A comparison of the binding characteristics of class I antiarrhythmic agents for human muscarinic m1-m3 receptors

The binding characteristics of the class 1 antiarrhythmic agents, cibenzoli ne, disopyramide, disopyramide metabolite (the main active metabolite of di sopyramide in humans), and pirmenol, for human muscarinic receptors (m1-m3) stably expressed in Chinese hamster ovary cells (CHO) were investigated by binding assay with [H-3]N-methylscopolamine ([H-3]NMS) as a ligand. All of these agents inhibited the specific [H-3]NMS binding to membrane preparati ons in a concentration-dependent manner. The potencies of affinity of these agents for mi, m2, and m3 receptors (compared by IC50) were disopyramide > pirmenol > disopyramide metabolite > cibenzoline, pirmenol > cibenzoline > disopyramide > disopyramide metabolite, and disopyramide > disopyramide me tabolite > pirmenol > cibenzoline, respectively. Some competition curves of cibenzoline, disopyramide, and pirmenol were shallow, and Hill coefficient s of these curves differed from unity, suggesting that these agents have al losteric binding characteristics for human muscarinic receptors. The m2-sel ective ratios to mi (IC50 m1/IC50 m2) and m3 (IC50 m3/IC50 m2) of cibenzoli ne were 4.0 and 16, and those of pirmenol were 6.5 and 43, respectively, wh ereas those of disopyramide and its metabolite ranged from 0.46 to 1.6, sug gesting that cibenzoline and pirmenol exerted high selectivity to the m2 re ceptor. We conclude that (a) all class 1 antiarrhythmic agents in this stud y have inhibitory effects on human mi, m2, and m3 receptors, and some of th ose binding may show allosteric characterization; (b) disopyramide and its metabolite have similar affinity to m1 to m3 receptors; and (c) cibenzoline and pirmenol have high m2-selective ratios to mi and m3.