-invasive cardiac monitoring) were assessed for 2 h on four consecutive days: one control day and after doses 1 (0 h), 7 (24 h) and 13 (48 h).

RESULTS

The maximum postprandial increases in mean blood velocity of the superior mesenteric artery (SMA-Vmean +72%), portal (PBF +52%) and total hepatic blood flow (HBF +50%) observed in the placebo group, were abolished after the first dose of octreotide (SMA-Vmean -23%, P<0.01; PBF -22%, P<0.01; HBF -21%, P<0.01). Postprandial hyperemia was restored at the end of the 48-h study period, but baseline SMA-Vmean (placebo 40+/-12, OCT 29+/-11 cm/s, P<0.05) and PBF (placebo 1200+/-971, OCT 743+/-449 mL/min, P<0.05) remained significantly lower in the octreotide group. The postprandial decrease of systemic vascular resistance and increase of cardiac index were prevented by octreotide for 48 h.

CONCLUSIONS

Repeated 4-hourly bolus injections of octreotide reduce splanchnic blood flow for at least 48 h, but the prevention of food-induced splanchnic hyperemia is short-lasting.

Endothelin-induced contraction of the portal vein in cirrhosis

Endothelin (ET) is one of the most potent vasoconstrictors known so far. It has been proposed that the ET-induced contraction of hepatic stellate cells (Ito, endothelial cells) is an important mechanism for the development of portal hypertension. The purpose of this study was to investigate in an in vitro model whether ET causes a contraction of the portal vein which can contribute to portal hypertension in cirrhosis. Portal veins from normal and cirrhotic rats were used for experiments. Measurements were performed in vitro for cumulative concentrations of ET-1 and ET-3 (1, 5, 10, 50 and 100 nM). Both ETs caused a dose-dependent increase in portal venous tension; the maximal tension (Tmax) was measured at 50 nM. The measured Tmax was higher for cirrhotic (ET-1: Tmax = 189%; ET-3: Tmax = 175%) than for normal rats (ET-1: Tmax = 130%; ET-3: Tmax = 151%). ET-3 produced a higher tension of portal veins in normal rats than ET-1. In conclusion, this study shows that portal veins from cirrhotic rats react more sensitively to ET than those from normal rats. Besides the ET-induced contraction of hepatic stellate cells, contraction of the portal vein and its intrahepatic branches, especially in cirrhotic individuals, has to be considered as a further mechanism of ET contributing to portal hypertension.

Hepatic blood flow and splanchnic oxygen consumption in patients with liver failure. Effect of high-volume plasmapheresis

Liver failure represents a major therapeutic challenge, and yet basic pathophysiological questions about hepatic perfusion and oxygenation in this condition have been poorly investigated. In this study, hepatic blood flow (HBF) and splanchnic oxygen delivery (DO2, sp) and oxygen consumption (VO2,sp) were assessed in patients with liver failure defined as hepatic encephalopathy grade II or more. Measurements were repeated after high-volume plasmapheresis (HVP) with exchange of 8 to 10 L of plasma. HBF was estimated by use of constant infusion of D-sorbitol and calculated according to Fick's principle from peripheral artery and hepatic vein concentrations. In 14 patients with acute liver failure (ALF), HBF (1.78 +/- 0.78 L/min) and VO2,sp (3.9 +/- 0.9 mmol/min) were higher than in 11 patients without liver disease (1.07 +/- 0.19 L/min, P <.01) and (2.3 +/- 0.7 mmol/min, P <.001). In 9 patients with acute on chronic liver disease (AOCLD), HBF (1.96 +/- 1.19 L/min) and VO2,sp (3.9 +/- 2.3 mmol/min) were higher than in 18 patients with stable cirrhosis (1.00 +/- 0.36 L/min, P <.005; and 2.0 +/- 0.6 mmol/min, P <.005). During HVP, HBF increased from 1.67 +/- 0.72 to 2.07 +/- 1.11 L/min (n=11) in ALF, and from 1.89 +/- 1.32 to 2.34 +/- 1.54 L/min (n=7) in AOCLD, P <.05 in both cases. In patients with ALF, cardiac output (thermodilution) was unchanged (6.7 +/- 2.5 vs. 6.6 +/- 2.2 L/min, NS) during HVP. Blood flow was redirected to the liver as the systemic vascular resistance index increased (1,587 +/- 650 vs. 2, 020 +/- 806 Dyne. s. cm-5. m2, P <.01) whereas splanchnic vascular resistance was unchanged. In AOCLD, neither systemic nor splanchnic vascular resistance was affected by HVP, but as cardiac output increased from 9.1 +/- 2.8 to 10.1 +/- 2.9 L/min (P <.01) more blood was directed to the splanchnic region. In all liver failure patients treated with HVP (n=18), DO2,sp increased by 15% (P <.05) whereas VO2,sp was unchanged. Endothelin-1 (ET-1) and ET-3 were determined before and after HVP. Changes of ET-1 were positively correlated with changes in HBF (P <.005) and VO2,sp (P <.05), indicating a role for ET-1 in splanchnic circulation and oxygenation. ET-3 was negatively correlated with systemic vascular resistance index before HVP (P <.05) but changes during HVP did not correlate. Our data suggest that liver failure is associated with increased HBF and VO2, sp. HVP further increased HBF and DO2,sp but VO2,sp was unchanged, indicating that splanchnic hypoxia was not present.

