High affinity agonist-binding (HAB) sites are formed from specific receptor interaction with guanine nucleotide-binding (Gi) proteins. To determine whether the release of endothelium-derived relaxing factor (EDRF) is regulated by specific receptor-Giprotein coupling, we treated bovine aortic endothelial cells with 100 ng/ml pertussis toxin (PTX) for 16 hours to effect receptor-Giprotein uncoupling. The degree of receptor uncoupling as measured by the loss of HAB sites for the α2-adrenergic receptor and bradykinin receptor was assessed by radioligand binding studies using partially purified bovine aortic endothelial cell membranes. The release of EDRF in response to UK14304 (an α2-adrenergic receptor agonist) and bradykinin stimulation was measured with a bioassay apparatus. The Giprotein isoforms were characterized by Western blotting, and complete ADP-ribosylation of these proteins was confirmed by PTX-catalyzed [32P]NAD ribosylation. PTX produced a greater inhibition of EDRF release via the α2-adrenergic receptor pathway compared with the bradykinin receptor pathway (80% versus 46%,p<0.01). This corresponded to the loss of HAB sites from the α2-adrenergic receptor and bradykinin receptor pathway (72% versus 46%,p<0.01) as compared with complete loss of both HAB sites in the presence of GppNHp (0.1 mM). Since loss of HAB sites from PTX-mediated receptor uncoupling parallels the inhibition of EDRF release, these data suggest that Giproteins contribute to a greater proportion of HAB sites derived from α2-adrenergic receptor rather than bradykinin receptor interaction and that the inhibition of EDRF release by PTX is mainly due to the loss of these HAB sites. The degree of HAB site formation from specific receptor-Giprotein coupling may serve as one mechanism for regulating EDRF release via different cell surface receptors. (Circulation Research1992;70:1018–1026)