The objective of this study was to determine whether the responsiveness of the mammary gland to prolactin (PRL) is affected by the concentration of the hormone. After 1 pre-experimental week (d −7 to −1), 18 Holstein cows in mid to late lactation were injected intramuscularly twice daily with either 0.5 mg of quinagolide (QN) or 2 mL of water (control) for 2 wk (d 1 to 14; treatment period). After the treatment period, all cows received daily subcutaneous injections of 300 mg of domperidone (DOMP) for 3 wk (d 15 to 35; DOMP period). The cows were monitored for an additional 2 wk as a posttreatment period (d 36 to 49). Blood and milk samples were collected 3 times per week. Additionally, blood samples were collected during the a.m. milking on d −4, 14, and 35. Milk production was not affected by QN during the treatment period but was increased during the DOMP and posttreatment periods in the QN cows. With respect to milk composition, the treatments affected only the protein content, which was greater in the QN cows during the treatment period. Blood PRL concentration declined during QN injections and was lower in the QN cows than in the control cows between d 5 and 14. The basal concentration of PRL was increased by DOMP injections during the DOMP and posttreatment periods but was not affected by previous QN injections. Prolactin concentration in milk was not affected by the QN treatments but was increased by DOMP injections during the DOMP and posttreatment periods. Milking-induced PRL release was decreased by QN on d 14. On d 35, milking did not induce a significant release of PRL above the baseline for both treatments. In conclusion, the results of this experiment support the contention that the mammary gland's responsiveness to PRL is modulated by the previous level of the hormone.

这项研究的目的是确定乳腺对催乳素（PRL）的反应性是否受激素浓度的影响。在实验前1周（d -7至-1）后，每天两次给肌肉内注射的荷斯坦奶牛18头，每天两次肌肉注射0.5 mg奎奴那利德（QN）或2 mL水（对照组），持续2 wk（d 1至14；治疗期）。治疗期后，所有母牛每天皮下注射300 mg多潘立酮（DOMP），连续3周（第15至35天； DOMP期）。作为后处理期，对母牛进行了另外2周的监测（第36至49天）。每周采集3次血液和牛奶样本。另外，在上午d -4、14和35挤奶期间收集了血液样本。在治疗期间，产奶量不受QN的影响，但在DOMP和后期处理期间，QN奶牛的产奶量增加了。关于牛奶成分，这些处理仅影响蛋白质含量，在处理期间，QN奶牛的蛋白质含量更高。在QN注射期间，血液PRL浓度下降，在第5到14天之间，QN母牛的血PRL浓度低于对照母牛。在DOMP和后期治疗期间，通过DOMP注射增加了PRL的基础浓度，但不受先前QN注射的影响。牛奶中的催乳素浓度不受QN处理的影响，但在DOMP和后期处理期间通过DOMP注射会增加。第14天的QN降低了挤奶诱导的PRL释放，第35天，两种方法的挤奶均未引起PRL明显高于基线释放。总之，该实验的结果支持这样的论点，即乳腺对PRL的反应能力受激素的先前水平调节。