Regulation of endothelin-1 synthesis by endothelin-converting enzyme-1 during wound healing

Endothelin-1 (ET-1) is involved in the pathogenesis of a number of diseases, including wound healing. In cirrhosis, the wounding response of the liver, circulating ET-1 levels are elevated; moreover, ET-1 has potent effects on hepatic stellate cells, the key effectors of cirrhosis. In this study, we have examined the regulatory role of ECE-1, a critical enzyme involved in ET-1 synthesis, in the two major cellular sources of hepatic ET-1. ET-1 release from normal hepatic endothelial cells was 25-fold higher than that from normal stellate cells. However, after liver injury, ET-1 release was increased in stellate cells but markedly decreased in endothelial cells. The two major isoforms of ECE-1, ECE-1alpha/1beta, made up 80% and 20%, respectively, of total ECE-1 in both stellate and endothelial cells. Following liver injury, ECE-1alpha mRNA was decreased by 44.2% in stellate cells, and by 16.1% in endothelial cells. ECE-1beta mRNA expression remained unchanged after injury. In contrast to ECE-1 mRNA, ECE-1 protein expression was increased by 43.9% in stellate cells but decreased in endothelial cells, while relative ECE-1 enzymatic activity was unchanged. In mRNA stability experiments, the half-life of ECE-1alpha mRNA in normal stellate cells was 13 h compared with 38 h in cells from injured livers. Thus, during hepatic wound healing, differential regulation of ECE-1 mRNA and protein appears to be critical in controlling ET-1 production.

Bacterial infection in the pathogenesis of variceal bleeding

Variceal bleeding is a life-threatening complication of cirrhosis. Potential risk factors include clinical, endoscopic, and haemodynamic factors, but why bleeding occurs unpredictably in individual patients is not known. We postulate that bacterial infections in patients with variceal haemorrhage may be the critical factor that triggers bleeding. In patients with large varices and a high wall tension, the release of endotoxin into the systemic circulation during episodes of bacterial infection results in a further increase in portal pressure through the induction of endothelin and possibly vasoconstrictive cyclo-oxygenase products. The subsequent contraction of hepatic stellate cells causes a rise in intrahepatic vascular resistance. Furthermore, endotoxin-induced nitric oxide and prostacyclin, and prostacyclin induced by endothelin could inhibit platelet aggregation, which may result in a further deterioration of primary haemostasis at the level of varix. We propose that the combination of these two effects leads to the onset of variceal haemorrhage.

Endothelin and other mediators in the pathophysiology of portal hypertension

Hyperdynamic circulation as well as increased hepatic resistance contribute to portal hypertension in cirrhosis of the liver [1]. Hyperdynamic circulation with increased cardiac output, heart rate and plasma volume and decreased arterial blood pressure and systemic <i>vasodilatation</i> is pivotal for the hyperdynamic circulation. This has prompted intense research of a number of endogenous neurohumoral mediators with vasodilating properties (NO, natriuretic peptides, glucagon, etc.) [2, 3].

