Ponchon B, Zhao X, Ollier S, Lacasse P
Department of Animal Science, McGill University, Sainte-Anne-de-Bellevue, QC, H9X 3V9, Canada.
Sherbrooke Research and Development Centre, Agriculture and Agri-Food Canada, Sherbrooke, QC, J1M 0C8, Canada.
J Dairy Sci. 2017 Feb;100(2):1521-1534. doi: 10.3168/jds.2016-11490. Epub 2016 Dec 21.
The objectives of this study were to determine the role of glucocorticoids in the regulation of prolactin (PRL) release induced by mammary gland stimulation and to investigate whether the milk depression induced by glucocorticoids in dairy cows is due to a decrease in PRL release. In experiment 1, 8 dairy cows were used in a 4 × 4 Latin square design. Four hours after the morning milking, the cows received 1 of the following treatments: (1) a 5-min manual stimulation of the mammary gland; (2) an i.v. injection of 1 mg of dexamethasone; (3) 2 infusions of 2.5 g of metyrapone (an inhibitor of cortisol biosynthesis) in the omasum 4 and 2 h before a 5-min stimulation of the mammary gland; or (4) no treatment. Sixty minutes later, the mammary gland of each cow was stimulated for 5 min. Blood samples were collected from 20 min before to 120 min after the start of the treatment. When the mammary gland was stimulated twice in 60 min, less PRL and cortisol were released during the second stimulation. Metyrapone did not affect PRL or cortisol release. Dexamethasone decreased serum cortisol concentration but did not affect PRL concentration. In experiment 2, 16 cows were used in a crossover experimental design consisting of 2 experimental weeks separated by 1 resting week. During the first week, cows were treated as follows: (1) 4 cows were injected with 0.5 g of domperidone (a PRL secretagogue) in canola oil on d 1 and 2 and 20 mg of dexamethasone on d 1; (2) 4 cows were injected with 0.5 g of domperidone on d 1 and 2; (3) 4 cows were injected with canola oil on d 1 and 2 and with 20 mg of dexamethasone on d 1; and (4) 4 cows were injected with canola oil on d 1 and 2. During the second experimental week, the same 4 treatments were repeated, except the cows that did not receive dexamethasone in the first week received it on d 1 of the second week, and cows that did receive it in the first week did not receive it in the second week. On d 1 and 2 of each week, blood samples were collected during morning milking for PRL determination. Dexamethasone reduced milk production and decreased both basal and milking-induced PRL release. It also increased milk fat and protein percentages and decreased milk lactose content. Domperidone increased basal PRL levels in serum and milk but did not affect milk yield. Although we cannot rule out the possibility that inhibition of PRL secretion or reduction of mammary gland PRL responsiveness play a role in the inhibition of milk production by glucocorticoids, the fact that enhancement of PRL secretion by domperidone could not prevent the depression of milk yield suggests that other mechanisms are involved.
本研究的目的是确定糖皮质激素在调节乳腺刺激诱导的催乳素（PRL）释放中的作用，并研究糖皮质激素导致奶牛产奶量下降是否是由于PRL释放减少所致。在实验1中，8头奶牛采用4×4拉丁方设计。早晨挤奶4小时后，奶牛接受以下处理之一：（1）对乳腺进行5分钟的手动刺激；（2）静脉注射1毫克地塞米松；（3）在对乳腺进行5分钟刺激前4小时和2小时，在皱胃中两次输注2.5克美替拉酮（一种皮质醇生物合成抑制剂）；或（4）不进行处理。60分钟后，对每头奶牛的乳腺进行5分钟刺激。在处理开始前20分钟至处理后120分钟采集血样。当乳腺在60分钟内被刺激两次时，第二次刺激期间释放的PRL和皮质醇较少。美替拉酮不影响PRL或皮质醇的释放。地塞米松降低了血清皮质醇浓度，但不影响PRL浓度。在实验2中，16头奶牛采用交叉实验设计，包括两个实验周，中间间隔1个休息周。在第一周，奶牛的处理如下：（1）4头奶牛在第1天和第2天注射0.5克多潘立酮（一种PRL促分泌剂）于菜籽油中，并在第1天注射20毫克地塞米松；（2）4头奶牛在第1天和第2天注射0.5克多潘立酮；（3）4头奶牛在第1天和第2天注射菜籽油，并在第1天注射20毫克地塞米松；（4）4头奶牛在第1天和第2天注射菜籽油。在第二个实验周重复相同的4种处理，但第一周未接受地塞米松的奶牛在第二周第1天接受地塞米松，第一周接受地塞米松的奶牛在第二周不接受。在每周的第1天和第2天，在早晨挤奶时采集血样用于PRL测定。地塞米松降低了产奶量，并降低了基础和挤奶诱导的PRL释放。它还提高了乳脂和蛋白质百分比，并降低了乳乳糖含量。多潘立酮提高了血清和乳汁中的基础PRL水平，但不影响产奶量。虽然我们不能排除PRL分泌抑制或乳腺PRL反应性降低在糖皮质激素抑制产奶中起作用的可能性，但多潘立酮增强PRL分泌不能防止产奶量下降这一事实表明还涉及其他机制。
