Effects of a lactation-induced and body condition-dependent mobilization of body reserves on steroid hormone and bile acid metabolism in high-yielding dairy cows

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In high-yielding dairy cows, physiological and metabolical processes are necessary for adapting to the increased metabolic stress around parturition. In order to meet the high-energy requirements for milk synthesis, cows mobilize large amounts of body reserves particularly from adipose and muscle tissue. The mobilization of adipose tissue post partum depends on the amount of fat reserves before calving. In the course of body fat mobilization, not only fatty acids but also other lipophilic compounds such as steroids, can be released into the circulation. Besides effects on reproductive organs and their hormonal regulation, these substances might significantly interact with metabolic processes particularly around parturition and at the onset of lactation. Local effects of synthesized or transformed steroids from adipose tissue might thus interact with the whole metabolism. Besides steroids and their metabolites, bile acids are synthesized from cholesterol. Bile acids are involved in lipid metabolism, therefore an increased peripartal lipolysis can lead to an imbalance of bile acid metabolism. The impact of an increased mobilization of body reserves in early lactating dairy cows on bile acid metabolism has not been investigated so far, however, glucose-, lipid-, lipoprotein- as well as the whole energy metabolism could be affected. In the current project, we aim to investigate the steroid hormone and bile acid metabolism from high-yielding dairy cows. In doing so, the impact of increased fat mobilization from pregnancy to lactation, provoked by a greater body condition before calving, will be compared to a „normal“ lactation-induced mobilization. Besides the classical zootechnical variables and blood parameters for characterizing the metabolism, we aim to assess the steroid hormone concentrations in blood and adipose tissue as well as circulating bile acids (via an „omics“ approach). Furthermore, the expression of steroidogenic and bile acid enzymes as well as their receptors and transporters will be investigated in adipose tissue and liver. Regarding the increased lipolysis during the transition of late pregnancy to early lactation in high-yielding dairy cows, we aim to elucidate the physiological relevance of steroid hormones (and metabolites) and bile acids with focus on adipose tissue. Results generated herein might help to develop strategies to improve cows` fertility and health aspects.