Effect of vasodilators on hepatic microcirculation: a study of the inhibition of norepinephrine-induced vasoconstriction in the isolated perfused rat liver

Saline-enhanced ultrasonography: prediction of X-ray appearance of hepatic venography in patients with cirrhosis

OBJECTIVE

To examine the efficacy of saline-enhanced ultrasound (US) in predicting the X-ray appearance of hepatic venography.

MATERIALS AND METHODS

This prospective study consisted of 50 cirrhosis patients (31 males and 19 females; mean age, 64.2±11.1 years). US patterns in the liver, after injection of agitated saline via balloon-occluded catheter, were evaluated with respect to the findings of CO2-enhanced hepatic venogram.

RESULTS

US demonstrated two patterns: type I showing positive parenchymal enhancement (40 patients) and type II showing negative parenchymal enhancement with detection of hepatic vein (10 patients). There were also two patterns shown by hepatic venography: type A showing retrograde detection of intrahepatic portal vein (41 patients) and type B showing hepatic venous enhancement via intrahepatic venous-venous communications with no detection of intrahepatic portal vein (9 patients). All patients with type I showed retrograde detection of intrahepatic portal vein via hepatic sinusoid on X-ray venograms (type A). Of the 10 patients with type II, nine showed type B and one showed type A. Sensitivity and specificity of type I US pattern to predict the detection of intrahepatic portal vein on the venogram were 100% and 90%, respectively. There was no significant difference in hepatic venous pressure gradient or wedged hepatic venous pressure between patients with type I and type II.

CONCLUSIONS

Saline-enhanced US is effective in predicting the findings of hepatic venogram. As type II strongly suggests the shunt-modified venogram, image taking in these cases would be superfluous with the added advantage of avoiding unnecessary radiation exposure.

Pentoxifylline in liver ischemia and reperfusion

Pentoxifylline is a methylxanthine compound which was first filed in 1973 and registered in 1974 in the United States by Sanofi-Aventis Deustchland Gmbh for the treatment of intermittent claudication for chronic occlusive arterial disease. This methylxanthine was later discovered to be a phosphodiesterase inhibitor. Furthermore, its hemorheological properties and its function as an inhibitor of inflammatory cytokines, like TNF-α, allowed researchers to study its effects in organ ischemia and reperfusion and transplantation. Although this drug has demonstrated beneficial effects, the mechanisms by which Pentoxifylline exerts a protective effect are not fully understood. This paper focuses on reviewing the literature to define the effect of Pentoxifylline when used in liver ischemia and reperfusion injury. Our research shows different animal models in which Pentoxifylline has been used as well as different doses and time of administration, as the ideal dose and timing have not yet been ascertained in liver ischemia and reperfusion. In conclusion, Pentoxifylline has shown positive effects in liver ischemia and reperfusion injury, and the main mechanism seems to be associated with the inhibition of TNF-α.

Intraportal versus Systemic Pentoxifylline Infusion after Normothermic Liver Ischemia: Effects on Regional Blood Flow Redistribution and Hepatic Ischemia-Reperfusion Injury

Pentoxifylline (PTX) has been shown to have beneficial effects on microcirculatory blood flow. In this study we evaluate the potential hemodynamic and metabolic benefits of PTX during hepatic ischemia. We also test the hypothesis that portal PTX infusion can minimize the I/R injury when compared to systemic infusion. Methods. Twenty-four dogs (18.1 ± 0.7 kg) were subjected to portal triad occlusion (PTO) for 45 min. The animals were assigned to 3 groups: CT (control, PTO, n = 8), PTX-syst (PTO + 25 mg/Kg of PTX IV, n = 8), and PTX-pv (PTO + 25 mg/Kg of PTX in the portal vein, n = 8). Animals were followed for 120 min. Systemic hemodynamics, gastrointestinal tract perfusion, oxygen-derived variables, and liver enzymes were evaluated throughout the experiment. Results. Animals treated with PTX presented significantly higher CO in the first hour after reperfusion, when compared to the CT (~3.7 vs. 2.1 L/min, P < 0.05). Alanine aminotransferase (ALT) was similar in the PTX groups two hours after reperfusion but significantly higher in the CT (227 vs. ~64 U/L, P < 0.05). Conclusion. PTX infusion was associated with hemodynamic benefits and was able to minimize liver injury during normothermic hepatic I/R. However, local PTX infusion was not associated with any significant advantage over systemic route.

Einfluss der Clonidin-induzierten systemischen Sympathikolyse auf die Oxygenierung und Perfusion der Leber

BACKGROUND

Increased sympathetic nervous activity which induces vasoconstriction and decreases perfusion may be an underlying mechanism behind the development of perioperative liver damage. This animal study was designed to assess how clonidine-induced systemic sympathicolysis affects liver oxygenation with respect to induced hypotension and vasodilatation under physiological conditions.

