Opiates inhibit adenylate cyclase by stimulating GTP hydrolysis.

Abstract

Specific, GTP hydrolysis catalyzed by membranes prepared from [rat] neuroblastoma-glioma (NG108-15) hybrid cells can be measured in the presence of adenosine-5''-[.beta.,.gamma.-imido]triphosphate (p[NH]ppA), ATP and a nucleoside triphosphate-regenerating system. Opiates [diprenorphine, etorphine, naltrexone, levorphanol, nalorphine, morphine] and opioid peptides ([D-Ala2,Met5]enkephalinamide) stimulate low Km GTP hydrolysis when measured in the presence of Na+ and Mg2+. Opiate stimulation is rapid, stereospecific and reversed by the antagonist naloxone. Potencies of opiates as stimulators of GTP hydrolysis and as inhibitors of adenylate cyclase are closely correlated. Agents that stimulate adenylate cyclase, including prostaglandin E1, 2-Cl-adenosine, secretin and NaF, have little or no effect upon the rate of GTP hydrolysis. Opiates have no effect upon either adenylate cyclase or GTPase activity in membranes prepared from [rat] C6-BU1 glioma cells, which lack opiate receptors. In view of the pivotal role of GTP in the activation of adenylate cyclase, it was concluded that receptor-mediated stimulation of GTP hydrolysis is the mechanism by which opiates and other inhibitory hormones lower adenylate cyclase activity in HG108-15 cell membranes.