Relation Between Myocardial Beta-adrenoceptor Density and Hemodynamic and Neurohumoral Changes in a Rat Model of Chronic Myocardial Infarction: Effects of Ibopamine and Captopril

Objectives: The purpose of this study was to investigate the changes in beta-adrenoceptor density (Bmax) and distribution in a model of chronic myocardial infarction in rats, and to relate possible changes to hemodynamic and neurohumoral abnormalities. In addition, we examined the effects of 8 weeks treatment with ibopamine and captopril.

Methods: There were 3 experiments: (1) Bmax and plasma catecholamines were examined (n = 46), (2) Bmax was compared in infarcted and non-infarcted tissue (n = 13), and (3) contractile function was evaluated by isolated heart perfusion (n = 40). Of rats in Expts. (1) and (3), 50% had myocardial infarction induced by coronary ligation and 50% were controls. Each group was divided between ibopamine, ibopamine and captopril, or standard (no drug) treatment.

Results: Bmax was not decreased in rats with myocardial infarction (10.8 +/- 0.8 fmol/mg protein), compared to normal rats (11.4 +/- 0.6 fmol/mg protein), and the ratio beta 1/beta 2 was also unaffected. In infarcted tissue, Bmax was significantly (P = 0.03) lower than in non-infarcted tissue. Baseline left ventricular pressure, systolic and diastolic dP/dT were all impaired (P < 0.001), and plasma norepinephrine levels were elevated in rats with myocardial infarction (16.03 +/- 230 vs. 1287 +/- 83 pg/ml; P < 0.05), compared to normals. Both ibopamine alone and in combination with captopril reduced the elevated plasma norepinephrine levels in infarcted rats (P < 0.001), but only the combination of the 2 drugs significantly increased Bmax in infarcted rats (14.7 +/- 0.8 fmol/mg protein; P = 0.03 vs. untreated myocardial infarction), while ibopamine alone had no significant effect (13.1 +/- 1.1 fmol/mg protein; p = ns). Also, active drug treatment had no significant effect on the hemodynamic changes.

Conclusions: In this coronary artery ligation model of myocardial infarction in rats, no beta-adrenoceptor down-regulation is observed, despite marked abnormalities in baseline left ventricular function and plasma norepinephrine levels. The combination of ibopamine and captopril significantly increases Bmax in infarcted rats, which is accompanied by a reduction in plasma norepinephrine levels, but not by an improvement in hemodynamic parameters.