Neonatal capsaicin treatment does not prevent splanchnic vasodilatation in portal-hypertensive rats

Molecular pathophysiology of portal hypertension

Over the past two decades the advances in molecular cell biology have led to significant discoveries about the pathophysiology of portal hypertension (PHT). In particular, great progress has been made in the study of the molecular and cellular mechanisms that regulate the increased intrahepatic vascular resistance (IHVR) in cirrhosis. We now know that the increased IHVR is not irreversible, but that both the structural component caused by fibrosis and the active component caused by hepatic sinusoidal constriction can be, at least partially, reversed. Indeed, it is now apparent that the activation of perisinusoidal hepatic stellate cells, which is a key event mediating the augmented IHVR, is regulated by multiple signal transduction pathways that could be potential therapeutic targets for PHT treatment. Furthermore, the complexity of the molecular physiology of PHT can also be appreciated when one considers the complex signals capable of inducing vasodilatation and hyporesponsiveness to vasoconstrictors in the splanchnic vascular bed, with several vasoactive molecules, controlled at multiple levels, working together to mediate these circulatory abnormalities. Added to the complexity is the occurrence of pathological angiogenesis during the course of disease progression, with recent emphasis given to understanding its molecular machinery and regulation. Although much remains to be learned, with the current availability of reagents and new technologies and the exchange of concepts and data among investigators, our knowledge of the molecular basis of PHT will doubtless continue to grow, accelerating the transfer of knowledge generated by basic research to clinical practice. This will hopefully permit a better future for patients with PHT.

Blockage of the afferent sensitive pathway prevents sympathetic atrophy and hemodynamic alterations in rat portal hypertension

BACKGROUND AND AIMS

Portal hypertension causes arterial vasodilation and sympathetic atrophy in the splanchnic area. We aimed to demonstrate a relationship between hemodynamic alterations and sympathetic atrophy by investigating a pathway from sensitive afferent signals to mesenteric sympathetic ganglia.

METHODS

Experiments were conducted in sham and portal vein ligated (PVL) adult and neonatal rats treated with vehicle or capsaicin. Hemodynamic parameters, and immunohistochemistry, immunofluorescence and Western blot of different tissues were analysed.

RESULTS

cFos expression in the brain supraoptic nuclei was used to confirm abrogation of the afferent signal in capsaicin-treated PVL rats (effectively afferent blocked). Neonatal and adult PVL afferent blocked rats showed simultaneous prevention of hemodynamic alterations and sympathetic atrophy (measured by tyrosine hydroxylase expression in nerve structures of splanchnic vasculature). Not effectively afferent blocked rats showed none of these effects, behaving as PVL vehicle. All capsaicin treated animals presented loss of calcitonin gene-related peptide in superior mesenteric artery and ganglia, whereas neuronal nitric oxide synthase remained unaffected. Neuronal markers semaphorin-3A, nerve growth factor, its precursor and p75 neurotrophic receptor, were significantly over-expressed in the PVL sympathetic ganglia compared with sham, but not in effectively afferent blocked rats. Semaphorin-3A staining in mesenteric ganglia co-localized with vesicular acetylcholine transporter, but not with adrenergic, nitrergic and sensory axons, suggesting that semaphorin-3A might originate in preganglionic neurons.

CONCLUSION

These results indicate that the nervous system has a central role in the genesis of the circulatory abnormalities of portal hypertension, and support that mesenteric sympathetic atrophy contributes to splanchnic arterial vasodilation.

