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

在高产奶牛中，生理和代谢过程对于适应分娩前后增加的代谢应激是必要的。为了满足牛奶合成的高能量需求，奶牛动员了大量的身体储备，特别是脂肪和肌肉组织。产后脂肪组织的动员取决于产犊前的脂肪储备量。在身体脂肪动员的过程中，不仅脂肪酸，而且其他亲脂性化合物如类固醇，可以释放到循环中。除了对生殖器官及其激素调节的影响外，这些物质还可能与代谢过程发生重大相互作用，特别是在分娩前后和哺乳期开始时。因此，脂肪组织合成或转化的类固醇的局部作用可能与整个代谢相互作用。除了类固醇及其代谢产物，胆汁酸也是由胆固醇合成的。胆汁酸参与脂质代谢，因此围产期脂解增加可导致胆汁酸代谢失衡。到目前为止，尚未研究过泌乳早期奶牛体内储备动员增加对胆汁酸代谢的影响，但可能会影响葡萄糖、脂质、脂蛋白以及整个能量代谢。本课题旨在研究高产奶牛的类固醇激素和胆汁酸代谢。在此过程中，将由产犊前更大的身体状况引起的从妊娠到哺乳的脂肪动员增加的影响与"正常”哺乳诱导的动员进行比较。除了用于表征代谢的经典畜牧学变量和血液参数外，我们还旨在评估血液和脂肪组织以及循环胆汁酸中的类固醇激素浓度（通过“组学”方法）。此外，将在脂肪组织和肝脏中研究类固醇生成酶和胆汁酸酶及其受体和转运蛋白的表达。关于高产奶牛妊娠晚期到泌乳早期过渡期间脂肪分解的增加，我们的目的是阐明类固醇激素（和代谢产物）和胆汁酸的生理相关性，重点是脂肪组织。本文产生的结果可能有助于制定改善奶牛生育力和健康方面的策略。

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.