Beta-adrenergic signal transduction and contractility in the canine heart after cardiopulmonary bypass

PPARγ Agonist Ameliorates the Impaired Fluidity of the Myocardial Cell Membrane and Cardiac Injury in Hypercholesterolemic Rats

Hypercholesterolemia can increase the risk of cardiac injury, but the underlying mechanisms are not fully understood. The present study aimed to determine whether changes in the fluidity of the cardiomyocyte membrane may contribute to the increased susceptibility to myocardial ischemia/reperfusion (MI/R) injury observed in hypercholesterolemic rats. Male Wistar rats were fed a normal (n = 24) or high-cholesterol diet (n = 32) for 10 weeks. At the 6th week, the rats in the high-cholesterol diet group were treated with vehicle (n = 16, HC + V) or pioglitazone (n = 16, HC + PIO), a peroxisome proliferator-activated receptor-γ (PPARγ) agonist, and treatment lasted for the next 4 weeks. Rats in HC + V group displayed less membrane fluidity, a greater membrane cholesterol-to-phospholipid ratio (C/P), less Na⁺-K⁺-ATPase activity, and less cAMP content in their myocardial cells than rats fed a normal diet. A strong positive correlation was observed between membrane fluidity and cardiac injury, i.e., the myocardial infarct size when subjected to MI/R (30 min/24 h). Treatment with PIO restored much of the lost hypercholesterolemia-induced myocardial cell membrane fluidity, decreased membrane C/P ratio, increased Na⁺-K⁺-ATPase activity and cardiac cell cAMP content, improved cardiac function, and reduced the sizes of myocardial infarcts. Results demonstrated that hypercholesterolemia-induced decreased myocardial cell membrane fluidity may contribute to the increased susceptibility to cardiac injury, and PPARγ agonists may have therapeutic value in patients with hypercholesterolemia.

Inhibition of phosphodiesterase type III before aortic cross-clamping preserves intramyocardial cyclic adenosine monophosphate during cardiopulmonary bypass

Inotropes are often used to treat myocardial dysfunction shortly after cardiopulmonary bypass (CPB). beta-Adrenergic agonists improve contractility, in part by increasing cyclic adenosine monophosphate (cAMP) production, whereas phosphodiesterase type III inhibitors prevent its breakdown. CPB is associated with abnormalities at the beta-receptor level and diminished adenyl cyclase activity, both of which tend to decrease cAMP. These effects may be increased in the presence of preexisting myocardial dysfunction. We tested the hypothesis that inhibition of phosphodiesterase type III before global myocardial ischemia and pharmacologic arrest results in the preservation of intramyocardial cAMP concentration during CPB. Twenty adult patients undergoing coronary artery bypass grafting with CPB were studied. After CPB was instituted, a myocardial biopsy was obtained from the apex of the left ventricle. Patients were randomized to receive either placebo or milrinone (50 micro/kg) through the bypass pump 10 min before aortic cross-clamping. Another myocardial biopsy was performed adjacent to the left ventricular apex just before weaning from CPB. Myocardial cAMP concentration was determined by radioimmunoassay. Myocyte protein content was determined by the Bradford method by using a commercial kit. There were no significant demographic differences between the groups; however, patients in the Milrinone group had a lower left ventricular ejection fraction than placebo (41% +/- 13% vs 53% +/- 7%; P < 0.05). Patients who received milrinone had larger cAMP concentrations at the end of CPB compared with placebo (21 +/- 12.5 pmol/mg protein versus 12.8 +/- 2.2 pmol/mg protein; P < 0.05). The administration of milrinone before aortic cross-clamping is associated with increased intramyocardial cAMP concentration at the end of CPB.

IMPLICATIONS

The administration of a single dose of milrinone before aortic cross-clamping resulted in significantly larger intramyocardial cyclic adenosine monophosphate concentration in myocardial biopsy specimens compared with controls.

Adenovirus-mediated gene transfer of the beta2-adrenergic receptor to donor hearts enhances cardiac function

Gene transfer to modify donor heart function during transplantation has significant therapeutic implications. Recent studies by our laboratory in transgenic mice have shown that overexpression of beta2-adrenergic receptors (beta2-ARs) leads to significantly enhanced cardiac function. Thus, we investigated the functional consequences of adenovirus-mediated gene transfer of the human beta2-AR in a rat heterotopic heart transplant model. Donor hearts received 1 ml of solution containing 1 x 1010 p.f.u. of adenovirus encoding the beta2-AR or an empty adenovirus as a control. Five days after transplantation, basal left ventricular (LV) pressure was measured using an isolated, isovolumic heart perfusion apparatus. A subset of hearts was stimulated with the beta2-AR agonist, zinterol. Treatment with the beta2-AR virus resulted in global myocardial gene transfer with a six-fold increase in mean beta-AR density which corresponded to a significant increase in basal contractility (LV + dP/dtmax, control: 3152.1 +/- 286 versus beta2-AR, 6250.6* +/- 432.5 mmHg/s; n = 10, *P < 0.02). beta2-AR overexpressing hearts also had higher contractility after zinterol administration compared with control hearts. Our results indicate that myocardial function of the transplanted heart can be enhanced by the adenovirus-mediated delivery of beta2-ARs. Thus, genetic manipulation may offer a novel therapeutic strategy to improve donor heart function in the post- operative setting.