Effect of chronic myocardial infarction on the distribution of β-adrenoceptors in heart:experiment with dogs

Abstract

To investigate the distribution of beta-adrenoceptors at different sites of heart after myocardial infarction (MI). 14 dogs were randomly divided into 2 equal groups: MI group, undergoing ligation of the left anterior descending coronary artery, and control group undergoing sham-operation. Four weeks later metoprolol, a beta 1-adrenergic receptor antagonist, was injected intravenously. Doppler tissue imaging (DTI) was used to evaluate the peak systolic myocardial velocity (Sm) of the regions apical and basal to the infarction region before and after the injection. Then the dogs were killed with their hearts taken out. Reverse-transcriptase polymerase chain reaction was used to examine the mRNA expression of beta 1-receptor and beta 2-receptor in the non-infracted myocardial tissues apical and basal to the infarction region. The Sm values at the regions apical and basal to the infarction region of the MI group were 3.93 +/- 0.47 and 0.81 +/- 0.19 cm/s respectively, both significantly lower than those of the control group (10.84 +/- 1.97 and 5.85 +/- 1.15 cm/s respectively, both P < 0.05). After injection of metoprolol, the Sm values at the regions apical and basal to the infarction region of the MI group were 3.43 +/- 0.37 and 0.73 +/- 0.14 cm/s respectively, not significantly different from those before the injection; however, the corresponding Sm values of the control group were 8.69 +/- 1.14 and 4.33 +/- 0.29 cm/s respectively, both significantly lower than those before the injection (both P < 0.05). The mRNA expression levels of beta 1-receptor decreased in both apical and basal regions in the MI group compared with those in the control group, and the degree of expression decrease at the apical region was significantly greater than that at the basal region. However, there was no significant difference in the expression level of beta 1-receptor mRNA between the apical and basal regions in the control group. There was no significant difference in the mRNA expression of beta 2-receptor in different regions of the heart in both groups. After MI regional variation occurs for the beta 1-receptor mRNA expression, but not to the beta 2-receptor.