OESTROGEN, PROGESTERONE AND CORTICOSTEROID CONCENTRATIONS IN PERIPHERAL PLASMA OF SOWS DURING PREGNANCY, PARTURITION, LACTATION AND AFTER WEANING R. W. ASH AND R. B. HEAP A.R.C. Institute of Animal Physiology, Babraham, Cambridge CB2 4AT

(Received

13

May 1974)

SUMMARY

Steroid hormone concentrations in plasma have been measured in blood samples taken from conscious sows with ear vein catheters. In late pregnancy, the plasma progesterone concentration ranged from 6 to 12 ng/ml and it decreased in all animals before the onset of parturition. Total unconjugated oestrogens increased to high values of up to about 3 ng/ml in late pregnancy and then declined after the onset of parturition. Oestrone was the predominant unconjugated oestrogen measured. Plasma corticosteroid (mainly cortisol) concentration was about 33 ng/ml and showed no consistent change at the time of parturition. During lactational anoestrum the plasma concentration of progesterone and total unconjugated oestrogens was very low, while that of corticosteroids was 21 ng/ml. When the piglets were weaned at 26\p=n-\31 days, sows came into oestrus 4\p=n-\12days later, and this was preceded, or accompanied by, an increase in plasma oestrogens. In the luteal phase, plasma progesterone concentrations rose to 20\p=n-\35ng/ml. A sow whose piglets were removed at birth, showed signs of oestrus (vulval enlargement and a lordosis response), but a lack of receptivity to the boar associated with no detectable changes in the plasma oestrogen concentration; however, ovulation probably occurred since plasma progesterone values increased in a manner comparable to that found after the formation of normal corpora lutea in other

sows.

After a second non-receptive cycle, the sow was mated and became pregnant

post-weaning oestrus. parturition the concentration of progesterone and total unconjugated oestrogens was greater in placental venous plasma than in maternal jugular plasma, which indicates placental synthesis of these hormones. A greater concentration of plasma corticosteroids in foetal blood than in placental venous or maternal jugular plasma suggests foetal synthesis in late at the third

At

pregnancy.

INTRODUCTION

Intensive methods of husbandry have resulted in an increased awareness of clinical problems associated with reproduction in the sow; dystocia, lactational disorders, and failure to conceive after early weaning are the main problems. Some of these conditions may derive from metabolic or endocrine imbalances but present knowledge of the normal changes in blood constituents at these critical stages of reproduction is fragmentary (Polge, 1972a, 6). During the first 14 days of pregnancy the plasma progesterone concentration is similar to that found during the luteal phase of the oestrous cycle, 20-30 ng/ml. After about day 15 it declines gradually to 10-15 ng/ml in pregnant sows, and to basal values in non-pregnant animals (Tillson & Erb, 1967; Stabenfeldt, Akins, Ewing & Morrisette, 1969; Edqvist & Lamm, 1971; Guthrie, Henricks & Handlin, 1972; Henricks, Guthrie & Handlin, 1972). The plasma concentrations of total unconjugated oestrogens during the first 24 days of pregnancy are within the range 10-28 pg/ml (Guthrie et al. 1972), and the urinary excretion of oestrogens reaches a peak at the end of the first month of pregnancy and, after a fall, reaches even higher values towards the end of gestation after day 80 (Lunaas, 1962; Rombauts, 1962; Raeside, 1963; Fèvre, Léglise & Rombauts, 1968). Furthermore, plasma luteinizing hormone concentration, which is high at oestrus, decreases and remains low during early pregnancy (Niswender, Reichert & Zimmerman, 1970; Tillson, Erb & Niswender, 1970; Rayford, Brinkley & Young, 1971; Guthrie et al. 1972). Recent investigations on the endocrinology of parturition have been concerned particularly with the sheep and goat, monotocous or ditocous species in which the foetus apparently plays a major role in the initiation of labour (see Bedford, Challis, Harrison & Heap, 1972a; Liggins, Grieves, Kendall & Knox, 1972; Thorburn, Nicol, Bassett, Shutt & Cox, 1972; Liggins, Fairclough, Grieves, Kendall & Knox, 1973 for recent reviews of the subject). The endocrine mechanisms that underlie the control of parturition in the pig have not yet been elucidated, though in another polytocous species, the rabbit, the removal of the inhibitory effect of a progesterone block of myometrial activity appears to be an essential component of parturition according to the studies of Csapo (1969) and other workers. Corticosteroids play an important part in the induction of parturition in sheep and goats, but their role in the pig is less clear. The purpose of this study was to follow the changes in the plasma concentrations of progesterone, total unconjugated oestrogens and corticosteroids during late pregnancy, parturition, lactation and after weaning in sows. In addition, blood samples were obtained from newborn piglets and from the umbilical vein for com¬ parison with those from the maternal circulation. A preliminary report of some of this work has been published (Ash, Banks, Bailes, Broad & Heap, 1973). MATERIALS AND METHODS

