Effects of portal hypertension on responsiveness of rat mesenteric artery and aorta

The vascular Ca2+-sensing receptor regulates blood vessel tone and blood pressure

The extracellular calcium-sensing receptor CaSR is expressed in blood vessels where its role is not completely understood. In this study, we tested the hypothesis that the CaSR expressed in vascular smooth muscle cells (VSMC) is directly involved in regulation of blood pressure and blood vessel tone. Mice with targeted CaSR gene ablation from vascular smooth muscle cells (VSMC) were generated by breeding exon 7 LoxP-CaSR mice with animals in which Cre recombinase is driven by a SM22α promoter (SM22α-Cre). Wire myography performed on Cre-negative [wild-type (WT)] and Cre-positive (SM22α)CaSR(Δflox/Δflox) [knockout (KO)] mice showed an endothelium-independent reduction in aorta and mesenteric artery contractility of KO compared with WT mice in response to KCl and to phenylephrine. Increasing extracellular calcium ion (Ca(2+)) concentrations (1-5 mM) evoked contraction in WT but only relaxation in KO aortas. Accordingly, diastolic and mean arterial blood pressures of KO animals were significantly reduced compared with WT, as measured by both tail cuff and radiotelemetry. This hypotension was mostly pronounced during the animals' active phase and was not rescued by either nitric oxide-synthase inhibition with nitro-l-arginine methyl ester or by a high-salt-supplemented diet. KO animals also exhibited cardiac remodeling, bradycardia, and reduced spontaneous activity in isolated hearts and cardiomyocyte-like cells. Our findings demonstrate a role for CaSR in the cardiovascular system and suggest that physiologically relevant changes in extracellular Ca(2+) concentrations could contribute to setting blood vessel tone levels and heart rate by directly acting on the cardiovascular CaSR.

Effects of portal hypertension on contractility of rat spleen

Portal hypertension induces changes in vascular responses to vasoconstrictors. However, the effects of portal hypertension on splenic contraction have not previously been investigated. In partial portal vein ligated (PVL) and sham-operated rats, we examined the splenic contractile responses to cumulative concentrations of noradrenaline and KCl. In PVL rats, the potency of noradrenaline in producing splenic contraction was significantly increased (pEC50 of 5.88 ± 0.08), as compared to sham (5.40 ± 0.06; p<0.001). In the presence of prazosin (10(-8)M), there was a significant rightward shift in the noradrenaline concentration response curve but the shift was greater for PVL, so that in the presence of prazosin there was no significant difference between PVL and sham animals in the potency of noradrenaline. Prazosin produced a significantly greater shift of noradrenaline potency in spleen from PVL (pKB of 8.88 ± 0.06) (n=6) than from sham animals (8.51 ± 0.08, n=6), demonstrating that the α1-adrenoceptor mediated component is greater in spleen from PVL. In the presence of prazosin (10(-8)M) the residual response is non-α1-adrenoceptor mediated, presumably α2-adrenoceptor mediated, and this response did not differ between sham and PVL. The maximum splenic contraction did not significantly differ between sham and PVL rats for either agonist. In conclusion, noradrenaline potency in contracting the rat spleen was significantly increased in tissues from PVL rats. The increased potency of prazosin suggests a greater predominance of α1-adrenoceptors in spleen of PVL rats, as prazosin has lower potency at α2-adrenoceptors.

Effect of short- and long-term portal hypertension on adrenergic, nitrergic and sensory functioning in rat mesenteric artery

