Milrinone, phosphodiesterase III inhibitor, has been used effectively in patients with right heart failure, especially resulted from pulmonary hypertension. However, milrinone is often used with α- and β-adrenergic receptor agonist to prevent severe systemic vasodilation and unfavorable hypotension. Furthermore, structural and functional vasacular changes are associated with aging and are greatest in the aorta. We evaluated the vasodilatory effects of milrinone and sodium nitroprusside (SNP) on young and old rat aortic rings preconstricted with various catecholamines.

Aortic rings of young and old rat were placed in 25 ml organ chamber and preconstricted with epinephrine (EPI, 10^-6 M), norepinephrine (NE, 10^-7 M) , phenylephrine 10^-7 M) , and U46619 (10^-8 M). Cummulative dose-responses to milrinone (10^-9-10^-5 M) and SNP (10^-9-10^-5 M) were obtained to characterize vasodilatory effects.

Relaxation response to milrinone was markedly enhanced in both young and old aortic rings preconstricted with U46619 compared with other vasoconstrictors. The maximal response of the young rat aortic rings preconstricted with NE is significantly reduced, compared with that of EPI. The maximal vasorelaxant response of SNP in young and old aortic rings are nearly identical.

We conclude that combined use of milrinone and epinephrine may be more useful in prevention and treatment of systemic hypotension.