Developmental changes in the cardiac effects of amrinone in the dog.

Amrinone, a bipyridine compound, has been shown to exert a positive inotropic effect on the heart, without producing cardiac arrhythmias. Because of preliminary observations suggesting that the actions of amrinone might change significantly with growth and development, we studied its effects on the contractility and electrophysiology of isolated cardiac muscle of 0- to 96-day-old beagles and Purkinje fibers of 5-year-old beagles. Amrinone's effects on ventricular muscle contractility were age-related. A significant decrease in contractility of right ventricular trabeculae and papillary muscles (not associated with changes in action potential characteristics) was observed in the 0- to 3-day newborn, whereas, by day 4-10, amrinone increased contractility. The magnitude of the increase became greater through day 96 of life. The negative inotropic effect of amrinone is unassociated with changes in the action potential plateau, suggesting that the slow inward current is not involved in this mechanism.