Measurement of functional cholinergic innervation in rat heart with a novel vesamicol receptor ligand

Regional differences in cholinergic activity in the cardiac conduction system have been difficult to study. We tested the utility of (+)-m-[I-125]iodobenzyl)trozamicol(+)-[I-125]MIBT), a novel radioligand ligand that binds to the vesamicol receptor located on the synaptic vesicle in presynaptic cholinergic neurons, as a functional marker of cholinergic activity in the conduction system. The (+)-[I-125]MIBT was injected intravenously into four rats. Three hours later, the rats were killed and their hearts were frozen. Quantitative autoradiography was performed on 20-micron-thick sections that were subsequently stained for acetylcholinesterase to identify specific conduction system elements. Marked similarities existed between (+)-[I-125]MIBT uptake and acetylcholinesterase positive regions. Optical densitometric analysis of regional (+)-[I-125]MIBT uptake revealed significantly greater (+)-[I-125]MIBT binding (nCi/mg) in the atrioventricular node (AVN) and His bundle regions compared with other conduction and contractile elements (AVN:3.43 +/- 0.37; His bundle: 2.16 +/- 0.30; right bundle branch: 0.95 +/- 0.13; right atrium: 0.68 +/- 0.05; right ventricle: 0.57 +/- 0.03; and left ventricle: 0.57 +/- 0.03; p < 0.05 comparing conduction elements with ventricular muscle). This study demonstrates that (+) [I-125]MIBT binds avidly to cholinergic nerve tissue innervating specific conduction system elements. Thus, (+)-[I-125]MIBT may be a useful functional marker in studies on cholinergic innervation in the cardiac conduction system. Copyright (C) 1996 Elsevier Science Inc.