Portal hypertension: from pathophysiology to clinical practice

Portal hypertension (PHT) is responsible for the more severe and often lethal complications of cirrhosis such as bleeding oesophageal varices, ascites, renal dysfunction and hepatic encephalopathy. Because of the combined impact of these complications, PHT remains the most important cause of morbidity and mortality in patients with cirrhosis. Over the years, it has become clear that a decrease in portal pressure is not only protective against the risk of variceal (re)bleeding but is also associated with a lower long-term risk of developing complications and an improved long-term survival. A milestone in therapy was the introduction of non-selective beta-blockers for the prevention of bleeding and rebleeding of gastro-esophageal varices. However, in practice, less than half the patients under beta-blockade are protected from these risks, supporting the overall demand for innovation and expansion of our therapeutic armamentarium. Recent advances in the knowledge of the pathophysiology of cirrhotic PHT have directed future therapy towards the increased intrahepatic vascular resistance, which, in part, is determined by an increased hepatic vascular tone. This increased vasculogenic component provides the rationale for the potential use of therapies aimed at increasing intrahepatic vasorelaxing capacity via gene therapy, liver-selective nitric oxide donors and statines on the one hand, and at antagonizing excessive intrahepatic vasoconstrictor force through the use of endothelin antagonists, angiotensin blockers, alpha(1) adrenergic antagonists or combined alpha(1)- and non-selective beta-blockers or somatostatin analogues on the other. The focus of this review is to give an update on the pathophysiology of PHT in order to elucidate these potential novel strategies subsequently.

Role of acute elevation of portal venous pressure by exogenous glucagon on gastric mucosal injury in rats with portal hypertension

Time-course studies revealed the increased susceptibility of the gastric mucosa to noxious injury in portal hypertension correlates with the level of elevated portal venous pressure and hyperglucagonemia. Whether acute elevation of portal venous pressure by exogenous glucagon aggravates such injury is not known. We tested the hypothesis that glucagon in a dose sufficient to acutely elevate portal venous pressure aggravates noxious injury of the gastric mucosa in rats with portal hypertension. Infusion of a portal hypotensive dose of somatostatin should reverse these changes. In anesthetized rats with portal vein ligation, glucagon, somatostatin or the combination was administered intravenously in a randomized, coded fashion. Acidified ethanol-induced gastric mucosal injury was determined. Portal venous pressure and gastric mucosal perfusion and oxygenation (reflectance spectrophotometry) were monitored to confirm the effects of the respective intravenous treatments. Exogenous glucagon exacerbated acidified ethanol-induced gastric mucosal injury. The exacerbation was attenuated by somatostatin. These changes paralleled the portal hypertensive and hypotensive effects of glucagon and somatostatin, respectively. Our data suggest that a unique mechanism is triggered with the onset of portal hypertension. In an antagonistic manner, glucagon and somatostatin modulate this novel mechanism that controls portal venous pressure and susceptibility of the gastric mucosa to noxious injury.

The insulin-mediated vascular and blood pressure responses are suppressed in CGRP-deficient normal and diabetic rats

BACKGROUND

Calcitonin gene-related peptide (CGRP) is extensively localized in the perivascular or periadventitia nerves throughout the body. CGRP is a potent vasodilator and its release is associated with dilation of these blood vessels. The present study investigated the contribution of the CGRP-mediated vasodilation to the insulin-induced vasodilatory response.

METHODS

Male Wistar rats were treated with capsaicin (50 mg/kg) at 1-3 days of age to ablate the CGRP-containing neurons. After 8 weeks some animals were made diabetic using streptozotocin. Vehicle-treated animals were used as controls. At 12-13 weeks the animals were fasted, anesthetized with chloralose/urethane and instrumented for recording of cardiovascular dynamics.

RESULTS

Body weights and basal, insulin, glucose, mean arterial pressure (MAP), heart rate (HR), and vascular flows were not different in CGRP-deficient rats versus controls. Insulin infusion significantly decreased the MAP in vehicle-treated controls but this response was completely attenuated in CGRP-deficient rats. The decreased response to insulin was associated with a diminished vascular dilatory response in the iliac, renal, and superior mesenteric vessel beds. When insulin was infused in CGRP-deficient diabetic animals there was also a diminished response. Diabetes resulted in an increased renal vascular flow in response to insulin.

CONCLUSIONS

From the present studies we conclude that the insulin-mediated vasodilation was due, in part, to the stimulation of perivascular nerves to release CGRP, and the action of CGRP on vascular smooth muscle enhanced directly or indirectly the vasodilatory response to insulin.

