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

Acute ruminal acidosis is a major metabolic disorder in agriculturally important ruminants, such as cattle and sheep. A common symptom of acute 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, twelve 9-month-old sheep were randomly assigned to two groups, six sheep per group. Sheep in 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 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. Acute ruminal acidosis in the sheep fed the concentrate diet was confirmed by ruminal pH < 5.0, elevated levels of free lipopolysaccharides (LPS), L- and D-lactic acids, and symptoms like loss of appetite and diarrhea. Concentrations of acetic, propionic, and butyric acids in rumen fluid and plasma were lower in the acidotic sheep (P < 0.05). Plasma concentrations of LPS, L-lactic acid and D-lactic acid were greater 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 greater in the acidotic sheep compared to controls (P < 0.05). These changes indicated systemic inflammation, which was associated with increased rumen fluid and plasma concentrations of LPS and L- and D-lactic acids. In the in vitro experiments, peripheral blood mononuclear cells (PBMC) and hepatocytes isolated from normal sheep were cultured with or without LPS, L-lactate or D-lactate. 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). 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 mRNA (P < 0.05), and L-lactate decreased the expression of IL6 mRNA (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.