α2-Adrenoceptor subsensitivity in mesenteric vascular bed of cholestatic rats: The role of nitric oxide and endogenous opioids

Cardiovascular abnormalities in obstructive cholestasis: the possible mechanisms

Cholestatic liver disease is associated with widespread derangements in the cardiovascular system, such as bradycardia, hypotension, QT prolongation and peripheral vasodilation; it is also associated with increased susceptibility to postoperative renal failure and haemorrhagic shock. A number of cellular signalling pathways have been shown to contribute to these abnormalities. In this article, we briefly review recent in vivo and in vitro findings in the field in an attempt to highlight the areas of agreement and areas of controversy. In this review, we will summarize pathogenic mechanisms underlying cardiac and vascular abnormalities in obstructive cholestasis. It seems that cardiovascular dysfunction is likely because of bile acids as one of the predominant factors. Other important factors which might play roles in these abnormalities are increased nitric oxide, endogenous opioids and endocannabinoids. These three factors interact with each other to exert vasodilation and impaired cardiovascular responses to sympathetic stimulation.

alpha(1)- and alpha(2)-Adrenoceptor hyporesponsiveness in isolated bisected vas deferens of bile duct-ligated rats

It has been suggested that cholestasis accompanied with changes in autonomic balance and hyporesponsiveness in muscarinic and adrenergic receptors of some organs, e.g. cardiovascular system. Increased plasma levels of epinephrine and norepinephrine has been shown during cholestasis suggesting augmented activity of sympathetic nervous system. In this study we evaluate both alpha(1) and alpha(2) responsiveness in isolated rat vas deferens, as a tissue with rich adrenergic innervations. Epididymal and prostatic halves of vas deferens responsiveness have been studied to phenylephrine and clonidine respectively in three groups of un-operated, sham-operated (sham), and bile duct-ligated (BDL) rats. Our results indicate that in vas deferens of BDL animals, the concentration-response curve of both phenylephrine and clonidine shifted to rightward compared to control group, while the position of concentration-response curve of sham group did not change significantly (P > 0.05). EC(50) of phenylephrine and IC(50) of clonidine were increased showing a decreased responsiveness of tissue to phenylephrine (P < 0.05) and clonidine (P < 0.001) in BDL rats. In this study, both subtype of alpha-adrenoceptors (alpha(1) and alpha(2)) has been studied in cholestatic rat vas deference. Our results showed that cholestasis induce hyporesponsiveness to phenylephrine and clonidine. These results are consistent with previous reports, suggesting the hyporesponsiveness of alpha(1)-adrenoceptors in pulmonary artery and papillary muscle and mesenteric beds. Our conclusion is that the cholestasis induces hyporesponsiveness to phenylephrine and clonidine in epididymal (alpha(1)-adrenoceptors) and prostatic (alpha(2)-adrenoceptors) halves of rat vas deferens respectively. Although the logical explanation to this hyporesponsiveness is the down regulation but it has been suggested that it is not because of down regulation.

Involvement of endogenous opioid peptides and nitric oxide in the blunted chronotropic and inotropic responses to beta-adrenergic stimulation in cirrhotic rats

It is well known that chronotropic and inotropic responses to beta-adrenergic stimulation are impaired in cirrhosis, but the exact reason is not clear. Considering the inhibitory effect of endogenous opioid peptides and nitric oxide (NO) on beta-adrenergic pathway, we examined their roles in hyporesponsiveness of isolated atria and papillary muscles to isoproterenol stimulation in cirrhotic rats. Cirrhosis was induced by chronic bile duct ligation. Four weeks after ligation or sham operation, the responses of the isolated atria and papillary muscles to isoproterenol stimulation were evaluated in the absence and presence of naltrexone HCl (10(-6) m), N(omega)-nitro-L-arginine methyl ester (L-NAME, 10(-4) m), and naltrexone plus L-NAME in the organ bath. Considering the role of inducible NOS (iNOS) in hemodynamic abnormalities of cirrhotic rats, the chronotropic and inotropic responses of cirrhotic rats to isoproterenol stimulation were also assessed in the presence of aminoguanidine (a selective inhibitor of iNOS, 3 x 10(-4) m). Sham operation had no significant effect on basal atrial beating rate, contractile force, and maximal time derivatives for the development and the dissipation of papillary muscle tension. The basal atrial beating rate of cirrhotic rats did not show any significant difference compared with the sham-operated ones; however, the basal contractile parameters were significantly decreased in cirrhosis. Although the maximum effects of isoproterenol on chronotropic and inotropic responses were significantly reduced in cirrhotic rats, there was no difference in half-maximal effective concentrations of isoproterenol in these concentration-response curves. The basal abnormalities and the attenuated chronotropic and inotropic responses to isoproterenol were completely corrected by the administration of naltrexone, L-NAME and aminoguanidine. Concurrent administration of naltrexone and L-NAME also restored to normal the basal abnormalities and the blunted responses to isoproterenol in cirrhotic rats, and did not show any antagonistic effect. Based on these findings, both the endogenous opioid peptides and NO may be involved in the attenuated chronotropic and inotropic responses to beta-adrenergic stimulation in cirrhosis. It seems that the iNOS activity results in NO-induced hyporesponsiveness to beta-adrenergic stimulation in cirrhosis.

