The Comparative Effects of Four Antihistamines on Isolated Rat Atria

It has been reported that some of H1 receptor antagonists have important effects on cardiovascular system. Terfenadine as a non-sedative H1 receptor antagonist has an arrhythmogenic activity. In this study we have shown the effects of four antihistamine drugs: terfenadine, loratadine, clemastine and diphenhydramine, on the rate and contractions of isolated rat atria. Terfenadine (1-10 μM) caused a negative chronotropic effect (19.5-80%) and arrhythmia after 10 min. followed by a decrease in the contractile force by (7.5%), and finally after 45 min. asystolia occurred. Loratadine (30-150 μM) decreased the rate of contractions (10-82%) after 10 min. the contractile force of atria was decreased (10-19%) after 20 min. Loratadine did not produce any arrhythmia. Diphenhydramine (5-20 μM) produced bradycardia (14.5-43%) after 20 min and decreased the contractile force (2.5-40%) after 40 min. Clemastine (3-10 μM) produced negative chronotropic and inotropic effect by (11.5-42%) and (10-58%) respectively. These findings indicate that all four drugs caused bradycardia and reduced contractile force, but in the case of terfenadine, it also had arrhythmogenic activity. Loratadine had the least cardiotoxic effect.