Comparison of the binding affinity of CGP-12177A at recombinant rat alpha(1D)-adrenoceptors expressed in BHK-21 cell membranes and alpha(1)-adrenoceptors present in rat cerebral cortex membranes

Recent in vitro studies, performed in rat aorta, mesenteric and intrapulmonary arteries, and human pulmonary artery, demonstrated that the beta-adrenoceptor ligand CGP-12177A (4-[3-[(1,1-dimethylethyl)amino]-2-hydroxypropoxy]-1,3-dihydro-2H-benzimidazol-2-one) is also provided with antagonist or partial agonist properties at alpha(1)-adrenoceptors. These observations were supported by estimates of CGP-12177A binding affinity at alpha(1)-adrenoceptors, which have been always performed in rat cerebral cortex membranes, as a surrogate of vascular tissue. Since alpha(1D)-adrenoceptors are predominant in both rat aorta and mesenteric artery, in the present study, we measured, for the first time, the binding affinity of CGP-12177A at recombinant rat alpha(1D)-adrenoceptors expressed in BHK-21 cell membranes. CGP-12177A binding affinity was also determined in rat cerebral cortex membranes, where various alpha(1)-adrenoceptor subtypes are present. By means of [(3)H]prazosin binding competition experiments, we found that CGP-12177A bound to alpha(1D)-adrenoceptor-expressing BHK-21 cell membranes, with a binding affinity (pK(i)=5.39+/-0.27) almost identical to that measured in cerebral membranes (pK(i)=5.44+/-0.07), indicating that it is a non-subtype selective alpha(1)-adrenoceptor ligand. Moreover, CGP-12177A binding affinity was very close to its functional affinity evaluated in rat aorta in terms of antagonist potency against phenylephrine-induced contraction (pK(B)=5.65+/-0.07). In conclusion, our results demonstrate that, in order to evaluate CGP-12177A binding affinity at aorta and mesenteric artery alpha(1)-adrenoceptors, estimates in rat cerebral membranes are as reliable as those in recombinant rat alpha(1D)-adrenoceptors, since both values are very close to CGP-12177A functional affinities in isolated vessels.