Role of nitric oxide in hyporeactivity to noradrenaline of isolated aortic rings in portal hypertensive rats

Hepatic injury induced by thioacetamide causes aortic endothelial dysfunction by a cyclooxygenase-dependent mechanism

Liver cirrhosis is associated with a wide range of cardiovascular abnormalities including hyperdynamic circulation and cirrhotic cardiomyopathy. The pathogenic mechanisms of these cardiovascular changes are multifactorial and include vascular dysregulations.

AIM

The present study tested the hypothesis that the systemic vascular hyporesponsiveness in thioacetamide (TAA)-induced liver injury model is dependent on nitric oxide (NO) and cyclooxygenase (COX) derivatives.

MAIN METHODS

Wistar rats were treated with TAA for eight weeks to induce liver injury.

KEY FINDINGS

The maximal contractile response in concentration-effect curves to phenylephrine was decreased in aorta from TAA-treated rats, but no differences were found in aorta without endothelium, suggesting an endothelium-dependent mechanism in decreased contractile response. There was no difference in the contractile response with and without L-NAME (N(ω)-nitro-l-arginine methyl ester) in rats with liver injury, showing that the TAA treatment impairs NO synthesis. Pre-incubation of the aorta with indomethacin, a COX-inhibitor, normalized the reduced contractile response to phenylephrine in arteries from TAA group. Also, COX-2 and iNOS (inducible nitric oxide syntase) protein expression was increased in aorta from TAA group compared to control group. Animals submitted to TAA treatment had a reduction in systolic blood pressure. Our findings demonstrated that liver injury induced by TAA caused a decrease in aortic contractile response by a COX-dependent mechanism but not by NO release. Also, it was demonstrated an inflammatory process in the aorta of TAA-treated rats by increased expression of COX-2 and iNOS.

SIGNIFICANCE

Therefore, there is an essential contribution of COX-2 activation in extra-hepatic vascular dysfunction and inflammation present in cirrhosis induced by TAA.

Glytan decreases portal pressure via mesentery vasoconstriction in portal hypertensive rats

AIM: To investigate the effects of Glytan on splanchnic hemodynamics and its reduction of portal pressure in portal hypertensive rats.

METHODS: Glytan (Ganluotong in Chinese), is composed of salvianolic acid B and diammonium glycyrrhizinate. Portal hypertension (PHT) was induced in the rats by common bile duct ligation (BDL). Hemodynamic studies were performed using the colored microsphere method. Radioimmunoassay (RIA) was used to determine endothelin (ET)-1 levels in the mesenteric circulation. Western blotting methods were used to investigate the effect of Glytan on ET A receptor (ETAR), ET B receptor (ETBR), endothelial NO synthase (eNOS), G-protein-coupled receptor kinase (GRK)2, and β-arrestin 2 expression in the mesentery. The mRNA of ETAR and ETBR was determined using real-time polymerase chain reaction.

RESULTS: Treatment with Glytan reduced portal pressure (PP) and portal territory blood flow (PTBF) and increased both mean arterial pressure (MAP) and splanchnic vascular resistance (SVR). Especially at 4 wk, PP decreased by about 40%, while MAP increased by 13%, SVR increased by 12%, and PTBF decreased by about 21%. The effect of blood flow reduction was greatest in the mesentery (about 33%) at 4 wk. The mesenteric circulation ET-1 levels of BDL rats were lower and negatively correlated with PP at 4 wk. Glytan can increase mesenteric ET-1 content and inhibit ETBR, eNOS, GRK2, and β-arrestin 2 expression in the mesentery. Moreover, Glytan showed no effect on the expression of ETAR protein and mRNA.

CONCLUSION: The decreased PP and PTBF observed after Glytan treatment were related to increased mesenteric vasoconstriction and increased receptor sensitivity to vasoconstrictor.

Nitric oxide and prostaglandin as mediators in the pathogenesis of hyperkinetic circulatory state in a model of endotoxemia-induced portal hypertension

AIMS

To evaluate the participation of nitric oxide (NO) and prostaglandin (PGI2) on hyperdynamic state in endotoxemia-induced portal hypertension (EIP) induced by chronic endotoxemia.

