Effects of isoprenaline and aminophylline on the chronotropic responses of the isolated guineapig heart to vagal stimulation and acetylcholine.

1. In isolated perfused guinea-pig hearts, isoprenaline (2 x 10(-8) g/ml) reduced the negative chronotropic responses to vagal stimulation and to exogenous acetylcholine.2. The beta-adrenoceptor antagonist LB46 (2 x 10(-8) g/ml) abolished the effect of isoprenaline in reducing vagal bradycardia. LB46 itself did not alter the responses to vagal stimulation.3. Increase in (86)Rb efflux induced by acetylcholine was not affected by isoprenaline.4. Aminophylline (2.3 x 10(-4) g/ml) almost abolished the negative chronotropic effect of vagal stimulation.5. The ;antivagal' effects of isoprenaline and aminophylline may be at a common site beyond the level of the cardiac beta-adrenoceptors, perhaps related to cyclic-3',5'-AMP and/or sodium transport.