Actions of terazosin and its enantiomers at subtypes of alpha(1)- and alpha(2)-adrenoceptors in vitro

Terazosin and its enantiomers, antagonists of alpha(1)-adrenoceptors, we re studied in radioligand binding and functional assays to determine relative potencies at subtypes of alpha(1)- and alpha(2)-adrenoceptors in vitro. The racemic compound and its enantiomers showed high and apparently equal affinity for subtypes of alpha(1)-adrenoceptors with K-I values in the low nanomolar range, and showed potent antagonism of alpha(1)-adrenoceptors in isolated tissues, with the enantiomers approximately equipotent to the racemate at each alpha(1)-adrenoceptor subtype. At alpha(2b) sites, R(+) terazosin bound less potently than either the S(-) enantiomer or racemate. R(+) terazosin was also less potent than the S(-) enantiomer or the racemate at rat atrial alpha(2B) receptors. These agents were not significantly different in their potencies at alpha(2a) or alpha(2A) sites. Since the high affinity for alpha(2B) sites of quinazoline-type alpha-adrenoceptor antagonists has been used to differentiate alpha(2)-adrenoceptor subtypes, the low affinity of R(+) terazosin for these sites was unexpected. Because terazosin or its enantiomers are approximately equipotent at alpha(1)-adrenoceptor subtypes, the lower potency of R(+) terazosin at alpha(2B) receptors indicates a somewhat greater selectivity for alpha(1)- compared to alpha(2B) adrenoceptor subtypes. The possible pharmacological significance of this observation is discussed.