TRANSITION AND HEAVY-METAL INHIBITION OF LIGAND-BINDING TO MUSCARINIC ACETYLCHOLINE RECEPTORS FROM RAT-BRAIN

Several divalent heavy metals interact with muscarinic acetylcholine receptors in neural membranes from rat forebrains in vitro to inhibit the binding of [3H]3-quinuclidinyl benzilate (QNB), a potent and specific receptor antagonist. Hg is the most potent inhibitor with an ID50 [median infective dose] value of 10-7 M for the inhibition of the binding of 10-10 M QNB. Several other metals are less effective with ID50 values between 10-5 and 2 .times. 10-4 M increasing in the following order: Fe < Ag < Cu < Pb < Cd < Tb < Zn. Co, Mn, La, Ni and Sn are less potent inhibitors (ID50 > 10-3 M). Binding inhibition by Cu, Hg and Cd is competitive and the inhibition by all the metals can be reversed by decreasing the free metal concentration through dilution or chelation by EDTA, dimercaprol or penicillamine. The effects of metals on muscarinic binding are largely independent of temperature. Chaotropic anions (100 mM iodide, thiocyanate, trichloroacetate or nitrate) are similarly without effect on the metal-receptor interaction. The limited effects of sulfhydryl reducing (dithiothreitol, 2-mercaptoethanol and glutathione) and alkylating (N-ethyl maleimide) reagents do not indicate a prominent role for sulfhydryl groups in mediating metal inhibition of muscarinic receptors.