ADP- and epinephrine-elicited release of [3H]guanylylimidodiphosphate from platelet membranes. Implications for receptor-Ni stoichiometry.

Epinephrine-promoted release of [3H]guanylylimidodiphosphate ( [3H]Gpp(NH)p) from human platelet membranes has been used to probe the interactions between alpha 2-adrenergic receptors and Ni, the guanine nucleotide binding protein that couples those receptors to an inhibition of adenylate cyclase activity. We show here that ADP, which also acts through specific platelet receptors to inhibit adenylate cyclase activity, also promotes the release of [3H]Gpp(NH). The amount of [3H]Gpp(NH)-release elicited by epinephrine and by ADP together is equal to the sum of the amounts released by the two agents acting individually. Furthermore the maximal amounts of [3H]Gpp(NH)-release elicited by each of the two agents approximates the numbers of receptors for ADP and epinephrine present in the platelet membranes. These results suggest that the two receptor types interact with distinct portions of the pool of Ni molecules and that each receptor initiates guanine-nucleotide exchange on a single molecule of Ni.