Effects of milking frequency and domperidone injections on milk production and prolactin signaling in the mammary gland of dairy cows

Eleven mid-lactation Holstein cows were milked twice daily during the first 2 experimental weeks. During wk 3 to 10, the cows were differentially milked: right quarters were milked thrice daily (3x) and left quarters were milked once daily (1x). During wk 11 to 14, all quarters were milked twice daily. After 4 wk of differential milking, the cows received daily i.m. injections of the dopamine antagonist domperidone (DUMP; 300 mg; n = 6) or of dimethyl sulfoxide as the control (CTL; n = 5) for 8 wk (wk 7-14). During the differential milking period (wk 3-6), milk production was greater for quarters milked 3x than for those milked 1x but did not differ between DUMP and CTL cows. During the differential milking + injection period (wk 7-10), milk production continued to differ according to milking frequency. However, DOMP injection did not have an effect or interact with milking frequency on milk production. During the injection period (wk 11-14), milk production remained greater in the quarters previously milked 3x and milk production increased in DUMP injected cows but not in CTL cows. Injections of DOMP increased prolactin concentration, which was greater in the serum of DOMP cows than in that of CTL cows during the differential milking + injection and the injection periods. The expression of genes that are directly related to milk synthesis (CSN2, LALBA, and ACACA) was greater in the 3x quarters than in the 1x quarters. In addition, DOMP increased CSN2 expression during the injection period. The expression of both isoforms of the PRLR gene was greater in the 3x quarters during the differential milking + injection and the injection periods. At the protein level, injections of DUMP tended to increase the number of long PRLR isoform during the differential milking + injection period. The number of short PRLR isoform was greater in the lx quarters than in the 3x quarters during the differential milking, the differential milking + injection, and the injection periods. The total amount of STAT3 protein was greater in the 1x quarters during the differential milking and the differential milking + injection periods. The amount of phosphorylated STAT3 protein was greater in the 1x quarters during the differential milking period. The total amount of phosphorylated STAT5 protein was greater in the 3x quarters during the differential milking and the differential milking + injection periods. The results of this experiment support the hypothesis that the responsiveness of the mammary gland to PRL is modulated by milking frequency, although the underlying mechanism remains to be determined.