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

The activation of different G protein subtypes by the rat adenosine A(1) receptor initiated by stimulation with the full agonist 2-chloro-N-6-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-[S-35]thio)triphosphate ([S-35]GTP gamma S) binding after reconstitution of G protein alpha subunits with the adenosine A(1) receptor in N-ethylmaleimide-treated membranes. 2-Chloro-N-6-cyclopentyladenosine and 3'-deoxy-N-6-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-N-6-cyclopentyladenosine (2'-d-CPA), 2-phenylaminoadenosine (CV 1808) and C8-aminopropyl-N-6-cyclopentyladenosine (C8-aminopropyl-CPA) did not exhibit higher potency for G(o) or any G(i) subtype. All partial agonists, although carrying structurally different modifications, showed higher relative intrinsic activities in activation of G(i) than of G(o), indicating that G(i)-coupled pathways may be activated selectively via the A(1) receptor by partial agonists, but not G(o)-mediated responses. BIOCHEM PHARMACOL 56;10:1287-1293, 1998. (C) 1998 Elsevier Science Inc.