Endothelin-1-induced vasoconstriction causes a significant increase in portal pressure of rat liver: localized constrictive effect on the distal segment of preterminal portal venules as revealed by light and electron microscopy and serial reconstruction

Intraportal infusion of endothelin-1 (ET-1), a potent vasoconstrictor, significantly elevates portal venous pressure. To determine the major site of vascular constriction in the intrahepatic porto-sinusoidal system, we performed an in situ perfusion of rat livers with 1 nmol/L ET-1 at a flow rate of 20 mL/min. Portal pressure rose from 22 cm H2O to 54 cm H2O within 25 minutes. Specimens were prepared for light-microscopic serial reconstruction and electron microscopy. The distal segment of preterminal portal venules (DS/PPV) with an inner diameter of 40 to 80 microm showed complete obliteration of the lumen over a 300-microm distance caused by the intense contraction of perivascular smooth muscle cells and protruding of endothelial cells into the lumen. The proximal segment of preterminal portal venules (PS/PPV) with a larger diameter up to 150 microm also underwent strong constriction, but still had luminal space for the flow, while the PS/PPV with a diameter of 150 to 400 microm showed moderate or mild constriction and retained a wide lumen. Neither terminal portal venules, inlet venules, sinusoids, nor central veins, however, exhibited demonstrable constriction. Liver parenchyma fed by the inlet venules that emerged from the PS/PPV exhibited a wide sinusoidal lumen and vacuolated hepatocytes caused by the influx of excess portal perfusate that escaped from the occlusive areas. The present study has revealed that the DS/PPV functions as a presinusoidal quasi-sphincter mechanism and is involved in the redistribution of intrahepatic portal flow under increased portal pressure.

Increased adrenomedullin levels in cirrhosis: relationship with hemodynamic abnormalities and vasoconstrictor systems

BACKGROUND & AIMS

Arterial vasodilation in cirrhosis may be related to increased circulating levels of vasodilators. This study was designed to assess the circulating levels of adrenomedullin, a recently described vasodilator peptide, in cirrhosis.

METHODS

Plasma adrenomedullin levels were measured in 17 healthy subjects and 34 cirrhotic patients. Hemodynamic parameters, renal function, and levels of vasoactive substances were also assessed.

RESULTS

Patients with ascites had increased adrenomedullin levels (289 +/- 47 pg/mL) compared with healthy subjects and patients without ascites (135 +/- 17 and 142 +/- 32 pg/mL, respectively; P < 0.05). Adrenomedullin levels correlated inversely with arterial pressure, glomerular filtration rate, and renal plasma flow and correlated directly with pulse rate, endothelin levels, and aldosterone and plasma renin activity. In cirrhotic patients, no significant differences in adrenomedullin levels were found between samples obtained from hepatic vein, renal vein, pulmonary artery, and femoral artery. Plasma expansion with albumin suppressed the renin-angiotensin system but did not affect adrenomedullin levels.

CONCLUSIONS

Circulating levels of adrenomedullin are increased in patients with ascites and correlate with hemodynamic and renal abnormalities and activation of vasoconstrictor systems. These increased levels seem to result from a generalized increase in adrenomedullin production from vascular tissue and are not suppressed by plasma expansion. Adrenomedullin may participate in the pathogenesis of arterial vasodilation in cirrhosis.

