Ritanserin decreases portal pressure in conscious and unrestrained cirrhotic rats

Targeting 5-Hydroxytryptamine Receptor 1A in the Portal Vein to Decrease Portal Hypertension

BACKGROUND & AIMS

Portal hypertension (PH) is one of the most frequent complications of chronic liver disease. The peripheral 5-hydroxytryptamine (5-HT) level was increased in cirrhotic patients. We aimed to elucidate the function and mechanism of 5-HT receptor 1A (HTR1A) in the portal vein (PV) on PH.

METHODS

PH models were induced by thioacetamide injection, bile duct ligation, or partial PV ligation. HTR1A expression was detected using real-time polymerase chain reaction, in situ hybridization, and immunofluorescence staining. In situ intraportal infusion was used to assess the effects of 5-HT, the HTR1A agonist 8-OH-DPAT, and the HTR1A antagonist WAY-100635 on portal pressure (PP). Htr1a-knockout (Htr1a-/-) rats and vascular smooth muscle cell (VSMC)-specific Htr1a-knockout (Htr1aΔVSMC) mice were used to confirm the regulatory role of HTR1A on PP.

RESULTS

HTR1A expression was significantly increased in the hypertensive PV of PH model rats and cirrhotic patients. Additionally, 8-OH-DPAT increased, but WAY-100635 decreased, the PP in rats without affecting liver fibrosis and systemic hemodynamics. Furthermore, 5-HT or 8-OH-DPAT directly induced the contraction of isolated PVs. Genetic deletion of Htr1a in rats and VSMC-specific Htr1a knockout in mice prevented the development of PH. Moreover, 5-HT triggered adenosine 3',5'-cyclic monophosphate pathway-mediated PV smooth muscle cell contraction via HTR1A in the PV. We also confirmed alverine as an HTR1A antagonist and demonstrated its capacity to decrease PP in rats with thioacetamide-, bile duct ligation-, and partial PV ligation-induced PH.

CONCLUSIONS

Our findings reveal that 5-HT promotes PH by inducing the contraction of the PV and identify HTR1A as a promising therapeutic target for attenuating PH. As an HTR1A antagonist, alverine is expected to become a candidate for clinical PH treatment.

The function of serotonin within the liver

Serotonin or 5-hydroxytryptamine (5-HT) is known to regulate several key aspects of liver biology and these functions include hepatic blood flow, innervation and wound healing. Given the importance of these functions it is surprising that relatively little time has been dedicated to studying the precise function and mechanisms of serotonin within the liver. Here we describe what is known about serotonin and the liver and those receptor types that mediate the observed effects with an aim to stimulating new interest in the field of serotonin and liver biology.

5-Hydroxytryptamine receptors in the human cardiovascular system

The human cardiovascular system is exposed to plasma 5-hydroxytryptamine (5-HT, serotonin), usually released from platelets. 5-HT can produce harmful acute and chronic effects. The acute cardiac effects of 5-HT consist of tachycardia (preceded on occasion by a brief reflex bradycardia), increased atrial contractility and production of atrial arrhythmias. Acute inotropic, lusitropic and arrhythmic effects of 5-HT on human ventricle become conspicuous after inhibition of phosphodiesterase (PDE) activity. Human cardiostimulation is mediated through 5-HT4 receptors. Atrial and ventricular PDE3 activity exerts a protective role against potentially harmful cardiostimulation. Chronic exposure to high levels of 5-HT (from metastatic carcinoid tumours), the anorectic drug fenfluramine and its metabolites, as well as the ecstasy drug 3,4-methylenedioxymethamphetamine (MDMA) and its metabolite 3,4-methylenedioxyamphetamine (MDA) are associated with proliferative disease and thickening of cardiac valves, mediated through 5-HT2B receptors. 5-HT2B receptors have an obligatory physiological role in murine cardiac embryology but whether this happens in humans requires research. Congenital heart block (CHB) is, on occasion, associated with autoantibodies against 5-HT4 receptors. Acute vascular constriction by 5-HT is usually shared by 5-HT1B and 5-HT2A receptors, except in intracranial arteries which constrict only through 5-HT1B receptors. Both 5-HT1B and 5-HT2A receptors can mediate coronary artery spasm but only 5-HT1B receptors appear involved in coronary spasm of patients treated with triptans or with Prinzmetal angina. 5-HT2A receptors constrict the portal venous system including oesophageal collaterals in cirrhosis. Chronic exposure to 5-HT can contribute to pulmonary hypertension through activation of constrictor 5-HT1B receptors and proliferative 5-HT2B receptors, and possibly through direct intracellular effects.