METHODS

Following ethical approval 17 anesthetized and acutely instrumented pigs were randomly assigned to 2 groups. Group 1 consisted of 8 animals receiving intravenous clonidine (2 microg x kg(-1) bolus and 2 microg x kg(-1) x h(-1) for induction of sympathicolysis and group 2 consisted of 9 animals serving as controls. After obtaining baseline values, measurements were repeated 90 and 250 min after starting to reduce systemic sympathetic nervous activity.

RESULTS

Clonidine-induced systemic sympathicolysis was associated with decreased mean arterial blood pressure, cardiac output and heart rate. Portal venous and hepatic arterial blood flow, oxygen delivery to the liver, oxygen uptake and liver tissue oxygen partial pressure remained unchanged. The plasma indocyanine green disappearance rate increased.

CONCLUSION

We concluded that despite decreased mean arterial pressure and cardiac output, clonidine-induced systemic sympathicolysis did not affect liver oxygenation or perfusion.

Buflomedil and pentoxifylline in the viability of dorsal cutaneous flaps of rats treated with nicotine

BACKGROUND

Nicotine reduces skin-flap survival. Pharmacologic therapy may represent an alternative treatment strategy to counteract nicotine effects in the flap surgery setting. In this study, we have compared the isolated and associated actions of the vasoactive drugs buflomedil and pentoxifylline in the viability of dorsal cutaneous flaps of rats treated with subcutaneous doses of nicotine.

METHODS

The survival of modified McFarlane skin flaps was assessed on post-operative day 7. Nicotine group received 4 mg/kg nicotine during 40 days pre-operatively and 7 days post-operatively. Nicotine+buflomedil group received nicotine and 6 mg/kg buflomedil 24 h pre-operatively and 7 days post-operatively. Nicotine+pentoxifylline group received nicotine and 20 mg/kg pentoxifylline in 15 pre-operatively and 7 post-operatively days. Nicotine+buflomedil+ pentoxifylline group received nicotine and both drugs administered as above. Control group received daily 1 ml normal saline during 40 days pre-operatively and 7 days post-operatively. Using image analysis, five different flap areas were quantified: Total, preserved, necrotic, ischaemic and viability. Viability areas comprised the sum of ischaemic and preserved areas.

RESULTS

Nicotine treated animals had lower percentage of viability areas (60.7% +/- 6.8) than the control group (73.7% +/- 9.5), p=0.016. The percentage of viability areas in the buflomedil (76.4% +/- 11.4), pentoxifylline (74.2% +/- 15.6) and buflomedil+ pentoxifylline (74.0% +/- 9.7) groups were larger than the nicotine group (p=0.002, p=0.011 and p=0.012, respectively). There were no significant differences in the viability areas when drugs were used isolated or in association. We further demonstrated that the increase in the viability area of the buflomedil and pentoxifylline groups (isolated or in association) was due to increase in ischaemic areas.

CONCLUSIONS

Both drugs equally increased flap survival in nicotine treated animals. Viability areas increased due to larger ischaemic areas, probably as a reflex of the action of these drugs in sites of partial circulatory deficit.

Kupffer cell-dependent TNF-alpha signaling mediates injury in the arterialized small-for-size liver transplantation in the mouse

Implantation of small liver grafts causes liver injury and defective regeneration leading to graft failure. We investigated whether Kupffer cell-dependent TNF-alpha signaling contributes to this poor outcome. Partial 30% liver transplantation was performed in C57BL/6 wild-type mice (control group), and in three groups with down-regulation of the TNF-alpha pathway: (i) TNF receptor 1 knockout [TNFR-1(-/-)] mice, and mice pretreated with (ii) gadolinium chloride or (iii) pentoxifylline (PTX). Fifty-percent partial liver transplantation, a model associated with full recovery, and transplantation in IL-6 knockout [IL-6(-/-)] mice were performed in some experiments. Graft injury, regeneration, portal flow, liver microcirculation, leukocyte adhesion, and animal survival were assessed. Animal survival rates were 14% in the control group vs. 43% in the gadolinium chloride group, 57% for the TNFR-1(-/-) group, and 86% in the PTX group (P < 0.001). Markers of liver injury were reduced in all treated groups when compared with controls. Each treated group disclosed better portal flow and sinusoid perfusion, decreased leukocyte adherence, particularly in the PTX group. Liver regeneration occurred only in the treated groups. IL-6 and IL-10 levels were dramatically up-regulated (50x) in the PTX group, and at lower levels in other experimental groups. The protective effect of PTX was lost in IL-6(-/-) mice and protection was restored by a single dose of r-IL-6. In conclusion, interruption of TNF-alpha signaling or depletion of Kupffer cells improves survival after 30% liver transplantation, reduces liver injury, and enhances regeneration. The superior effects of PTX are mediated by IL-6.

