Involvement of purine compounds in the inotropic action of milrinone

In spontaneously beating atria from reserpine-treated guinea pigs, milrinone (1-100 micrograms/ml) induced a positive inotropic and chronotropic effect but was ineffective in preparations preincubated with adenosine deaminase (1 U/ml). Both in spontaneously beating and in electrically driven atria, ATP and adenosine evoked a dual effect: a first negative phase characterized by a reduction in contractile force, followed by a positive phase of increased inotropism. In these preparations milrinone inhibited the early negative influence exerted by purine compounds and amplified the following positive phase. These data suggest that the positive inotropic and chronotropic effect of milrinone may originate from its interference with endogenous purines.