We determined the binding affinity of tamsulosin, a selective α 1 -adrenoceptor antagonist, for human α 1 -adrenoceptor subtypes in comparison with those of other α 1 -adrenoceptor antagonists including silodosin, prazosin, 5-methylurapidil, terazosin, alfuzosin, nafopidil, urapidil and BMY7378. The association and dissociation kinetics of [ 3 H]tamsulosin for recombinant human α 1 -adrenoceptor subtypes were compared with those of [ 3 H]prazosin. Tamsulosin competitively inhibited [ 3

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... n binding to human α 1A -, α 1B -and α 1D -adrenoceptors (pK i values were 10.38, 9.33, 9.85) indicating 11 and 3.4-fold higher affinities for human α 1A -adrenoceptor than those for α 1B -and α 1D -adrenoceptors, respectively. The affinity of tamsulosin for the human α 1A -adrenoceptor was, respectively, 5, 9.9, 38, 120, 280, 400, 1200 and 10000 fold higher than those of silodosin, prazosin, 5-methylurapidil, terazosin, alfuzosin, naftopidil, urapidil and BMY7378, respectively. [ 3 H]Tamsulosin dissociated from the α 1A -adrenoceptor slower than from the α 1B -and α 1D -adrenoceptors (α 1B >α 1D >α 1A ). Moreover, [ 3 H]tamsulosin dissociated slower than [ 3 H]prazosin from the α 1A -adrenoceptor and faster from the α 1B -and α 1D -adrenoceptors. In conclusion, tamsulosin potently and selectively antagonized α 1A/1D -adrenoceptor ligand binding, and slowly dissociated from the α 1A -adrenoceptor subtype.