Effects of norepinephrine and acetylcholine on portal-systemic collaterals of common bile duct-ligated cirrhotic rat

BACKGROUND

Catecholamine exerts profound vascular effects on vascular smooth muscle, and serum norepinephrine (Nepi) and sympathetic nervous activity are increased in cirrhosis. The vascular response of Nepi and acetylcholine (Ach) in portal-systemic collaterals of cirrhotic rats is unknown. The purpose of the present study was to investigate the effects of Nepi and Ach on portal-systemic collaterals of common bile duct-ligated (BDL) cirrhotic rats.

METHODS

Perfusion studies were performed 6 weeks after BDL when hepatic cirrhosis was induced. Three series of experiments were performed in BDL rats: (i) the Nepi (10(-10) mol/L-10(-5) mol/L) vasoconstrictory responses with (n = 6) and without (n = 5) alpha1- (phentolamine, 5 x 10(-6) mol/L) or beta- (propranolol, 10(-5) mol/L) receptor antagonist (n = 6 and 5, respectively); (ii) the Ach (10(-8) mol/L-10(-5) mol/L) vasodilatory responses in Nepi- preconstricted portal-systemic collaterals in the absence (n = 7) or presence (n = 8) of N(pi)- L-nitro-arginine (NNA, 10(-4) mol/L); and (iii) the effect of indomethacin (10(-5) mol/L; n = 6) on Nepi responses. The collateral vascular responses were evaluated by the in situ collateral perfusion.

RESULTS

Norepinephrine significantly increased the perfusion pressure and was inhibited by phentolamine (P < 0.05). The constrictive responses of Nepi were not significantly modified in the presence of propranolol (P > 0.05). Acetylcholine produced >50% relaxation of the Nepi-preconstricted collateral vessels and was inhibited by NNA (P < 0.05). In addition, indomethacin significantly enhanced the constrictive response of Nepi (P < 0.05).

CONCLUSION

Norepinephrine has a vasoconstrictive effect on the portal-systemic collaterals of cirrhotic rats and this effect was mediated by alpha1-receptor. Both nitric oxide and prostaglandin are endogenous modulators in the collateral circulation of cirrhotic rats.

A pilot study to investigate the use of oxpentifylline (pentoxifylline) and thalidomide in portal hypertension secondary to alcoholic cirrhosis

BACKGROUND

Tumour necrosis factor-alpha is thought to be important in the pathogenesis of portal hypertension. Oxpentifylline (pentoxifylline) and thalidomide inhibit endotoxin-induced tumour necrosis factor-alpha production in vitro.

AIMS

To assess the toxicity of oxpentifylline (pentoxifylline) and thalidomide in cirrhosis and their effect on the hepatic venous pressure gradient and tumour necrosis factor-alpha production.

METHODS

In an open-label pilot study, 20 abstinent patients with stable alcoholic cirrhosis and oesophageal varices were recruited; 12 patients completed haemodynamic measurements before and after treatment with oxpentifylline (pentoxifylline) 1800 mg (n=6) or thalidomide 200 mg (n=6) daily for 2 weeks. Tumour necrosis factor-alpha production was assessed in ex vivo monocyte cultures stimulated with endotoxin.

RESULTS

Thalidomide reduced the hepatic venous pressure gradient from 19.7 mmHg (9.3-23.5 mmHg) to 12.2 mmHg (4.7-19.5 mmHg) (P=0.03) without reducing the hepatic blood flow or altering systemic haemodynamic parameters. Thalidomide reduced ex vivo tumour necrosis factor-alpha production by approximately 50%. Oxpentifylline (pentoxifylline) had no significant effect on any of the parameters measured. Side-effects led to dose reduction or treatment withdrawal in 40% of patients.

CONCLUSION

Thalidomide, but not oxpentifylline (pentoxifylline), reduces the hepatic venous pressure gradient in stable alcoholic cirrhotics, an effect that may be mediated by the inhibition of tumour necrosis factor-alpha production. The role of tumour necrosis factor-alpha inhibitory drugs in the therapy of portal hypertension should be investigated in a randomized controlled trial.

Beneficial effect of pentoxifylline on microvesicular steatotic livers submitted to a prolonged cold ischemia

BACKGROUND

The deleterious effect of steatosis on transplanted livers is mainly related to a microcirculation impairment. We investigated the effect of preservation duration on the recovery of isolated perfused rat steatotic livers and tested the effect of pentoxifylline (PTX), known to have a beneficial effect on hepatic microcirculation.

MATERIALS AND METHODS

Fatty rat livers were obtained using a diet able to induce an 80% to 100% microvesicular steatosis within 7 days. We studied the effect of the duration of preservation (12 hr, 18 hr, and 24 hr) on fatty and normal isolated perfused rat liver. PTX was added to University of Wisconsin solution during cold storage (30 mM/kg of weight) and at reperfusion (3 mM) (n=5 livers in each group). Lactate dehydrogenase, aspartate aminotransferase, alanine aminotransferase, bile production, and vascular resistance were evaluated. The liver injury at the end of perfusion was assessed by optical and electron microscopy.

