We examined the diversity of single receptor function by measuring receptor–G protein coupling in the baculovirus–Sf21 expression system. In comparative studies using Sf21 cell membranes expressing κ-opioid receptor (KOR) plus Gαi1β1γ2 or KOR plus GαoAβ1γ2, there was no significant difference between both preparations in the K i values of various κ-opioid ligands for the displacement of [3H]U69593 binding. However, a marked difference in the rank order of agonists to stimulate [35S]GTPγS binding was observed between both preparations. These findings suggest that agonist efficacy is dependent on the population of different G proteins expressed in various tissues.