In the present study, we analysed possible alterations in adrenergic, nitrergic and sensory functioning in mesenteric arteries from rats at 1 and 21 months after partial portal vein ligation, and the mechanisms involved in these alterations, if any. For this purpose, we analysed the vasoconstrictor response to EFS (electrical field stimulation) and the effect of the α-antagonist phentolamine, the NOS (nitric oxide synthase) inhibitor L-NAME (N(G)-nitro-L-arginine methyl ester) and the CGRP (calcitonin gene-related peptide) receptor antagonist CGRP-(8-37) in mesenteric segments from ST (short-term; 1 month) and LT (long-term; 21 months) SO (sham-operated) and pre-hepatic PH (portal hypertensive) rats. The vasomotor responses to NA (noradrenaline), the NO donor DEA-NO (diethylamine NONOate) and CGRP were analysed. NA, NO and CGRP releases were measured. Phospho-nNOS (neuronal NOS) expression was studied. The vasoconstrictor response to EFS was decreased in STPH animals. Phentolamine decreased this vasoconstrictor response more strongly in SO animals. Both L-NAME and CGRP-(8-37) increased vasoconstrictor response to EFS more strongly in PH than SO segments. PH did not modify vasomotor responses to NA, DEA-NO or CGRP, but it decreased NA release while increasing those of NO and CGRP. Phospho-nNOS expression was increased by PH. In LTPH, no differences were observed in vasoconstrictor response to EFS, vasomotor responses or neurotransmitter release when compared with age-matched SO animals. In conclusion, the mesenteric innervation may participate in the development of the characteristic hyperdynamic circulation observed in STPH through the joint action of decreased adrenergic influence, and increased nitrergic and sensory innervations influences. The participation of each innervation normalizes under conditions of LTPH.

Long-term portal hypertension increases the vasodilator response to acetylcholine in rat aorta: role of prostaglandin I2

In the present study, we have analysed both the effect of long-term portal hypertension on the vasomotor response to acetylcholine in rat aorta and the mechanism involved in this response. For this purpose, sham-operated rats and rats with pre-hepatic PH (portal hypertension; triple partial portal vein ligation) were used at 21 months after surgery. The participation of NO and COX (cyclo-oxygenase) derivatives in the vasodilator response elicited by acetylcholine after incubation with L-NAME (NG-nitro-L-arginine methyl ester), indomethacin, SC-560, NS-398, tranylcypromine and furegrelate, was analysed. NO, TXB2 (thromboxane B2) and 6-keto PGF1alpha (prostaglandin F1alpha) release were measured. In addition, SNP (sodium nitroprusside), U-46619, PGI2 and forskolin vasomotor responses were analysed. COX-1 and COX-2 expression was also determined. The acetylcholine-induced vasodilating response was higher in rats with PH. TXA2 and NO release, and SNP and U-46619 sensitivity were similar in both groups. PGI2 release was not modified by portal hypertension, but vasodilator responses to this prostanoid and to forskolin were higher in rats with PH. COX-1 and COX-2 expression remained unmodified by surgery. In conclusion, increased vasodilation to acetylcholine is maintained in long-term PH. Although the participation of endothelial NO remained unmodified, the COX-2 derivative PGI2 does participate through an increased vasodilator response.

Vascular actions of the prostacyclin receptor antagonist BAY 73-1449 in the portal hypertensive rat

We have investigated the actions of the postacyclin receptor antagonist BAY 73-1449 on shunt vessel development and shunt flow in the portal vein ligated portal hypertensive Wistar rat in vivo. BAY 73-1449 (0.1-1 mg/kg), given intravenously, did not significantly reduce mesenteric inflow, but significantly reduced splenic shunt vessel outflow, compared to the effects of vehicle, in anaesthetized portal vein ligated rats as measured by shunt vessel conductance. There were no differences between portal vein ligated animals treated, beginning just before portal vein ligation, with vehicle for 7 days and animals treated for 7 days with BAY 73-1449 (1-5 mg/kg, s.c.) in the degree of porto-systemic shunting, as measured by the radioactive microsphere technique in anaesthetized rats. Portal pressure was similar in animals treated with vehicle or BAY 73-1449. It is concluded that the prostacyclin receptor antagonist BAY 73-1449 can acutely reduce shunt vessel blood flow in portal hypertensive rats.

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.