RESULTS

For a 24-hr preservation period, fatty livers demonstrated increased enzymatic release (aspartate aminotransferase: 42+/-16 vs. 17+/-5 IU/L/g of liver, P<0.005; alanine aminotransferase: 32+/-13 vs. 13+/-3 IU/L/g of liver, P<0.005; lactate dehydrogenase: 1,207+/-497 vs. 291+/-195 IU/L/g of liver, P<0.001). Vascular resistance (0.32 vs. 0.15 cm H(2)O/min/mL, P<0.0005) and bile output (67+/-24 vs. 141+/-61 mg/g of liver, P<0.05) were decreased. Peliosis appeared after an 18-hr preservation period for fatty livers compared with a 24-hr preservation period for controls. All these negative effects were suppressed by PTX.

CONCLUSION

Diffuse microvesicular steatosis became deleterious only after long preservation times (24 hr). PTX prevented this effect.

Management of portal hypertension after variceal hemorrhage

Each variceal bleed is associated with 20% to 30% risk of dying. Management of portal hypertension after a bleed consists of (1) control of bleeding and (2) prevention of rebleeding. Effective control of bleeding can be achieved either pharmacologically by administering somatostatin or octreotide or endoscopically via sclerotherapy or variceal band ligation. In practice, both pharmacologic and endoscopic therapy are used concomitantly. Rebleeding can be prevented by endoscopic obliteration of varices. In this setting, variceal ligation is the preferred endoscopic modality. B-blockade is as effective as endoscopic therapy and, in combination, the two modalities may be additive.

Esophageal varices. I. Clinical presentation, medical therapy, and endoscopic therapy

The last half century has witnessed great advances in the understanding of the pathogenesis and natural history of portal hypertension in cirrhotics. Several pharmacologic and endoscopic techniques have been developed for the treatment of portal hypertension. The use of these agents in a given patient must be based on an understanding of the stage in the natural history of the disease and the relative efficacy and safety of the available treatment options.

Affinity of isoxsuprine for adrenoreceptors in equine digital artery and implications for vasodilatory action

We used isolated equine digital arteries to study the vasodilatory mechanism of isoxsuprine, and fowl caecum preparations to investigate the affinity of the drug for beta-adrenoceptors. Isoxsuprine is a potent vasodilator of arterial smooth muscle that has been precontracted by an alpha-adrenoceptor agonist such as noradrenaline (log EC50 = -6.33 [-5.98; -6.68]). The present study indicates that its effect is due to alpha-adrenoceptor blockade since: (1) after a long lasting exposure to cumulative doses of isoxsuprine the vasoconstricting action of noradrenaline cannot be restored; (2) isoxsuprine does not promote relaxation on preparations precontracted by PGF2alpha; (3) isoxsuprine shifts the dose-response curve of noradrenaline to the right; and (4) its affinity (pK(B) = 6.90 [6.60; 7.20]) in this experiment is comparable to that in noradrenaline-precontracted preparations and is 14 times lower than that of the selective alpha1-adrenergic antagonist prazosin [pK(B) = 8.04 (7.40; 8.68]). The affinity of isoxsuprine for beta-adrenoceptors was 100 times lower than that of isoprenaline when tested on fowl caecum. This preparation has a large beta-adrenoceptor and negligible alpha-adrenoceptor population concerned with the control of smooth muscle motility. Our data suggest that the alpha-mediated effect of isoxsuprine on horse arterial smooth muscle is due to higher affinity of the drug for alpha- than beta-adrenoceptors rather than low concentration or functionality of beta-sites at this site. According to these data, pure beta2-agonists seem to be more profitable tools to determine vasodilation of the arterial bed in horses legs.

Investigations on the stereoselective action of isoxsuprine on alpha- and beta-adrenoceptors in equine common digital artery

The affinity and functional effects of isoxsuprine enantiomers were investigated to determine the enantiospecificity of the beta-agonistic and alpha-blocking effects. Functional assays on isolated smooth muscle preparations from equine common digital artery were performed to determine the apparent affinity (pD(2)) and intrinsic activity (alpha(E)) of (-)erythro-isoxsuprine (alphaS, betaR, gammaR) and (+)erythro-isoxsuprine (alphaR, betaS, gammaS). The affinity of two enantiomers for the different adrenoceptor types was studied by radioligand binding assays on membrane preparations from the same tissue, using (-)[(3)H]CGP12177 and [(3)H]prazosin. On noradrenaline-precontracted artery preparations (-)isoxsuprine was markedly more potent than (+)isoxsuprine in dilating preparations, indicating that the laevorotatory enantiomer has a very high apparent affinity for alpha-adrenoceptors. Binding studies confirmed that (-)isoxsuprine has a higher affinity than (+)isoxsuprine for alpha-adrenoceptors, while the (+) isomer competes for beta-adrenoceptors with an affinity similar to that of propranolol. As described for other beta-phenylethylamines, the two isoxsuprine enantiomers studied have different efficacies for alpha- and beta-adrenoceptors and the effects of the commercially available mixture of stereoisomers therefore depend on the density and functional importance of the adrenoceptor types present in the tissue studied. 1999 Academic Press.

