Characterization of beta 1- and beta 2-adrenoceptor subtypes in the rat atrioventricular node by quantitative...

We characterized beta-adrenoceptor subtypes in the atrioventricular node of the rat heart by quantitative autoradiography. Consecutive 16-microns-thick sections from single rat hearts containing the atrioventricular node were incubated with increasing concentrations of [125I]iodocyanopindolol. After exposure to [3H]Ultrofilm, optical densities corresponding to the atrioventricular node were determined by computerized densitometry after comparison with [125I]standards. The computer program LIGAND was used for analysis of receptor subtypes. Delineation of beta-adrenoceptor subtypes was achieved by incubating consecutive tissue sections with 50 pM [125I]iodocyanopindolol in the presence of increasing concentrations of the beta 1-selective antagonist atenolol or the beta 2-selective antagonist ICI 118,551. The atrioventricular node contains a higher concentration of beta-adrenoceptors than the adjacent interventricular septum. We estimated that the proportions of beta 1- and beta 2-adrenoceptors in the atrioventricular node were about 56% and 44% of the total binding capacity respectively.