SUMMARYThe purpose of this study was to determine if proinflammatory cytokines are up-regulated during reperfusion following sublethal ischaemia, and whether concurrent up-regulation of antioxidant enzymes occurs. Open-chest rats were subjected to 15 min of ischaemia followed by 1 or 3 h reperfusion (R). Myocardium from the ischaemic zone showed a significantly higher (P < 0 . 01) generation of thiobarbituric acid-reactive substances at 1 h and 3 h R. Northern blots showed a weak signal in controls for IL-6 mRNA (0 . 13 Ϯ 0 . 01); this was elevated to 0 . 68 Ϯ 0 . 12 at 1 h and 0 . 69 Ϯ 0 . 10 at 3 h R. Neither IL-1b nor tumour necrosis factor-alpha (TNF-a) were detectable in controls. IL-1b rose to 0 . 78 Ϯ 0 . 07 at 1 h and 0 . 51 Ϯ 0 . 08 at 3 h R, and TNF-a rose to 0 . 69 Ϯ 0 . 10 at 1 h and 0 . 38 Ϯ 0 . 15 at 3 h R. Western blotting showed no signals in the control, but readily detectable signals at 1 h R; these remained high (IL-6) or decreased (IL-1b and TNF-a) at 3 h R. mRNA analysis for antioxidant enzymes revealed a weak signal in controls for catalase (CAT; 0 . 16 Ϯ 0 . 08), glutathione peroxidase (GSH-Px; 0 . 15 Ϯ 0 . 06) and superoxide dismutase (SOD; 0 . 21 Ϯ 0 . 05). After 1 h R, levels increased significantly for CAT (0 . 46 Ϯ 0 . 10; P < 0 . 025) and GSH-Px (0 . 51 Ϯ 0 . 13; P < 0 . 01), but remained similar to controls for SOD (0 . 26 Ϯ 0 . 15). At 3 h R the mRNA levels were significantly elevated for the three enzymes (CAT 0 . 48 Ϯ 0 . 13; GSH-Px 0 . 47 Ϯ 0 . 10; SOD 0 . 54 Ϯ 0 . 08). We conclude that mRNA for proinflammatory cytokines is expressed early in reperfusion, and that the proteins are present in heart tissue. Also, reperfusion is associated with rapid expression of genes for antioxidant enzymes, which may enhance reactive oxygen intermediate (ROI) scavenging.