Drug treatment for bleeding oesophageal varices

At the time of diagnosis of cirrhosis, varices are present in about 60% of decompensated and 30% of compensated patients. The risk factors for the first episode of variceal bleeding in cirrhotic patients are the severity of liver dysfunction, a large size of the varices and the presence of endoscopic red colour signs, but only a third of patients who suffer variceal haemorrhage demonstrate the above risk factors. The only treatment that does not require sophisticated equipment or the skills of a specialist, and is immediately available, is vasoactive drug therapy. Hence, drug therapy should be considered to be the initial treatment of choice and can be administered while the patient is transferred to hospital, as has been done in one recent study. Moreover, drug therapy is no longer considered to be only a 'stop-gap' therapy until definitive endoscopic therapy is performed. Several recent trials have reported an efficacy similar to that of emergency sclerotherapy in the control of variceal bleeding. Furthermore, recent evidence suggests that those patients with high variceal or portal pressure are likely to continue to bleed or re-bleed early, implying that prolonged therapy lowering the portal pressure over several days may be the optimal treatment. Pharmacological treatment with beta-blockers is safe, effective and the standard long-term treatment for the prevention of recurrence of variceal bleeding. The combination of beta-blockers with isosorbide-5-mononitrate needs further testing in randomized controlled trials. The use of haemodynamic targets for the reduction of the HVPG response needs further study, and surrogate markers of the pressure response need evaluation. Ligation has recently been compared with beta-blockers for primary prophylaxis, but there is as yet no good evidence to recommend banding for primary prophylaxis if beta-blockers can be given.

Studies on smaller (approximately 315 microM) microcapsules: IV. Feasibility and safety of intrahepatic implantations of small alginate poly-L-lysine microcapsules

The most successful transplantation site of nonencapsulated islets of Langerhans is the liver. Because usual alginate poly-L-lysine microcapsules were too large (700-1200 microm diameter) for intravascular implantations and were almost exclusively implanted intraperitoneally, the question of the preferred implantation site of microencapsulated islets has received little attention. The feasibility of implanting smaller (approximately 315 microm) alginate poly-L-lysine microcapsules into the liver and the effect of such implantations on portal pressure and liver histology was evaluated in Wistar rats. A bolus of 10,000 microcapsules of 315 microm diameter was injected intraportally (group 1; n = 22). The portal pressure increased from 6.4 +/- 1.8 mmHg to a maximum of 19 mmHg, returned to basal levels within 2 h, and remained normal after 2 months. In group 2 (n = 3), following the injection of 10,000 larger microcapsules (420 microm), the portal pressure increased to > 60 mmHg and two out of the three rats died within 3 h. When 5,000 microcapsules of 420-microm diameter were injected (group 3; n = 5), the portal pressure peaked to 30 +/- 8 mmHg and remained elevated after 4 h (12 +/- 3 mmHg), but returned to normal (8 +/- 1 mmHg) after 2 weeks. Histological studies showed normal hepatic architecture without collagen deposition into portal tracts occupied by microcapsules.

CONCLUSION

intrahepatic implantations of approximately 315-microm alginate poly-L-lysine microcapsules are feasible and safe. These results justify further investigation of this potential implantation site for microencapsulated islets.

Prevention of first variceal bleed: an appraisal of current therapies

Primary prophylaxis of esophageal variceal hemorrhage (EVH) is an important issue in the management of patients with portal hypertension. Given the high rates of initial variceal hemorrhage and mortality in patients who have not experienced bleeding from varices, there is an urgent need for some form of primary prophylaxis in all patients with large esophageal varices. The aim of this article is to review the various therapies that have been clinically assessed in randomized controlled trials for their efficacy in prevention of initial EVH. Beta-blockers have been found to be useful in primary prophylaxis of EVH, and the consensus at present is that they should be offered to all patients with portal hypertension who are at high risk for EVH. Nitrates and other newer agents are under evaluation. Surgery is not recommended for primary prophylaxis of EVH. Endoscopic sclerotherapy has not been shown unequivocally to be efficacious, and may even be deleterious, possibly related to an unacceptably high complication rate in this clinical setting. However, it may merit further clinical evaluation in light of recent reports of benefit in certain subgroups of patients with portal hypertension. On the other hand, endoscopic variceal ligation, which has an inherently low complication rate and brings about rapid obliteration of varices, may be a better option for primary prophylaxis of EVH. In the future, preprimary prophylaxis, an attractive concept, may be considered. This would involve intervention with pharmacologic agents even before the development of portal hypertension or esophageal varices.

