DDAH1 and bronchopulmonary dysplasia: RT-qPCR results indicated that, in comparison to air group, miR-21 expression was upregulated and DDAH1 expression was downregulated in lung tissues of BPD rats (1.00 ± 0.03 vs 2.26 ± 0.21, 1.00 ± 0.03 vs 0.39 ± 0.03, respectively, p < 0.01) (Figure 6a and b).