The inotropic effects of amrinone and milrinone on neonatal and young canine cardiac muscle.

Standard techniques were used to study developmental changes in the effects of amrinone and milrinone on contractile properties of isolated canine cardiac papillary and trabecular muscle. In contrast to milrinone, which induced a positive inotropic effect, amrinone had a negative inotropic effect on the neonatal canine muscles studied. For both drugs there was an age-dependent increase in contractility beyond the neonatal period. The negative inotropic effect of amrinone was not related to a change in phosphodiesterase inhibition, although developmental changes in phosphodiesterase inhibition did occur. These results highlight the differences in the mechanism of action of two similar molecules. They also suggest that use of amrinone as an inotropic agent in the early neonatal period should be viewed with caution.