Effects of long-term heat exposure on cholesterol metabolism and immune responses in growing pigs

Heat stress can greatly influence metabolic activities in pigs. In heat-stressed pigs, lipid metabolism was altered in a way which is distinct from animals experiencing the same level of feed restriction at thermoneutrality. Besides stress hormones, damaged intestinal barrier functions in heat-stressed pigs can alter insulin sensitivity by inducing systemic inflammation. This study was conducted to investigate the impacts of long-term exposure to an environmental temperature of 33 degrees C on lipid metabolism, tissue damages and cytokine productions in growing pigs. Twenty-seven Large White pigs (40.8 +/- 2.7 kg) were randomly allocated to the control (CON, n = 9; kept at 23 degrees C and fed ad libitum), heat-stressed (HS, n = 9; kept at 33 degrees C and fed ad libitum) and pair-fed groups (PF, n = 9; kept at 23 degrees C and fed the same amount as the HS group) for 21 days. Serum parameters were determined on d3, 7, 14, 21. Cholesterol and expression of genes associated with its metabolism were quantified in the liver on d21. The results showed serum biomarkers for liver functions, such as aspartate aminotransferase (AST, P = 0.514), lactate dehydrogenase (LDH, P = 0.792), total protein (TP, P = 0.379), and blood urea nitrogen (BUN, P = 0.997), did not differ among all treatments. But activities of serum alanine aminotransferase (ALT, P < 0.001) and alkaline phosphatase (ALP, P = 0.001) were decreased in the HS pigs from d7 to 21. Serum total cholesterol (TC), low-density lipoprotein-cholesterol (LDL-C) and triglycerides (TG) were temporally increased in the HS pigs on d3 (P < 0.05). Serum cytokines quantification revealed a decrease in IL-12 (P = 0.048) and IFN-gamma (P = 0.086) in the HS pigs compared with the other two groups. In the liver, LDLR (P = 0.018) and SREBP2 (P = 0.003) were down-regulated in the HS pigs, but HMGCR and acute phase proteins, HP, pig-MAP and SAA3, remained unchanged. For the PF pigs, serum cholesterol tended to be lower than the CON pigs on d21 (P = 0.068). Liver cholesterol was also reduced (P < 0.005) in the PF pigs; while LDLR (P = 0.003), SREBP2 (P = 0.006) and HMGCR (P = 0.024) were decreased. Liver HP was up-regulated in the PF pigs (P = 0.050). Collectively, exposure to 33 degrees C can influence cholesterol metabolism in growing pigs without causing severe tissue damages. Reduced feed intake did not completely explain those changes in the cholesterol metabolism during long-term heat exposure.