Progressive changes in plasma progesterone, prolactin and corticosteroid levels during late pregnancy and the initiation of lactose synthesis in the rat.

The relationship between progesterone, prolactin, corticosteroids and corticosteroid binding globulin (CBG) activity in plasma and the initiation of lactation were studied in normal parturient rats, and rats either ovariohysterectomized or Caesarean-sectioned on day 19 of gestation. In chronically cannulated rats the decline in plasma progesterone to low values (less than 10 micrograms/l) in normal parturient rats 20 h before term and in Caesarean-sectioned rats 10-14 h after surgery was closely related to an increase in plasma prolactin. However, in ovariohysterectomized rats the levels of progesterone declined abruptly to 20 micrograms/l within 30 min of surgery and prolactin remained low (5 micrograms/l) for 4-8 h and then gradually increased during the subsequent 16-18 h. Lactose concentrations in mammary tissue of rats killed at 0, 12, 18, 24, 36 and 48 h after surgery was low (less than 0.14 mg/g tissue) up to 12 h, and increased to reach maximum values at 36h and 48 h after surgery in ovariohysterectomized and Caesarean-sectioned rats respectively. The concentration of corticosteroids and CBG capacity in the plasma was 250-550 micrograms/l and 250-480 micrograms corticosterone bound per litre, respectively, in rats killed during the last 4 days of gestation. However, both the concentration of corticosteroids and the CBG capacity, within the individual cannulated rats, remained relatively constant during late gestation. These findings support the proposal that progesterone withdrawal is the lactogenic trigger and suggest the sequential involvement of prolactin. An increase in the concentration of free corticosteroids in late pregnancy and a related stimulatory role in the lactogenic mechanism were not established.