Ability of δ-opioid receptors to interact with multiple G-proteins is independent of receptor density

To determine whether the previously demonstrated ability of δ-opioid receptors to interact simultaneously with multiple G-proteins was a function of high receptor levels, this interaction was investigated in Chinese hamster ovary cells stably expressing 10 different levels of cloned δ-opioid receptors, ranging from 18,000 to 1.6 x 10⁶ receptors/cell. The opioid agonist D-Ala²,D-Leu⁵-enkephalin (DADLE) inhibited forskolin-stimulated adenylyl cyclase activity in all 10 clones with variable maximal inhibitory levels. Furthermore, opioid agonists altered incorporation of [α- ³²P]azidoanilido-GTP into at least four G-protein α-subunits in all 10 clones, three of which were determined to be G(i3α), G(i2α) and G(o2α). This effect was concentration-dependent, naloxone-reversible, and δ-opioid agonist-specific and was blocked by pretreatment with pertussis toxin. Although DADLE induced an increase in the incorporation of [α- ³²P]azidoanilido-GTP into three of the four G(α) proteins that was independent of receptor density, the magnitude of this response was greater as receptor density increased. In addition, concentrations of DADLE required to promote 50% maximal labeling were similar for all four G(α) proteins within each clone and did not appear to be affected by receptor density. Therefore, the ability of δ-opioid receptors to interact with multiple G- proteins is independent of receptor density and there is also no apparent correlation between the amount of G-opioid activated and the maximal effect of an agonist.