Further evidence implicating a cytochrome P-450-mediated reaction in the contractile tension of the lamb ductus arteriosus

The authors have recently provided evidence implicating the cytochrome P-450 system in the generation of contractile tension of the ductus arteriosus. To confirm this possibility, carbon monoxide (CO/O₂ ratio, 0.27) and cytochrome P-450 inhibitors [4-phenylimidazole; 14-isocyano,15-(methoxymethyleneoxy)-5Z,8Z,11Z-eicosatrienoic acid; 9-hydroxyellipticine; α-naphthoflavone] were tested on the isolated ductus arteriosus from mature fetal lambs equilibrated at low (4-26 mm Hg) or high (229-694 mm Hg) O₂ partial pressure (P(O₂)). Carbon monoxide completely relaxed intact vessel wall preparations and preparations consisting of only the muscle. Carbon monoxide relaxation was reversed by illumination with monochromatic light and the peak for the photoactivated contraction occurred at 450 nm. 4-Phenylimidazole (100 and 1,000 μM) was also a relaxant agent, and its action was manifest at both low and high P(O₂). Unlike 4-phenylimidazole, the isonitrile compound (5 μM) and 9-hydroxyellipticine (10 and 100 μM) were relaxant only at low P(O₂) and were also less potent. At the same P(O₂), α-naphthoflavone (10 μM) barely reduced ductal tension. Treatment of the ductus with either a combination of superoxide dismutase (60 or 150 U/ml) and catalase (40 or 1,000 U/ml) or mannitol alone (80 mM) failed to alter the steady-state tone at low P(O₂) and the contractile response to O₂. Arachidonic acid was tested on tissues pretreated with the dual cyclooxygenase and lipoxygenase inhibitor, BW775C (10 μM), and produced a weak relaxation at a concentration of 1 μM or higher. 5.6-Epoxytrienoic acid relaxed the untreated tissue, and its action was abolished by indomethacin (2.8 μM). Other arachidonic acid epoxides produced either a marginal relaxation (14,15-epoxytrienoic acid) or had no effect at all (8,9- and 11,12-epoxytrienoic acids). These findings confirm the existence in the lamb ductus arteriosus of a cytochrome P-450-linked mechanism sustaining muscle tone and possibly the contraction of O₂. The actual constrictor could not be identified with monooxygenase-derived product of arachidonic acid and remains to be characterized.