Cellular localization of endothelin-1 and increased production in liver injury in the rat: potential for autocrine and paracrine effects on stellate cells

Endothelin (ET) peptides have been implicated in the pathogenesis of several biological processes within the liver. ET levels are elevated in the circulation of patients with cirrhosis, and recent data suggest that ET may be overproduced in the liver itself in this condition. The aims of the current study were to elucidate the cellular source and expression of endothelin-1 (ET-1) in normal and injured liver, and to investigate its biological effects on stellate cells, the primary target of ETs in the liver. In normal hepatic cells, preproET-1 messenger RNA (mRNA) was detected in only nonparenchymal cells, predominantly in sinusoidal endothelial cells. After biliary fibrosis and early cirrhosis induced by bile duct ligation, preproET-1 mRNA and immunoreactive ET levels increased with progressive injury in whole liver extracts, as well as in isolated stellate and endothelial cell fractions. Eight days after bile duct ligation, the relative increase in preproET-1 mRNA was 1.6- and 7.6-fold above normal in sinusoidal endothelial and stellate cells, respectively. Additionally, immunoreactive ET peptide levels increased by 60% +/- 27% over basal values in sinusoidal endothelial cells and 98% +/- 40% in stellate cells. Cultured stellate cells responded dramatically to exogenous ET-1 by the spreading and up-regulation of smooth muscle alpha actin expression. Furthermore, in early culture before cellular activation, ET-1 (10 nmol/L) caused over a twofold increase in [3H]thymidine incorporation, while activated cells (i.e., those cultured for >1 week) exposed to ET-1 exhibited up to a fivefold decrease in [3H]thymidine incorporation. The data indicate that not only is ET-1 overproduced by both sinusoidal endothelial and stellate cells during liver injury, but that it also has potent effects on features of stellate cell activation. We conclude that autocrine and paracrine production of ET-1 is prominent and is likely to be important in the pathogenesis of hepatic diseases.

Urinary endothelin-like immunoreactivity in patients with cirrhosis

BACKGROUND/AIMS

To investigate a possible relationship between the renal production of endothelin and the presence of renal dysfunction and activation of vasoactive systems in cirrhosis, the urinary excretion and the circulating plasma levels of immunoreactive endothelin (irET) and the plasma levels of vasoactive hormones were measured in 19 healthy subjects, 12 cirrhotic patients without ascites and 39 patients with ascites and different degrees of renal dysfunction.

METHODS

The urinary excretion and the circulating levels of irET were assessed after 5 days on a 40 mEq sodium diet and off diuretics. Renal function parameters and the plasma levels of vasoactive hormones were also measured.

RESULTS

Patients with and without ascites had similar values of urinary irET as compared with healthy subjects (30+/-3, 31+/-3 and 29+/-2 ng/day, respectively, p>0.10). By contrast, patients with ascites had higher circulating levels of irET (15+/-1.2 pg/ml) than patients without ascites and healthy subjects (11+/-1.6 and 5+/-0.4 pg/ml, p<0.01). In patients with cirrhosis, no correlation was found between urinary irET and circulating irET. Moreover, urinary irET did not correlate with liver tests, serum and urine sodium, glomerular filtration rate or vasoactive substances. Patients with hepatorenal syndrome had similar urinary irET to patients with ascites without hepatorenal syndrome.

CONCLUSIONS

Urinary excretion of irET is not increased in cirrhotic patients with ascites and does not correlate with abnormalities in renal function.

Responsiveness to growth factors in aortic vascular smooth muscle cells from rats with cirrhosis

Hemodynamic changes in cirrhosis may be associated with alterations in aortic vascular smooth muscle cell (AVSMC) function. The present study compared the proliferative response to serum and growth factors in cirrhotic and control AVSMC. Serum from cirrhotic rats, cirrhotic cell lysates, and the conditioned medium of cultured cirrhotic AVSMC induced an increase in [3H]thymidine incorporation in control but not in cirrhotic AVSMC. Platelet-derived growth factor-beta (PDGF-BB) induced a greater increase in [3H]thymidine incorporation in cirrhotic than in control cells. [3H]thymidine incorporation induced by cirrhotic conditioned medium was blocked by anti-PDGF antibody. Immunoblot studies showed that the anti-PDGF antibody recognized a 30-kDa protein in the conditioned medium of cirrhotic AVSMC culture, a protein corresponding to PDGF. Binding studies of PDGF-BB indicated a twofold increase in receptor density in cirrhotic AVSMC with no alteration in affinity for PDGF-BB. We conclude that an increased responsiveness of cirrhotic AVSMC to the PDGF could contribute to alterations in AVSMC and muscle cell tone that may play a role in the hemodynamic changes in cirrhosis.

