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PRL involvement in periparturient mammary development and onset of milk secretion was studied in 17 multiparous, monotocous Holstein cows. Mammary glands were obtained 10 days prepartum from untreated cows and 10 days postpartum from untreated cows and cows treated with CB154 (2-Br-α-ergokryptin) or CB154 plus PRL. CB154 was administered for 12 days before expected parturition through 10 days postpartum. PRL was infused continuously for 6 days immediately before parturition in cows receiving CB154 plus PRL. Treatment with CB154 reduced basal serum PRL concentrations approximately 80% and blocked the normal surges in serum PRL concentrations at parturition and during milking. Average milk production in cows given CB154 alone was 11.4 kg/day lower than in controls. Periparturient infusion of PRL in cows treated with CB154 (CB154 plus PRL) prevented reductions in milk production. Treatment with CB154 had no effect on feed intake or periparturient serum concentrations of GH, progesterone, and glucocorticoids....

Hormonal control of α-lactalbumin secretion was studied in mammary tissue obtained from nonlactating multiparous Holstein cows pregnant approximately 8 months. Mammary explants were incubated for 72 h in medium containing insulin, T3 and various combinations or concentrations of PRL, cortisol, progesterone, 17β-estradiol, and GH. Concentrations ofα-lactalbumin in the medium and in homogenates of tissue were measured by RIA or sodium dodecyl sulfate-gel electrophoresis as an index of lactogenesis. Increasing quantities of PRL (25 to 100 ng/ml) progressively increased the release of α-lactalbumin into the medium, whereas concentrations of α-lactalbumin in explants exposed to PRL were similar to those in controls. Mammary explants exposed for 3 h to [3H]leucine plus PRL incorporated approximately 2, 11, and 18 times more label into α-lactalbumin 21–24, 45–48, and 69–72 h after initiation of culture, respectively, than explants cultured in the absence of PRL. Cortisol alone had no effect on α-lactalbumin secr...