Sympathectomy reveals alpha 1A- and alpha 1D-adrenoceptor components to contractions to noradrenaline in rat vas deferens

We have previously demonstrated that contractions of rat vas deferens to exogenous noradrenaline involve predominantly alpha(1A)-adrenoceptors, but that contractions to endogenous noradrenaline involve predominantly alpha(1D)-adrenoceptors. In this study, we have examined the effects of sympathectomy on the subtypes of alpha(1)-adrenoceptor in rat vas deferens in radioligand binding and functional studies. In vehicle-treated tissues, antagonist displacement of [(3)H]prazosin binding to alpha(1)-adrenoceptors was consistent with a single population of alpha(1)-adrenoceptors. Binding affinities for a range of alpha(1)-adrenoceptor antagonists were expressed as pK(i) values and correlated with known affinities for alpha(1)-adrenoceptor subtypes. The correlation was significant only with alpha(1A)-adrenoceptors. In tissues from rats sympathectomised with 6-hydroxy-dopamine (2 x 100 mg kg(-1) i.p.), binding affinity for the alpha(1D)-adrenoceptor antagonist BMY 7378 fitted best with a two-site model. In functional studies, the potency of noradrenaline at producing total (phasic plus tonic) but not tonic contractions was increased in tissues from sympathectomised rats. Results obtained from sympathectomised rats suggest that phasic contractions are mainly alpha(1D)-adrenoceptor mediated, whereas tonic contractions are mainly alpha(1A)-adrenoceptor mediated, based on the effects of BMY 7378 and the alpha(1A)-adrenoceptor antagonist RS 100329. It is concluded that the predominant alpha(1)-adrenoceptor in vehicle-treated rat vas deferens is the alpha(1A)-adrenoceptor, both in terms of ligand binding and contractions to exogenous agonists. The alpha(1D)-adrenoceptor is only detectable by ligand binding following chemical sympathectomy, but is involved in noradrenaline-evoked contractions, particularly phasic contractions, of rat vas deferens.

Deletion of inducible nitric oxide synthase decreases mesenteric vascular responsiveness in portal hypertensive mice

The effects of pre-hepatic portal hypertension were examined on the responsiveness of aorta and mesenteric artery from wild-type, inducible nitric oxide synthase knockout (iNOS-KO) and endothelial nitric oxide synthase knockout (eNOS-KO) mice. Mice were sham-operated or made portal hypertensive by creating a calibrated portal vein stenosis. Acetylcholine produced marked relaxations in phenylephrine (10 microM) contracted aorta and mesenteric artery from wild-type and iNOS-KO, both sham and portal hypertensive, but relaxations were abolished in vessels from eNOS-KO mice. There were no significant differences between sham and portal hypertensive animals within groups in the effects of acetylcholine. The potency of KCl was significantly increased in aorta and mesenteric artery from eNOS-KO mice. The maximum contraction to the alpha(1)-adrenoceptor agonist phenylephrine was significantly increased in aorta from eNOS-KO, as compared with wild-type mice. There were no significant differences between sham and portal hypertensive animals within each group in contractions of aorta to KCl or phenylephrine. However, in mesenteric artery, although portal hypertension did not change responsiveness in wild-type or eNOS-KO as compared to sham animals, the potency of phenylephrine was significantly reduced in portal hypertensive iNOS-KO mice as compared to shams. Hence, portal hypertension as compared to sham operation did not affect responses to vasoconstrictors in mouse aorta, but in mouse mesenteric artery portal hypertension affected vascular responses in iNOS-KO mice, suggesting that iNOS is involved in the mesenteric vascular response to portal vein ligation.

