Cardiotonic Activity of Milrinone, a New and Potent Cardiac Bipyridine, on the Normal and Failing Heart of Experimental Animals

Summary Milrinone (Win 47203) is a potent cardiac bipyridine with inotropic and vasodilator properties. Its effects were studied in anesthetized and unanesthetized dogs and in isolated cardiac tissues from guinea pigs. In the anesthetized dog, the intravenous injection of milrinone (0.01–0.1 mg/kg) increased cardiac contractile force (CF) (23 ± 6.1 to 87 ± 8.9%), maximum left ventricular pressure development (24 ± 5.8 to 119 ± 16.1%), and cardiac output (16 ± 4.5 to 33 ± 8.9%), with less than a 30% increase in heart rate (HR). Significant decreases in systolic and diastolic blood pressures were seen at 0.3–3 mg/kg i.v. Oral doses of milrinone (0.1–1.0 mg/kg), in unanesthetized dogs, increased cardiac CF by 35 ± 7.0 to 99 ± 17.0%, with a maximum increase in HR of 40 ± 7.1% and no significant change in blood pressure. Milrinone was effective in the presence of ouabain and dopamine without enhancing their arrhythmogenic properties. It was also effective in reversing propranolol-, verapamil-, or pentobarbital-induced heart failure. The inotropic response does not seem to involve the stimulation of the autonomic receptors, the release of endogenous catecholamines, histamine, or prostaglandins, or the inhibition of Na+,K+-adenosine triphosphatase. Milrinone is an inhibitor of cardiac adenosine 3‘,5’-monophosphate (cAMP) phosphodiesterase, with resultant increases in cardiac cAMP levels. However, the time course for this increase does not seem to correspond to the increase in muscle developed tension, and, therefore, it is unlikely to be responsible for the initiation of the inotropic response. Milrinone is a potent cardioactive agent which should be beneficial in the treatment of acute and chronic congestive heart failure.