Cardiac plasma membrane physical properties and β-adrenergic receptor function are unaltered in portal-hypertensive rats

Abstract

We have previously demonstrated that impairment of cardiac contractility in a rat model of cirrhosis may be secondary to altered cardiac plasma membrane physical properties affecting beta-adrenergic receptor function. It is unclear whether this is caused by the cirrhosis or by the portal hypertension in this model, so we studied cardiomyocyte plasma membrane physical properties, lipid composition, and beta-adrenergic receptor function in a rat model of prehepatic portal hypertension without cirrhosis. Portal hypertension was induced by graded portal vein stenosis, whereas controls had a sham operation. Cardiac sarcolemmal plasma membrane physical properties were studied by fluorescent polarization methods using diphenylhexatriene and a series of anthroyloxy-stearic acids to estimate, respectively, the static and dynamic components of membrane fluidity. beta-Adrenergic receptor density and binding affinity were measured using 3H-dihydroalprenolol as a radioligand. Isoproterenol-stimulated adenylyl cyclase activity was measured by radioimmunoassay. Neither the static nor the dynamic components of membrane fluidity were significantly different in portal-hypertensive rats compared with the controls. The membrane cholesterol, phospholipid content, and the cholesterol/phospholipid ratio did not differ between the two groups. beta-Adrenergic receptor density and binding affinity in the portal-hypertensive rats (respectively, 1.65 +/- 0.06 pmol/mg protein and 6.78 +/- 0.80 nmol/L) were not different from controls (1.76 +/- 0.17 pmol/mg protein and 7.25 +/- 1.48 nmol/L). Isoproterenol-stimulated adenylyl cyclase activity in membranes from portal-hypertensive rats did not significantly differ from controls. We conclude that cardiac plasma membrane physical properties and beta-adrenoceptor function in portal hypertensive rats remains unimpaired, suggesting that the cardiomyopathy of cirrhosis is not caused by portal hypertension per se.