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

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.

Hormones, IgG and lactose changes around parturition in plasma, and colostrum or saliva of multiparous sows

“…The increase in concentrations of lactose in the colostrum occurred under low P4 plasma concentrations but high E2 and PRL concentrations. This was in agreement with the suggested inhibitory action of P4 [4,47,48] and stimulatory actions of E2 and PRL [34,35,49] on lactose synthesis in the mammary glands. The decrease in the ratio of lactose concentrations in colostrum over those in plasma before and during parturition is in accordance with the passage of lactose from the colostrum to the blood circulation.…”

The Foetoplacental Unit and the Initiation of Lactation in the Rat

“…However, we have found that the administration of ovine prolactin to rats immediately after lactogenesis had been induced in late pregnancy by either Caesarean section or ovariohysterectomy, significantly increased the accumulation of lactose in the mammary gland above control values (Tables 1 and 2). In this connection it was observed that a decline in progesterone levels to below 10·0 /lgjl in late pregnancy and after either Caesarean section or ovariohysterectomy was followed by a rapid increase in the concentrations of prolactin in the plasma and of lactose in the mammary gland (Nicholas and Hartmann 1981b). Taken collectively, these data suggest that mammary tissue can respond to prolactin only after the removal of progesterone.…”

“…Lactose content of the mammary gland was determined as previously described (Nicholas and Hartmann 1981a). The concentration of corticosteroids in the plasma was measured by the competitive protein-binding assay of Martin et al (1977), and the concentration of prolactin by a double antibody radioimmunoassay using reagents supplied by the Rat Pituitary Hormone Program, National Institute of Arthritis and Metabolic Diseases (Nicholas and Hartmann 1981b).…”

Time of birth and daily activity mediated by feeding rhythms in the pregnant rat

“…1) (Bosc, 1981). The 25-hour latency observed between luteolysis and birth (Bartholomeusz et al, 1976 ;Nicholas and Hartmann, 1981 ;Puri and Garfield, 1982 ;Sherwood et al, 1983) also supports such a mechanism. In this model applied to environmental conditions and birth time in rats, a third component must be included.…”

“…The still-birth rate, which increases with the duration of expulsion (Bosc and Nicolle, 1979), was low in all groups (table 2) and thus provides an indirect argument for this statement. In contrast, it is known in this species that the interval (about 25 h) between the necessary decline in blood progesterone concentration and the onset of birth remains constant under photoperiodic conditions which have been studied (Bartholomeusz et al, 1976 ;Nicholas and Hartmann, 1981 ;Puri and Garfield, 1982 ;Sherwood et a/., 1983). It is also worth noting that, in this species, the increase of EMG activity of the uterus starts 20 to 24 h before the first fetus is expulsed (Legrand and Maltier, 1981 ;Germain et al, 1985).…”