METHODS

The portal pressure (PP) and mean arterial pressure (MAP) were recorded, in vivo before and after administration of L-NAME (NOS inhibitor) and indomethacin (specific blocker of COX). The vasoactive responses to acetylcholine of thoracic rat aortic rings were studied in vitro before and after nitric oxide and cyclooxygenase blockade using multichannel organ bath. The mRNA expression for isoforms of (cyclooxygenase) COX and nitric oxide synthase (NOS) were analyzed using RT-PCR.

RESULTS

Administration of both L-NAME and indomethacin in EIP rabbits significantly reduced (p < 0.05) the PP and reversed the MAP to normal as compared to sham-operated (SO) rabbits. There was impaired vasodilatory response to acetylcholine in EIP rabbits. L-NAME caused a significant reduction in acetylcholine-induced vasorelaxation in SO rabbits than EIP due to preexisting hyperemia in EIP. Indomethacin partially restored vasoresponsiveness to acetylcholine in EIP group. The mRNA expression of eNOS (endothelial NOS) and COX-1 (constitutive COX) were significantly higher in SO than EIP rabbits. iNOS (inducible NOS) and COX-2 (inducible COX) mRNA expression was seen only in EIP rabbits.

CONCLUSIONS

A significant component of acetylcholine-mediated vasorelaxation in EIP model is modulated by eNOS. There was increased production of contractile prostaglandin in EIP rabbits. iNOS and COX-2 play an important role in the hemodynamic abnormalities of PHT. This novel model of PHT produced by chronic splanchnic endotoxemia in rabbit, mimics impaired vasodilation and vasoreactivity akin to other models of PHT.

Hemodynamic changes in splanchnic blood vessels in portal hypertension

Portal hypertension (PHT) is associated with a hyperdynamic state characterized by a high cardiac output, increased total blood volume, and a decreased splanchnic vascular resistance. This splanchnic vasodilation is a result of an important increase in local and systemic vasodilators (nitric oxide, carbon monoxide, prostacyclin, endocannabinoids, and so on), the presence of a splanchnic vascular hyporesponsiveness toward vasoconstrictors, and the development of mesenteric angiogenesis. All these mechanisms will be discussed in this review. To decompress the portal circulation in PHT, portosystemic collaterals will develop. The presence of these portosystemic shunts are responsible for major complications of PHT, namely bleeding from gastrointestinal varices, encephalopathy, and sepsis. Until recently, it was accepted that the formation of collaterals was due to opening of preexisting vascular channels, however, recent data suggest also the role of vascular remodeling and angiogenesis. These points are also discussed in detail.

Altered alpha adrenergic vasoresponsiveness in a non-cirrhotic portal hypertension model of E. coli injection

BACKGROUND AND AIM

Portal hypertension is associated with decreased vascular responsiveness to vasoconstrictors, which may contribute to the hyperdynamic circulation in cirrhosis. Animal models of cirrhosis and portal vein ligation have helped in our understanding of portal hypertension. The etiopathogenesis of non-cirrhotic portal fibrosis (NCPF), a common cause of portal hypertension, is still poorly understood. The aim of this study was to investigate the pathophysiology of NCPF in a rabbit model.

METHODS

An indwelling cannula was inserted into the gastrosplenic vein of rabbits. Animals were randomly injected with saline (Group I, n = 13) or lipopolysaccharide (Group II, n = 13) from heat killed Escherichia coli at 0, 1, 2, 7, 14 and 28 days. Portal pressure was measured at 3 months and vasoresponsiveness studied in isolated aortic rings in intact and in endothelium-denuded tissues from both groups.

RESULTS

In all group II compared with group I animals, the splenic weight (0.89 +/- 0.16 vs 0.62 +/- 0.1 g, P < 0.05) and the portal pressure (14.99 +/- 0.56 vs 7.04 +/- 0.42 mmHg, P < 0.05) were higher at 3 months. The group II animals showed reduced responsiveness to phenylephrine showing maximal contraction of 1.25 +/- 0.08 at 10(-4) mol/L as compared to 2.85 +/- 0.33 g tension in Group I (P < 0.05). Endothelium denudation of aortic rings had no effect on reduced reactivity in Group II animals. Acetylcholine induced an increase in vasorelaxation at lower concentrations in preconstricted aortic rings in Group II compared to Group I animals, but this decreased in higher concentrations. Nifedipine produced comparable vasodilatation in preconstricted rings in both the groups of animals.

