The effect of certain β-adrenoceptor antagonists on overdrive suppression

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Abstract
1 Propranolol, as the racemate and the (+)- and (–)-isomers (400 μg/l.) and practolol (50 mg/l.) were tested for their effects on atrial and ventricular rates and on the duration of overdrive suppression (ODI) in isolated perfused cat hearts with surgically-induced heart block. 2 Racemic propranolol and the (+)- and (–)-isomers prolonged ODI and slowed the rate of the ventricular pacemaker; the (+)- and (–)-isomers also reduced the rate of the atrial pacemaker. Practolol shortened ODI and increased the rate of the atrial and ventricular pacemakers. 3 The (+)- and (–)-isomers were more potent than the racemate; the (–)-isomer was more potent than the (+)-isomer. The results suggest there is a stereospecific mechanism involved in the biological distribution of propranolol. 4 Twenty-four hours after reserpine treatment (5 mg/kg, i.p.) practolol continued to increase the rate of the atrial and ventricular pacemakers but did not shorten ODI. 5 The mechanism by which these agents affect myocardial excitability and automaticity is discussed.