8-NH2-boldine, an antagonist of α1A and α1B adrenoceptors without affinity for the α1D subtype: Structural requirements for aporphines at α1-adrenoceptor subtypes

Structure-activity analysis of 21 aporphine derivatives was performed by examining their affinities for cloned human α1A, α1B and α1D adrenoceptors (AR) using membranes prepared from rat-1 fibroblasts stably expressing each α1-AR subtype. All the compounds tested competed for [125 I]-HEAT binding with steep and monophasic curves. The most interesting compound was 8-NH2-boldine, which retains the selective affinity for α 1A-AR (pKi = 6.37 ± 0.21) vs. α 1B-AR (pKi = 5.53 ± 0.11) exhibited by 1,2,9,10-tetraoxygenated aporphines, but shows low affinity for α 1D-AR (pKi < 2.5). Binding studies on native adrenoceptors present in rat cerebral cortex confirms the results obtained for human cloned α1-AR subtypes. The compounds selective for the α1A subtype discriminate two binding sites in rat cerebral cortex confirming a mixed population of α1A- and α1B-AR in this tissue. All compounds are more selective as inhibitors of [3 H]-prazosin binding than of [3 H]-diltiazem binding to rat cerebral cortical membranes. A close relationship was found between affinities obtained for cloned α1A-AR and inhibitory potencies on noradrenaline-induced contraction or inositol phosphate accumulation in tail artery, confirming that there is a homogeneous functional population of α 1A-AR in this vessel. On the contrary, a poor correlation seems to exist between the affinity of 8-NH2-boldine for cloned α1D-AR and its potency as an inhibitor of noradrenaline-induced contraction or inositol phosphate accumulation in rat aorta, which confirms that a heterogeneous population of α1-AR mediates the adrenergic response in this vessel.