Noradrenaline-induced contraction of mice aorta is enhanced in schistosomiasis

Schistosomiasis, an intravascular parasitic disease caused by Schistosoma mansoni, is related to alterations of murine vascular reactivity in the mesenteric bed, characterized by an impairment of the l-arginine/NO pathway and an increased potency of 5-hydroxytryptamine. The current study was performed to test the hypothesis that a similar alteration of reactivity also occurs in the aorta and to identify the mechanism behind such an increase. We found that aorta from mice infected with male S. mansoni exhibited an enhanced contraction in response to noradrenaline and 100 mM KCl. The inhibition of nitric oxide synthase increased aortic maximal contraction in response to noradrenaline in both groups, but the effect was less pronounced in infected mice than in control mice. Endothelium-dependent relaxation induced by acetylcholine was also smaller in infected mice compared to control mice, while endothelial-independent relaxation induced by sodium nitroprusside and forskolin was similar in both groups. The inhibition of voltage-dependent L-type Ca(2+) channels reduced the maximal contraction in response to noradrenaline more effectively in infected than in control mice. Conversely the inhibition of K(ATP) channels had a smaller effect in the infected group. As a conclusion, our data indicate that schistosomiasis also alters murine vascular reactivity outside the mesenteric bed, due to a partial impairment of NO signaling, a reduced contribution of K(ATP) channels and an increased Ca(2+) influx through L-type Ca(2+) channels.

Modulated hemodynamic response to clonidine in bile duct-ligated rats: the role of nitric oxide

Despite the well-known involvement of the peripheral sympathetic abnormalities in the development of cardiovascular complications of cholestasis, the role of the central sympathetic system is still elusive. The goal of this study was to evaluate the effects of central sympathetic tone reduction, through clonidine administration, on hemodynamic parameters of 7-day bile duct-ligated rats. The contributions of nitric oxide and endogenous opioids were also examined by acute intravenous (10 min before clonidine) or chronic daily subcutaneous administrations of N(omega)-nitro-L-arginine methyl ester (L-NAME, 3 mg/kg) or naltrexone (20 mg/kg). Seven days after bile duct ligation or sham operation, animals were anesthetized with sodium pentobarbital. After hemodynamic stabilization, clonidine (10 microg/kg) was injected intravenously, which elicited an initial hypertension (the peripheral effect) followed by persistent hypotension and bradycardia (the central effects). Cholestatic rats demonstrated significant basal bradycardia (P<0.001) and hypotension (P<0.05), which were corrected by chronic naltrexone but not L-NAME treatment. While the peripheral effect of clonidine was blunted, the central effects were exaggerated in cholestatic rats (P<0.01). Acute L-NAME treatment accentuated the hypertensive phase in sham-operated and cholestatic rats (P<0.05). However, the difference between the two groups was preserved (P<0.01). This treatment attenuated the central effects in both sham-operated and cholestatic rats to the same level (P<0.001). Chronic L-NAME treatment resulted in exaggeration of the peripheral response in cholestatic and central responses in sham-operated rats (P<0.05), and abolished the difference between the groups. Naltrexone treatment had no significant effect on either the central or the peripheral responses to clonidine. This study shows that both central and peripheral hemodynamic responses to clonidine are altered in cholestasis. It also provides evidence that nitric oxide contributes to the development of these abnormalities.