Shifts in the affinity distribution of one class of seven-transmembrane receptors by activation of a separate class of seven-transmembrane receptors

Abstract

We have demonstrated previously that activation of thrombin receptors causes increased Gα q coupling to thromboxane A 2 receptors and increased thromboxane A 2 receptor ligand affinity. These results led to the hypothesis that thrombin receptor activation stimulates Gα q redistribution to thromboxane A 2 receptors, thereby shifting them to a higher affinity state. The present study investigated three questions regarding this inter-receptor signaling phenomenon: (i) does activation of thrombin receptors cause a redistribution of thromboxane A 2 receptor subpopulations; (ii) does inter-receptor signaling require that participating receptors couple to the same family of G-protein α-subunits; and (iii) does inter-receptor signaling occur in cell types other than platelets? It was found that thrombin receptor activation caused a shift in the thromboxane A 2 receptor binding data from a one-site model to a two-site model ( K i = 0.5 μM vs K i = 10 nM and 1.1 μM for the antagonist 4-[2-[[(4-chlorophenyl)sulfonyl]amino]ethyl]benzeneacetic acid (BM13.505) and K i = 2.5 μM vs K i = 29.5 nM and 2.6 μM for the agonist 9,11-dideoxy-9α,11α-methanoepoxy prostaglandin F 2α (U46619). It also was found that activation of prostaglandin D 2 receptors also caused a shift of prostacyclin receptor binding data from a one-site model ( ic 50 = 10.1 nM) to a two-site model ( ic 50 = 3.3 and 12.5 nM). The physiological manifestation of this inter-receptor signaling between prostacyclin and prostaglandin D 2 receptors was a synergistic inhibition of human platelet aggregation. Finally, the present results established that activation of endothelial cell thrombin receptors shifts thromboxane A 2 receptor affinity from K i = 0.8 μM (control) to K i = 0.2 μM (thrombin receptor-activating peptide), indicating that cells other than platelets have the capability to signal between seven-transmembrane receptors.