CANNABINOID INHIBITION OF ADENYLATE-CYCLASE - BIOCHEMISTRY OF THE RESPONSE IN NEURO-BLASTOMA CELL-MEMBRANES

Abstract

The inhibition of adenylate cyclase activity by cannabimimetic compounds in a membrane fraction from cultured neuroblastoma cells was examined. The inhibition was concentration-dependent over a nM range for both .DELTA.9-tetrahydrocannabinol and its synthetic analog, desacetyllevonantradol. Inhibition was rapid and reversible. The cannabimimetic compounds caused a decrease in Vmax of the enzyme, with no alteration in the Km for substrate. The effects of these compounds were related to the ability of the enzyme to be regulated by divalent cations and guanine nucleotides. The inhibition was greatest at .mu.M Mg2+ or Mn2+ concentrations and was abolished at < 1 mM MnCl2. In the hormone-stimulated state, the enzyme appeared to be regulated by one Mg2+ site. The addition of cannabimimetic or muscarinic cholinergic agents transformed the enzyme into one in which more complex regulation by divalent cations was observed. Half-maximal inhibition of adenylate cyclase was observed at 800 nM GTP for both cannabimimetic and muscarinic cholinergic agents. The substitution for GTP of a nonhydrolyzable analog resulted in activation of the enzyme and failure to respond to either class of inhibitory agents. If the Mg2+ concentration was reduced and exposure to the GTP analog was of short duration, inhibition by both cannabimimetic and muscarinic agents could be observed in the presence of forskolin. This study points to the similarities between the enzyme inhibition by cannabimimetic compounds and by muscarinic cholinergic compounds. The cannabimimetic compounds must act via regulatory mechanisms similar to those operating for receptor-mediated inhibition of adenylate cyclase.