Vasopressin reduces taurochenodeoxycholate-induced hepatotoxicity by lowering the hepatocyte taurochenodeoxycholate content

BACKGROUND/AIMS

Vasopressin has been reported to reduce bile flow, but its effects on bile acid secretion and bile acid-related hepatotoxicity are still unclear. We therefore investigated the influence of vasopressin on the hepatotoxicity and biliary excretion of taurochenodeoxycholic acid in primary cultured rat hepatocytes and isolated perfused rat liver models.

METHODS/RESULTS

1) Addition of vasopressin to hepatocyte cultures significantly decreased lactate dehydrogenase release as compared to cultures exposed to 1 mM taurochenodeoxycholic acid alone, and also reduced intracellular taurochenodeoxycholic acid content from 19.3 +/- 2.2 to 13.0 +/- 1.6 nmol/mg protein. After 30 min of preincubation with 1 mM taurochenodeoxycholic acid, rinsing and reculture of hepatocytes in bile acid-free medium resulted in gradual decrease in the intracellular level of the bile acid, and addition of vasopressin (10(-9) M) to the reculture medium accelerated this process. 2) Superimposition of vasopressin (330 pmol/l) for 10 min on taurochenodeoxycholic acid infusion (1.0 mumol/min: 25 mumol/l) caused a rapid increase in bile flow and biliary excretion of taurochenodeoxycholic acid (697 +/- 42 vs 584 +/- 27 nmol/10 min per g liver) from perfused rat livers, and significantly reduced lactate dehydrogenase release. 3) Superimposition of the PKC blocker H-7 (5 mumol/l) on taurochenodeoxycholic acid infusion (1.0 mumol/min: 25 mumol/l) caused a gradual increase in bile flow and biliary excretion of taurochenodeoxycholic acid. Furthermore, an additional infusion of vasopressin (100 pmol/l) for 10 min in the presence of H-7 produced a greater increase in bile flow and biliary excretion of taurochenodeoxycholic acid as compared with H-7 alone (754 +/- 71 vs. 657 +/- 26 nmol/g liver). 4) Continuous infusion of vasopressin (330 pmol/l) significantly increased the late peak (10-50 min) of horseradish peroxidase excretion from perfused livers (from 8.48 +/- 1.02 to 21.7 +/- 6.02 ng/g liver).

CONCLUSIONS

These findings suggest that vasopressin exerts a protective effect against taurochenodeoxycholic acid-induced hepatotoxicity by stimulating the secretion of this bile acid via intracellular vesicular transport systems.

Role of pentobarbitone anaesthesia and sympathetic tone in the haemodynamic effects of isosorbide dinitrate in rats with cirrhosis

The haemodynamic effects of nitrovasodilators and their mechanisms of action on portal hypertension remain unclear. The splanchnic and systemic haemodynamic response to the infusion of isosorbide dinitrate (100 micrograms/kg per min), a nitrovasodilator, was investigated in cirrhotic rats. The role of the conscious state in the haemodynamic response to isosorbide dinitrate was examined using rats that were anaesthetized with pentobarbitone. The role of sympathetic tone in the haemodynamic response to isosorbide dinitrate was examined using rats pretreated with the ganglion blocker hexamethonium. Isosorbide dinitrate had no haemodynamic effects in conscious, unblocked normal and cirrhotic rats. Isosorbide dinitrate had no haemodynamic effects in normal and cirrhotic rats treated with hexamethonium. In normal anaesthetized rats, isosorbide dinitrate significantly decreased systemic vascular resistance (414 +/- 25 vs 290 +/- 26 dyn.s/cm-5 per 100 g). In cirrhotic anaesthetized rats, isosorbide dinitrate significantly decreased mean arterial pressure (98 +/- 6 vs 79 +/- 7 mmHg), systemic vascular resistance (318 +/- 30 vs 207 +/- 10 dyn.s/cm-5 per 100 g), portal pressure (14.0 +/- 1.0 vs 11.3 +/- 0.9 mmHg) and portal territory vascular resistance (1362 +/- 163 vs 1031 +/- 182 dyn.s/cm5 per 100 g). In conclusion, this study shows that the portal hypotensive effects of isosorbide dinitrate depend upon the alterations of vascular tone by pentobarbitone.

