Milrinone is a phosphodiesterase III inhibitor. It increases cyclic adenosine monophosphate by inhibiting which enzyme class?

Milrinone is a phosphodiesterase III inhibitor. It increases cyclic adenosine monophosphate by inhibiting which enzyme class?

• A. PDE I
• B. PDE II
• C. PDE III
• D. PDE IV

Answer: (C)

Milrinone works by blocking phosphodiesterase-3, which normally breaks down cAMP inside cells. By inhibiting PDE-3, milrinone prevents the breakdown of cAMP, causing its levels to rise. Higher cAMP activates protein kinase A, increasing calcium availability in cardiac myocytes and enhancing contractility (positive inotropy). At the same time, in vascular smooth muscle, increased cAMP promotes relaxation, leading to vasodilation and lower afterload. The net effect is an inodilator action—improved cardiac output with decreased afterload. This mechanism is specific to PDE-3, unlike the other PDE families, which have different substrates and tissue distributions.

Milrinone works by blocking phosphodiesterase-3, which normally breaks down cAMP inside cells. By inhibiting PDE-3, milrinone prevents the breakdown of cAMP, causing its levels to rise. Higher cAMP activates protein kinase A, increasing calcium availability in cardiac myocytes and enhancing contractility (positive inotropy). At the same time, in vascular smooth muscle, increased cAMP promotes relaxation, leading to vasodilation and lower afterload. The net effect is an inodilator action—improved cardiac output with decreased afterload. This mechanism is specific to PDE-3, unlike the other PDE families, which have different substrates and tissue distributions.