The action of nitric oxide on hepatic haemodynamics during secondary biliary cirrhosis in the rat

The role of nitric oxide (NO) in portal hypertension is poorly understood. The role of NO upon hepatic arterial and portal venous vasoconstrictor responses to noradrenaline and ATP in rats with secondary biliary cirrhosis was evaluated. Cirrhosis was induced by bile duct ligation after which livers were excised and dual-perfused in vitro. Concentration-dependent dose-response curves were then constructed to hepatic arterial and portal venous noradrenaline and ATP. Hepatic arterial responses to noradrenaline and ATP were significantly attenuated in cirrhotic rats. 100 microM N(G)-nitro-L-arginine methyl ester (L-NAME) restored attenuated hepatic arterial responses to noradrenaline and ATP in cirrhotic rats. Portal venous responses to noradrenaline in cirrhotic rats were significantly increased compared to controls and were not affected by L-NAME. However, portal venous responses to ATP were significantly attenuated in cirrhotic rats and were also not restored by L-NAME. Hepatic arterial or portal venous responses to noradrenaline did not change after infusion of L-NAME. Hepatic arterial responses to noradrenaline and ATP were significantly attenuated in cirrhotic rats, possibly due to increased production of NO. However, portal venous responses in cirrhotic rats were increased to noradrenaline and attenuated to ATP, and were not related to increased NO production.

The role of angiotensin II in hypertension due to adenosine receptors blockade

The renin-angiotensin system may be involved in hypertension induced by adenosine receptors blockade with 1,3-dipropyl-8-sulfophenylxanthine (DPSPX). Contractions of the mesenteric vasculature to angiotensin II, noradrenaline and potassium chloride were studied in DPSPX-induced hypertension. Male Wistar rats received infusions of saline or DPSPX (90 microg kg(-1) h(-1), i.p.) for 3 or 7 days. Blood pressure was determined by the tail-cuff method. On days 3 or 14, concentration-response curves were obtained on mesenteric arteries and veins. Plasma angiotensin II levels, measured by radioimmunoassay, were higher in DPSPX-hypertensive rats. The maximum contractile effect of angiotensin II was lower in vessels from DPSPX-hypertensive rats while that for noradrenaline was higher. Potassium chloride-induced contractions were larger in veins from DPSPX-hypertensive rats but similar in arteries, when compared with control rats. We conclude that raised angiotensin II levels and altered vascular reactivity are consistent with a renin-angiotensin-mediated hypertension.

Endothelial dysfunction in cirrhosis and portal hypertension

Portal hypertension (PHT) is a common clinical syndrome associated with chronic liver diseases; it is characterized by a pathological increase in portal pressure. Pharmacotherapy for PHT is aimed at reducing both intrahepatic vascular tone and elevated splanchnic blood flow. Due to the altered hemodynamic profile in PHT, dramatic changes in mechanical forces, both pressure and flow, may play a pivotal role in controlling endothelial and vascular smooth muscle cell signaling, structure, and function in cirrhotics. Nitric oxide, prostacyclin, endothelial-derived contracting factors, and endothelial-derived hyperpolarizing factor are powerful vasoactive substances released from the endothelium in response to both humoral and mechanical stimuli that can profoundly affect both the function and structure of the underlying vascular smooth muscle. This review will examine the contributory role of hormonal- and mechanical force-induced changes in endothelial function and signaling and the consequence of these changes on the structural and functional response of the underlying vascular smooth muscle. It will focus on the pivotal role of hormonal and mechanical force-induced endothelial release of vasoactive substances in dictating the reactivity of the underlying vascular smooth muscle, i.e., whether hyporeactive or hyperreactive, and will examine the extent to which these substances may exert a protective and/or detrimental influence on the structure of the underlying vascular smooth muscle in both a normal hemodynamic environment and following hemodynamic perturbations typical of PHT and cirrhosis. Finally, it will discuss the intracellular processes that regulate the release/expression of these vasoactive substances and that control the transformation of this normally protective cell to one that may promote the development of vasculopathy in PHT.

Altered adrenergic responsiveness of endothelium-denuded hepatic arteries and portal veins in patients with cirrhosis

BACKGROUND & AIMS

Patients with cirrhosis are characterized by a reduced splanchnic vascular resistance and a hyporeactivity to adrenergic vasoconstrictors. So far, their adrenergic splanchnic vascular responsiveness has not been evaluated in vitro. We compared responses to alpha1- and beta2-adrenoceptor stimulation of hepatic arteries and portal veins of patients with cirrhosis undergoing transplantation with those of organ donors.

