<prompt> Tamsulosin, a competitive α adrenoceptor antagonist has an affinity for which of the following receptorsα1Aα1DNone of the aboveBoth a and b  <response>: The actions of the α1-adrenoceptor antagonist tamsulosin have been examined at functional α1-adrenoceptor subtypes and compared with those at the human prostate receptor.At the α1D-adrenoceptors of the rat aorta, tamsulosin acted as a competitive antagonist with a high affinity pKB10.1.At the α1B-adrenoceptor of the rat spleen and rabbit corpus cavernosum penis, tamsulosin again acted as a competitive antagonist but with a significantly lower affinity pKB8.99.2.Tamsulosin acted as an insurmountable antagonist of the α1A-adrenoceptor-mediated responses of the rat and human vas deferens, reducing maximal responses to phenylephrine by 20 and 50, respectively, at an antagonist concentration of 1 nm. Responses of depolarized 100 mm KCl rat vas deferens preparations were unaffected by 10 nm tamsulosin but this concentration reduced maximal responses to 5-hydroxytryptamine 5-HT in this tissue.When longer antagonist incubation periods 60 min were used, tamsulosin behaved as a competitive antagonist on the human prostate with a significantly higher affinity pKB10.0 than obtained at the α1B-adrenoceptor.The data demonstrate that tamsulosin is a high-affinity antagonist at functional α1-adrenoceptors with a selectivity α1Dα1Aα1B. In some tissues, the compound exhibits an additional unsurmountable antagonist action, the clinical significance of which is unknown. 