NRG‐1 down‐regulates the expression of NOX4 by activating ERK1/2. The mRNA expression levels of Nox4 (A), Gpx‐1 (B), and Cat (C) were detected using quantitative real‐time polymerase chain reaction. (D) Representative paraffin‐embedded cardiac sections subjected to immunohistochemical staining to detect NOX4 at the risk area. The cellular nuclei are shown in blue, whereas NOX4‐positive areas are shown in brown. E. Representative Western blots showing the levels of NOX4, P‐ERK1/2, T‐ERK1/2, GPX‐1 and GAPDH. Semi‐quantification of NOX4 level (F), the P‐ERK1/2 density/T‐ERK1/2 density ratio (G), and GPX‐1 level (H). The expression levels of target proteins were normalized to those of GAPDH Data are represented as mean ± standard error of mean (n = 6; *P < .05, **P < .001 vs. IR). CON, sham‐operated control; IR, ischaemia‐reperfusion; NRG‐1, IR + NRG‐1