Reciprocal modulation of agonist and antagonist binding to inhibitory adenosine receptors by 5'-guanylylimidodiphosphate and monovalent cations.

Previous work from this laboratory showed that rat hippocampal membranes contain adenosine receptors that mediate GTP-dependent inhibition of adenylate cyclase activity (Yeung, S. -M. H., and R. D. Green (1983) J. Biol. Chem. 258: 2334-2339). Furthermore, we reported that guanine nucleotides decrease agonist and increase antagonist binding to these adenosine receptors. The present study examines the effects of monovalent cations and guanine nucleotides, alone and in combination, on the binding of agonist [( 3H]N6(L-phenylisopropyl)adenosine) and antagonist [( 3H]diethylphenylxanthine) radioligands to adenosine receptors in rat hippocampal membranes. Low concentrations of monovalent cations (less than or equal to 100 mM) did not affect agonist binding. 5'-Guanylylimidodiphosphate (Gpp(NH)p) alone increased the KD of the agonist without affecting the maximal number of sites labeled by the agonist (Bmax); in the presence of monovalent cations, Gpp(NH)p both increased the KD and decreased the number of sites labeled by the agonist. In contradistinction, Gpp(NH)p increased the maximal number of sites to which the antagonist bound without affecting its KD, while monovalent cations decreased the KD of the antagonist both in the absence and the presence of Gpp(NH)p. It is proposed that both agonist and antagonist-receptor complexes exist in three distinct affinity states and that the transitions between these states are modulated by guanine nucleotides and monovalent cations.(ABSTRACT TRUNCATED AT 250 WORDS)