Differential alpha1-adrenoceptor labeling by [3H]prazosin and [3H]tamsulosin

The radioligand binding properties of [3H]prazosin and [3H]tamsulosin at alpha1-adrenoceptors of several rat tissues, human prostate and cloned rat and human alpha1-adrenoceptor subtypes were compared in Tris/EDTA buffer unless otherwise indicated. The affinity of [3H]tamsulosin at tissue and cloned alpha1A- and alpha1B-adrenoceptors was somewhat greater and smaller, respectively, than that of [3H]prazosin. In most rat tissues and at cloned rat alpha1A- and alpha1B-adrenoceptors, [3H]tamsulosin had a smaller Bmax than [3H]prazosin. Studies with rat liver showed that this was due to considerably poorer labeling of agonist low affinity sites, while both radioligands detected similar numbers of agonist high affinity sites. Statistically significant differences in the number of binding sites for both ligands were not detected in HEPES or glycylglycine buffer, as the detectable receptor number for [3H]prazosin and [3H]tamsulosin tended to be smaller and greater, respectively, in these than in Tris/EDTA buffer. Among human alpha1-adrenoceptor subtypes [3H]tamsulosin labeled fewer sites than [3H]prazosin for alpha1B- but more sites for alpha1A- and alpha1D-adrenoceptors. We conclude that [3H]prazosin and [3H]tamsulosin do not detect the same number of alpha1-adrenoceptors under a variety of conditions. This should be taken into account in the interpretation of data obtained with either radioligand.