CARBON MONOXIDE-INDUCED RELAXATION OF THE DUCTUS-ARTERIOSUS IN THE LAMB - EVIDENCE AGAINST THE PRIME ROLE OF GUANYLYL CYCLASE

1 We have previously found that carbon monoxide (CO) potently relaxes the lamb ductus arteriosus and have ascribed this response to inhibiti on of a cytochrome P450-based mono-oxygenase reaction which sustains c ontractile tone. Our proposal, however, has been questioned on the evi dence of findings in other blood vessels implicating the guanylyl cycl ase-based relaxing mechanism as the target for CO. To investigate this issue further, we have carried out experiments in the isolated ductus from near-term foetal lambs and have examined the effect of CO concom itantly on muscle tone and cyclic GMP content, both in the absence and presence of guanylyl cyclase inhibitors, or during exposure to monoch romatic light at 450 nm. 2 CO (65 mu M) reversed completely, or nearly completely, the tone developed by the vessel in the presence of oxyge n (30%) and indomethacin (2.8 mu M). Cyclic GMP content tended to incr ease with the relaxation, but the change did not reach significance. S odium nitroprusside (SNP), a NO donor, mimicked CO in relaxing the duc tus. Contrary to CO, however, SNP caused a marked accumulation of cycl ic GMP with levels being positively correlated with the relaxation. 3 Methylene blue (10 mu M) reduced marginally the CO relaxation, whilst LY-83583 (10 mu M) had an obvious, albeit variable, inhibitory effect. Basal cyclic GMP content was lower in tissues treated with either com pound and rose upon exposure to CO. However, the levels attained were still within the range of values for tissues prior to any treatment. F urthermore, the elevation in cyclic GMP was not related to the magnitu de of the CO relaxation. 4 Illumination of the ductus with monochromat ic light at 450 nm reversed the CO relaxation and any concomitant incr ease in cyclic GMP. In the absence of CO, light by itself had no effec t. 5 Ductal preparations with only muscle behaved as the intact prepar ations in reacting to CO, both in the absence and presence of guanylyl cyclase inhibitors, or during illumination. 6 We conclude that the pr imary action of CO in the ductus arteriosus is not exerted on the guan ylyl cyclase heme and that cyclic GMP may only have an accessory role in the relaxation to this agent. This finding reasserts the importance of a cytochrom P450-based mono-oxygenase reaction for generation of t one and as a target for CO in the ductus.