[Atenolol, its cardioselective property for adrenergic beta-receptor blocking action and effect on the cardiac...

Cardioselective property of the beta-adrenoceptor blocking action of atenolol and its effect on the AV conduction, the atrial muscle refractory period and the cardiac contractility were studied in comparison with these of propranolol. A ratio of pA2 values obtained in the isolated right-atrial preparations and the isolated tracheal preparations of guinea pigs demonstrated that atenolol was highly cardioselective, while propranolol was non-selective. This cardioselective property of atenolol was confirmed in in vivo experiments using guinea pigs. Atenolol increased the AV conduction time and decreased the cardiac contractility dose-dependently in anesthetized dogs of which the heart was electrically driven at fixed rates. At 140 beats/min, AV block was observed at 10 mg/kg, i.v. in 3 of 8 dogs, and further increase in a dose ti 30 mg/kg resulted in acute heart failure in one of remaining 5 dogs, while propranolol, though showing a tendency to be less potent in depressing AV conduction in a dose range less than 0.3 mg/kg, produced a state of heart failure in 2 of 5 dogs at 3 mg/kg, i.v. and AV block in all of the remainder at 10 mg/kg, i.v. Functional refractory period of AV node as well as functional and effective refractory period the the atrial muscle were increased dose-dependently by atenolol and propranolol. There were no significant differences between both drugs regarding these effects. Further increase in a pacing rate to 160 and 180 beats/min augmented the depressing effect of both drugs on the AV conduction.