Autoradiographic Localization and Quantitation of β₁- and β₂-Adrenoceptors in the Human Atrioventricular Conducting System: A Comparison of Patients with Idiopathic Dilated Cardiomyopathy and Ischemic Heart Disease

The density and distribution of β₁- and β₂-adrenoceptors in the atrioventricular conducting system and interatrial and interventricular septa from human hearts with idiopathic dilated cardiomyopathy and ischemic heart disease was determined by quantitative autoradiography using (-) [₁₂₅I]cyanopindolol and the selective β₁-adrenoceptor antagonist CGP 20712A and the selective β₂-adrenoceptor antagonist ICI 118 551. Both β₁- and β₂-adrenoceptors were present in the atrioventricular node, bundle of His, interatrial and interventricular septa. No differences in the density or proportions of β₁- and β₂-adrenoceptors in the atrioventricular node, bundle of His, interatrial septum and interventricular septum were observed between hearts with idiopathic dilated cardiomyopathy or ischemic heart disease (P > 0.05) so further analysis did not distinguish between the two aetiologies. The density of β₁-adrenoceptors was lower in the bundle of His (5.0 ± 1.7 fmol/mg protein) than in the atrioventricular node (22.2 ± 5.7 fmol/mg protein, P < 0.05), the interatrial septum (29.6 ± 4.5 fmol/mg protein, P < 0.001) and interventricular septum (24.9 ± 5.2 fmol/mg protein, P < 0.005, n = 8 for all values). The atrioventricular node, interatrial and interventricular septa had similar densities of β₁-adrenoceptors (P = 0.060, ANOVA). The distribution of β₂-adrenoceptors in the atrioventricular node (21.5 ± 4.1 fmol/mg protein), bundle of His (12.9 ± 2.6 fmol/mg protein) and atrial (16.7 ± 2.3 fmol/mg protein) and septal myocardium (13.8 ± 2.5 fmol/mg protein, n = 8 for all values) was uniform (P = 0.18, ANOVA). The percentage of β₁- and β₂-adrenoceptors in the atrioventricular node, bundle of His, interatrial and interventricular septa was uneven (P < 0.001, ANOVA). There was a higher proportion of β₂-adrenoceptors in the bundle of His (72 ± 6%) than in the atrioventricular node (51 ± 3%, P < 0.01), interatrial septum (36 ± 1%, P < 0.001) and interventricular septum (36 ± 1%, P < 0.001).