Metaiodobenzylguanidine as an index of atrioventricular nodal adrenergic activity

Despite its importance, little is known about the uptake, storage and release of catecholamines in the atrioventricular (AV) node and His bundle. Previous in vitro studies have been limited by metabolism of norepinephrine. Metaiodobenzylguanidine (MIBG) shares many transport properties with norepinephrine and is considered a functional marker of adrenergic activity. Methods: We used [¹²⁵I]MIBG ± ^99mTc-sestamibi (^99mc-MIBI) and [¹²³I]MIBG ± ²⁰¹Tl to evaluate regional differences in adrenergic activity between cardiac conductive and contractile elements in rats. Histological localization of the AV node and His bundle was performed using stains for acetylcholinesterase. Results: Densitometric evaluation of autoradiographs, obtained from 20-μ thick sections of hearts from rats injected with either [¹²⁵I]MIBG ± ^99mTc-MIBI (n = 4) and [¹²³I]MIBG ± ²⁰¹Tl (n = 6), revealed that there was approximately 30% more MIBG uptake in the AV node and His bundle compared to atrial or ventricular muscle (p < 0.05). Color-coded functional maps, generated by computer to simultaneously display ¹²³I or [¹²⁵I]MIBG and perfusion markers, revealed that the heterogeneous distribution of MIBG was independent of myocardial blood flow. Conclusion: When used as a selective functional marker of adrenergic activity in the cardiac conduction system, ¹²³I- or [¹²⁵I]MIBG autoradiography demonstrates increased adrenergic activity in the AV node and His bundle compared with the left ventricle. MIBG imaging provides a new research technique to probe in vivo modulation of AV nodal and His bundle sympathetic activity.