Effect of 4-AP on the basal tone of pulmonary artery rings from control (A) and PPHN lambs (B). Data are mean±SD for 15 rings from 5 animals each for 4-AP alone (-◆-), tiron+4AP (-○-), xanthine+xanthine oxidase+4-AP (-■-), peroxynitrite+4AP (-△-) and menadione+4-AP (-◇-). * Indicates P<0.05 from -5M conc. of 4-AP and § from 4-AP alone. The increase in basal tone in response to 4-AP in control rings was attenuated by xanthine+xanthine oxidase, menadione, and peroxynitrite (A). The attenuated response to 4-AP in PPHN rings was improved by superoxide scavenger, tiron and not altered further by xanthine+xanthine oxidase, menadione, or peroxynitrite (B).