CYTOCHROME-P450 DURING ONTOGENIC DEVELOPMENT - OCCURRENCE IN THE DUCTUS-ARTERIOSUS AND OTHER TISSUES

Our previous investigations have implicated a cytochrome P450 mechanis m in the oxygen contraction of the ductus arteriosus and, by extension , in the closure of the vessel at birth. This study was undertaken in fetal and newborn sheep to characterize the ductal cytochrome and gath er insight into its operation. Other tissues, vascular and extravascul ar, were used as a reference. Benzo[a]pyrene hydroxylation (a marker f or the 3-methylcholanthrene-inducible isozyme) and aminopyrine N-demet hylation (a marker for the glucocorticoid-inducible isozyme) had insig nificant activity in the ductus and aorta from fetal sheep, and no inc rease was noted after exposing the animal in utero to beta-naphthoflav one or dexamethasone, both alone and in combination with phenobarbital . However, dexamethasone and, particularly, dexamethasone plus phenoba rbital produced a variable constriction of the fetal ductus. No monoox ygenase activity was found in the naturally closing ductus of the newb orn. Conversely, both enzyme reactions were measurable in the fetal li ver, and they became more active after treatment with the inducers or at birth. Scanning immunoelectron microscopy of cultured endothelial a nd muscle cells from both ductus and aorta showed specific gold labell ing for the glucocorticoid-inducible cytochrome P450 only in ductal mu scle. By transmission electron microscopy, this immunoreactivity was l ocated along the sarcolemma and in the sarcoplasmic reticulum. These f indings indicate the presence in the ductus arteriosus of a cytochrome P450 belonging to the 3A subfamily. However, considering the uneven a ction of the inducers on enzyme activity in ductal tissue and muscle t one, the role of this cytochrome in closure of the vessel at birth rem ains to be ascertained.