Serotonin and serotonin antagonism in cardiovascular and non-cardiovascular disease

Serotonin, or 5-hydroxytryptamine, is a naturally-occurring vasoactive substance found primarily in the brain, enterochromaffin tissue, and blood platelets. It has diffuse cardiophysiologic effects. The multiple effects of serotonin on blood vessels can be explained by the existence of 2 serotonergic receptor subtypes (the S1 receptor mediates vasodilation, and the S2 receptor vasoconstriction). Serotonin via the S2 receptor also augments the actions of several other vasoconstricting substances. Serotonin may be responsible for causing, or at least perpetuating, some forms of systemic hypertension through peripheral and central nervous system (CNS) actions. Ketanserin is a highly selective S2-serotonergic antagonist with additional alpha-adrenergic blocking activity, which has been proposed as a therapy for various cardiovascular diseases including hypertension. It has been shown to be more effective than placebo in treating hypertension and comparable in effectiveness to other antihypertensive drugs. Its major side effects relate to the CNS, and prolongation of the electrocardiogram QT interval has been described. Caution must be used when using ketanserin in patients receiving potassium- and magnesium-losing agents, because of the risk of torsades de pointes. Ketanserin has potential utility in the treatment of eclampsia, peripheral vascular disease, carcinoid syndrome, and "shock lung." The drug is not yet approved for clinical use in the United States.

Blood and plasma 5-hydroxytryptamine levels in patients with cirrhosis

Serotoninergic mechanisms are thought to play a role in portal hypertension. Because this biomine is metabolized by the liver, peripheral blood and plasma levels of 5-hydroxytryptamine and 5-hydroxyindole acetic acid (the main metabolite of 5-hydroxytryptamine) were measured in 30 patients with cirrhosis. Whole-blood 5-hydroxytryptamine levels were significantly lower in patients with cirrhosis (158 +/- 28 nM) than in age-matched controls (332 +/- 19 nM), and no correlation was found between these levels and the severity of cirrhosis. Unconjugated plasma 5-hydroxytryptamine levels, an indication of the active form of 5-hydroxytryptamine, were significantly higher in patients with cirrhosis than in controls (6.8 +/- 1.7 nM and 3.4 +/- 0.5 nM, respectively), and in patients with cirrhosis these levels were higher in Pugh grade A than in Pugh grade C patients. Conjugated-plasma 5-hydroxytryptamine levels were not significantly different between patients with cirrhosis (32.2 +/- 8.1 nmol/L) and controls (16.4 +/- 1.4 nmol/L). Plasma 5-hydroxyindole acetic acid was significantly lower in patients with cirrhosis than in controls (1.5 +/- 0.1 nmol/L and 2.3 +/- 0.1 nmol/L, respectively). In conclusion, this study shows that serotoninergic mechanisms are altered in patients with cirrhosis.

Liver function improvement following increased portal blood flow in cirrhotic rats

BACKGROUND/AIMS

Liver microcirculation in cirrhosis is characterized by development of intrahepatic shunts and capillarization of sinusoids secondary to cell necrosis and deposition of new collagen, resulting in both decreased drug elimination and increased vascular resistance with portal hypertension. The aim of this study was to examine the effects of increased portal blood flow on hepatic microcirculation and drug elimination in 13 perfused livers from cirrhotic rats.

METHODS

Intrahepatic resistance was assessed under basal conditions (21.2 +/- 0.3 mL/min) and 1 hour after doubling the flow (41.6 +/- 1.0 mL/min). A multiple indicator dilution technique was used at both flow rates to measure sinusoidal volume, albumin and sucrose extravascular volumes, and cellular water volume. Hepatic elimination of labeled taurocholate and propranolol was also measured, and the recovery of 15-microns microspheres was used to evaluate large intrahepatic shunts.

RESULTS

After doubling the flow, intrahepatic resistance decreased by 31%. Sinusoidal and extravascular volume increased significantly without a change in microsphere recovery. However, there was a marked increase in taurocholate and propranolol elimination by cirrhotic livers. Moreover, during high flow, significant correlations were found between changes in albumin extravascular volume and taurocholate and propranolol elimination.

