Effect of Continuous Lipopolysaccharide Induction on Oxidative Stress and Heart Injury in Weaned Piglets

After weaning, piglets no longer consume breast milk, and their immune system is not yet fully developed. At this time, if weaned piglets are infected with E. coli, their subsequent growth will be seriously affected. In the present study, 48 healthy 28-day-old weaned piglets (6.65 ± 1.19 kg, Duroc × Landrace × Large White) were randomly divided into an LPS group and control group. Piglets in the LPS group were intraperitoneally injected with an LPS solution (LPS was dissolved in sterile saline to form a solution of 100 μg/mL and injected at a dose of 1 mL per kilogram of body weight) for 13 consecutive days. Piglets in the control group were injected with the same volume of sterile saline. On days 1, 5, 9, and 13 of the experiment, six piglets from each group were randomly selected for dissection, the blood and heart samples were collected, and then cardiac function-related indicators were detected. A portion of the heart tissue was fixed in 4% paraformaldehyde and further used to make paraffin sections; then, hematoxylin-eosin (H&E) staining was performed. Masson staining was used to detect the changes in collagen fibers in the hearts. The other parts of the heart tissues were frozen in liquid nitrogen and stored in a refrigerator at -80 °C for the detection of tissue antioxidant indices. The mRNA expression levels of the toll-like receptor 4 (TLR4) signaling pathway, transforming growth factor-β (TGF-β) signaling pathway, and inflammatory cytokines in heart tissues were detected by real-time PCR. The results showed that catalase (CAT) and superoxide dismutase (SOD) contents in the heart tissue homogenates increased significantly on days 1 and 5 in LPS-induced piglets (p < 0.01, p < 0.05), while total antioxidant capacity (T-AOC) and glutathione peroxidase (GSH-Px) contents decreased significantly on day 5 (p < 0.05). On day 5, the contents of serum cardiac function indicators lactate dehydrogenase (LDH), creatine kinase isoenzymes (CK-MB), and cardiac troponin I (cTn-I) were significantly increased in LPS-induced piglets (p < 0.01). On the 1st and 5th days, the heart tissue showed obvious pathological damage, which was manifested as the disordered arrangement of myocardial fibers, depression of myocardial cells, infiltration of inflammatory factors, congestion of capillaries, and significant increase in cardiac collagen fibers. On the 1st day, the mRNA expression levels of tumor necrosis factor-alpha (TNF-α) and interleukin 6 (IL-6) were significantly increased in LPS-induced piglets with heart injury (p < 0.01). On the 5th day, the mRNA expression levels of the TLR4 signaling pathway [TLR4, myeloid differentiation primary response gene 88 (MyD88), nuclear factor kappa-B (NF-κB)], TNF-α, and interleukin 10 (IL-10) were also significantly increased in LPS-induced piglets with heart injury (p < 0.01, p < 0.05). The mRNA expression levels of the TGF-β signaling pathway (TGF-β, Smad2, and Smad4) in cardiac fibrosis-related genes were significantly increased on days 5 and 9 (p < 0.01, p < 0.05). The mRNA expression levels of Smad3 and Smad7 in cardiac fibrosis-related genes were also significantly increased on day 9 (p < 0.01). These results indicate that oxidative stress occurs in the heart tissue of LPS-induced piglets on the 1st and 5th days, leading to cardiac tissue damage. However, on the 9th and 13th days, the degree of heart damage in the piglets was less than that on the 1st and 5th days, which may be due to the tolerance of piglets' tissues and organs because of multiple same-dose LPS stimulations.