Activation of G protein by opioid receptors: Role of receptor number and G-protein concentration

The collision-coupling model for receptor-G-protein interaction predicts that the rate of G-protein activation is dependent on receptor density, but not G-protein levels. C6 cells expressing μ- or δ-opioid receptors, or SH-SY5Y cells, were treated with β-funaltrexamine (μ) or naltrindole-5'-isothiocyanate (δ) to decrease receptor number. The time course of full or partial agonist-stimulated [³⁵S]GTPγS binding did not vary in C6 cell membranes containing <1-25 pmol/mg μ-opioid receptor, or 1.4-4.3 pmol/mg δ-opioid receptor, or in SHSY5Y cells containing 0.16-0.39 pmol/mg receptor. The association of [³⁵S]GTPγS binding was faster in membranes from C6μ cells than from C6δ cells. A 10-fold reduction in functional G-protein, following pertussis toxin treatment, lowered the maximal level of [³⁵S]GTPγS binding but not the association rate. These data indicate a compartmentalization of opioid receptors and G protein at the cell membrane. Copyright (C) 2000 Elsevier Science B.V.