CONCLUSIONS

Increased portal blood flow in cirrhotic rats induces a decrease in intrahepatic resistance without changes in intrahepatic shunting and improves drug elimination by the liver without deleterious effects on hepatocyte viability.

Pharmacological treatment of portal hypertension: hemodynamic effects and prevention of bleeding

In the past 10 years, it has been clearly shown that vasoactive substances reduce portal pressure in patients or animals with portal hypertension. Some of these substances act by inducing splanchnic vasoconstriction, while others reduce hepatic and porto-systemic collateral vascular resistance and, thus, induce a portal hypotensive effect. Still others induce arterial hypotension, which causes a vasoconstrictive effect in the splanchnic territory. Since these drugs act on different vascular receptors, their combination should have a more marked effect on portal hypertension. Up to now, only nonselective beta-blockers have been used in the prevention of first gastrointestinal bleeding in patients with portal hypertension and esophageal varices and in the prevention of recurrent gastrointestinal bleeding. These trials have shown that propranolol or nadolol significantly reduce either a first episode of bleeding or recurrent bleeding. This pharmacological treatment also improves the survival rate in these patients. All of these studies have helped us to understand, in part, why gastrointestinal hemorrhage occurs in certain patients. Additional studies of beta-blockers or other substances are, nevertheless, necessary to select patients who will respond to this type of treatment. Finally, it is possible that the pharmacological treatment of portal hypertension may also be used before esophageal varices occur.

Effects of ritanserin, a selective and specific S2-serotonergic antagonist, on portal pressure and splanchnic hemodynamics in rats with long-term bile duct ligation

This study investigated the short-term effects of ritanserin, a selective and specific S2-serotonergic antagonist, in an experimental model of cirrhosis and intrahepatic portal hypertension caused by long-term bile duct ligation and division and in normal control rats. The rats subjected to bile duct ligation were randomized under blind conditions into two groups to receive ritanserin (0.7 mg/kg body wt, intravenously; n = 10) or the same volume of placebo (isotonic saline solution; n = 10). We performed hemodynamic studies with radiolabeled microspheres 60 min after drug administration. Two groups of normal rats (n = 6) were studied after they received ritanserin or placebo. Ritanserin administration to rats subjected to bile duct ligation significantly reduced portal pressure (from 16.2 +/- 1.3 mm Hg to 12.3 +/- 0.7 mm Hg; mean decrease, 22% +/- 5%; p < 0.05). This reduction was associated with lower portal venous resistance (4.3 +/- 0.5 mm Hg.min.100gm/ml in the placebo group vs. 3.1 +/- 0.3 mm Hg.min.100 gm/ml in rats given ritanserin; mean decrease, 28%; p = 0.069), but we saw no changes in portal vein inflow (3.9 +/- 0.5 ml/min.100 gm vs. 4.4 +/- 0.4 ml/min.100 gm), mean arterial pressure (110 +/- 9 mm Hg vs. 102 +/- 9 mm Hg) and cardiac index (32.9 +/- 2.7 ml/min.100 gm vs. 40.5 +/- 6.7 ml/min.100 gm). Hepatic arterial and kidney blood flows were not modified by ritanserin. Ritanserin had no systemic or splanchnic effects in normal rats. Our results demonstrate that ritanserin infusion decreases portal pressure without any systemic hemodynamic change in rats with secondary biliary cirrhosis and portal hypertension.(ABSTRACT TRUNCATED AT 250 WORDS)

New approaches in the pharmacologic treatment of portal hypertension

To alleviate the risk of variceal bleeding, the portal pressure gradient--usually evaluated as the hepatic venous pressure gradient (HVPG)--must be reduced to < or = 12 mmHg. Although beta-blocking agents are accepted therapy for preventing first or subsequent bleeding episodes, propranolol therapy decreases final HVPG to < or = 12 mmHg in only 12% of patients, while only 24% of patients have a > or = 20% reduction in HVPG and nearly 40% show no reduction in HVPG. This has stimulated research on alternative or additional treatments. Nitrates such as isosorbide dinitrate reduce portal pressure by decreasing resistance to portal and collateral blood flow and by promoting reflex splanchnic vasoconstriction. However, while nitrates are effective in the acute situation, tolerance leading to refractoriness develops over the long term unless they are combined with diuretics or other agents in the treatment of portal hypertension. Propranolol and isosorbide-5-mononitrate combined cause a substantially greater reduction in HVPG than monotherapy with either drug in both acute and long-term use. Presumably concomitant isosorbide-5-mononitrate administration opposes the increase in portal resistance induced by propranolol. Spironolactone, which has been shown to lower HVPG in patients with cirrhosis, produces a reduction in plasma volume that attenuates the increased cardiac output associated with cirrhosis and triggers vasoactive mechanisms that decrease splanchnic blood flow. Potentially, spironolactone may maintain and enhance the decrease in portal pressure achieved by nitrates or propranolol. Triple therapy with a beta-blocker, a nitrate and spironolactone may be feasible.(ABSTRACT TRUNCATED AT 250 WORDS)

