Effects of altered cardiac membrane fluidity on β-adrenergic receptor signalling in rats with cirrhotic cardiomyopathy

Background/Aims: The relationship between cardiac plasma membrane physical properties and β-adrenergic receptor signalling function in cirrhotic cardiomyopathy remains unclear. We aimed to clarify this issue by examining the effect of altering membrane fluidity on β-adrenergic receptor signalling in cirrhotic rats. Methods: Cirrhosis was induced by chronic bile duct ligation, while controls were sham-operated. Left ventricular papillary muscle contractility was measured in an organ bath containing 95% oxygen saturated Tyrodeʹs buffer. Cardiac plasma membrane physical properties, represented by membrane fluidity, were evaluated by fluorescent depolarization, using 1,6-diphenyl-1,3,5,-hexatriene as a probe. Membrane cAMP levels were measured after stimulation at the β-adrenoceptor level by isoproterenol (10−4 mol/l), the G-protein level with AIF−4 (10 mmol/l) and the adenylyl cyclase level with forskolin (100 μmol/l) before and after membrane fluidization with 2-(2-methoxyethoxy)ethyl 8-(cis-2-n-octylcyclopropyl)octanoate (A2C). Results: Maximum papillary muscle contractile force under isoproterenol stimulation was 0.48±0.05 and 0.34±0.03 N/cm2 in sham-operated and bile duct ligated rats, respectively (p<0.05). Cardiac membranes from cirrhosis were rigid and this was associated with diminished cAMP generation. After the fluidity of membranes from cirrhotic rats was restored to control values with A2C, cAMP production stimulated with isoproterenol was significantly increased. However, cAMP production stimulated by AIF−4 and forskolin did not differ before and after membrane fluidization. Conclusions: The blunted cardiac contractility of cirrhosis was associated with decreased membrane fluidity and diminished β-adrenergic receptor signalling. The results suggest that in cirrhotic cardiomyopathy, the rigid plasma membrane interferes with the β-adrenoceptor and G-protein coupling process.