CONCLUSIONS

Repeated injection of lipopolysaccharide into the gastrosplenic vein leads to the development of portal hypertension. This non-cirrhotic model of portal hypertension is characterized by generalized arterial hyporeactivity to vasoconstrictors akin to other models of portal hypertension.

Gender differences in vascular reactivity of aortas from rats with and without portal hypertension

BACKGROUND

Inflammatory responses related to portal hypertension may be different in male and female rats. Most experimental studies of portal hypertension have involved male animals, and little information is available on gender differences in this setting. The aim of the present study was to compare aortic reactivity in female and male rats with and without portal hypertension.

METHODS

Contraction response curves to phenylephrine were studied with aortic rings, with and without endothelium. For relaxation studies, rings were precontracted with phenylephrine 10(-7) mol/L and then exposed to acetylcholine 10(-4) mol/L. Portal hypertension was provoked by calibrated portal stenosis performed 2 weeks before experiments.

RESULTS

In non-hypertensive conditions, the contractile response to increasing phenylephrine concentrations was significantly stronger in rings from male than female rats, both with and without endothelium. In male rats with portal hypertension, the phenylephrine concentration-response curves were lowered and shifted to the right in aortic rings both with and without endothelium. In female rats, portal hypertension did not induce significant changes in the phenylephrine concentration-response curves. In female rats, portal hypertension induced a marked increase in relaxation (157 +/- 123% vs 81 +/- 64% in controls); the increase was also stronger than that in male rats with portal hypertension (95 +/- 6%; P < 0.01).

CONCLUSION

Clear gender differences were observed in vasoconstrictor responsiveness of aortic rings from rats with and without portal hypertension. Contrary that in male rats, portal hypertension did not induce vascular hyporesponsiveness in female rats. Further investigations are required to explain these differences.

Systemic and splanchnic haemodynamic effects of sildenafil in an in vivo animal model of cirrhosis support for a risk in cirrhotic patients

OBJECTIVES

Sildenafil is a selective inhibitor of the cGMP-specific phosphodiesterase type V (PDE-V) in the corpus cavernosum. PDE-V is also present in the mesenteric artery. Cirrhosis is complicated by a splanchnic vasodilation attributed to a local overproduction of nitric oxide (NO). As sildenafil potentiates the effects of NO, it may further decrease mesenteric vascular tone and increase portal venous blood flow. The aim is to evaluate the effects of sildenafil on the systemic and splanchnic haemodynamics in an experimental model of cirrhosis.

METHODS

Secondary biliary cirrhosis was induced in male Wistar rats by common bile duct ligation (CBDL, n=8); control rats were sham-operated (sham, n=7). The mean arterial pressure (MAP), portal venous pressure (PVP) and arterial mesenteric blood flow (MBF) were measured after intramesenteric (0.01-10 mg/kg) and after intravenous (i.v.) (0.01-10 mg/kg) administration of sildenafil.

RESULTS

Baseline PVP was significantly higher in CBDL than in sham rats, whereas baseline MAP tended to be lower and MBF tended to be higher in CBDL compared with sham rats. Both intramesenteric and i.v. injection of sildenafil significantly decreased MAP and increased MBF and PVP in a dose-dependent way. The decrease in MAP was significantly less important in CBDL than in sham rats. The increase in MBF was importantly lower in CBDL than in sham rats. PVP tended to increase more significantly in sham rats than in CBDL.

CONCLUSION

Sildenafil increases MBF and PVP and induces systemic hypotension. The effects are less pronounced in cirrhosis, suggesting vascular hyporesponsiveness to sildenafil. Although the rise in PVP in cirrhotic animals is smaller than in controls, it may present a risk for haemorrhagic complications. Further studies are necessary before prescribing sildenafil to patients with cirrhosis.

