A comparison of the inotropic effects of milrinone and piroximone

Patients with low cardiac output syndrome often have a reduced sensitivity to inotropes acting via the beta-adrenergic receptors. In this situation, drugs such as milrinone and piroximone may have a therapeutic advantage. Strips of human right atrial appendage were used to examine the inotropic actions of phosphodiesterase (PDE) inhibitors, measured as an increase in isometric tension in response to electrical stimulation at a frequency of 1 Hz. Cumulative concentration response curves were established for piroximone (an imidazalone derivative) and Milrinone (a bipyridine derivative.) Concentration response curves were then recorded to adrenaline in the presence or absence of the EC50 of each PDE inhibitor. The results indicate that these drugs, under the conditions employed in these experiments, have a significant inotropic effect when used alone. They also have a significant additive and potentiating effect when used in combination with adrenaline. Milrinone produced a significantly greater maximum tension as a percentage of basal tension, and has a lower EC50 than Piroximone.