Hypoxia stimulates prostacyclin synthesis in newborn pulmonary artery endothelium by increasing cyclooxygenase-1 protein

In newborn lambs, pulmonary prostacyclin (PGI₂) production increases acutely in response to low oxygen. We tested the hypothesis that decreased oxygenation directly stimulates PGI₂ synthesis in arterial segments and cultured endothelial cells from newborn lamb intrapulmonary arteries. In segments studied at PO₂ of 680 mm Hg, the synthesis of PGI₂ exceeded prostaglandin E₂ (PGE₂) by 73%. Endothelium removal lowered PGI₂ by 77% and PGE₂ by 66%. At low oxygen tension (PO₂, 40 mm Hg), PGI₂ and PGE₂ synthesis rose by 96% and 102%, respectively. Similarly, in endothelial cells studied at PO₂ of 680 mm Hg, the synthesis of PGI₂ exceeded PGE₂ by 50%, and at low oxygen tension both PGI₂ and PGE₂ increased (89% and 64%, respectively). Endothelial cell PGI₂ synthesis maximally stimulated by bradykinin, A23187, or arachidonic acid was also increased at low PO₂ by 50%, 66%, and 48%, respectively. PGE₂ synthesis was similarly altered, increasing by 33%, 37%, and 41%, respectively. In contrast, lowering oxygen had minimal effect on PGI₂ and PGE₂ synthesis with exogenous PGH₂, which is the product of cyclooxygenase. Immunoblot analyses revealed that there was a 2.6-fold greater abundance of cyclooxygenase-1 protein at PO₂ of 40 versus 680 mm Hg, and the increase at lower oxygen tension was inhibited by cycloheximide. The cyclooxygenase-2 isoform was not detected. Thus, attenuated oxygenation directly stimulates PGI₂ and PGE₂ synthesis in intrapulmonary arterial segments and endothelial cells from newborn lambs. This process is due to enhanced cyclooxygenase activity related to increased abundance of the cyclooxygenase-1 protein, and this effect may be due to increased synthesis of the enzyme protein.