Amrinone relaxing effect on the isolated guinea pig trachea and its interaction with aminophylline

The relaxing effect of Amrinone, an inotropic and vasodilating agent, was investigated on the isolated guinea pig tracheal smooth muscle in comparison and in association with Aminophylline. Amrinone (pD2 = 5.02 +/- 0.07) resulted more active than Aminophylline (pD2 = 4.25 +/- 0.08) in relaxing the isolated guinea pig trachea. The interaction curves of the two drugs showed the pattern typical for "competitive synergism", thus suggesting that Amrinone may act on tracheal muscle through inhibition of phosphodiesterase.