Contribution of nitric oxide to the pathogenesis of cirrhotic cardiomyopathy in bile duct-ligated rats

Increased formation of S-nitrothiols and nitrotyrosine in cirrhotic rats during endotoxemia

Plasma S-nitrosothiols are believed to function as a circulating form of nitric oxide that affects both vascular function and platelet aggregation. However, the formation of circulating S-nitrosothiols in relation to acute and chronic disease is largely unknown. Plasma S-nitrosothiols were measured by chemiluminescence in rats with biliary cirrhosis or controls, and the effect of lipopolysaccharide (LPS) on their formation was determined. Plasma S-nitrosothiols were increased in rats with cirrhosis (206 +/- 59 nM) compared to controls (51 +/- 6 nM, p <.001). Two hours following injection of LPS (0.5 mg/kg) plasma S-nitrosothiols increased to 108 +/- 23 nM in controls (p <.01) and to 1335 +/- 423 nM in cirrhotic rats (p <.001). The plasma clearance and half-life of S-nitrosoalbumin, the predominant circulating S-nitrosothiol, were similar in control and cirrhotic rats, confirming that the increased plasma concentrations were due to increased synthesis. Because reactive nitrogen species, such as peroxynitrite, may cause the formation of S-nitrosothiols in vivo, we determined the levels of nitrotyrosine by gas chromatography/mass spectrometry as an index for these nitrating and nitrosating radicals. Hepatic nitrotyrosine levels were increased at 7.0 +/- 1.2 ng/mg in cirrhotic rats compared to controls (2.0 +/- 0.2 ng/mg, p <.01). Hepatic nitrotyrosine levels increased by 2.3-fold and 1.5-fold in control and cirrhotic rats, respectively, at 2 h following injection of LPS (p <.01). Strong positive staining for nitrotyrosine was shown by immunohistochemistry in all the livers of the rats with cirrhosis. We conclude that there is increased formation of S-nitrosothiols and nitrotyrosine in biliary cirrhosis, and this is markedly upregulated during endotoxemia.

Role of heme oxygenase-carbon monoxide pathway in pathogenesis of cirrhotic cardiomyopathy in the rat

The enzyme heme oxygenase (HO), which exists in inducible (HO-1) and constitutive (HO-2) isoforms, degrades heme to biliverdin and CO. CO depresses cardiac contraction via cGMP. We aimed to clarify a possible role for the HO-CO pathway in the pathogenesis of cirrhotic cardiomyopathy in bile duct-ligated rats. Four weeks after bile duct ligation or sham operation, rat ventricles were examined for HO-1 and HO-2 mRNA by RT-PCR and for protein expression by Western blotting. Total HO enzyme activity and cGMP levels were also measured. The effects of a HO inhibitor, zinc protoporphyrin IX (ZnPP), on ventricular cGMP levels and isolated papillary muscle contractility were studied. We found that HO-1 mRNA transcription and protein expression were significantly augmented in cirrhotic hearts compared with sham-operated controls, whereas there was no difference in HO-2 mRNA or protein levels. Total HO activity and cGMP levels were significantly increased in cirrhotic ventricles vs. controls. In cirrhotic ventricles, treatment with ZnPP significantly decreased cGMP production and improved the blunted papillary muscle contractility, whereas it had no effect on control muscles. CO perfusion inhibited papillary muscle contractility, an effect completely blocked by methylene blue and partially blocked by ZnPP. These results indicate that activation of the HO-CO-cGMP pathway is involved in the pathogenesis of cirrhotic cardiomyopathy.