Gastric mucosal resistance to acute injury in experimental portal hypertension

1. The gastric mucosa of portal hypertensive rats exhibits important microvascular changes and a nitric oxide (NO)-dependent hyperemia. This study analyses whether portal hypertensive mucosa exhibits changes in its ability to withstand aggression. 2. Portal hypertension was induced by partial portal vein ligation (PPVL) or common bile duct ligation (CBDL) and gastric damage was induced by oral administration of ethanol or aspirin. Experiments were performed in conscious or anaesthetized rats and some animals were pre-treated with the NO-synthesis inhibitor L-NAME. 3. Conscious PPVL or CBDL rats showed an increased resistance to the damaging effects of ethanol. Oral administration of aspirin produced less gastric damage in PPVL conscious rats than in the control group. 4. The protective effects of portal hypertension were maintained in animals anaesthetized with ketamine and absent when pentobarbital was employed. 5. Pre-treatment with L-NAME restored the damaging effects of ethanol and aspirin in PPVL rats without modifying the level of damage in control animals. 6. Gastric bleeding induced by oral aspirin, as measured by the luminal release of (51)Cr-labelled erythrocytes, was significantly greater in PPVL rats than in control animals. 7. Semi-quantitative analysis by RT--PCR of the mRNA for endothelial NO-synthase (eNOS), neuronal NOS (nNOS) and inducible NOS (iNOS) levels showed that the expression of iNOS was slightly increased in both the gastric mucosa and smooth muscle of PPVL rats. No changes were observed in eNOS and nNOS expression. 8. Conscious portal hypertensive rats exhibit an enhanced resistance to acute gastric damage which is absent under the influence of some types of anaesthesia and seems related to an increased synthesis of nitric oxide. However, mucosal lesions in these animals show an augmented bleeding per area of injury.

Complications of cirrhosis. I. Portal hypertension

Increased resistance to portal blood flow is the primary factor in the pathophysiology of portal hypertension, and is mainly determined by the morphological changes occurring in chronic liver diseases. This is aggravated by a dynamic component, due to the active-reversible- contraction of different elements of the porto-hepatic bed. A decreased synthesis of NO in the intrahepatic circulation is the main determinant of this dynamic component. This provides a rationale for the use of vasodilators to reduce intrahepatic resistance and portal pressure. Another factor contributing to aggravate the portal hypertension is a significant increase in portal blood flow, caused by arteriolar splanchnic vasodilation and hyperkinetic circulation. Splanchnic arteriolar vasodilation is a multifactorial phenomenon, which may involve local (endothelial) mechanisms as well as neurogenic and humoral pathways. Most pharmacological treatments have been aimed at correcting the increased portal blood inflow by the use of splanchnic vasoconstrictors, such as beta-blockers, vasopressin derivatives and somatostatin. Several studies have demonstrated that changes in the hepatic venous pressure gradient (HVPG) during maintenance therapy are useful to identify those patients who are going to have a variceal bleeding or rebleeding. The wide individual variation in the HVPG response to pharmacological treatment makes it desirable to schedule follow-up measurements of HVPG during pharmacological therapy. A priority for research in the forthcoming years is to develop accurate non-invasive methods to assess prognosis, which can be used to substitute or as surrogate indicators of the HVPG response. In the clinical management of portal hypertension, beta-blockers are at present the only accepted treatment for the prevention of variceal bleeding. Whether the association of isosorbide-5-mononitrate will improve the high efficacy of beta-blockers is questionable. The efficacy of more aggressive techniques, such as endoscopic band ligation, should be further tested against beta-blockers in patients with a high risk of bleeding. In the treatment of acute variceal bleeding, administration of somatostatin or terlipressin is an established therapy. It may be used alone or, preferably, as an initial treatment before sclerotherapy or endoscopic band ligation. No more than two sessions of endoscopic treatment should be used to control the bleeding. If the bleeding is not easily controlled, other alternatives such as transjugular intrahepatic portosystemic shunts (TIPS) or derivative surgery should be considered, the former being the best in patients with poor liver function. Recent studies suggest that early measurement of HVPG during variceal bleeding may be used as a guide for therapeutic decisions in the treatment of patients with acute variceal bleeding. Those patients with a high HVPG have a high risk of poor evolution, and may be candidates for more intensive and aggressive therapy, such as surgery or TIPS. Those with lower HVPG have a very high probability of an uneventful evolution, and may thus be managed more conservatively using medical and endoscopic treatments. Pharmacological agents (propranolol or nadolol), endoscopic treatment (preferably banding ligation) or surgery can be used to prevent rebleeding. A pending task for the new millennium is to assess whether the early treatment of asymptomatic, compensated cirrhotic patients with portal pressure reducing agents can prevent the development of esophageal varices and of other complications of portal hypertension.

