Isolation of rat DA and Ao. Rats were mated for 24 hours and the end of this period was marked as day zero of pregnancy. Near-term fetuses (gestational age, 19 days) were delivered by Cesarean section from a normoxic or hyperoxic dam, while newborns were used 3 hours after vaginal delivery. Pregnant rats were made hyperoxic to reproduce the neonatal condition in utero. They breathed 100% oxygen inside a box for 3 hours. All animals were killed by cervical dislocation and, to isolate the DA and Ao (arch and thoracic sections), were secured inside a chamber filled with ice-cold Krebs medium through which was bubbled a gas mixture with 2.5 (fetus) or 12.5% (newborn) oxygen. Special care was taken to obtain the blood vessels free of any adhering tissue.
