Notes: Esmolol acid is an active metabolite of the β1-adrenergic receptor (β1-AR) antagonist esmolol (Item No. 22581).{54302} It is formed from esmolol via hydrolysis by a red blood cell esterase. Esmolol acid inhibits isoproterenol-induced increases in the beating rate of isolated guinea pig right atria (pA2 = 3.73) and prevents tachycardia and hypotension induced by isoproterenol (Item No. 15592) in anesthetized dogs in a dose-dependent manner.{64615} It is also a potential impurity in commercial preparations of esmolol.{64616}