Cell-permeable. A potent, cAMP-specific phosphodiesterase (PDEIII ) inhibitor (IC₅₀ = 0.45 and 1.0 µM for PDEIIIA and PDEIIIB, respectively). It inhibits other PDE isoforms at much higher concentrations. Milrinone has positive inotropic (stimulates cardiac muscle contractions) and vasodilatory effects when administered in vivo.