In open-chest dogs the heart rate was controlled at 150 beats/min and drugs were given intravenously. Propranolol (30 μg/kg—1 mg/kg) prolonged the atrio-ventricular (A-V) conduction time and functional refractory period of the A-V conduction system (FRP) by blockade of the existing tone of the sympathetic nerves to the heart. The prolongation of the two parameters by the non-specific depressant action of propranolol was evident only at 3 mg/kg. Propranolol (3-30 μg/kg) shortened the A-V conduction time in the heart deprived of the vagal and sympathetic tone, suggesting some sort of sympathomimetic effect. Pindolol in a wide range of doses (0.3-300 μg/kg) exerted virtually no effect on the A-V conduction time and FRP, and its non-specific depressant action was apparent only at 3 mg/kg. Carteolol slightly prolonged the A-V conduction time and FRP only in low doses (1-10 μg/kg), and in high doses (30 μg/kg—1 mg/kg) it shortened the two parameters, reflecting its predominant sympathomimetic action.
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