Vascular hyporesponsiveness in the mesenteric artery of anaesthetized rats with cirrhosis and portal hypertension: an in-vivo study

INTRODUCTION

Cirrhosis is complicated by splanchnic vasodilation. Nitric oxide (NO) and prostacyclin contribute to this. Vascular hyporesponsiveness has been reported, but the underlying pathophysiological mechanisms are unclear.

OBJECTIVE

This in-vivo study examined the contribution of NO and prostacyclin to the development of vascular hyporesponsiveness in the mesenteric circulation of animals with cirrhosis and portal hypertension.

METHODS

Rats underwent common bile duct ligation (CBDL) (n = 11), partial portal vein ligation (PPVL) (n = 12) and sham-operation (sham) (n = 11). Blood flow in the mesenteric artery (MBF) was measured during intramesenteric infusion of endothelium-dependent (acetylcholine) and endothelium-independent vasodilators (deta-NONOate, pinacidil) and a vasoconstrictor (L-phenylephrine). The measurements were repeated after systemic infusion of L-NAME (NO synthase inhibition) and indomethacin (cyclo-oxygenase inhibition).

RESULTS

The MBF response to acetylcholine was significantly lower in CBDL and tended to be lower in PPVL than in sham. L-NAME and indomethacin significantly decreased the MBF response to acetylcholine in all groups. The hyporeactivity to acetylcholine in CBDL and PPVL was maintained after L-NAME and indomethacin. The MBF response to pinacidil, deta-NONOate and phenylephrine, before and after NO synthase and cyclo-oxygenase inhibition, was lower in CBDL and PPVL than in sham.

CONCLUSION

This is the first in-vivo study demonstrating an impaired response to endothelium-dependent and endothelium-independent vasodilators as well as vasoconstrictors in the mesenteric artery of animals with cirrhosis and portal hypertension. The generalised hyporeactivity suggests an abnormality on the vascular smooth muscle cell level. The hyporesponsiveness persisted after combined NO synthase and cyclo-oxygenase inhibition.

Portal hypertension is associated with increased mRNA levels of vasopressor G-protein-coupled receptors in human hepatic arteries

BACKGROUND

The contractile response of human splanchnic vessels to different vasoconstrictors is attenuated in cirrhosis. Functional studies indicate a cellular signalling defect upstream of the G-protein level. The aim of the present study was to analyze expression and mRNA levels of the following most relevant vasopressor receptors in the smooth musculature of human hepatic arteries: alpha1 adrenoceptor (AR) subtypes a, b and d, angiotensin II type 1 receptor (AT1), arginine vasopressin receptor type 1a (V1a), endothelin receptor type A (ETA) and B (ETB).

MATERIALS AND METHODS

Hepatic arteries were collected from 10 donors (noncirrhotic) and 14 recipients (cirrhotic) at liver transplantations. Real-time-PCR was performed to quantify steady-state levels of receptor mRNAs.

RESULTS

alpha 1aAR mRNA levels showed no significant difference between the cirrhotic arteries and the controls while the mRNA levels of the other vasoactive receptors were significantly higher in the cirrhotic hepatic arteries (alpha 1bAR: 4-fold, P = 0.013; AT1: 16-fold, P = 0.024; V1a: 23-fold, P = 0.001; ETA: 4-fold, P = 0.02; ETB: 8-fold, P = 0.008). No mRNA for the alpha 1dAR was detected either in the donor or recipient hepatic arteries.

CONCLUSION

We conclude that vascular hyporeactivity to the most relevant endogenous vasoconstrictors of cirrhotic hepatic arteries is not caused by a receptor down-regulation at mRNA levels. In contrast they were up-regulated.

Role of small-conductance Ca2+-dependent K+ channels in in vitro nitric oxide-mediated aortic hyporeactivity to alpha-adrenergic vasoconstriction in rats with cirrhosis

BACKGROUND/AIMS

In vitro studies have shown that cirrhotic aortas are hyporeactive to the contractile effect of vasoconstrictors because upregulated endothelial nitric oxide-synthase (NOS) overproduces nitric oxide (NO). Although stimulation of endothelial small-conductance Ca2+-dependent K+ (SK(Ca)) channels may elicit vasorelaxation in normal arteries, the role of these channels in cirrhosis-induced hyporeactivity is unknown. Thus, the aim of the present study was to investigate the role of endothelial SK(Ca) channels in cirrhosis-induced, NO-mediated, in vitro aortic hyporeactivity to alpha1-adrenergic vasoconstrictors.

