Relationship between beta-adrenoceptors and coronary blood flow heterogeneity

The purpose of this study was to investigate the hypothesis that the heterogeneous distribution of β adrenoceptors contributes to the control of flow heterogeneity in the canine myocardium. β adrenoceptor density and affinity were measured simultaneously with coronary blood flow in multiple sections of the left ventricle of 14 anesthetized open chest dogs. Radioactive microspheres were used for the measurement of blood flow. The left ventricle was cut into 15 subepicardial (EPI) and 15 subendocardial (ENDO) sections. Receptor density (Bmax) and dissociation constant (Kd) were measured using [¹²⁵I]- iodopindolol. The average control myocardial blood flow (MBF) was 86±15 ml/min/100 g. Isoproterenol (0.5 μg/kg/min) increased MBF by 82%, whereas propranol (2 mg/kg) reduced MBF by 13%. The mean value for Bmax was unaltered by either treatment. Under control conditions, a significant positive correlation (r=0.26, p<0.0001) was observed between Bmax and blood flow. In the isoproterenol treatment group, this correlation was enhanced (r = 0.49, p< 0.0001). Beta adrenoceptor blockade led to a negative correlation. Kd showed no overall correlation with blood flow. Kd but not Bmax was significantly higher in the EPI than in the ENDO and in the base compared to the apex. There appears to be a direct linear relationship between the distribution of beta adrenoceptors and MBF distribution which is enhanced under conditions of high beta adrenergic activity. There is a correlation between beta adrenoceptor activity and blood flow distribution in the canine myocardium.