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

摘要

Eleven mid-lactation Holstein cows were milkedtwice daily during the first 2 experimental weeks. Duringwk 3 to 10, the cows were differentially milked:right quarters were milked thrice daily (3×) and leftquarters were milked once daily (1×). During wk 11to 14, all quarters were milked twice daily. After 4 wkof differential milking, the cows received daily i.m.injections of the dopamine antagonist domperidone(DOMP; 300 mg; n = 6) or of dimethyl sulfoxide asthe control (CTL; n = 5) for 8 wk (wk 7–14). Duringthe differential milking period (wk 3–6), milk productionwas greater for quarters milked 3× than for thosemilked 1× but did not differ between DOMP and CTLcows. During the differential milking + injection period(wk 7–10), milk production continued to differ accordingto milking frequency. However, DOMP injection didnot have an effect or interact with milking frequency onmilk production. During the injection period (wk 11–14), milk production remained greater in the quarterspreviously milked 3× and milk production increased inDOMP injected cows but not in CTL cows. Injectionsof DOMP increased prolactin concentration, which wasgreater in the serum of DOMP cows than in that ofCTL cows during the differential milking + injectionand the injection periods. The expression of genes thatare directly related to milk synthesis (CSN2, LALBA,and ACACA) was greater in the 3× quarters than inthe 1× quarters. In addition, DOMP increased CSN2expression during the injection period. The expressionof both isoforms of the PRLR gene was greater in the3× quarters during the differential milking + injectionand the injection periods. At the protein level, injectionsof DOMP tended to increase the number of longPRLR isoform during the differential milking + injectionperiod. The number of short PRLR isoform wasgreater in the 1× quarters than in the 3× quarters duringthe differential milking, the differential milking +injection, and the injection periods. The total amountof STAT3 protein was greater in the 1× quarters duringthe differential milking and the differential milking+ injection periods. The amount of phosphorylatedSTAT3 protein was greater in the 1× quarters duringthe differential milking period. The total amountof phosphorylated STAT5 protein was greater in the3× quarters during the differential milking and the differentialmilking + injection periods. The results of thisexperiment support the hypothesis that the responsivenessof the mammary gland to PRL is modulated bymilking frequency, although the underlying mechanismremains to be determined.