METHODS

Isolated thoracic aortas from cirrhotic and normal rats were used. The effects of apamin, a selective SK(Ca) channel blocker, were measured on the vascular reactivity to phenylephrine. In addition, SK(Ca) channel protein expression was studied. The effects of iberiotoxin and charybdotoxin, blockers of other K(Ca) channels, were also studied in cirrhotic aortas.

RESULTS

Apamin suppressed cirrhosis-induced aortic hyporeactivity to phenylephrine in an endothelium-dependent, NOS-inhibitor-sensitive manner. SK(Ca) channel protein was overexpressed in cirrhotic aortic walls. Iberiotoxin abolished cirrhosis-induced aortic hyporeactivity to phenylephrine in an endothelium-dependent but NOS-inhibitor-resistant manner. Charybdotoxin did not induce any significant increase in phenylephrine-elicited contraction.

CONCLUSIONS

In cirrhotic aortas, the overexpression and overactivity of endothelial SK(Ca) channels contributes to in vitro NO-mediated hyporeactivity to the contractile action of alpha1-adrenergic agonists.

Cirrhosis serum induces a nitric oxide-associated vascular hyporeactivity of aortic segments from healthy rats in vitro

OBJECTIVE

Arterial vasodilation with concomitant hyperdynamic circulation are common findings in liver cirrhosis. Nitric oxide acting at a local level has been suggested to be pathophysiologically relevant in this context. Several systemic factors in conjunction with nitric oxide might interfere with the observed phenomena.

DESIGN

The study has been designed to demonstrate the influence of cirrhotic serum on the nitric oxide system and vascular contractility.

METHODS

The contractile response of aortic segments from healthy rats was studied in vitro after incubation with serum of healthy and cirrhosis-induced rats (1 week, 2 weeks, 3 weeks and 4 weeks after bile duct ligation). A cumulative dose response curve to phenylephrine (10--10-4 mol) was established before and after incubation with nitric oxide synthesis blocker N(omega)-nitro-L-arginine, the more selective aminoguanidine (nitric oxide synthase [NOS]-2 inhibitor) and W7 (NOS-3 inhibitor). NOS-2 expression in incubated aortic rings was evaluated by Western blot analysis.

RESULTS

A 4-hour incubation with serum of cirrhosis-induced rats reduced the maximum contractile response to phenylephrine to 66.8 +/- 9.1% after 1 week, 50.4 +/- 7.8% after 2 weeks, 43.2 +/- 2.8% after 3 weeks and 35 +/- 5.2% after 4 weeks of bile duct ligation. This reduction in the contractility response to phenylephrine was completely reversed by blocking nitric oxide synthesis with N(omega)-nitro-L-arginine and aminoguanidine, but not after W7. Incubation with cirrhotic serum induced NOS-2 expression in aortic rings. In Western blot analysis, the most intensive signal for NOS-2 protein was obtained in rings incubated with serum from rats 3 weeks and 4 weeks after induction of cirrhosis.

CONCLUSIONS

Cirrhotic serum decreases the contractile response to phenylephrine even in an early stage of secondary cirrhosis. Reversibility of this effect after nitric oxide synthesis blockade suggests an induction of nitric oxide synthesis by systemic factors as a major point in vascular hyporeactivity to vasoconstrictors in cirrhosis.

