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1 Prenatal patency of the ductus arteriosus is maintained by prostaglandin (PG) E 2 , conceivably in concert with nitric oxide (NO). Local PGE 2 formation is sustained by cyclooxygenase-1 (COX1) and cyclooxygenase-2 (COX2), a possible exception being the mouse in which COX1, or both COXs, are reportedly absent. Here, we have examined the occurrence of functional COX isoforms in the nearterm mouse ductus and the possibility of COX deletion causing NO upregulation. 2 COX1 and COX2 were detected in smooth muscle cells by immunogold electronmicroscopy, both being located primarily in the perinuclear region. Cytosolic and microsomal PGE synthases (cPGES and mPGES) were also found, but they occurred diffusely across the cytosol. COX1 and, far more frequently, COX2 were colocalised with mPGES, while neither COX appeared to be colocalized with cPGES. 3 The isolated ductus from wild-type and COX1À/À mice contracted promptly to indomethacin (2.8 mm). Conversely, the contraction of COX2À/À ductus to the same inhibitor started only after a delay and was slower. 4 N G -nitro-l-arginine methyl ester (l-NAME, 100 mm) weakly contracted the isolated wild-type ductus. Its effect, however, increased three-to four-fold after deleting either COX, hence equalling that of indomethacin. 5 In vivo, the ductus was patent in all mice foetuses, whether wild-type or COX-deleted. Likewise, no genotype-related difference was noted in its postnatal closure. 6 We conclude that the mouse ductus has a complete system for PGE 2 synthesis comprising both COX1 and COX2. The two enzymes respond differently to indomethacin but, nevertheless, deletion of either one results in NO upregulation. PGE 2 and NO can function synergistically in keeping the ductus patent.

Background and purpose: Prenatal patency of ductus arteriosus is maintained by prostaglandin (PG) E 2 , possibly along with nitric oxide (NO) and carbon monoxide (CO), and cyclooxygenase (COX) deletion upregulates NO. Here, we have examined enzyme source and action of NO for ductus patency and whether NO and CO are upregulated by deletion of, respectively, heme oxygenase 2 (HO-2) and COX1 or COX2. Experimental approach: Experiments were performed in vitro and in vivo with wild-type and gene-deleted, near-term mouse fetuses. Key results: N G -nitro-L-arginine methyl ester (L-NAME) contracted the isolated ductus and its effect was reduced by eNOS, but not iNOS, deletion. L-NAME contraction was not modified by HO-2 deletion. Zinc protoporphyrin (ZnPP) also contracted the ductus, an action unaffected by deletion of either COX isoform. Bradykinin (BK) relaxed indomethacin-contracted ductus similarly in wild-type and eNOSÀ/À or iNOSÀ/À. BK relaxation was suppressed by either L-NAME or ZnPP. However, it reappeared with combined L-NAME and ZnPP to subside again with K þ increase or K þ channel inhibition. In vivo, the ductus was patent in wild-type and NOS-deleted fetuses. Likewise, no genotype-related difference was noted in postnatal closure. Conclusions and implications: NO, formed mainly via eNOS, regulates ductal tone. NO and CO cooperatively mediate BKinduced relaxation in the absence of PGE 2 . However, in the absence of PGE 2 , NO and CO, BK induces a relaxant substance behaving as an endothelium-derived hyperpolarizing factor. Ductus patency is, therefore, sustained by a cohort of agents with PGE 2 and NO being preferentially coupled for reciprocal compensation.

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