Adaptations of Glucose and Long-Chain Fatty Acid Metabolism in Liver of Dairy Cows during the Periparturient Period

Tremendous metabolic and endocrine adjustments must be made as dairy cows move from late gestation to early lactation. Requirements for glucose and metabolizable energy increase two- to threefold from 21 d before to 21 d after parturition. The liver must adapt quickly to provide the increased glucose needed to support high milk production, and to process the flood of nonesterified fatty acids taken up from extensive mobilization of adipose triglycerides. While the end results of these adaptations are well known, much less is known about the cellular and molecular mechanisms underpinning hepatic adaptation to lactation. Increases in metabolic activity per gram of liver tissue, not just increased liver mass, are responsible for increased metabolism. Compared with activities present at 21 d before parturition, the capacity of liver tissue isolated at 1 d postpartum to convert alanine (an important glucogenic amino acid) to glucose increases more on a percentage basis than does gluconeogenic capacity from propionate. Likewise, hepatic abundance of mRNA for pyruvate carboxylase increases around calving, whereas mRNA for phosphoenolpyruvate carboxykinase does not. These changes in gluconeogenic enzymes suggest that amino acids from body and feed protein may be critically important sources of glucose for peripartal cows. Hepatic tissue from cows 1 d postpartum has greater rates of palmitate esterification, total and peroxisomal beta-oxidation of palmitate, and activity of mitochondrial carnitine palmitoyltransferase than hepatic tissue from the same cows 21 d prepartum. Prepartal nutrition has been shown to modulate some of these metabolic adaptations in the liver. Effects of hormones and cytokines that mediate adaptive responses to environmental and infectious stressors (or the lack of “cow comfort”) have not been investigated. Techniques of modern biochemistry promise to further our understanding of the mechanisms of metabolic adaptation during the peripartal period, and to quantify the effects of nutrition and environment during pre- and postpartum periods on hepatic glucose and lipid metabolism.