Enhancement of hepatic glucose release and bile flow by the phosphodiesterase-III-inhibitor enoximone in the perfused rat liver

The use of phosphodiesterase-III-inhibitors (PDI) as inotropic substances in the treatment of cardiac failure can be associated with hyperglycaemia. This phenomenon could be caused by hepatic events induced by PDI. The purpose of our study was to investigate the effects of the PDI enoximone on hepatic carbohydrate metabolism and bile flow. In the rat liver perfusion model, hepatic glucose and lactate production, portal flow and bile flow were determined. Administration of enoximone (1, 10, 100 microM) increased hepatic glucose output and bile acid-independent bile flow in a dose-dependent manner. The PDI enhanced the glycogenolytic effects of glucagon (from 15.7 to 38.6 mumol glucose/g/20 min), of epinephrine (from 7.1 to 38.7 mumol glucose/g/20 min), of norepinephrine (from 9.8 to 32 mumol/g/20 min) and of phenylephrine (from 25.5 to 40.8 mumol glucose/g/20 min). Furthermore, lactate production was significantly reduced by enoximone. The effect of epinephrine and phenylephrine on portal flow was blocked or diminished by enoximone administration. In summary, it was shown that the PDI enoximone is able to enhance hepatic glucose production. Bile acid-independent bile flow was increased by the inhibition of phosphodiesterase-III. The effects of enoximone and glycogenolytic hormones on glucose release were synergistic. The vasoconstrictive action of catecholamines was reduced or completely prevented by enoximone. In conclusion, enoximone has glycogenolytic, vasodilatory and choleretic properties in the liver.

Prolonged antagonism of alpha 1-adrenergic vasoconstriction in the rat liver by atrial natriuretic peptide

BACKGROUND/AIMS

Vasodilator hormones that regulate hepatic circulation at physiological concentrations have not been sufficiently identified. The presence of atrial natriuretic peptide (ANP) and its receptors in the hepatic vascular bed suggest such vasorelaxing potential.

METHODS

Livers of male Sprague-Dawley rats were perfused in a flow-constant fashion. The selective alpha 1-adrenergic agonist phenylephrine (PE) (1.5 mumol/L) was infused from 30 to 36 minutes and again from 70 to 76 minutes after starting perfusion (n = 5). ANP (0.1 pmol/L to 200 nmol/L), des-(Gln18, Ser19, Gly20, Leu21, Gly22)-ANP fragment (C-ANP) (20 nmol/L), or 8-bromoguanosine 3',5'-cyclic monophosphate (8-Br-cGMP) (50 mumol/L) (each n = 4) were added from 20 to 40 minutes.

RESULTS

During the first infusion of PE, portal pressure increased from 3.7 +/- 0.5 to 12.1 +/- 0.8 cm H2O maximally (mean +/- SD) and increased again to 11.5 +/- 2.0 during the second PE infusion. ANP at physiological concentrations reduced both PE-induced increases of portal pressure in a dose-dependent fashion, reaching half-maximal effects around 20 pmol/L and maximal effects (about 50% inhibition of PE-induced vasoconstriction) at 40 pmol/L. The cGMP analogue 8-Br-cGMP showed the same long-lasting vasodilating effect as ANP. In contrast, C-ANP, which binds only to the ANP C-receptor, had no effects.

CONCLUSIONS

Physiological concentrations of ANP antagonize alpha 1-adrenergic vasoconstriction in the liver, suggesting an important function in the humoral regulation of hepatic circulation. The prolonged hemodynamic effect of ANP seems to be ANP A-receptor/guanylyl cyclase/cGMP-mediated.

Morphometric study of centrilobular vessels in the rat liver after continuous hypothermic perfusion with Euro-Collins and University of Wisconsin solution

Damage to the hepatic microcirculation plays an important part in the preservation-related loss of graft viability. In a study on the isolated, hypothermically preserved rat liver perfused continuously with Euro-Collins and University of Wisconsin solution (gluconate substituted for lactobionate) at different flow rates we determined changes in diameter of central veins and pericentral sinusoids and related them to weight changes. While post-perfusion liver weight increased with Euro-Collins and decreased with University of Wisconsin solution, weight changes did not vary significantly with different flow rates. Changes in sinusoidal diameter depended on the type of solution as well as on the flow rate. Cell swelling under Euro-Collins perfusion leads to compression of pericentral sinusoids, which varies in degree with different flow rates. Perfusion with University of Wisconsin solution under near-normal flow, due to slight shrinking of parenchymal cells, keeps the sinusoidal lumina wide open. We conclude that there is a causal relationship between perfusion-dependent cell swelling and reduction in sectional area of the hepatic microvasculature. Continuous hypothermic perfusion with University of Wisconsin solution at physiologic flow rates excellently preserves the size and trabecular architecture of hepatocytes and thereby the sinusoidal lumina within the hepatic lobules.

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.