Potassium channels participate in gastric mucosal protection in rats with partial portal vein ligation

Glybenclamide, an adenosine triphosphate-dependent potassium (K+(ATP)) channel blocker, lowered portal pressure and attenuated the hyperdynamic splanchnic circulation in rats with partial portal vein ligation (PPVL). The purpose of this report was to confirm these observations and to test the hypothesis that glybenclamide could reduce acidified ethanol-induced gastric mucosal injury in rats with PPVL. Gastric mucosal blood flow (hydrogen gas clearance), systemic blood pressure, and portal pressure were monitored in rats with PPVL or sham operation (SO). Intravenous glybenclamide (20 mg/kg) or vehicle was administered, followed by intragastric acidified ethanol (0.15 N HCl and 15% ethanol). The area of gastric mucosal lesions was assessed by image analysis. In contrast to published findings, there was no significant elevation of portal pressure after glybenclamide administration in rats with PPVL. Glybenclamide did not alter the gastric mucosal hyperemia in these rats. Glybenclamide significantly increased mucosal injury. The data are consistent with the hypothesis that K+(ATP) channels play a role in protecting the gastric mucosa in rats with PPVL.

Role of calcitonin gene-related peptide in the vascular system on the development of the hyperdynamic circulation in conscious cirrhotic rats

BACKGROUND/AIMS

Calcitonin gene-related peptide (CGRP), a potent vasodilator; plays an important role in modulating vascular tone, acting as a noncholinergic nonadrenergic neurotransmitter. The aim of this study was to assess the role of CGRP, present in the vascular system, in the development of the hyperdynamic circulation observed in liver cirrhosis.

METHODS

Two doses of human alpha-CGRP [8-37], a specific antagonist of CGRP, were administered to cirrhotic and controls rats. Hemodynamics were evaluated using radioactive microspheres in conscious animals. To investigate the arterial depressor effect of exogenous CGRP, we constructed a dose-response curve for mean arterial pressure in cirrhotic and control rats by administering human alpha-CGRP.

RESULTS

The administration of high-dose human alpha-CGRP [8-37] (300 nmol.kg body weight-1.min-1) significantly increased both the mean arterial pressure (21 +/- 2 vs. 13 +/- 1%, p < 0.01) and total vascular resistance (76 +/- 5 vs. 54 +/- 5%, p < 0.01) in cirrhotic rats, compared to control rats. The splanchnic hemodynamic effects induced by human alpha-CGRP [8-37] were a significant decrease in percent change of portal venous inflow -42 +/- 3 vs. -33 +/- 3%, p < 0.05) and a significant increase in percent change of splanchnic arterial resistance (110 +/- 9 vs. 76 +/- 5%, p < 0.01) in cirrhotic rats, compared to control rats. Low-dose human alpha-CGRP [8-37] (60 nmol.kg body weight-1. min-1) caused similar hemodynamic changes, but the degree of change was much less than for the high-dose administration. The vascular response to human alpha-CGRP was significantly reduced in cirrhotic rats as compared to controls (ANOVA, p < 0.01). Plasma concentrations of CGRP were significantly elevated in cirrhotic rats.

CONCLUSIONS

CGRP in the vascular system was involved in the modulation of vasodilatation in rats with liver cirrhosis, as demonstrated by the administration of a selective CGRP antagonist and exogenous CGRP.