Review article: the pathophysiology and pharmacological treatment of portal hypertension

The pathogenesis of portal hypertension remains poorly understood. Similarly, pharmacological manipulation for the prevention and treatment of variceal haemorrhage has not fulfilled the promise of the 1980s. This article reviews current concepts in the pathophysiology of portal hypertension and considers pharmacotherapy for the treatment of variceal bleeding.

Splanchnic haemodynamic effects of ketanserin in anaesthetized cirrhotic rats

The effects of ketanserin, atenolol and their association on portal vein pressure (PVP), portal and caval bile acid concentrations (PBA and CBA), and extraction ratio of bile acids (ER) after oral loading with chenodeoxycholic acid were investigated in anaesthetized cirrhotic rats. PVP was significantly reduced by ketanserin and by the association ketanserin plus atenolol; PBA was significantly decreased in all the treated groups, whereas the reduction of CBA was significant only in the rats treated with ketanserin and atenolol alone. ER was increased by atenolol and ketanserin, but it was not modified by their association. Moreover, a significant correlation was observed between PBA and CBA and between PVP and ER. These results suggest that, in anaesthetized cirrhotic rats, ketanserin reduces PVP probably through more complex mechanisms than the simple indirect reduction of portal flow, and that the association of ketanserin plus atenolol has no additive effect in reducing PVP.

Effects of alpha 1 and beta-adrenergic antagonists and 5-hydroxytryptamine receptor antagonist on portal-systemic collateral vascular resistance in conscious rats with portal hypertension

In order to study the acute effects of pharmacological agents on the vascular resistance of portal-systemic collaterals, a model of total portal vein occlusion with 100% portal-systemic shunts was developed in the conscious rat. The haemodynamic effects of several vaso-active substances were evaluated in this model and compared with those obtained after saline administration. Prazosin (0.5 mg), an alpha 1-adrenergic antagonist, significantly reduced mean arterial pressure by 29%, portal pressure from 13.8 +/- 1.0(mean +/- s.e.m.) to 10.1 +/- 0.4 mmHg and portal tributary blood flow (radioactive microspheres) from 13.6 +/- 2.1 to 11.7 +/- 1.2 mL/min. It also decreased portal-systemic vascular resistance from 95 +/- 16 to 73 +/- 9 dyn s/cm5 x 10(3). Propranolol (4 mg), a beta-adrenergic antagonist, significantly reduced mean arterial pressure by 12% and portal pressure from 15.5 +/- 1.2 to 13.3 +/- 0.9 mmHg while reducing portal tributary blood flow from 14.6 +/- 1.5 to 11.0 +/- 1.7 mL/min and increasing portal systemic collateral vascular resistance from 88 +/- 7 to 103 +/- 8 dyn s/cm5 x 10(3). Ketanserin (0.25 mg/kg), a 5-hydroxytryptamine receptor antagonist, reduced portal pressure from 15.8 +/- 1.0 to 13.3 +/- 0.7 mmHg at a dose that did not alter mean arterial pressure or portal tributary blood flow. It achieved this by reducing portal-systemic collateral vascular resistance from 90 +/- 14 to 74 +/- 13 dyn s/cm5 x 10(3). Saline had no significant effect on systemic and splanchnic haemodynamics. This study shows that ketanserin decreases vascular resistance of portal-systemic collaterals while propranolol increases it. Thus, it is suggested that collateral vascular resistance is accessible to pharmacological manipulation.