Papaverine inhibits transcytotic vesicle transport and lipid excretion into bile in isolated perfused rat liver

Papaverine is a nonspecific smooth muscle relaxant and a phosphodiesterase inhibitor. Its effects on biliary excretion of lipids and horseradish peroxidase were investigated in a single-pass isolated perfused rat liver model. A constant infusion of papaverine (1.6 mumol/min; 40 mumol/L) significantly increased bile flow (microliters per minute per gram of liver) before (2.03 +/- 0.09 vs. 1.0 +/- 0.06) and after sodium taurocholate infusion (2.77 +/- 0.10 vs. 1.88 +/- 0.11). However, papaverine significantly and reversibly reduced biliary excretion of phospholipids and cholesterol (nanomoles per minute per gram of liver) after a 1.0 mumol/min sodium taurocholate infusion, from 7.45 +/- 0.83 and 1.42 +/- 0.15 to 1.75 +/- 0.18 and 0.39 +/- 0.06, respectively (p less than 0.01), whereas secretion of bile acids was unaffected. When a 1-min pulse of horseradish peroxidase (25 mg) was infused in isolated perfused rat liver after a continuous infusion of N6,O-2'-dibutyryladenosine 3',5'-cyclic monophosphate (0.25 mumol/min; 6.25 mumol/L), horseradish peroxidase appeared in bile in an early (4 to 6 min) and late (20 to 25 min) peak. Papaverine significantly reduced the late peak, from 1.211 +/- 0.264 to 0.498 +/- 0.107 (p less than 0.01). Papaverine had no significant effects on either cyclic AMP or cyclic GMP in the liver and bile, although it has been reported that papaverine is a phosphodiesterase inhibitor. These findings indicate that papaverine inhibits biliary excretion of lipids but not bile acids, and they suggest that papaverine has an inhibitory effect on transcytotic vesicle transport independent of an increase of cyclic nucleotides in hepatocytes.

Effects of vaso-active agents on hepatic function and blood gases in patients with cirrhosis: a study of vasopressin and nitroglycerin

The effects of vaso-active agents on hepatic function and splanchnic oxygenation were studied in 17 patients with cirrhosis and portal hypertension. Eight patients received vasopressin (0.3 iu/min) and nine patients received nitroglycerin (50 micrograms/min). Both drugs caused a significant reduction in the portal venous pressure gradient. Vasopressin infusion significantly decreased intrinsic clearance of indocyanine green (-23%, P less than 0.01). This may be due to a decreased hepatic perfusion (-28%, P less than 0.01) and portal venous oxygenation (-15% in portal venous oxygen tension, P less than 0.05). In contrast, no changes in hepatic perfusion and portal venous oxygenation were observed after nitroglycerin infusion. Nitroglycerin did not decrease intrinsic clearance of indocyanine green. These results suggest that vasodilators, rather than vasoconstrictors, might be welcome in the treatment of patients with cirrhosis and portal hypertension.

A study of the inhibition of adrenaline-induced vasoconstriction in the isolated perfused liver of rabbit

We have studied the action of a series of vasoactive and antispasmodic agents on the intrahepatic vasoconstriction induced by adrenaline in the isolated perfused liver of rabbits. The arterial and portal venous resistance, oxygen consumption, liver weight and bile flow were investigated. The drugs used were as follows: nonspecific alpha-adrenergic antagonists (DH-ergocristine, dibenamine, phenoxybenzamine), vasodilators with a direct miscellaneous action (theophylline, papaverine, dipyridamole, glucagon, Aiu-cor by Instituto Gentilli, Italy [inosine, ATP, IPI, UTP]) and antispasmodics (piperylone, tropenziline, noraminophenazone). Adrenaline increased arterial and portal venous resistance followed by a diminution of oxygen consumption, liver weight and bile flow. alpha-Adrenergic antagonists inhibited the effects of adrenaline on portal venous resistance and oxygen consumption and especially the effects on hepatic arterial resistance. The most potent agent was phenoxybenzamine. In contrast to alpha-adrenoceptor blockade, the effects of other vasoactive agents were without a sustained influence on hepatic arterial resistance (excepting those of glucagon and dipyridamole). Some of them were effective as antagonists on responses in the portal venous bed (papaverine, Aiu-cor). Moreover, there were drugs exerting an enhancement of the vasoconstrictor responses of hepatic artery to low concentrations of adrenaline with no effect on the portal venous bed (piperylone, tropenziline). Theophylline and noraminophenazone exerted no effect either on the arterial or portal venous bed. No vasodilator agent antagonized the changes of the bile flow after adrenaline administration.

Hepatic circulation: potential for therapeutic intervention

In recent years, knowledge of the physiology and pharmacology of hepatic circulation has grown rapidly. Liver microcirculation has a unique design that allows very efficient exchange processes between plasma and liver cells, even when severe constraints are imposed upon the system, i.e. in stressful situations. Furthermore, it has been recognized recently that sinusoids and their associated cells can no longer be considered only as passive structures ensuring the dispersion of molecules in the liver, but represent a very sophisticated network that protects and regulates parenchymal cells through a variety of mediators. Finally, vascular abnormalities are a prominent feature of a number of liver pathological processes, including cirrhosis and liver cell necrosis whether induced by alcohol, ischemia, endotoxins, virus or chemicals. Although it is not clear whether vascular lesions can be the primary events that lead to hepatocyte injury, the main interest of these findings is that liver microcirculation could represent a potential target for drug action in these conditions.

