Abstract

We have tested the hypothesis that smaller α 1B-adrenoceptor labeling by [ 3H]tamsulosin compared to [ 3H]prazosin is related to differential recognition of agonist low affinity states. Paired saturation binding experiments with [ 3H]prazosin and [ 3H]tamsulosin were performed in membrane preparations from rat liver and Rat-1 fibroblasts stably transfected with wild-type hamster α 1B-adrenoceptors or a constitutively active mutant thereof. In all three settings [ 3H]tamsulosin labeled significantly fewer α 1B-adrenoceptors than [ 3H]prazosin. In noradrenaline competition binding experiments, the percentage of agonist low affinity sites was smallest for the constitutively active α 1B-adrenoceptor but the percentage of agonist low affinity sites recognized by [ 3H]tamsulosin and [ 3H]prazosin did not differ significantly. We conclude that [ 3H]tamsulosin labels fewer α 1B-adrenoceptors than [ 3H]prazosin but this is not fully explained by a poorer labeling of agonist low affinity sites.

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