Liver metabolism of Holstein cows is altered by nutrient supply but not by lipopolysaccharide in vitro

Abstract Text:

Recent work suggests liver function is reduced during inflammation.  To our knowledge, no studies have examined whether increased amino acid or propionate supply improves hepatic function during inflammation and how this is altered by stage of lactation.  The objective of the current study was to characterize the effect of nutrient supply and lipopolysaccharide (LPS) challenge on hepatic intermediate metabolism for cows in early and mid-lactation by employing gas chromatography–mass spectrometry (GC-MS) and stable isotope tracer ([¹³C₃] pyruvate).  Liver samples were collected via biopsy from healthy early (n = 6, < 21 DIM) and mid-lactation (n = 6, < 110 DIM) Holstein cows.  Liver slices were incubated for 2 h at 37°C in 3 mL of Krebs-Ringer bicarbonate buffer containing 2 mM of a 50:50 mix of labeled and unlabeled [¹³C₃]pyruvate with addition of either phosphate buffered saline (control), 2 mM propionate, or 2 mM amino acids, with or without addition of LPS (200 ng/mL).  Krebs cycle intermediates were determined by GC-MS by monitoring alanine, aspartate, and glutamate, which are in metabolic equilibrium with Krebs cycle intermediates. Data were analyzed as a randomized block design in a 3×2×2 factorial arrangement. Significance was declared at P < 0.05.  Overall, LPS challenge had minimal effects on intermediary metabolism. Regardless of nutrient supply, the ¹³C kinetics demonstrated that the liver of early lactation cows exhibits higher relative activity of phosphoenolpyruvate carboxykinase (PEPCK), a tendency for greater flux of pyruvate to oxaloacetate (P = 0.07, 82 vs. 74%), and a lower relative activity of pyruvate dehydrogenase (PDH) compared to pyruvate carboxylase (PC) when compared to cows in mid-lactation.  Regardless of stage of lactation, propionate and amino acids increased the proportion of acetyl-CoA derived from pyruvate (13 and 10%, respectively) compared to control (7%), with no difference in the flux of oxaloacetate that derived from pyruvate.  Moreover, propionate increased the relative activity of PDH compared to PC.  In conclusion, results suggest that the LPS endotoxin exposure in vitro does not impair liver metabolic function and that cows in early lactation had a greater gluconeogenic capacity than cows in mid-lactation.

Keywords:

Cows, liver, nutrient flux