Opioid inhibition of adenylyl cyclase remaining after pertussis toxin treatment of NG108-15 cells: Possible involvement of tightly bound GTP.

Abstract

G i/G o proteins are uncoupled from receptors by ADP-ribosylation with pertussis toxin (PTX). However, PTX treatment of the δ opioid receptor-containing NG108-15 cell line often reduces but does not eliminate opioid inhibition of adenylyl cyclase, even when opioid stimulation of low K m GTPase is abolished. In our laboratory, treatment of NG108-15 cells with 100 ng/ml PTX eliminated both PTX-catalyzed 32P-labeling of G proteins and agonist stimulation of low K m GTPase in membranes. Although PTX decreased maximal opioid inhibition of adenylyl cyclase by 50–65%, the inhibition that remained was concentration-dependent, saturable, and antagonist-reversible. This inhibition persisted in the absence of GTP, even though opioid inhibition of adenylyl cyclase in untreated membranes was absolutely GTP-dependent. Opioid inhibition of adenylyl cyclase was eliminated by the nonhydrolyzable GTP analog, Gpp(NH)p, in membranes from both control and PTX-treated cells. This effect was not due to the stimulatory effect of Gpp(NH)p on adenylyl cyclase per se, since opioid inhibition of PGE 1-stimulated adenylyl cyclase remained in membranes from both control and PTX-treated cells. These results indicate that in membranes from PTX-treated cells, a subpopulation of inhibitory G proteins that tightly bind but do not hydrolyze GTP are able to transduce an inhibitory signal from opioid receptors.