Fig. 3. In vivo effects of muscle relaxants on vagal nerve–induced bradycardia in guinea pigs to measure antagonism of cardiac M2 muscarinic receptor. The magnitude of vagal nerve stimulation (VNS)-induced bradycardia before muscle relaxants was expressed as 100%. Antagonism of cardiac M2 muscarinic receptors by muscle relaxants resulted in reduced bradycardia during subsequent VNS challenges, which is expressed as a percent of the initial baseline VNS-induced bradycardia. Cumulatively increasing concentrations of intravenous muscle relaxants were administered with measurements of VNS-induced bradycardia between each dose (n = 6). After each completed dose response, each animal received a single intravenous dose of rapacuronium as a positive control. Subsequently, each animal received atropine to confirm the muscarinic origin of measured responses. CW002 did not block vagal nerve–induced bradycardia until a dose of 0.4 mg/kg, 33 times the ED95for twitch suppression in guinea pigs, which showed a weak response (20% block). Gantacurium did not block vagal nerve–induced bradycardia until a dose of 1.5 mg/kg (23 times the ED95for twitch suppression in guinea pigs) produced a weak response (20% block). Cisatracurium did not block vagal nerve-induced bradycardia until a dose of 0.5 mg/kg (five times the ED95for twitch suppression in guinea pigs) produced a weak response (30% block). Rapacuronium (8 mg/kg; 26 times ED95for twitch suppression in guinea pigs) profoundly blocked VNS-induced bradycardia (90–95% block) in all animals (n = 18) indicative of cardiac M2 muscarinic receptor antagonism. *P < 0.05 compared with initial baseline, **P < 0.01 compared with initial baseline, # P < 0.05 compared with rapacuronium in absence of atropine.