METHODS

Isometric contractions of endothelium-denuded vessel rings were induced cumulatively by methoxamine and relaxations by isoproterenol. Results are expressed as percentage of the contraction obtained by 85 mmol/L KCl or of the relaxation obtained by 100 micromol/L papaverine, respectively.

RESULTS

Maximal methoxamine-induced contractions were reduced in cirrhotic hepatic arteries (cirrhosis, 51.8% +/- 6.8%; donor, 89.9% +/- 6.6%; P < 0.01) and portal veins (cirrhosis, 49.2% +/- 6.4%; donor, 94.0% +/- 5.3%; P < 0.01). In cirrhosis, isoproterenol induced a less marked relaxation of hepatic arteries (cirrhosis, 46.6% +/- 3.2%; donor, 100.3% +/- 4.4%; P < 0. 01) but an increased relaxation of portal veins (cirrhosis, 41.9% +/- 6.2%; donor, 26.2% +/- 2.8%; P < 0.01).

CONCLUSIONS

In cirrhosis, endothelium-free hepatic arteries are hyporeactive to alpha1- and beta2-adrenoceptor agonists, and portal veins are hyporeactive to alpha1- but hyperreactive to beta2-adrenoceptor agonists. These findings support the in vivo findings of a hyporesponsiveness to adrenergic vasoconstrictors in patients with cirrhosis.

Cardiovascular actions of nitric oxide synthase inhibition in the portal hypertensive rat

We have investigated the actions of the nitric oxide synthase inhibitor L-NMMA in the portal hypertensive Wistar rat in vivo. Resting blood pressure in the anaesthetised portal hypertensive rat was 107.8+/-11.0 / 79.2+/-11.7 mmHg (n = 12), which was significantly lower than in sham-operated animals (143.0+/-3.8 / 114.0+/-4.0 mmHg, n = 19; P < 0.01). Cardiac output was significantly higher in portal hypertensive (30.2+/-1.0 ml min(-1) per 100 g, n = 12) than sham-operated animals (23.7+/-2.2 ml min(-1) per 100 g, n = 13; P < 0.01). Intravenous injection of L-NMMA (10 mg kg(-1)) significantly increased systemic blood pressure in both portal hypertensive and sham-operated animals to 123.0+/-15.0 / 93.4+/-14.0 mmHg and 162.1+/-5.7 / 131.6+/-6.0 mmHg, respectively. The magnitude of the changes were similar in both groups. This increase in blood pressure was accompanied by a decrease in cardiac output to 88.5+/-2.8% and 91.5+/-2.4% of control in portal hypertensive and sham-operated animals, respectively (no significant difference). L-NMMA (10 mg kg(-1)) had similar effects on small mesenteric arterial conductance in both portal hypertensive and sham operated animals, reducing conductance to 84.4+/-3.6% (n = 6) and 82.7+/-1.2% (n = 4) of control, respectively. It is concluded that L-NMMA has similar effects in vivo in portal hypertensive as compared with sham-operated rats. Hence, an enhancement of endothelium-derived nitric oxide is not involved in the hyperdynamic state following portal hypertension in the rat.