Hepatic stellate cells and intralobular innervation in human liver cirrhosis

In normal and cirrhotic human liver tissues, we examined immunolocalization of alpha-smooth muscle actin (alpha-SMA), endothelin-1 receptor (ET-1R), and S-100 protein, with special emphasis on the intralobular spaces, using immunohistochemical methods. The ratio of the number of hepatic stellate cells (HSCs) with closely apposing nerve endings to the total number of HSCs in normal livers was compared with that in cirrhotic livers by electron microscopy. Immunolocalization of alpha-SMA and ET-1R was obviously recognized along the sinusoidal walls in cirrhotic liver and was significantly increased in cirrhotic liver compared with that in normal liver. Immunoreactive products for these substances were mainly localized in HSCs. However, immunolocalization of S-100 protein in intralobular spaces was markedly decreased in cirrhotic liver compared with that in normal liver. Nerve fibers were ultrastructurally hardly visible in intralobular spaces of cirrhotic livers. The ratio of the number of HSCs with closely apposing nerve endings to the total number of HSCs was significantly reduced in cirrhotic liver compared with that in normal liver. These results indicate that in liver cirrhosis, alpha-SMA-positive HSCs may play an important role in hepatic sinusoidal microcirculation through vasoactive agents such as ET-1 rather than through intralobular innervation.

Ascites and renal functional abnormalities in cirrhosis. Pathogenesis and treatment

In the past few years, there have been important advances in the field of pathogenesis and management of ascites and hepatorenal syndrome in cirrhosis. A new pathogenic theory of ascites and renal dysfunction in cirrhosis has been presented and previously ill-defined conditions, such as refractory ascites and hepatorenal syndrome, have been defined precisely. The link between the diseased liver and the disturbances in renal function and vasoactive systems is not completely known, but a large body of evidence indicates that it consists of a circulatory dysfunction that affects mainly the arterial circulation and is characterized by an inability to maintain an effective arterial blood volume within normal limits. The research on the mechanisms of this circulatory dysfunction will give valuable information in the design of more pathophysiologically oriented therapeutic approaches to the management of ascites.

New concepts in the pathogenesis of portal hypertension: hepatic wounding and stellate cell contractility

The pathogenesis of portal hypertension is multifactorial, and appears to result from interplay between fixed and dynamically modulable elements; the stellate cell is a newly recognized example of the latter. This perisinusoidal, pericyte-like cell has contractile features that are most prominent after liver injury, concomitant with their activation. These data imply an exaggerated contractile phenotype in the cirrhotic liver. This cell may contribute to increased intrahepatic portal hypertension via perisinusoidal constriction of the sinusoid or by contraction of fibrous extracellular matrix rich in type I collagen with concomitant disruption of lobular architecture. Endothelins and NO play a major role in the modulation of stellate cell contractility, and are therefore important in the pathogenesis of intrahepatic portal hypertension. These new data provide potential areas for therapeutic intervention in this clinical entity.

Daily profile of plasma endothelin-1 and -3 in pre-ascitic cirrhosis: relationships with the arterial pressure and renal function

BACKGROUND/AIMS

Measurements of plasma endothelin-1 and -3 in pre-ascitic cirrhosis have provided controversial results. Similarly, the role of the endothelin system in the pathogenesis of volume and hemodynamic disturbances of cirrhosis is still debated. To provide a further insight into this issue, we assessed the daily fluctuations of plasma endothelins and their relationship with arterial pressure and renal function in pre-ascitic cirrhosis.

METHODS

Endothelin-1 and -3, plasma renin activity, atrial natriuretic peptide, noradrenaline and mean arterial pressure were measured at 11 pm, 7 am, 9 am and 6 pm in 10 patients with pre-ascitic cirrhosis and in 10 healthy subjects on normal sodium diet and carrying on their usual activities (supine from 10 pm to 7 am, standing and mobile after 7 am). Glomerular filtration rate and daily renal sodium excretion were assessed during the supine period, and from 7 am to 12 am and from 12 am to 10 pm during the standing period.

