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 th at the rate of G-protein activation is dependent on receptor density, but n ot G-protein levels. C6 cells expressing mu- or delta-opioid receptors, or SH-SY5Y cells, were treated with beta-funaltrexamine (mu) or naltrindole-5' -isothiocyanate (delta) to decrease receptor number. The time course of ful l or partial agonist-stimulated [S-35]GTP gamma S binding did not vary in C 6 eel membranes containing < 1-25 pmol/mg mu-opioid receptor, or 1.4-4.3 pm ol/mg F-opioid receptor, or in SHSY5Y cells containing 0.16-0.39 pmol/mg re ceptor. The association of [S-35]GTP gamma S binding was faster in membrane s from C6 mu cells than from C6 delta cells. A 10-fold reduction in functio nal G-protein, following pertussis toxin treatment, lowered the maximal lev el of [S-35]GTP gamma S binding but not the association rate. These data in dicate a compartmentalization of opioid receptors and G protein at the cell membrane. (C) 2000 Elsevier Science B.V. All rights reserved.