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Glucocorticoids secreted by the foetal adrenal cortex play a major causative role in initiating parturition in the sheep and goat (Liggins, 1968, 1969a; Bassett & Thorburn, 1969; Comline, Nathanielsz, Paisey & Silver, 1970; Thorburn, Nicol, Bassett, Shutt & Cox, 1972). However, Liggins (1969b) failed to precipitate premature parturition with dexamethasone (4 mg/h) infused into the maternal circulation of the pregnant ewe. Fylling (1971) produced delivery with 6–10 mg dexamethasone/day in the pregnant ewe. Since it is uncertain what role glucocorticoids may play in polytocous species, an attempt was made to initiate parturition by infusing cortisol into the maternal circulation of the pregnant rabbit. Cortisol was chosen as the glucocorticoid for infusion since in the newborn rabbit the plasma cortisol: corticosterone ratio is about 3·0 (K. W. Malinowska, R. N. Hardy & P. W. Nathanielsz, unpublished observations).
Pregnant rabbits of known gestational age were anaesthetized with sodium pentobarbitone (30–45
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SUMMARY
Continuous infusion of cortisol at rates from 0·31 to 1·24 mg/h into pregnant rabbits from day 21 of pregnancy onwards induced delivery in approximately 72 h. Similar rates of infusion into the foetal amniotic sac induced delivery with a shorter latency. Maternal infusion of 1·24 mg cortisol/h produced parturition providing it was continued for at least 24 h. If after this period the infusion was terminated, delivery still occurred after a further 48 h (i.e. 72 h in all). Progesterone administration to the mother blocked the effect of cortisol administered by the maternal or foetal route.
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It is now well established that several hormones are secreted in an episodic or pulsatile fashion (McNatty, Cashmore & Young, 1972; Murray & Corker, 1973). When plasma concentrations of certain hormones are examined, peaks occur at intervals related to different features of the 24-h cycle, circadian rhythms (Retiene, Zimmerman, Schindler, Neuenschwander & Lipscomb, 1968). Circadian and episodic patterns in thyroid function have been claimed by certain workers (Bakke & Lawrence, 1965; Blum, Greenspan & Magnum, 1968) and refuted by others (Schatz & Volpe, 1959; Odell, Wilber & Utiger, 1967). A partial explanation of these conflicting results probably lies in functional differences in the level of the hypothalamo-pituitary-thyroidal-peripheral target tissue axis under investigation by different techniques.
Five Jersey bull calves aged between 2 and 80 days were investigated on a total of nine occasions. Five experiments were conducted under natural lighting conditions (daylight from 04.30 to 19.30 h). Calves in the
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Prostaglandin F2α (PGF2α) has been shown to possess luteolytic properties in several species including the rabbit (Duncan & Pharriss, 1970; O'Grady, Kohorn, Glass, Caldwell, Brock & Speroff, 1972). Surgical removal of the corpus luteum leads to parturition in the rabbit. Thus, PGF2α should initiate parturition in this species. In primates, controversy has arisen over the ability of PGF2α to produce luteolysis (Kirton, Pharriss & Forbes, 1970). Since prostaglandins are very rapidly cleared by the pulmonary circulation, observed discrepancies may be due to the actual route of administration and dosage used.
Experiments have been carried out to measure the effect of PGF2α in producing parturition in the pregnant rabbit. PGF2α was infused continuously into rabbits with an indwelling aortic catheter (Nathanielsz & Abel, 1972). Infusions were started on day 21 and continued until delivery. PGF2α was infused at dose rates varying from 1·125 ng/h
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Concentrations of prostaglandin F2α (PGF2α) and prostaglandin E (PGE) were measured in endometrium from 18 women with ectopic pregnancies. In the nine pregnancies not associated with vaginal bleeding or an intra-uterine contraceptive device (IUCD; intact ectopics), concentrations of PGF2α (12·8 ± 7·4 (s.e.m.) ng/g) and PGE (4·7 ± 3·0 ng/g) were similar to those in decidua from nine intra-uterine pregnancies of comparable gestational age (14·4 ± 4·4 and 8·2 ± 2·2 ng/g respectively). In both ectopic and intra-uterine pregnancies concentrations of prostaglandins were significantly lower than those found in endometrium throughout the normal menstrual cycle (P < 0·01). In nine ectopic pregnancies with associated vaginal bleeding and/or an IUCD, concentrations of PGF2α and PGE were significantly higher than in the intact group (P < 0·05), although the concentration of PGF2α remained significantly lower than levels in normal secretory endometrium (P < 0·05).
These results suggested that suppression of endometrial synthesis of prostaglandin during early pregnancy may be mediated systemically rather than through a local action of the conceptus.