The role of nitric oxide in hepatic metabolism

Nitric oxide (NO) may regulate hepatic metabolism directly by causing alterations in hepatocellular (hepatocyte and Kupffer cell) metabolism and function or indirectly as a result of its vasodilator properties. Its release from the endothelium can be elicited by numerous autacoids such as histamine, vasoactive intestinal peptide, adenosine, ATP, 5-HT, substance P, bradykinin, and calcitonin gene-related peptide. In addition, NO may be released from the hepatic vascular endothelium, platelets, nerve endings, mast cells, and Kupffer cells as a response to various stimuli such as endotoxemia, ischemia-reperfusion injury, and circulatory shock. It is synthesized by nitric oxide synthase (NOS), which has three distinguishable isoforms: NOS-1 (ncNOS), a constitutive isoform originally isolated from neuronal sources; NOS-2 (iNOS), an inducible isoform that may generate large quantities of NO and may be induced in a variety of cell types throughout the body by the action of inflammatory stimuli such as tumor necrosis factor and interleukin (IL)-1 and -6; and NOS-3 (ecNOS), a constitutive isoform originally located in endothelial cells. Another basis for differentiation between the constitutive and inducible enzymes is the requirement for calcium binding to calmodulin in the former. NO is vulnerable to a plethora of biologic reactions, the most important being those involving higher nitrogen oxides (NO2-), nitrosothiol, and nitrosyl iron-cysteine complexes, the products of which (for example, peroxynitrite), are believed to be highly cytotoxic. The ability of NO to react with iron complexes renders the cytochrome P450 series of microsomal enzymes natural targets for inhibition by NO. It is believed that this mechanism provides negative feedback control of NO synthesis. In addition, NO may regulate prostaglandin synthesis because the cyclooxygenases are other hem-containing enzymes. It may also be possible that NO-induced release of IL-1 inhibits cytochrome P450 production, which ultimately renders the liver less resistant to trauma. It is believed that Kupffer cells are the main source of NO during endotoxemic shock and that selective inhibition of this stimulation may have future beneficial therapeutic implications. NO release in small quantities may be beneficial because it has been shown to decrease tumor cell growth and levels of prostaglandin E2 and F2 alpha (proinflammatory products) and to increase protein synthesis and DNA-repair enzymes in isolated hepatocytes. NO may possess both cytoprotective and cytotoxic properties depending on the amount and the isoform of NOS by which it is produced. The mechanisms by which these properties are regulated are important in the maintenance of whole body homeostasis and remain to be elucidated.

Vascular actions of octreotide in the portal hypertensive rat

1. We have investigated the actions of the somatostatin analogue octreotide in the portal hypertensive Wistar rat in vivo and in rat small mesenteric artery and aorta in vitro. 2. In small mesenteric artery, octreotide (0.1-0.3 microM) failed to produce any direct contraction, nor did it affect contractions to noradrenaline (NA, 10 microM) or endothelium-dependent relaxations to acetylcholine. 3. In rat aorta, octreotide (0.3 microM) and somatostatin (1 microM) failed to affect contractions to NA (1 microM), or concentration-contractile response curves to NA. 4. In rat vas deferens, octreotide and somatostatin significantly reduced contractile responses to electrical stimulation with pD2 values (-log IC50) of 8.19 +/- 0.10 (n = 4) and 8.16 +/- 0.26 (n = 4), respectively. Hence, the lack of effect of these agents in aorta or mesenteric artery was not due to lack of efficacy or inappropriate choice of concentration. 5. In the anaesthetized portal hypertensive rat, intravenous injection of octreotide (1-100 microg kg[-1]) did not significantly affect systemic blood pressure, nor did it affect mesenteric vascular conductance as measured by laser doppler flow probes. However, octreotide (100 microg kg[-1]) significantly reduced vascular conductance to 74.2 +/- 7.7% of control (n = 6) in porto-systemic shunt vessels as measured by laser doppler flow probes. 6. Phenylephrine (1 microg kg[-1]) significantly raised blood pressure and significantly decreased vascular conductance in both mesenteric (66.6 +/- 3.7% of control) and porto-systemic shunt vessels (58.7 +/- 10.0% of control). 7. It was concluded that octreotide has selective effects on porto-systemic shunt vessles in vivo in the portal hypertensive rat.

