Activation of various subtypes of G-protein alpha subunits by partial agonists of the adenosine A1 receptor.

The activation of different G protein subtypes by the rat adenosine A1 receptor initiated by stimulation with the full agonist 2-chloro-N6-cyclopentyladenosine (CCPA) and by six structurally distinct partial agonists of this receptor was investigated. Endogenous G protein alpha subunits in rat cortical membranes were inactivated by N-ethylmaleimide (NEM). Activation of rat recombinant myristoylated alpha(o), alpha(i1), alpha(i2) and alpha(i3) by partial agonists in comparison to the full agonist was assessed by guanosine-5'-(gamma-[35S]thio)triphosphate ([35S]GTPgammaS) binding after reconstitution of G protein alpha subunits with the adenosine A1 receptor in N-ethylmaleimide-treated membranes. 2-Chloro-N6-cyclopentyladenosine and 3' -deoxy-N6-cyclopentyladenosine (3'-d-CPA), the partial agonist with the highest intrinsic activity, were significantly more potent in activation of alpha(i) subtypes than alpha(o). In contrast, 5'-methylthioadenosine (MeSA), 2'-deoxy-2-chloroadenosine (cladribine), 2'-deoxy-N6-cyclopentyladenosine (2'-d-CPA), 2-phenylaminoadenosine (CV 1808) and C8-aminopropyl-N6-cyclopentyladenosine (C8-aminopropyl-CPA) did not exhibit higher potency for Go or any Gi subtype. All partial agonists, although carrying structurally different modifications, showed higher relative intrinsic activities in activation of Gi than of Go, indicating that Gi-coupled pathways may be activated selectively via the A1 receptor by partial agonists, but not Go-mediated responses.