The hepatic glutathione system--influences of xenobiotics

The hepatic glutathione (GSH) system and the influences of xenobiotics have been reviewed. Key steps in the regulation of hepatic GSH are GSH biosynthesis, the GSH-peroxidase/reductase cycle, the cystathionine pathway, and the carrier-mediated export processes. Influences of xenobiotics on these different pathways are discussed. Xenobiotics may lead to liver injury after biotransformation to highly reactive electrophilic metabolites (mainly cytochrome P-450 mediated), which easily conjugate with GSH, thus producing GSH depletion. This GSH depletion and probably an additional loss of protein sulfhydryl groups cause a disturbance of the intracellular calcium homeostasis which leads to an irreversible cell injury. The different acinar distribution of cytochromes P-450 and of GSH and GSH-related detoxication pathways points to a greater susceptibility of perivenous hepatocytes to xenobiotic-induced damage. Also, the intracellular compartmentation of GSH is important for the understanding of hepatocellular injury induced by several xenobiotics.

Hemodynamic and metabolic responses to leukotriene C4 in isolated perfused rat liver

Responses of isolated perfused rat liver to leukotriene C4 were studied in order to assess the mechanisms involved in leukotriene-mediated liver injury. Infusion of leukotriene C4 (11 and 44 pmoles per min per gm liver weight) into the portal vein resulted in a rise in portal pressure, a decrease in oxygen consumption, an increase in hepatic glucose and lactate efflux and lactate/pyruvate ratio in the perfusate and a small decrease in bile flow. Isoproterenol (1 microM) counteracted the effects of leukotriene C4 on respiration and portal pressure, whereas bile flow and glucose efflux were reversibly stimulated. The same changes were observed upon withdrawal of leukotriene C4. The release of glucose was correlated with the increase in oxygen consumption upon both isoproterenol addition and withdrawal of leukotriene C4. These results are indicative of leukotriene C4-induced microcirculatory redistribution of perfusate flow. Since, in the presence of nitroprusside (50 microM), both the effects of leukotriene C4 and their reversal by isoproterenol were diminished, a vascular site of action can be assumed. Accordingly, the accompanying metabolic responses can be explained by gradual changes in oxygen supply to parts of the liver. Reversibility of the leukotriene C4 effects and lack of short-term impairment of viability of the isolated liver suggest that leukotriene-mediated liver injury is a long-term effect related to events subsequent to microcirculatory changes.

Effect of vasodilators on hepatic microcirculation in cirrhosis: a study in the isolated perfused rat liver

We studied the effects of a series of vasodilators on intrahepatic vascular resistance of isolated perfused cirrhotic rat livers in basal conditions and during norepinephrine-induced vasoconstriction. Cirrhosis was induced by repeated intraperitoneal injections of carbon tetrachloride. The vasodilators were a nonselective beta-adrenergic antagonist (propranolol), an alpha 1-adrenergic antagonist (prazosin), a nonselective beta-adrenergic agonist (isoproterenol), an alpha 2-agonist (clonidine), nitrovasodilators (nitroglycerin and nitroprusside), calcium channel blockers (verapamil, diltiazem, nifedipine), papaverine, diazoxide and pentoxifylline. In basal conditions, isoproterenol, nitroglycerin, papaverine, pentoxifylline and nitroprusside demonstrated significant vasodilatory activity. However, the response was weak and isoproterenol was the only drug active in the therapeutic range of concentrations. Propranolol, prazosin, verapamil, diltiazem, nifedipine and diazoxide were ineffective. Prazosin, papaverine and pentoxifylline reduced norepinephrine-induced vasoconstriction, whereas isoproterenol, clonidine and propranolol were ineffective. We conclude that several vasodilators can reduce resistance in the cirrhotic rat liver, but their potency is low and few are effective at therapeutic concentrations. Furthermore, their activity may be blunted when resistance is increased by norepinephrine.

Protective effect of vasodilators on liver function after long hypothermic preservation: a study in the isolated perfused rat liver

The effects of two vasodilators, papaverine and pentoxifylline (a methylxanthine derivative), on liver function after 19 hr hypothermic preservation were investigated. Hypothermic preservation was performed according to the standard technique, and liver hemodynamics and function were studied during 70 min immediately after reperfusion in an isolated perfused rat liver system. No significant changes occurred after hypothermic storage for 5 hr. However, when the storage was prolonged to 19 hr, bile flow and taurocholate intrinsic clearance were significantly reduced; transaminase release was markedly increased and histological studies demonstrated centrilobular necrosis. Concomitantly, liver blood flow was significantly reduced and intrahepatic vascular resistance was increased. Papaverine and pentoxifylline administered during preservation and at the time of reperfusion significantly improved all parameters. The improvement was more pronounced after pentoxifylline, and this group showed no significant difference in any of the studied parameters from the control livers. The results show that two vasodilators significantly protect the liver during long hypothermic preservation. The data suggest that abnormalities of liver microcirculation are of major importance in the pathogenesis of liver injury after hypothermic storage.