Nitric oxide-dependent and -independent vascular hyporeactivity in mesenteric arteries of portal hypertensive rats

1. Increased production of nitric oxide (NO) has been suggested to underlie both the vascular hyporeactivity to vasoconstrictors and the splanchnic vasodilatation seen in portal hypertension. This study assessed the role of NO in the vasoconstrictor hyporeactivity of portal vein-ligated (PVL) rats in isolated and in situ perfused mesenteric arterial beds. 2. Isolated perfused mesenteric arteries of PVL rats were significantly less reactive to noradrenaline (NA), methoxamine (METH), arginine vasopressin (AVP) and endothelin-1 (ET-1) than those from sham-operated (Sham) rats. 3. Blockade of NO synthesis with NG-nitro-L-arginine methyl ester (L-NAME, 100 microM) in isolated perfused mesenteric arteries from PVL rats restored the reactivity to bolus injections of AVP and ET-1, but had little effect on the hyporeactivity to NA or METH. Cyclo-oxygenase inhibition with indomethacin (5 microM) likewise did not restore reactivity to METH of isolated perfused mesenteric arteries of PVL rats. 4. The hyporeactivity to METH seen in isolated perfused mesenteric arteries from PVL rats was reduced by low concentrations of AVP (20 nM) or ET-1 (1 nM) which per se caused only a slight increase in perfusion pressure. When L-NAME (100 microM) was combined with AVP (20 nM) or ET-1 (1 nM), respectively, reactivity to METH of isolated perfused mesenteric arteries of PVL rats was restored to the level seen in Sham rats. These effects of AVP and ET-1 were not mimicked by precontracting the vessels with 5-hydroxytryptamine (5 microM). 5. The differential effects of L-NAME and AVP on the hyporesponsiveness to methoxamine and AVP were corroborated by experiments performed with the in situ perfused mesenteric vascular bed preparation. 6. These data indicate that both NO-dependent and NO-dependent mechanisms are involved in the vasoconstrictor hyporesponsiveness of mesenteric arteries from portal hypertensive rats. The hyporeactivity to AVP and ET-1 is mediated by NO whereas the reduced responsiveness to adrenoceptor agonists appears to be predominantly NO-independent AVP and ET-1, in addition, seem to inhibit the NO-independent mechanism of vascular hyporeactivity, since the hyporesponsiveness to METH was reduced in the presence of AVP or ET-1 and abolished by the combination of these peptides with L-NAME.

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.

Role of protein kinase C in mesenteric pressor responses of rats with portal hypertension

1. Hyporesponsiveness to vasoconstrictors is a characteristic abnormality of liver diseases of uncertain origin. In the present study, we have evaluated the involvement of protein kinase C (PKC) in the reduced pressor response to methoxamine (MTX) of a rat model of portal hypertension induced by partial portal vein ligation (PVL). Experiments were performed in the isolated and perfused mesentery. 2. The pressor response to MTX was reduced in PVL compared to that of control animals (Sham) and pretreatment with NG-nitro-L-arginine (L-NOARG, 10(-4) M) or removal of the endothelium potentiated the response of both groups. However, only removal of the endothelium completely eliminated the reduced pressor response to MTX of the PVL vessels. 3. Pretreatment of the mesentric vessels with calphostin C (10(-6) M), a PKC inhibitor, reduced the response to MTX of Sham to a level similar to that of untreated PVL vessels, but did not change that of PVL animals. 4. Mesenteric pressor responses to a PKC activator, phorbol 12,13-dibutyrate (PDBu), were similar in vessels from both PVL and Sham rats and pretreatment with L-NOARG or removal of the endothelium enhanced those responses while indomethacin (10(-5) M) decreased them. In all cases, the responses to PDBU were similar in PVL vessels compared to Sham. 5. These results indicate that the reduced pressor response to MTX of the mesenteric vascular bed of PVL rats is due to an endothelial alteration, compatible with an enhanced production of nitric oxide. The lack of response to calphostin C in PVL vessels suggests an impairment in agonist-induced PKC activation. Since direct activation of PKC induces a normal pressor response, it is concluded that the endothelial alteration interacts with the mechanism producing PKC activation, which results in a lower pressor response of the PVL mesenteric vaculature.