Cardiac M2 receptors consist of two different types, both regulated by GTP

Cardiac muscarinic receptors are predominantly M2 receptors, and have three agonist binding sites (super-high(SH), high(H) and low(L) affinity agonist binding sites). Treatment of cardiac membranes with 50 nM propylbenzilyl choline mustard (PrBCM) caused 88% loss of binding sites for [3H]QNB. Carbamyl choline (CCh) inhibits this alkylation dose dependently and, theoretically, generates uneven alkylation of multiple agonist binding sites. Pretreatment of the membranes with 50 nM PrBCM and 0.5 mM CCh resulted in almost complete disappearance of L sites with similar degrees of conservation of H sites and SH sites. In these pretreated membranes, guanine nucleotide and sulfhydryl reagent caused a change in the ratio of residual SH and H sites but not of L sites though previous studies showed that, in intact membranes, these reagents affected the ratio of SH and L sites without significantly changing that of the H site. These results indicate the existence of two equilibria regulated by guanine nucleotide and sulfhydryl reagent in cardiac muscarinic receptors: one between SH and H sites and the other between H and L sites. The participation of GTP binding protein(s) in all cardiac muscarinic responses is suggested.