Heterogeneous receptor binding of classical quaternary muscarinic antagonists. II. Modulation in bovine brain

The heterogeneous binding behavior exhibited by classical quaternary muscarinic antagonists was further investigated in order to establish possible molecular and/or environmental differences between the high (Q1) and low (Q2) affinity binding sites. Using agents that are known to modulate (muscarinic) receptor binding (Na+ and Mg2+, 5'-guanylylimido diphosphate, dithiothreitol, N-ethylmaleimide), no evidence was obtained that the Q1 and Q2 binding site populations are molecularly different muscarinic receptor subtypes. Using membrane modulating agents (sodium dodecyl sulphate, digitonin, polyethylene glycol 6000, cholesteryl hemisuccinate), no evidence was obtained that the Q2 binding site population represents muscarinic receptors located in a different membrane environment less accessible to quaternary compounds. However, a qualitative correlation was found between lipid solubility and the appearance of Q1/Q2 binding heterogeneity for six quaternary muscarinic antagonists, suggesting that the low affinity binding sites may be (normal) muscarinic receptors located in a hydrophobic membrane domain.