Evidence for an endothelium-derived hyperpolarizing factor in the superior mesenteric artery from rats with cirrhosis

In cirrhosis, in splanchnic arteries, endothelium-dependent relaxation may persist even if overactive nitric oxide synthase (NOS) and cyclooxygenase (COX) are inhibited. In normal arteries, a significant endothelium-dependent relaxation to acetylcholine persists after NOS/COX inhibition. This relaxation is caused by smooth muscle cell (SMC) membrane hyperpolarization, which is sensitive to a combination of the potassium channel blockers apamin and charybdotoxin, and is mediated by an endothelium-derived hyperpolarizing factor (EDHF). The aim of this study was to detect EDHF and evaluate its pathophysiologic role in isolated superior mesenteric arteries from cirrhotic rats. Arterial rings were obtained and exposed to N(w)-nitro-L-arginine (L-NNA, a NOS inhibitor) and indomethacin (a COX inhibitor). Acetylcholine-induced membrane potential responses and concentration-response curves to the relaxant of acetylcholine were obtained with and without apamin plus charybdotoxin. Acetylcholine-induced responses were measured in certain rings from endothelium-denuded arteries. Contractions caused by the alpha(1)-adrenoceptor agonist phenylephrine were obtained in cirrhotic and normal rings with and without apamin and charybdotoxin. Significant acetylcholine-induced, endothelium-dependent, apamin- and charybdotoxin-sensitive, SMC membrane hyperpolarization and relaxation were found. An apamin- and charybdotoxin-sensitive hyporesponsiveness to the contractile action of phenylephrine was found in cirrhotic rings. In conclusion, in cirrhotic rats, in the superior mesenteric artery exposed to NOS/COX-inhibitors, an EDHF exists that may replace NOS/COX products to induce endothelium-dependent arterial relaxation.

Role of iNOS in the vasodilator responses induced by L-arginine in the middle cerebral artery from normotensive and hypertensive rats

1. The substrate of nitric oxide synthase (NOS), L-arginine (L-Arg, 0.01 microM - 1 mM), induced endothelium-independent relaxations in segments of middle cerebral arteries (MCAs) from normotensive Wistar-Kyoto (WKY) and hypertensive rats (SHR) precontracted with prostaglandin F2alpha (PGF2alpha). These relaxations were higher in SHR than WKY arteries. 2. L-N(G)-nitroarginine methyl ester (L-NAME) and 2-amine-5,6-dihydro-6-methyl-4H-1,3-tiazine (AMT), unspecific and inducible NOS (iNOS) inhibitors, respectively, reduced those relaxations, specially in SHR. 3. Four- and seven-hours incubation with dexamethasone reduced the relaxations in MCAs from WKY and SHR, respectively. 4. Polymyxin B and calphostin C, protein kinase C (PKC) inhibitors, reduced the L-Arg-induced relaxation. 5. Lipopolysaccharide (LPS, 7 h incubation) unaltered and inhibited these relaxations in WKY and SHR segments, respectively. LPS antagonized the effect polymyxin B in WKY and potentiated L-Arg-induced relaxations in SHR in the presence of polymyxin B. 6. The contraction induced by PGF2alpha was greater in SHR than WKY arteries. This contraction was potentiated by dexamethasone and polymyxin B although the effect of polymyxin B was higher in SHR segments. LPS reduced that contraction and antagonized dexamethasone- and polymyxin B-induced potentiation, these effects being greater in arteries from SHR. 7. These results suggest that in MCAs: (1) the induction of iNOS participates in the L-Arg relaxation and modulates the contraction to PGF2alpha; (2) that induction is partially mediated by a PKC-dependent mechanism; and (3) the involvement of iNOS in such responses is greater in the hypertensive strain.

Vasopressin reverses mesenteric hyperemia and vasoconstrictor hyporesponsiveness in anesthetized portal hypertensive rats

We recently reported that vasopressin analogues correct the in vitro vascular hyporeactivity to adrenergic vasoconstrictors in portal hypertensive rats. The aim of the present study was to determine whether vasopressin reduces splanchnic blood flow in portal vein-ligated (PVL) rats by restoring vasoconstrictor responsiveness in vivo. The ultrasonic transit time-shift technique was used for blood flow measurements. At basal conditions, blood flow through the superior mesenteric artery was elevated 1.6-fold in PVL rats as compared with sham-operated (SHAM) control rats. PVL rats also exhibited blunted mesenteric constrictor responses to the adrenoceptor agonist, phenylephrine (0.03-1 micromol x min(-1) x kg(-1)). Terlipressin (2-20 microg x k(-1)) and arginine vasopressin (3-300 pmol x min(-1) x kg(-1)) dose-dependently reduced, and at the highest doses, even abolished, the difference in mesenteric blood flow (MBF) between PVL and SHAM rats. When expressed as percent changes relative to baseline, mesenteric arterial responses to terlipressin and arginine vasopressin were found to be enhanced in PVL rats as compared with SHAM rats. Moreover, pretreatment with terlipressin (20 microg x kg(-1)) reversed the mesenteric hyporesponsiveness to phenylephrine of PVL rats. These vasopressin effects were independent of the nitric oxide (NO) pathway, because they were not mimicked by inhibition of NO synthesis with N(G)-nitro-L-arginine methyl ester (L-NAME) (0.1-10 mg x kg(-1)). These data indicate that pharmacological doses of vasopressin reverse the splanchnic hyperemia by restoring the responsiveness to adrenergic vasoconstrictors in portal hypertensive rats.

