Inverse agonism and neutral antagonism at alpha(1a)- and alpha(1b)-adrenergic receptor subtypes

We have characterized the pharmacological antagonism, i.e., neutral antagon ism or inverse agonism, displayed by a number of alpha-blockers at two alph a 1-adrenergic receptor (AR) subtypes, alpha(1a)- and alpha(1b)-AR. Constit utively activating mutations were introduced into the alpha(1a)-AR at the p osition homologous to A293 of the alpha(1b)-AR where activating mutations w ere previously described. Twenty-four alpha-blockers differing in their che mical structures were initially tested for their effect on the agonist-inde pendent inositol phosphate response mediated by the constitutively active A 271E and A293E mutants expressed in COS-7 cells. A selected number of drugs also were tested for their effect on the small, but measurable spontaneous activity of the wild-type alpha(1a)- and alpha(1b)-AR expressed in COS-7 c ells. The results of our study demonstrate that a large number of structura lly different alpha-blockers display profound negative efficacy at both the alpha(1a)- and alpha(1b)-AR subtypes. For other drugs, the negative effica cy varied at the different constitutively active mutants. The most striking difference concerns a group of N-arylpiperazines, including 8-[2-[4-(5-chl oro-2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro[4,5]decane-7,9-dione ( REC 15/3039), REC 15/2739, and REC 15/3011, which are inverse agonists with profound negative efficacy at the wild-type alpha(1b)-AR, but not at the a lpha(1a)-AR.