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

Abstract

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.)

Role of Oxygen Free Radicals, Nitric Oxide and Mitochondria in Mediating Cardiac Alterations During Liver Cirrhosis Induced by Thioacetamide

“…Simultaneous elevations in NO and superoxide can result in formation of peroxynitrite, which compromises protein function by formation of nitrotyrosine [35]. It has been demonstrated that nitration of cardiac proteins is associated with abnormal cardiac chronotropic responses in rats with biliary cirrhosis [36]. Our data now show elevated nitrotyrosine levels in the heart of TAAtreated animals within a month after treatment indicating that formation of peroxynitrite and nitrosative stress in the heart is also an early event in conjunction with oxidant stress.…”

Decreased heart rate variability in patients with cirrhosis relates to the presence and degree of hepatic encephalopathy

“…Although this hypothesis is attractive within the context of "cirrhotic cardiomyopathy" (18,61), recent studies have shown that decreased HRV following endotoxin challenge is not related to alterations in cardiac ␤-adrenergic signaling in endotoxemic mice (36). In addition, loss of HRV in a rat model of cirrhosis occurs independently of any impairment in cardiac ␤-adrenergic responsiveness (35).…”

Cirrhotic Cardiomyopathy

“…Peroxynitrite is a reactive oxygen species, formed by the reaction of NO with superoxide anion (O 2Ϫ ), that may inhibit cardiac function through nitration (or S-nitrosation) of cardiac contractile proteins, such as actin (67). In a rat model of bile duct ligated cirrhosis, increased protein nitration in cardiac tissue was associated with reduced chronotropic function (68). In a separate study of L-NAME (NG-L-nitro-arginine methyl ester) and N-acetylcysteine, decreased cardiac nitrotyrosine levels favored normalization of cardiac function and further confirmed the inhibitory effect of nitration.…”