title: Modulation of muscarinic cholinergic receptor affinity for antagonists in rat heart.
abstract: Modulation of the affinity of agonists and antagonists at muscarinic cholinergic receptors in rat heart membranes was investigated using the radiolabeled antagonist, [3H]quinuclidinyl benzilate ([3H]QNB), and the radiolabeled agonist, [methyl-3H]oxotremorine acetate ([3H]OXO). Receptor affinity for oxotremorine measured in competition binding assays with [3H]QNB or by equilibrium binding of [3H]OXO was increased when the incubation temperature was reduced to 4 degrees C. In contrast, the receptor affinity for [3H]QNB was decreased at lower incubation temperatures and a marked effect of guanine nucleotides on the affinity for [3H]QNB was revealed. Guanine nucleotides increased receptor affinity for [3H]QNB without changing the total number of binding sites. The GTP-induced increase in the affinity for [3H]QNB was reflected by an increase in the rate constant for association of [3H]QNB. At subsaturating ligand concentrations, guanine nucleotides increased [3H]QNB binding and decreased [3H]OXO binding with the same order of potency: GppNHp = GTP gamma S greater than GTP greater than guanosine 5'-diphosphate greater than GMP. Free Mg++ ion was required to observe guanine nucleotide effects on antagonist binding. Pretreatment of heart membranes with N-ethyl-maleimide increased [3H]QNB affinity and blocked the effects of guanine nucleotides. N-Ethylmaleimide also decreased [3H]OXO binding and increased [3H]QNB binding with a similar concentration-effect relationship. Thus, antagonist and agonist binding to muscarinic cholinergic receptors is modulated in a reciprocal manner by a number of factors; this modulation appears to reflect interaction of agonist and antagonist-occupied receptors with a guanine nucleotide regulatory protein, Ni.
mesh_terms: D000818:Animals; D001667:Binding, Competitive; D003080:Cold Temperature; D003201:Computers; D005033:Ethylmaleimide / Q000494:pharmacology; D005260:Female; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D006160:Guanosine Triphosphate / Q000031:analogs & derivatives / Q000494:pharmacology; D006165:Guanylyl Imidodiphosphate / Q000494:pharmacology; D007700:Kinetics; D008274:Magnesium / Q000378:metabolism; D009206:Myocardium / Q000378:metabolism*; D010095:Oxotremorine / Q000378:metabolism; D011813:Quinuclidinyl Benzilate / Q000378:metabolism; D051381:Rats; D011919:Rats, Inbred Strains; D011976:Receptors, Muscarinic / Q000378:metabolism*; D013873:Thionucleotides / Q000494:pharmacology
chemical_list: D011976:Receptors, Muscarinic; D013873:Thionucleotides; D006165:Guanylyl Imidodiphosphate; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D010095:Oxotremorine; D011813:Quinuclidinyl Benzilate; D006160:Guanosine Triphosphate; D008274:Magnesium; D005033:Ethylmaleimide