Atropine sulfate is a competitive muscarinic acetylcholine receptor antagonist.(In Vitro):Atropine (Tropine tropate; 1 μM; pulmonary veins and arteries) sulfate inhibits ACh-induced relaxations in human pulmonary veins.(In Vivo):Atropine (Tropine tropate; 10 mg/kg; i.p.; once, for 40 minutes; Peromyscus sp.) sulfate inhibits the cardiac arrhythmia which normally occurs throughout torpor.