Vasoactivity of magnesium sulfate in chicken ductus arteriosus.

Prenatal treatment with magnesium sulfate (MgSO4) has neuroprotective effects in very preterm infants but its use has been associated with an increased rate of patent ductus arteriosus (DA). MgSO4 is a vasodilator and thus may exert a direct relaxant effect in the DA. We aimed to investigate the vasoactive effects of MgSO4 in the DA using the chicken embryo as experimental model. DA rings from 15-d (E15), 17-d (E17) and 19-d (E19) chicken embryos (total incubation: 21-d) were mounted in a wire myograph for isometric tension recordings. Exposure of DA rings to 21% O2 induced a tonic contraction which was relaxed by MgSO4 (2.4 - 7.2 mmol L-1) in a concentration-dependent manner (mean maximal relaxation E19: 51.4%, SE 6.3; EC50: 3.5 mmol L-1, SE 0.7). The relaxation evoked by MgSO4 was not significantly different between E15, E17 and E19 DA and was not affected by removal of the endothelium or by the presence of the nitric oxide synthase inhibitor L-NAME, the soluble guanylate cyclase inhibitor ODQ, or the cyclooxygenase inhibitor indomethacin. In contrast, when the DA rings were incubated in Ca2+-free solution, or in the presence of inhibitors of L-type Ca2+ channels (nifedipine), or large-conductance Ca2+-activated K+ (BKCa) channels (iberiotoxin), MgSO4-induced relaxation was impaired. Preincubation of the DA rings with MgSO4 concentrations ranging from 0 to 6.0 mmol L-1 did not significantly affect O2-induced contraction that was only impaired by a concentration of 7.2 mmol L-1. In conclusion, MgSO4 induced endothelium-independent relaxation of chicken DA and this relaxation appeared to be mediated through stimulation of BKCa channels and blockade of transmembrane flux of extracellular Ca2+. However, O2-induced DA contraction was only impaired by suprapharmacological concentrations of MgSO4 (> 6.0 mmol L-1). Therefore, our data suggest that the higher incidence of patent DA in preterm infants exposed to MgSO4 is unlikely to be due to a direct pharmacological effect of the drug on the DA.

Prenatal exposure to hyperoxia modifies the thromboxane prostanoid receptor-mediated response to H2O2 in the ductus arteriosus of the chicken embryo.

O2 tension plays a critical role in the control of prenatal patency and postnatal closure of the ductus arteriosus (DA). We hypothesized that exposure of chicken embryos to hyperoxia alters the morphology and function of DA. Hyperoxia was induced by incubating fertilized eggs at 60% O2 from day 15 to 19 of the 21-d incubation period. DA reactivity (assessed by wire myography), morphometry and mRNA expression of antioxidant enzymes were studied on day 19. Hyperoxic incubation neither affected embryonic growth nor induced signs of DA constriction or changed the mRNA expression of superoxide dismutase and catalase. The contractions induced by O2 (21%), KCl, 4-aminopyridine, phenylephrine, and endothelin-1 and the relaxations induced by acetylcholine (ACh), sodium nitroprusside, isoproterenol, and hydroxyfasudil were similar in DA from embryos incubated under normoxic or hyperoxic conditions. In contrast, hyperoxic incubation impaired the thromboxane prostanoid (TP) receptor-mediated contractions evoked by U46619, 15-E2t-Isoprostane and high concentrations (≥3 µM) of ACh. Exogenous hydrogen peroxide (H2O2) evoked endothelium-dependent contraction in the normoxic DA and endothelium-dependent relaxation in the hyperoxic group. The presence of the TP receptor antagonist SQ 29548 unmasked a relaxant response to H2O2 in the normoxic DA and the cyclooxygenase (COX) inhibitor indomethacin blocked H2O2-induced contraction (in the normoxic group) and relaxation (in the hyperoxic group). Altogether our functional data suggest that, in the chicken DA, exogenous H2O2 induces the release of endothelium-derived COX metabolite(s) with contractile and relaxant properties. Under normal conditions H2O2-induced contraction prevails and relaxation is unmasked after pharmacological or functional (i.e.hyperoxia) TP receptor impairment.

Developmental changes in endothelium-dependent relaxation of the chicken ductus arteriosus.

We tested the hypothesis that endothelium-dependent relaxation in the chicken ductus arteriosus (DA) is developmentally regulated. Isolated DA rings from 15-, 19- and 21-day-old (externally pipped) chicken embryos relaxed to acetylcholine (ACh). This relaxation was unaffected by indomethacin but impaired by endothelium removal, by the NO synthase inhibitor L-NAME, and by the soluble guanylate cyclase inhibitor ODQ, suggesting the involvement of NO. This NO production was confirmed with the fluorescent probe DAF-2DA. The combination of apamin and charybdotoxin with L-NAME produced a further inhibition of ACh-induced relaxation, suggesting the participation of a putative EDHF. In the 21-day DA, the relaxations induced by ACh and sodium nitroprusside (SNP) were markedly reduced and scanning electron microscopy demonstrated an irregular endothelial lining with protrusion and detachment of endothelial cells. The relaxations induced by BAY 41-2272 and 8-Br cGMP were not affected by age. When compared with 5%, lower (0%) and higher (21, 95%) O(2) concentrations impaired ACh-induced relaxation. In summary, we found that ACh induces endothelium-dependent relaxation of the chicken DA and that NO and EDHF are involved in this response. During chicken DA closure, endothelial cells undergo morphologic and functional alterations that result in the lack of endothelium-dependent relaxation.