The goal of this proposal is to reveal the novel contribution of actin polymerization to regulating patency and closure of the ductus arteriosus (DA). The DA is an essential O2-sensitive fetal blood vessel that provides a right-to-left shunt of blood away from the unventilated fetal lungs and to the aorta. During the vascular transition at birth, the DA closes rapidly to provide blood flow and oxygenation at the lungs. In the pre-term neonate, mechanisms governing DA O2-induced contraction and remodeling do not produce strong vasoconstriction and the DA does not close properly in up to 70% of pre-term neonates. This leads to numerous neonatal pathologies associated with a patent DA (PDA) requiring unwanted intervention. Our current understanding of cellular signaling pathways regulating DA tone and closure is incomplete. Studies of O2-induced DA contraction have focused DA relaxation by prostaglandins and nitric oxide or regulation of DA contraction at the myosin light chain. This view ignores the important unexplored role of actin polymerization in regulating DA smooth muscle tone. Actin polymerization is involved in all regulating smooth muscle tones studied to date and has recently been found to be involved in chronic hypoxic pulmonary hypertension. Preliminary myograph studies in our lab have begun to reveal actin polymerization as an importance pathway for regulating DA tone and closure. Both basal tone and tension generated during O2-induced contraction decrease in the presence of actin polymerization antagonists. Additionally, the gradual tension DA increase observed in response to extended exposure to elevated O2 is absent when inhibiting actin polymerization. These data strongly support a novel role for actin polymerization in regulating DA constriction and closure at birth. The proposed work will test the hypothesis that actin polymerization is a major regulator of tone and constriction in the preterm and full-term DA. Aim 1 will determine the extent actin polymerization plays in DA tone, closure, and constriction in preterm and term embryos. Aim 2 will determine the cellular signaling pathways regulating actin polymerization in the O2-sensitive DA. Molecular and pharmacological techniques will be used to determine the role of actin polymerization in regulate DA patency and closure in the chicken DA during the transition from in ovo to ex ovo life. The chicken is an excellent model for studying DA physiology because all contractile pathways studied to date are shared with the mammalian DA. Given the importance of actin polymerization in regulating other vascular smooth muscle contraction and tone, completion of the proposed aims will reveal actin polymerization as a previously unidentified factor regulating the DA, thus providing novel targets for PDA treatment. Furthermore, findings from this study will further our understanding of the role actin polymerization plays in other O2-sensitive vessels, such as pulmonary arteries.