defective myocardial contractility is seen under various phar-The pathogenesis of cirrhotic cardiomyopathy remacological and physiological stimuli. This condition has mains unclear. Because ventricular contractility is debeen termed cirrhotic cardiomyopathy, but the mechanisms pendent on the interplay of stimulatory b-adrenergic underlying these cardiac abnormalities remain poorly underand inhibitory muscarinic receptors, we aimed to examstood. Various hypotheses have been proposed, including abine a possible role of muscarinic M2 receptor overactinormalities in neural factors, mechanoreceptor-mediated revity in a rat model of cirrhotic cardiomyopathy. Cirrhosponses, and the cardiac muscle itself (reviewed in Lee, 4 Ma, 5 sis was induced by bile duct ligation (BDL), while and Ramond 6 ). controls underwent sham operations. Contractile re-Cardiac contractile function is regulated mainly by the sponses to the muscarinic agonist carbachol were measympathetic and parasympathetic branches of the autonomic sured in situ in the autonomic-denervated pithed rat nervous system through the guanine nucleotide binding reguand in vitro in isolated ventricular papillary muscles.
latory protein (G-protein)-coupled b-adrenergic and musca-Ventricular sarcolemmal plasma membranes were isorinic receptors. Attenuation in the b-adrenergic-stimulated lated by sucrose density gradients, and muscarinic chronotropic and inotropic responses has been shown in cirreceptor characteristics were studied using 1-[N-methylrhotic patients and animal models of cirrhosis. 6-9
3 H]scopolamine (NMS). Membrane adenylyl cyclase
Because ventricular contractility is dependent on the interactivity was tested by a protein binding assay. Maximum play of stimulatory b-adrenergic and inhibitory muscarinic first time derivative of peak ventricular systolic presreceptor function, we hypothesized that the abnormalities sure (/dP/dt) for sham-operated and cirrhotic rats at seen in the cirrhotic heart could be caused by changes in baseline was 3,599 { 296 versus 1,226 { 63 mm Hg/sec muscarinic receptor activity. Specifically, the blunted ven-(P Γ΅ .01). Maximum first time derivative of ventricular tricular responsiveness of cirrhotic cardiomyopathy might be diastolic relaxation (0dP/dt) for sham and cirrhotic rats caused by overactivity of inhibitory muscarinic receptors. at basal levels was 03,040 { 235 versus 0864 { 59 (P Γ΅ However, this hypothesis has not previously been examined. .01). The /dP/dt max , and 0dP/dt max responses to carba-Therefore, we aimed to clarify this issue, using different in chol were blunted in the cirrhotic rats. The cirrhotic situ and in vitro protocols to test muscarinic receptor function papillary muscles showed significantly less inhibition to and characteristics in a rat model of cirrhotic cardiomyopaincremental doses of carbachol than control rat muscles.
thy.
Likewise, isoproterenol-stimulated membrane adenylyl cyclase activity was significantly less inhibited by carba-MATERIALS AND METHODS
chol doses in the cirrhotic rats. Membrane M2 receptor density and binding affinity in cirrhotic rat hearts were Animal Model. Male Sprague-Dawley rats (BioScience, Calgary, similar to controls. We conclude that muscarinic respon-Alberta, Canada) weighing 250 to 350 g were divided randomly into siveness was blunted in cirrhotic hearts, but this was two groups. The animals were housed in an environmentally con- not caused by receptor down-regulation, suggesting trolled vivarium with a 12-hour light/dark cycle and allowed access changes in postreceptor factors. These changes in musto rat chow and water ad libitum. The animals were treated ac- carinic function are likely compensatory, and M2 recepcording to guidelines established by the Canadian Council of Animal Care, and the protocol was approved by the Animal Care Committee Abbreviations: BDL, bile duct ligation; /dP/dt, first time derivative of peak ventricular BDL group was studied 24 to 28 days after the surgery, and cirrhosis systolic pressure; 0dP/dt, first time derivative of ventricular diastolic relaxation; cAMP, was verified by the presence of hepatosplenomegaly, severe jaundice, adenosine 3,5-cyclic monophosphate; NMS, [N-methyl-3 H]scopolamine; Bmax, receptor denand ascites. isty; Kd, binding affinity.