Nitration of cardiac proteins is associated with abnormal cardiac chronotropic responses in rats with biliary cirrhosis

✍ Scribed by Ali R. Mani; Silvia Ippolito; Richard Ollosson; Kevin P. Moore

• Publisher: John Wiley and Sons
• Year: 2006
• Tongue: English
• Weight: 455 KB
• Volume: 43
• Category: Article
• ISSN: 0270-9139
• DOI: 10.1002/hep.21115

✦ Synopsis

Acceleration of the heart rate in response to catecholamines is impaired in cirrhosis. In this study, we tested the hypothesis that increased formation of reactive nitrogen species in biliary cirrhosis causes nitration of cardiac proteins and leads to impaired chronotropic function. Bile duct-ligated (rats with cirrhosis) or sham-operated rats were injected daily with either saline, N G -L-nitro-arginine methyl ester (L-NAME), or N-acetylcysteine for 7 days from week 3 to week 4 after surgery. Cardiac chronotropic responsiveness to ␤-adrenergic stimulation was assessed in vitro using spontaneous beating isolated atria. Nitration of cardiac proteins was measured by mass spectrometry and located by immunogold electron microscopy. Marked impairment of chronotropic responses of isolated atria to isoproterenol was seen in rats with cirrhosis, which normalized after the administration of N-acetylcysteine or L-NAME. The levels of protein-bound nitrotyrosine in atrial tissue increased from 16 ؎ 1 to 23 ؎ 3 pg/g tyrosine in rats with cirrhosis, and decreased to 15 ؎ 1 and 17 ؎ 1 pg/g after treatment with L-NAME and N-acetylcysteine, respectively (P < .05). Immunogold electron microscopy demonstrated increased nitration of mitochondrial proteins in the atria of rats with cirrhosis. The plasma nitrite/nitrate levels were elevated in rats with biliary cirrhosis, and decreased after administration of L-NAME but were unchanged by N-acetylcysteine. In conclusion, abnormal cardiac chronotropic function in cirrhosis is associated with increased nitration of cardiac proteins. Two independent treatments (N-acetylcysteine and L-NAME) that decrease nitration of cardiac proteins led to normalization of cardiac responses. Nitration of critical proteins in cardiac tissue may lead to abnormal cardiac function. (HEPATOLOGY 2006;43:847-856.)

C ardiac responses are impaired in cirrhosis, with a significant diminution of both chronotropic and inotropic changes after physiological or pharmacological stimulation. 1 Thus, maneuvers leading to sympathetic activation, such as tilting, physical exercise, and pharmacological stimulation, do not evoke an adequate acceleration of heart rate in patients with cirrhosis compared with healthy subjects. [2][3][4] Current evidence suggests that this is attributable to inherent abnormalities of the cardiac tissue, a phenomenon that has been controversially termed cirrhotic cardiomyopathy. 1 The contribution of nitric oxide (NO) to the pathophysiology of cardiac dysfunction in cirrhosis was first described by Van Obbergh et al. in 1996. 5 Subsequent studies by Lee and colleagues 6 have shown that serum cytokine levels are increased in rats with cirrhosis, and that the negative inotropic effect of interleukin-1␤ (IL-1␤) could be reversed by pre-incubation of isolated papillary muscle with an NO synthase (NOS) inhibitor. Similarly, we have shown that increased NO synthesis in bile ductligated (BDL) rats causes bradycardia by causing impaired responsiveness of cardiac pacemaker cells to adrenergic stimuli. 7,8 Nitric oxide is involved in the modulation of cardiac function in the normal heart, as well as in various cardiac diseases via mechanisms involving NO that are both dependent on and independent of guanylyl cy-