Effects of ICI 118.551, a selective beta-2 adrenergic blocking agent on the guinea pig cardiac excitability and ventricular fibrillation threshold.

In isolated guinea pig perfused hearts ICI 118.551, a selective beta 2 adrenoceptor antagonist, induced transient ventricular extrasystoles. Following the termination of the perfusion, a very significant increase of both the ventricular fibrillation threshold and the refractory periods were measured. In guanethidine pretreated hearts, ICI 118.551 failed to induce premature beats. At the same time the fibrillation threshold and refractory periods exhibited a very significant decrease. The perfusion of equimolecular concentration of metoprolol, a beta-1-adrenoceptor antagonist, and (+) propranolol, a quinidine-like compound, induced, in most experimental settings, similar results as ICI 118.551. Thus, besides its beta-2-adrenoceptor antagonist properties, ICI 118.551 presented other pharmacological actions.