Progressive Changes in Plasma Progesterone, Prolactin and Corticosteroid Levels During Late Pregnancy and the Initiation of Lactose Synthesis in the Rat

Publisher Website
Google Scholar
Abstract
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 in rats with either ovariohysterectomy or cesarean-section on day 19 of gestation. In chronically cannulated rats the decline in plasma progesterone to low values (< 10 .mu.g/l) in normal parturient rats 20 h before term and in rats with cesarean-section 10-14 h after surgery was closely related to an increase in plamsa prolactin. In rats with ovariohysterectomy the levels of progesterone declined abruptly to 20 .mu.g/l within 30 min of surgery, and prolactin remained low (5 .mu.g/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 (< 0.14 mg/g tissue) up to 12 h, and increased to reach maximum values at 36 h and 48 h after surgery in rats with ovariohysterectomy and cesarean-section, respectively. The concentration of corticosteroids and CBG capacity in the plasma was 250-550 .mu.g/l and 250-480 .mu.g corticosterone bound/l, respectively, in rats killed during the last 4 days of gestation. Both the concentration of corticosteroids and the CBG capacity, within the individual cannulated rats, remained relatively constant during late gestation. Progesterone withdrawal apparently is the lactogenic trigger and there probably is 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.