Animals

Large White sows of approximately 200 kg body weight from the Institute's breeding herd of enzootic pneumonia-free pigs were used. Six sows with a prominent

central ear vein were selected and indwelling ear vein catheters were introduced and their patency maintained as described previously (Ash, Banks, Broad & Heap, 1972). The catheters were established 7-21 days before parturition and they re¬ mained patent for the remainder of pregnancy, during parturition, and for the postpartum period. In four sows, blood samples were also taken during lactation and the weaning period, and during the post-weaning oestrus. The gestation period of the six sows was 112-115 days, parturition was normal, and the litter size, 9-15 live

piglets. Approximately 50 ml of blood was taken each morning, usually at 09.00 h before feeding; during sampling a small amount of food was offered to attract the sow's

attention and to obviate the need for restraint. The volume of blood removed never exceeded 2 % of the calculated blood volume, even at parturition when sampling was more frequent. The removal of these amounts of blood did not affect the haematocrit, erythrocyte or reticulocyte counts. Iron fumarate was given at frequent intervals to ensure an adequate supply of iron. Plasma was separated from hepar¬ inized blood and stored at —10 °C before assay. After thawing at room temperature, portions were assayed immediately for oestrogens; portions for progesterone and cortisol determinations were re-frozen and stored at 10 °C for subsequent assay, a procedure that had no detrimental effect on the results. —

Assays measured by a radioimmunoassay described unconjugated oestrogens previously (Challis, Heap & Illingworth, 19716) using antiserum SLC-6X kindly supplied by Dr Burton Caldwell, Yale University. The relative binding affinities of this antiserum for a range of steroids have been reported (Challis, Harrison & Heap, 1973a); a high cross-reaction with oestrone (90%) relative to that of oestradiol-17/? (100%) was a notable feature of this particular preparation. Progesterone and corticosteroids were determined by competitive protein binding using the methods of Thorburn, Bassett & Smith (1969) and Bassett & Hinks (1969) as described by Challis et al. (19716). In some experiments progesterone was also measured by a modified radioimmunoassay (Abraham, Swerdloff, Tulchinsky & Odell, 1971). The method departed from the original procedure only in respect of the extraction of plasma (0-1 ml) which was carried out as described by Challis et al. (19716). A onetenth portion of the washed petroleum ether extract was assayed directly using antiserum S49 No. 6 (Abraham et al. 1971). Total

were

RESULTS

Hormone

Progesterone

changes in late pregnancy

In three sows plasma progesterone concentrations ranged from about 8-10 ng/ml during the last 18 days of pregnancy (Figs 1, 2 and 6) but in three other animals a gradual decline in concentration was observed during the last 2-4 days of gestation (Fig. 3). A comparison was made between two techniques used to measure peripheral progesterone concentrations, competitive protein binding and radioimmunoassay. Results obtained for samples from sow 4424 (from 8 days before parturition to the

day of parturition) showed that there was no significant difference between the values measured by the two techniques (paired i-test, > 0-05). When a more specific antiserum was used (S257 No. 2) the concentration measured was only slightly lower.

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1. Plasma concentrations of progesterone, total unconjugated oestrogens and corticosteroids in sow 3356 (5th pregnancy) during late pregnancy, parturition (14 piglets) and early lactation. Day 0 and the arrow denote the day of parturition.

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Fig. 2. Plasma concentrations of progesterone, total unconjugated oestrogens and corticosteroids in sow 3214 (5th pregnancy) during late pregnancy, parturition (11 piglets) and lactation and after weaning. Parturition occurred on day 0, weaning was completed by day 31, and behavioural oestrus was observed

on

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34.

