Blood prolactin concentrations affect prolactin transfer into goat milk: implications for maintenance of lactation

Effect of the concentration of circulating prolactin on dairy cows' responsiveness to domperidone injection

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.

The dopamine antagonist domperidone increases prolactin concentration and enhances milk production in dairy cows

In previous studies, our team showed that the inhibition of prolactin (PRL) secretion by the dopamine agonist quinagolide reduces milk production in dairy cows. The objective of this study was to determine the effects of administration of a dopamine antagonist on basal and milking-induced PRL concentrations in blood and on milk production during positive energy balance and feed restriction in dairy cows. Eighteen mid-lactation Holstein cows received daily s.c. injections of either domperidone (300 mg, DOMP, n=9) or the vehicle, canola oil (CTL, n=9), for 5 wk. During wk 5, all cows were fed at 65% of their dry matter intake in the previous week. Blood and milk samples were collected before (for blood) and during (for milk) the a.m. milking thrice weekly from d -9 to 41 (8d after the last injection). In addition, blood samples were collected during the a.m. milking on d -1 (before the first injection), and on d 1, 28, and 34. Basal PRL concentration was similar in both groups before the start of the treatments. Domperidone injections caused a gradual increase in basal PRL concentration. Feed restriction reduced basal PRL concentration in both the CTL and DOMP cows, but PRL concentration remained higher in the DOMP cows. Prolactin concentration remained elevated in the DOMP cows 7d after the last injection. The milk concentration of PRL increased during the DOMP treatment, but the increase was smaller than that observed in serum. In the CTL cows, the milking-induced PRL release above the premilking concentration was similar on d -1, 1, and 28 but was reduced during feed restriction. In the DOMP cows, the milking-induced PRL release was similar on d -1 and 1 but was reduced on d 28 and 34. Milk production was similar for both groups before the treatments started but was greater in the DOMP cows during the treatment period, at 2.9 ± 0.6 and 2.4 ± 0.6 kg/d greater during wk 3 and 4 of treatment, respectively. Milk production declined in both groups during feed restriction but remained higher in the DOMP cows. Milk production became similar again for both groups after the last injection. In addition, dry matter intake was increased by DOMP. These results support the hypothesis that PRL is galactopoietic in dairy cattle.

Transferrin and prolactin transcytosis in the lactating mammary epithelial cell

The mammary epithelial cell ferries constituents originating from blood and from stromal cells, into milk, by transcytosis. Morphological analysis of a membrane marker of transcytosis in the lactating mammary epithelial cell showed that very rapid endocytosis of surface membrane occurs from both the basolateral and the apical side of the cell. In both cases, membrane trafficking between endosomes and the Golgi complex allows communication between the endocytic and the biosynthetic pathways. Transferrin and prolactin are internalized in mammary cells and transported through multivesicular bodies and Golgi stacks. They are released into milk via different types of secretory vesicles, prolactin being carried in secretory vesicles containing casein micelles. Consequences of the intracellular transport of these proteins and physiological benefits for cell function are discussed.

Interactions of prolactin and growth hormone (GH) in the regulation of mammary gland function and epithelial cell survival

The relative importance of GH and prolactin in mammary gland function varies between species with prolactin playing a major role in rodents and GH taking lead role in ruminants. In rodents, however, GH appears to play a vital role in maintaining a high-fat/low volume milk in the absence of prolactin and a similar finding has been demonstrated in goats where prolactin deficiency causes a more modest (15%) decrease in milk yield. Surprisingly GH-deficiency in goats induced no further decline in milk yield whereas exogenous GH or prolactin both stimulated milk output considerably. Although direct effects of prolactin on mammary epithelial cells are well-documented effects of GH are believed to be mediated indirectly via IGF-1 production from the liver. We have been unable to confirm this hypothesis in rats and believe this to be because it is too simplistic. By considering prolactin and GH to be survival factors for the mammary gland we now propose a mechanism by which they interact through the IGF system. Involution of the mammary gland involves apoptosis and, in rats, it is induced by prolactin-deficiency or milk accumulation. Coincidentally with this process mammary epithelial cells synthesize and secrete and IGF binding protein, IGFBP-5. We hypothesize that GH stimulates IGF-1 production, possibly from the mammary parenchyma. IGF-1 then acts as a survival factor for the mammary gland. Prolactin plays an essential role since it suppresses the secretion of IGFBP-5 which would otherwise inhibit IGF-1 action and lead to the induction of cell death.

Changes in prolactin bioactivity of milk following isolation from litter and oxytocin administration in rats

Changes in prolactin (PRL)-like bioactivity in rat milk after a longer isolation of litters and the effects of oxytocin (OT) were determined by Nb2 lymphoma cell bioassay and compared with the immunoreactivity using an enzyme immunoassay. First, we confirmed the appropriate dilution rates of rat milk which could neutralize the influence of an antimitogenic factor(s) in milk to Nb2 lymphoma cell proliferation. To prolong the litter removal, i.e. 6 h to 20 h, resulted in the increase in an antimitogenic factor(s) for Nb2 lymphoma cells in milk and in the significant decrease in milk PRL concentration measured by both assays, and plasma bioactive PRL level increased. The bioactivity/immunoreactivity ratio of PRL in milk accumulated in the mammary gland was 0.65 in rats after a 20-h isolation, but 1.18 after a 6-h isolation. The milk PRL concentration in rats isolated for 6 h decreased significantly within 45 min after the administration of OT and the plasma PRL concentration increased only slightly, but OT had no effect in rats after isolation for 20 h. The present findings indicate that milk PRL might transfer to plasma with the excessive engorgement of milk in the mammary gland and with OT administration. Moreover, PRL trapped in milk for a long period loses its biological activity more rapidly than its immunological activity, since bioactivity/immunoreactivity ratio in the 20-h isolation group was significantly lower than in the 6-h isolation group.