Effects of oxygen on active tension in the Immature ductus arteriosus. Ductus were studied in the presence or absence of Panel A: Krebs solution (Control, ◻, n=13), K+-Krebs solution (∎, n=13), calcium-free Krebs solution (▵, n=13), or calcium-free/High K+-Krebs solution (▾, n=13); Panel B: nifedipine (◯, n=12); Panel C: BAY K8644 (●, n=12). * p<0.05, experimental condition versus Control (◻). Ductus tension at 6, 15, 30 and 95% O2 differed significantly (p<0.05) from the tension at 2% O2, except for rings incubated with calcium-free Krebs solution, in calcium-free/High K+-Krebs solution, and in nifedipine where tensions at 6% O2 were not significantly different from those at 2% O2.