Tetranitromethane modification of the muscarinic receptors in bovine adrenal medulla.

Muscarinic receptor binding was examined in bovine adrenal medullary microsomes following exposure to tetranitromethane (TNM) that modifies tyrosine and cysteine residues in proteins. The TNM (10-100 microM) treatment of adrenal medullary microsomes caused a concentration-dependent and irreversible reduction in the maximum number of binding sites (Bmax) for l-(3H)quinuclidinyl benzilate (QNB), with a slight increase in the equilibrium dissociation constant (KD). Typically, about a 36% decrease and a 1.3-fold increase in the corresponding values were obtained at 50 microM of TNM. The alteration in the Bmax was partially prevented by atropine but not carbamylcholine, and it was not reversed by subsequent treatment with dithiothreitol, a disulfide reducing agent. The change in the KD was unaffected by these agents. The TNM (50 microM) treatment also caused a slight decrease in the affinity of atropine and pirenzepine (for both the high and low affinity sites), and it caused a slight decrease in the affinity of carbamylcholine at the high affinity site, with a large loss of the low affinity site. Thus, the results indicate that TNM causes a loss of muscarinic binding sites and a decrease in the binding affinity of muscarinic receptors in bovine adrenal medulla, probably through modifications of functional groups such as tyrosine residues.