The effects of propranolol stereoisomers on nomotopic and ectopic cardiac automaticity in the rat.

The effect of the two isomers of propranolol was tested on nomotopic and ectopic cardiac automaticity in the rat. The spontaneously beating isolated right atria of the rat was used as a model for studying the effect of these drugs on nomotopic automaticity. Ectopic automaticity was induced by local injury of the isolated right ventricle. Both dextro and levopropranolol reduce nomotopic and abolish ectopic automaticity (ID50 was 0.17 microM for d-propranolol and 0.96 microM for 1-propranolol) in the rat heart. We conclude that the membrane stabilizing activity, which is present functionally in both stereoisomers, plays a significant role in the effect of these drugs on cardiac automaticity. On the other hand, the property of competitive blocking of beta-adrenergic receptors, which mainly resides in the levoisomer of propranolol does not seem to be essential for this effect.