Vascular hyporeactivity persists despite increased contractility after long-term administration of isosorbide dinitrate in portal hypertensive rats

BACKGROUND/AIMS

Portal hypertension is associated with decreased vascular responsiveness to vasoconstrictors, which may contribute to the hyperdynamics. Isosorbide dinitrate is an effective portal hypotensive drug. The present study aimed to investigate whether chronic administration of isosorbide dinitrate could affect vascular responsiveness in portal hypertensive rats.

METHODS

Portal hypertension was induced by partial portal vein ligation. Sham-operated (Sham) rats served as controls. There were four animal groups for this study: portal vein ligation-isosorbide dinitrate group, portal vein ligation-vehicle (Veh) group, Sham-isosorbide dinitrate group and Sham-Veh group. Isosorbide dinitrate (5 mg x kg(-1) x 12 h(-1) was given by gavage for 8 days starting 1 day before ligation and continuing thereafter. Mesenteric arteries were removed for contractile study after hemodynamic measurement.

RESULTS

Contractile responses to KCI (15-90 mM) and phenylephrine (10(-9)-10(-4) M) were recorded. Both vascular reactivity and sensitivity were significantly reduced in portal vein ligation rats as compared to Sham rats. Chronic isosorbide dinitrate treatment reduced portal venous pressure in portal vein ligation rats. Moreover, the maximal contractile responses to KCl and phenylephrine were significantly enhanced in both portal vein ligation and Sham rats after isosorbide dinitrate treatment, but relative hyporeactivity persisted in portal vein ligation rats. In contrast, a single dose of isosorbide dinitrate did not alter the contractile sensitivity or reactivity to KCl or phenylephrine in either portal vein ligation or Sham rats.

CONCLUSION

Our results show that long-term administration of isosorbide dinitrate enhanced vascular contractility in both portal vein ligation and Sham rats, but relative hyporeactivity persisted in portal vein ligation rats.

Endothelial calcium-calmodulin dependent nitric oxide synthase in the in vitro vascular hyporeactivity of portal hypertensive rats

BACKGROUND/AIMS

Increased nitric oxide production has been implicated in impaired vascular responsiveness to vasoconstrictors in portal hypertension. However, there is no firm evidence concerning the involved nitric oxide synthase isoform. The present study investigated the possible contribution of one nitric oxide synthase isoform, the endothelial constitutive Ca2+-calmodulin dependent, in the overproduction of nitric oxide in portal hypertension.

METHODS

Vascular responses to norepinephrine and acetylcholine were evaluated in isolated thoracic aortic rings from normal and portal vein stenosed rats.

RESULTS

An impaired concentration-dependent contraction to norepinephrine was observed in intact rings from portal hypertensive rats compared to controls. The hyporeactivity to norepinephrine was reversed after endothelium denudation, the inhibition of nitric oxide synthase with L-NOARG or the inhibition of calmodulin with W-7, but not after pre-incubation with indomethacin. Stimulation of intact rings with norepinephrine after the inhibition of calmodulin with calmidazolium was followed by a decreased vascular response in vessels from normal rats but not in those from portal hypertensive rats. Stimulation of intact rings with norepinephrine in a Ca2+-free medium was followed by a decreased vascular response in vessels from both portal hypertensive and normal rats. No difference in vasoconstrictive responses was observed between the two groups after calmidazolium or in a Ca2+-free medium. Relaxation induced by acetylcholine in norepinephrine-precontracted rings was more marked in rings from portal hypertensive rats than in controls. No differences in the vasodilator responses were observed after relaxations had been inhibited by the removal of the endothelium, pre-incubation with L-NOARG, indomethacin, W-7 or calmidazolium and in a Ca2+-free medium.

