420 Both elevated lipopolysaccharide and D-lactic acid concentrations in blood contribute to systemic inflammation in acute ruminal acidosis.

Abstract Ruminal acidosis is a major metabolic disorder in agriculturally important ruminants, such as cattle and sheep. A common symptom of ruminal acidosis is systemic inflammation. This study aimed to identify the underlying causes of systemic inflammation during acute ruminal acidosis using both in vivo and in vitro experiments. In the in vivo experiments, 12, 9-month-old sheep were randomly assigned to two groups, 6 sheep per group. Sheep in one group (the acidotic group) were fasted for 24 h and then provided ad libitum access to a 1:1 mix of whole corn and wheat flour to induce acute ruminal acidosis. Sheep in the other group (the control group) were not fasted and had ad libitum access to alfalfa hay. Rumen fluid and blood samples were collected every 12 h, and all sheep were euthanized 48 h after the introduction of the concentrate diet for the liver tissue collection. Data were analyzed using a linear-mixed effects model, with treatment and time point as fixed effects and animal as a random effect. Acute ruminal acidosis in the sheep fed the concentrate diet was confirmed by ruminal pH values below 5.0, marked increases in ruminal concentrations of free lipopolysaccharides (LPS), L- and D-lactic acids, and symptoms such as loss of appetite and diarrhea. Concentrations of major volatile fatty acids (acetic, propionic and butyric acids) in the rumen and plasma were lower in the acidotic sheep compared to the controls (P < 0.05). Plasma concentrations of LPS, L-lactic acid and D-lactic acid were higher in the acidotic sheep (P < 0.05). Hepatic mRNA expression of haptoglobin (HP), serum amyloid a2 (SAA2), interleukin 1 alpha (IL1A) and interleukin 6 (IL6) as well as plasma concentrations of IL1A and IL6 were higher in the acidotic sheep compared to controls (P < 0.05). These differences demonstrated that the acidotic sheep developed systemic inflammation, which was associated with increased ruminal and plasma concentrations of LPS and L- and D-lactic acids. In the in vitro experiments, hepatocytes and peripheral blood mononuclear cells (PBMC) isolated from normal sheep were cultured with or without LPS, L-lactate or D-lactate. In hepatocyte cultures, LPS increased the expression of HP, SAA2, IL1A, IL1B, IL6, and tumor necrosis factor (TNF) mRNAs (P < 0.05), while D-lactate increased the expression of HP and IL6 mRNAs (P < 0.05), and L-lactate decreased the expression of IL6 mRNA (P < 0.05). In PBMC cultures, both LPS and D-lactate increased the expression of HP, SAA2, IL1A, IL1B and IL6 mRNAs (P < 0.05), while L-lactate increased the expression of HP mRNA only (P < 0.05). Collectively, these in vivo and in vitro findings support the conclusion that both increased concentrations of LPS and D-lactic acid contribute to systemic inflammation during acute ruminal acidosis.