Effects Of Chromium Supplementation On Lipid Metabolism And Immune Response In Heat-Stressed Dairy Cows

The aim of this study was to study the effects of chromium (Cr) picolinate supplementation on production performance, lipid metabolism, antioxidant function and immunity response of dairy cows under different hot conditions, and to select the optimized conditions for proliferation and apoptosis of bovine peripheral blood mononuclear cell (PBMC), then provide theoretical basis for prevention and control of heat stress and Cr supplementation in dairy cow production.Based on the temperature-humidity index (THI) values, the experiment was prearranged as3periods:low THI period (LTHI; THI=56.4±2.5), moderate THI period (MTHI; THI=73.9±1.7), and high THI period (HTHI; THI=80.3±1.0). The whole experiment period was60d. Twenty-four clinically healthy uniparous mid-lactation Holstein cows were selected and fed with the same basal diet. Twelve cows randomly selected were fed the basal diet with chromium picolinate (3.5mg Cr/head·d).Moderate and high THI caused experimental dairy cows in mild and moderate heat stress status. The data of production performance showed that period and Cr supplementation had no effect on dry matter intake and milk yield of lactating dairy cows. The number of dairy cow with higher milk somatic cell count (SCC) in heat stress condition increased.There were lower level of white blood cell count and blood glucose (GLU), and higher concentration of blood urea nitrogen (BUN) and creatinine (CRE) in the serum of midlactation cows under hot summer conditions. No differences were found in serum cholesterol (CHO), aspartate aminotransferase (AST), alanine aminotransferase (ALT) and γ-glutamyltransferase (GGT). The results indicate that heat stress inhibited the immune response in dairy cows and had no effect on the function of liver.Heat stress resulted in the increase of serum superoxide dismutase (SOD) activity and the decrease of serum malondialdehyde (MDA) concentration, while no significant changes in total antioxidant capacity (T-AOC). Our results imply that heat stress stimulates production of enzymatic antioxidants in dairy cows to ameliorate oxidative stress and maintain total antioxidant activity. The lower concentration of serum MDA in both MTHI and HTHI indicates that oxidative stress did not develop, which suggests that oxidative stress induced by hyperthermia may be related to the degree of heat stress and the efficacy of the antioxidant defense systems. At the same time, the increases in the concentrations of the pro-inflammatory cytokine tumor necrosis factor-a (TNF-α) and the anti-inflammatory cytokine interleukin (IL)-10were found in HTHI, indicating that oxidative stress did not contribute to the inflammatory response to HTHI. Western blot analysis showed that the expression of heat shock protein72(Hsp72) protein in milk somatic cells was higher in cows with a SCC>200,000cells/mL than in cows with a SCC<200,000cells/mL. Since heat stress had no effect on the expression of Hsp72in PBMC, we hypothesize that the observed increase in Hsp72expression in milk somatic cells can be attributed primarily to the udder epithelial cells.Chromium supplementation led to a significant reduction in the concentration of serum CHO and an elevation in the levels of serum IL-10and Hsp72. Western blot analysis showed an increase in the expression of inhibitor of nuclear factor kappa B alpha (IκBα) protein in the PBMC of heat-stressed cows supplemented with Cr. Our findings thus suggest that Cr regulate lipid metabolism and play an anti-inflammatory role in lactating cows under hot conditions during the summer.In order to reveal the apoptosis mechanism of bovine PBMC induced by high temperature, we conducted experiments in vitro and determined the optimal conditions for bovine PBMC proliferation response, and initially found the alteration of apoptosis ratio while PBMC exposed to40℃for different time.In summary, our data indicate that the inflammatory response occurring in dairy cows under HTHI conditions is not due to oxidative stress, and that Cr supplementation regulate lipid metabolism and plays an anti-inflammatory role in heat-stressed dairy cows by promoting the release of Hsp72, inhibiting the degradation of IκBα protein, and regulating the expression of pro-inflammatory cytokines.