8-NH2-Boldine, an Antagonist of alA and a18 Adrenoceptors without Affinity for the alD Subtype: Structural Requirements for Aporphines at al-Adrenoceptor Subtypes

Abstract

5tructure-activity analysis of 21 aporphine derivatives was performed by examining their affinities for c10ned human alA. alB and alD adrenoceptors (AR) using membranes prepared from rat -1 fibroblasts stably expressing each a¡-AR subtype. AIl the compounds tested competed for [I25I]-HEATbinding with steep and monophasic curves. The most interesting compound was 8NHrboldine. which retains the selective affinity for a¡A-AR (pKi = 6.37 ± 0.21) vs. a¡B-AR(pKi = 5.53 ± 0.11) exhibited by 1.2.9,lO-tetraoxygenated aporphines. but shows low affinity for alD-AR(pKi< 2.5). Binding studies on native adrenoceptors present in rat cerebral cortex confirms the results obtained for human c10ned a¡-AR subtypes. The compounds selective for the alA subtype discriminate two binding sites in rat cerebral cortex confirming a mixed population of a¡A- and a¡B-ARin this tissue. AIl compounds are more selective as inhibitors of [W]-prazosin binding than of [3Hj-diltiazem binding to rat cerebral cortical membranes. A c10se relationship was found between affinities obtained for c10ned a¡A-ARand inhibitory potencies on noradresubnaline- induced contraction or inositol phosphate accumulation in tail artery. confirming that there is a homogeneous functional population of a¡A-ARin this vessel. On the contrary, a poor correlation seems to exist between the affinity of 8-NH2-boldine for c10ned alD-ARand its potency as an inhibitor of noradrenalineinduced contraction or inositol phosphate accumulation in rat aorta, which confirms that a heterogeneous population of a¡-AR mediates the adrenergic response in this vessel.