The effect of alcohol-induced hepatomegaly on portal hypertension in cirrhotic rats

Male Sprague-Dawley rats with CCl4-induced cirrhosis (confirmed by increased collagen content and light microscopy) were fed either ethanol (Group A, n = 9) or isocaloric carbohydrate diet (Group B, n = 8) for 4 weeks. Histologic and hemodynamic measurements were obtained in the awake state before (time 1) and after the 4 weeks of diet (time 2). Portal-systemic shunts were evaluated using radiolabelled microspheres. Liver weight was increased in Group A (16.5 +/- 0.5 vs. 14.2 +/- 0.5 g, mean +/- SE, p less than 0.005) as was the ratio of liver weight over total body weight (3.41 +/- 0.05 vs. 2.86 +/- 0.09%, p less than 0.0001, +19.2%). Hepatocytes surface area was increased in the ethanol group (357 +/- 9 vs. 294 +/- 7 microns 2, p less than 0.0001). In Group B, only 9 +/- 2% of hepatocytes had steatosis as opposed to 69 +/- 3% of centronodular and 34 +/- 3% of perinodular hepatocytes in Group A (p less than 0.001). Portal pressure remained stable in both groups (time 1 (A) 16.9 +/- 0.8, (B) 15.8 +/- 1.1 mmHg, n.s.; time 2 (A) 15.9 +/- 0.7, (B) 15.8 +/- 0.6 mmHg, n.s.). Portal-systemic shunts did not change with time or diet (time 1 (A) 10.6 +/- 3.7%, (B) 4.1 +/- 2.1%, n.s.; time 2 (A) 13.4 +/- 5.9%, (B) 10.8 +/- 4.3%, n.s.).(ABSTRACT TRUNCATED AT 250 WORDS)

Combined treatment of portal hypertension with ritanserin and propranolol in conscious and unrestrained cirrhotic rats

We recently reported that ritanserin, a 5-hydroxytryptamine receptor antagonist, induced significant reduction of portal pressure in cirrhotic rats. In this study, we investigated the hemodynamic effects of a combination of propranolol and ritanserin in conscious and unrestrained cirrhotic rats. Heparinized catheters exiting from the neck were placed into the portal vein, inferior vena cava, aorta and left ventricle. Cardiac output and regional blood flows were measured with radiolabeled microspheres and the reference-sample method. Serial hemodynamic studies were performed 4 hr after rats awakened (basal), 1 hr after administration of ritanserin (0.63 mg/kg body wt, intravenously) and after intravenous propranolol infusion (0.33 mg/kg/min for 15 min) in nine cirrhotic rats. Similar measurements were obtained in a control group of eight cirrhotic rats treated with the solvents of ritanserin and propranolol. Ritanserin caused significant reduction of portal pressure (-19%). Portal-venous inflow and splanchnic arteriolar resistances remained unchanged, whereas portal-venous resistances were slightly but significantly lowered (-17%); and ritanserin had no effects on systemic hemodynamics. The addition of propranolol resulted in further reduction of portal pressure (-24%); the final reduction after combined therapy was -38%. Propranolol induced a marked decrease in cardiac output (-31%) and portal-venous inflow (-30%). It also caused a significant increase in splanchnic arteriolar resistance (+39%), but did not magnify the ritanserin-induced decrease of portal-venous resistance. The combined therapy did not modify the mean arterial pressure. Our results show that the effects of ritanserin on portal pressure--probably mediated by a reduction of intrahepatic and/or portocollateral resistances--can be potentiated by propranolol, which lowers the portal-venous inflow.

Endotoxin and the hyperdynamic circulation of portal vein-ligated rats

Humoral factors may be responsible for the hyperdynamic circulation seen in portal hypertension. Endotoxin, a peripheral arteriolar vasodilator, has been proposed to mediate this hemodynamic picture. We examined the pathogenic role of endotoxin in portal vein-ligated rats, a prehepatic portal hypertensive model with a well-developed hyperdynamic circulation. To this end, we (a) administered oral neomycin, a poorly absorbable antibiotic, at doses of 50 and 100 mg/day for 7 days and found no evident splanchnic hemodynamic effects of a 2-log-fold reduction of cecal aerobic bacterial flora as assessed by the radioactive microsphere technique in portal vein-ligated rats studied in the postanesthesia awake state; (b) assayed endotoxin in arterial samples using a quantitative limulus assay and found no evidence of endotoxinemia in PVL rats; (c) induced a state of endotoxin tolerance by repeated daily intraperitoneal injections of low-dose endotoxin and found no amelioration of the hyperdynamic state in portal vein-ligated rats. Our results do not support the hypothesis that endotoxin plays a major pathogenic role in the hyperdynamic circulation of this experimental model.