RESULTS

Endothelin-1 was higher in patients than in control subjects (p=0.000) and did not change during the study. Endothelin-3 was also higher in patients (p=0.002) and showed slight fluctuation in control subjects. The mean daily level of plasma renin activity was lower (p=0.016) and that of atrial natriuretic peptide higher (p=0.000) in patients with cirrhosis. Norepinephrine and mean arterial pressure did not differ significantly between the two groups. No correlations were found between endothelins and either hemodynamic or neuro-hormonal and renal function parameters in the two groups.

CONCLUSIONS

Despite the presence of increased effective volemia (as suggested by the reduced plasma renin activity and elevated atrial natriuretic peptide) and normal adrenergic tone, patients with pre-ascitic cirrhosis show elevated levels of endothelin-1 and endothelin-3 throughout the day. In early cirrhosis circulating endothelins, although elevated, do not appear to play a more prominent role in setting arterial pressure than in normal subjects, and endothelin elevation is not detrimental to renal function.

Endothelin-induced vasoconstriction in isolated perfused liver preparations from normal and cirrhotic rats

Isolated, perfused rat liver preparations (IPRL), obtained from rats with carbon tetrachloride-induced cirrhosis and normal controls, were used to investigate responses to the vasoactive peptide endothelin-1 (ET-1). The mean perfusion resistance (R) of cirrhotic IPRL was significantly greater than that of controls (2.63 +/- 0.24 vs 1.54 +/- 0.14 mmHg/mL per min per g; P < 0.01). Both control and cirrhotic IPRL demonstrated a concentration-related increase in resistance (delta R) in response to ET-1, with a minimum effective concentration of approximately 3 x 10(-11) mol/L. The EC50 (-log of the 50% effective concentration) was not significantly different between cirrhotic and control IPRL (8.48 +/- 0.19 and 8.79 +/- 0.11, respectively); however, the maximum response to ET-1 was significantly greater in cirrhotic preparations (R: 10.4 +/- 2.2 vs 4.4 +/- 0.5 mmHg/mL per min per g, P < 0.01; DR, 7.8 +/- 2.1 vs 2.8 +/- 0.4 mmHg/mL per min per g, P < 0.01). Following maximal stimulation by ET-1, the mean portal-hepatic venous pressure gradient at a physiological flow rate of 1 mL/min per g was approximately 90% greater across cirrhotic IPRL than that across normal IPRL (11.2 +/- 2.0 vs 5.9 +/- 0.9 mmHg, respectively; P < 0.05). These results support the hypothesis that endogenously released ET-1 has a significant influence on the portal vascular resistance of cirrhotic liver in vivo and has an important role in the pathogenesis of portal hypertension.

Endothelins in chronic liver disease

This review describes recent progress in the accumulation of knowledge about the endothelins (ETs), a family of vasoactive 21-amino acid polypeptides, in chronic liver disease. Particular prominence is given to the dynamics of ET-1 and ET-3 and their possible relation to the disturbed circulation and neurohumoral dysregulation found in cirrhosis. Recent studies have shown that the ET system is highly activated in most cirrhotic patients. Circulating ET-1 and ET-3 levels have a positive relation to the severity of the disease and fluid retention, with the highest values recorded in patients with functional renal failure. Studies on liver biopsies have revealed synthesis of ET-1 in hepatic endothelial and other cells, and recent investigations have identified the hepatosplanchnic system as a major source of ET-1 and ET-3 spillover into the circulation, with a direct relation to portal venous hypertension. In addition, marked associations with disturbance of systemic haemodynamics and with abnormal distribution of blood volume have been reported. Although the pathophysiological importance of the ET system in chronic liver disease is not completely understood, similarities to other vasopressive and antinatriuretic regulatory systems (i.e. the sympathetic nervous system, renin-angiotensin-aldosterone and vasopressin) are apparent, with respect to kinetics and haemodynamic dysregulation. Cirrhosis seems to be a pathophysiological condition with indications of the occurrence of ETs, not only as local modulators, but also as a system with potential importance for systemic regulation.