Abstract

The effect of GTP on the binding affinity of alpha-adrenergic receptors for alpha-adrenergic agents was studied in human platelet lysates using direct ligand binding methods with [³H]dihydroergocryptine. GTP at a concentration of 0.1 mM markedly decreased the binding affinity of the agonist (-)epinephrine for the receptors (more than 10-fold) but had no effect on the binding of antagonists. The half maximal effect of GTP on epinephrine binding occurred at a concentration of 4 µM. Gpp(NH)p was as effective as GTP at the same concentration, whereas GDP was only 80% as effective. Other nucleotides such as ATP and ITP were less effective. The extent of the GTP-induced reduction in the affinity of alpha-adrenergic agents for the receptors was directly related to the intrinsic activity of these agents for inhibition of PGE₁-stimulated adenylate cyclse. The effect of GTP appears to depend on the concurrent presence of Mg⁺⁺. In the absence of Mg⁺⁺, GTP caused only a slight decrease in the agonist binding affinity. When Mg⁺⁺ was present without GTP, the binding affinity of the agonist (-)epinephrine was increased by 5-fold. GTP in the presence of Mg⁺⁺ induces a state of diminished affinity of the receptor for the agonist which is lower than that induced by the nucleotide in the absence of MgCl₂. The relationship between GTP and MgCl₂ in the regulation of platelet alpha-adrenergic receptors which are inhibitory to adenylate cyclase activity appears to be analogous to their role in regulating beta-adrenergic receptors which are stimulatory for the enzyme in other tissues.