Prostaglandin E2-mediated relaxation of the ductus arteriosus -: Effects of gestational age on G protein-coupled receptor expression, signaling, and vasomotor control

Background - In the preterm newborn, a patent ductus arteriosus is in large part a result of the increased sensitivity of the immature ductus to prostaglandin E-2 (PGE(2)). PGE(2) acts through 3 G protein - coupled receptors (EP2, EP3, and EP4) that activate both adenyl cyclase and K-ATP channels. We explored these pathways to identify the mechanisms responsible for the increased sensitivity of the immature ductus to PGE(2). Methods and Results - We measured EP receptor content (mRNA and protein), receptor binding, cAMP production, and isometric tension in rings of ductus taken from immature (65% gestation) and mature (95% gestation) sheep and baboon fetuses. Ductus relaxation and cAMP generation were augmented in response to selective EP receptor agonists in the immature ductus. 8-Br-cAMP, a stable cAMP analogue, produced greater relaxation in the immature ductus. In the presence of a selective protein kinase A inhibitor, Rp-8-CPT cAMPS, the developmental differences in sensitivity to PGE(2) could no longer be demonstrated. EP2, EP3, and EP4 receptor densities were higher in immature ductus, despite similar receptor mRNA and protein contents at the 2 gestational ages. In contrast, forskolin and NaF, direct activators of adenyl cyclase and G(s), respectively, elicited comparable increases in cAMP in both age groups. K-ATP channel inhibition also had similar effects on PGE(2)-induced relaxation in both age groups. Conclusions - Two mechanisms explain the increased sensitivity of the immature ductus to PGE(2): ( 1) increased cAMP production because of increased binding of PGE(2) to the individual EP receptors and ( 2) increased potency of cAMP on protein kinase A - regulated pathways.