Sildenafil reverses O2 constriction of the rabbit ductus arteriosus by inhibiting type 5 phosphodiesterase and activating BKCa Channels

Oxygen constriction causes functional closure of the ductus arteriosus (DA) at birth. Although DA closure is crucial for postnatal adaptation, patency of the DA is critical for survival of newborns with duct-dependent cardiac malformations. In these cases, DA patency is achieved by i.v. infusion of prostaglandin El, which, though effective, is often associated with complications. We hypothesized that sildenafil, a specific phosphodiesterase type 5 inhibitor, is an effective DA vasodilator. In isolated DA rings from term (d 30) fetal rabbits, sildenafil (10(-6)-10(-4) M) and diethylamine NONOate (10(-7)-10(-5) M) induced dose-dependent relaxation of oxygen-constricted DA (-52 +/- 4% and -51 +/- 6%, respectively) that was inhibited by the soluble glucanylyl-cyclase inhibitor, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (5 X 10(-5) M). Sildenafil increased cyclic GMP levels. Iberiotoxin (200 nM), an inhibitor of calcium-sensitive potassium channels, decreased the vasodilatory effect of sildenafil and diethylamine NONOate (-30 +/- 2% and -27 +/- 4%, respectively). Oxygen inhibition of whole-cell K+ current and membrane depolarization were partially restored by sildenafil, and this was inhibited by iberiotoxin. Immunohistochemistry and immunoblotting confirmed the presence of phosphodiesterase type 5 and calcium-sensitive potassium channels in the DA smooth muscle cells. This is the first study to demonstrate that sildenafil dilates the DA by increasing soluble guanylyl-cyclase-derived cGMP levels and thereby activating calcium-sensitive potassium channels, causing membrane hyperpolarization. Sildenafil, already approved for human usage, might be an alternative or a useful adjunct to prostaglandin El as a bridge to cardiac surgery.