No abstract

The relationship between progesterone, prolactin, corticosteroids and corticosteroid binding globulin . (CBO) 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 « 10 p.g/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 p.g/l within 30 min of surgery and prolactin remained low (5 p.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 hand 48 h after surgery in ovariohysterectomized and Caesarean-sectioned rats respectively.The concentration of corticosteroids and CBO capacity in the plasma was 250--550 p.g/l and 250-480 p.g corticosterone bound per litre, respectively, in rats killed during the last 4 days of,gestation. However, both the concentration of corticosteroids and the CBO capacity, within the individual cannulated rats, remained relatively constant during late gestation.These findings support the proposal that progesterone withdrawal is the lactogenic trigger ana 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.

The in vitro incorporation of ['4C]glucose into lactose in mammary tissue, the concentration of lactose in the mammary tissue and the concentration of lactose in the mammary secretion were determined during late pregnancy and lactation in the rat. These changes were related to the decline in blood progesterone during late pregnancy. The incorporation of [,4C]glucose into lactose was detected on day 20 of pregnancy; it increased gradually until day 22 and then increased rapidly just prior to term (day 23 of pregnancy) to reach mean ± s.e.m. maximum hourly values of 12·0 ± 1· 5 cpm x 10 4 1 g by day 3 of lactation, and then declined to lower values (6· 1 ± 0·6 cpm x 10 4 /g) by day 20 of lactation. The concentration of lactose both in the mammary tissue and in the mammary secretion increased rapidly over the last 24 h of pregnancy and then more gradually after birth to reach mean ± s.e.m. maximum values of 6·85 ± 1 ·11 mg/g tissue and 43·1 ± 2·1 gil respectively on day 15 of lactation. The decline in plasma progesterone to low levels between days 21 and 22 of pregnancy preceded the rapid increase in the concentration of lactose in the mammary tissue. A similar relationship was observed between the decline in progesterone and the increase in lactose in mammary tissue in rats Caesarean-sectioned on day 19 of pregnancy, and the administration of progesterone immediately following Caesarean section significantly depressed the accumulation of lactose in the mammary tissue. The results support the involvement of progesterone withdrawal in lactogenesis in the rat and indicate that ['4C]glucose incorporation into lactose and the concentration of lactose both in the mammary tissue and in mammary secretion are useful indicators of the synthetic activity of the mammary gland.

No abstract

Rats were either Caesarean-sectioned, ovariohysterectomized or ovariectomized on day 19 of gestation anti the role of prolactin and corticosteroids in the initiation of lactation was studied by administering 2~bromo-lX-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 JIg/l plasma, 21·9-65·3 JIg/l plasma and 1·21--4·50mg/g tissue, respectively, in the Caesareansectioned, 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 Jig/I. Whereas CB 154 significantly depressed the lactose content of the mammary tissue in Caesarean-sectioned and ovariohysterectomized rats (0·42 and o· 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 both 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 JIg/l plasma and the lactose content of mammary tissue from 1 . 31 to O· 19 mg/g tissue. Whereas adrenalectomy of ovariectomized rats decreased corticosteroid concentrations to 53 Jig/I, the lactose content of mammary tissue (1·82 mg/g tissue) remained within the range for ovariohysterectomized rats.These findings 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.