CONCLUSIONS

This study demonstrates the involvement of the endothelial constitutive Ca2+-calmodulin dependent nitric oxide synthase isoform in the overproduction of nitric oxide in portal hypertension.

Vascular hyporesponsiveness to vasodilators in rats with cirrhosis

BACKGROUND/AIMS

In cirrhosis, the activation of nitric oxide and prostacyclin contributes to vasodilation, and ATP-sensitive K+ (KATP) channel activation or L-type calcium (Ca2+) channel inhibition may also play a role in this process. At the same time in cirrhosis, certain endogenous mechanisms may be stimulated which limit the influence of vasodilator mechanisms on vascular tone, thus altering vascular responses to exogenous substances such as nitric oxide donors, exogenous prostacyclin, KATP channel openers or L-type Ca2+ channel blockers. The aim of the present study was to examine the arterial depressor to these exogenous substances in normal rats and in rats with secondary biliary cirrhosis.

METHODS

Arterial depressor dose-response curves to nitroprusside (a nitric oxide donor, 5-60 micrograms.kg-1.min-1), prostacyclin (0.5-5 micrograms.kg-1) and aprikalim (a KATP channel opener, 10-200 micrograms.kg-1) were obtained in both groups. The effects of different L-type Ca2+ channel blockers, i.e. nicardipine (a dihydropyridine, 0.02-0.5 mg.kg-1), diltiazem (a benzothiazepine, 0.5-5 mg.kg-1) and verapamil (a phenylalkylamine, 0.02-0.2 mg.kg-1. min-1), were also studied.

RESULTS

Cirrhosis produced hyporeactivity to the arterial depressor effect of all doses of nitroprusside, the lowest dose of prostacyclin and the highest doses of aprikalim or diltiazem. Cirrhosis did not significantly change depressor responses to nicardipine or verapamil.

CONCLUSIONS

Rats with cirrhosis are hyporeactive to exogenous nitric oxide, prostacyclin, KATP channel opener and benzothiazepine (an L-type Ca2+ channel blocker). Therefore, cirrhosis-induced mechanisms seem to limit the decrease in vascular tone by most vasodilators. However, these mechanisms appear to be more marked in nitric oxide-mediated vasodilation than in other vasorelaxation mechanisms.

The role of increased nitric oxide in the vascular hyporeactivity to noradrenaline in long-term portal vein ligated rats

To test the possible role of nitric oxide production in long-term portal vein ligation in the rat, where the hyperdynamic circulation was reported to be absent, in vivo experiments on isolated thoracic aortic rings from partial portal vein ligated or sham-operated rats were performed, 6 months postoperatively. The concentration-response curves to noradrenaline of both intact and endothelium-denuded rings from portal hypertensive rats were significantly shifted to the right as compared to those from sham-operated animals. In intact rings, addition of NG-nitro-L-arginine, a specific inhibitor of nitric oxide synthase, resulted in a significant shift of the curves to the left in sham-operated and portal vein ligated rats. In endothelium-denuded rings, addition of NG-nitro-L-arginine resulted in a significant shift of the curves to the left in portal vein ligated but not in sham-operated animals. After blockade of the nitric oxide biosynthesis with NG-nitro-L-arginine, the negative logarithm of the concentration of nonadrenaline causing half-maximal response did not significantly differ any more between portal vein ligated and sham-operated rats; in endothelium-denuded rings hyporeactivity to noradrenaline persisted in portal vein ligated rats. Only in the intact rings did NG-nitro-L-arginine significantly increase the maximal contractions. No differences were demonstrated in endothelium-dependent relaxations to acetylcholine between sham-operated and portal hypertensive animals. From these results, it can be concluded that in vitro aortic hyporeactivity to noradrenaline is still present in long-term portal vein ligated rats, and that it results at least partially from activation of the L-arginine: nitric oxide pathway in the aortic vascular wall.