The foetoplacental unit and the initiation of lactation in the rat

Rats were either Caesarean-sectioned, ovariohysterectomized or ovariectomized on day 19 of gestation and the role of prolactin and corticosteroids in the initiation of lactation was studied by administering 2-bromo-alpha-ergocryptine (CB 154) and prolactin, and by using adrenalectomy and foetectomy. The concentrations of corticosteroids and prolactin in the plasma and the weight and lactose content of the inguinal mammary glands were determined 48 h after Caesarean section, and 24 h after ovariohysterectomy and ovariectomy. The ranges in concentrations of corticosteroids, prolactin and mammary lactose were 207-348 micrograms/l plasma, 21.9-65.3 micrograms/l plasma and 1.21-4.50 mg/g tissue, respectively, the Caesarean-sectioned, ovariohysterectomized and ovariectomized rats. The administration of CB 154 after either Caesarean section, ovariohysterectomy or ovariectomy decreased the concentration of prolactin to less than 8.8 micrograms/l. Whereas CB 154 significantly depressed the lactose content of the mammary tissue in Caesarean-sectioned and ovariohysterectomized rats (0.42 and 0.31 mg/g tissue, respectively), no effect was observed in ovariectomized rats (1.39 mg/g tissue). The administration of ovine prolactin reversed the inhibitory effects of CB 154. Furthermore, in two rats which were not ovariectomized and foetectomized, CB 154 did not inhibit lactose accumulation in the mammary tissue (2.02 mg/g tissue). Adrenalectomy of ovariohysterectomized rats decreased corticosteroid concentrations from 348 to 14 microgram/l plasma and the lactose content of mammary tissue from 1.31 to 0.19 mg/g tissue. Whereas adrenalectomy of ovariectomized rats decreased corticosteroid concentrations to 53 micrograms/l, the lactose content of mammary tissue (1.82 mg/g tissue) remained within the range for ovariohysterectomized rats. These finding show that hormones produced in late pregnancy by the foetus and placenta can support lactogenesis in the rat in the absence of maternal prolactin and corticosteroids.