Effects of isoprenaline and aminophylline on the chronotropic responses of the isolated guinea‐pig heart to vagal stimulation and acetylcholine

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Abstract
Summary: In isolated perfused guinea‐pig hearts, isoprenaline (2 × 10−8 g/ml) reduced the negative chronotropic responses to vagal stimulation and to exogenous acetylcholine. The β‐adrenoceptor antagonist LB46 (2 × 10−8 g/ml) abolished the effect of isoprenaline in reducing vagal bradycardia. LB46 itself did not alter the responses to vagal stimulation. Increase in 86Rb efflux induced by acetylcholine was not affected by isoprenaline. Aminophylline (2·3 × 10−4 g/ml) almost abolished the negative chronotropic effect of vagal stimulation. The ‘antivagal’ effects of isoprenaline and aminophylline may be at a common site beyond the level of the cardiac β‐adrenoceptors, perhaps related to cyclic‐3′,5′‐AMP and/or sodium transport.