Oestrogens Seven to nine

days before parturition,

the concentration of plasma

unconjugated

oestrogens increased steadily in all sows (Figs 1, 3 and 6). Figure 3 shows that in sows 4424 and 4459 (both second pregnancies) and sow 3457 (fourth pregnancy),

plasma oestrogen concentration reached its highest value on the day of par¬ turition, though the absolute value differed between animals. For example, in sows 4424 and 4459 the peak concentration was about 1 ng/ml, whereas in sow 3457 it was about 3 ng/ml. However, the highest concentration of plasma oestrogens did not coincide with the day of parturition in all animals (Figs 1 and 6). There appeared to be a relationship between the mean plasma oestrogen concentration during the 7 days before the day of parturition and the parity of the sow (P < 0-001) and/or the number of foetuses (Table 1). Experiments were carried out to investigate the identity of the oestrogens found in peripheral plasma in high concentration during late pregnancy. Plasma samples from sow 3457 at parturition (total unconjugated oestrogens, 3-1 ng/ml) and sow the

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16 24 32 40 48 4 12 20 28 36 44 Pregnancy After weaning of 3. total Plasma concentrations progesterone, Fig. unconjugated oestrogens and corticosteroids in sows 4424 and 4459 (2nd pregnancies) and sow 3457 (4th pregnancy), during late pregnancy, parturition (day 0), lactation (L) and in sow 4424 after weaning, oestrus, fertile mating and early pregnancy. Arrow denotes time of fertile mating; Oe, duration of oestrus. Sow 4424 delivered 9 piglets; sow 4459, 10 piglets; and sow 3457, 12 piglets.

Table 1. The relation between the parity of the sow, the number of piglets born, and the mean plasma concentration of total unconjugated oestrogens during the last 7 days of

gestation

No. of No. of sow 4424 4459 4448 3214 3457 3356

previous pregnancies

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unconjugated oestrogens (means + s.e.m.) 0-5 ±003

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during lactation (20 pg/ml) were extracted and purified by thin-layer chromatography (t.l.c.) (Bedford, Harrison & Heap, 19726; Challis, Harrison & Heap, 1973a,6). The solvent system for t.l.c. was cyclohexane:ethyl acetate (1:1) in which the RF value of progesterone was 0-575; oestrone, 0-72; oestradiol-17a, 0-62; and oestradiol-17/?, 0-54. In both samples the concentration of progesterone, which was not separated from oestradiol-17/?, was low (0-2 and 1-0 ng/ml) and its cross-reaction with antiserum SLC-6X was negligible. After Chromatographie separation, each lane of the plate was divided into 0-5 cm bands which were eluted (Bedford et al. 19726), assayed using antiserum SLC-6X, and the results corrected for background values determined for blank lanes. A trace amount of activity was detected in the area corresponding to oestradiol-17/? in sow 3214 during lactation. In sow 3457 at 3214

least one-half of the total amount of oestrogen recovered (2-8 ng/ml) was associated with the oestrone fraction and less than one tenth with oestradiol-17/?.

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Fig. 4. Parturition and the plasma concentration of progesterone, total unconjugated oestrogens and corticosteroids in sows 3356 and 4424. Black bar between the arrows indicates the duration of delivery from the birth of the first piglet to the expulsion of the last placenta.

Hormone

Progesterone Blood samples

changes on the day of parturition

were taken more frequently to investigate whether plasma pro¬ decrease before the onset of parturition. An example of concentrations gesterone the rapid changes observed during the last 120 h of gestation is given in Fig. 4. In sow 3356, plasma progesterone concentration remained at about 10 ng/ml until within 6 h of onset of parturition, though it had decreased to a low value when the first piglet was delivered. A different picture was seen in sow 4424 in which the progesterone concentration was low for about 24 h before delivery of the first piglet; there was no clinical evidence of dystocia or uterine inertia.

Oestrogens Plasma oestrogen concentration was high until the onset of delivery; thereafter it declined gradually, reaching low values only after the delivery of all piglets and

placentae (second arrow, Fig. 4).

Corticosteroids Plasma corticosteroid concentrations varied appreciably from day to day. The mean concentration during late pregnancy, and including the values obtained on the day of parturition, was 33 ± 1-5 (s.e.m.) ng/ml; at the time of parturition there was no consistent elevation. The identity of the maternal plasma corticosteroids was examined in a sample from sow 3356 which contained a concentration of 58-9 ng/ml. After t.l.c. (chloroform:acetone, 80:20) to separate cortisol (RF value, 0-30) and corticosterone (0-48), at least 70% of the total corticosteroid concentration was associated with the area of the plate that corresponded to cortisol. A small quantity of corticosterone was detected. 60

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5. Foetal and maternal plasma concentrations of progesterone, total unconjugated oestro¬ gens and corticosteroids measured in the maternal jugular vein, foetal heart, and the umbilical vein at the time of the expulsion of the piglets (sow 4459).

Fig.

Comparison of maternal, foetal and placental hormones Blood samples were collected during parturition from the maternal jugular vein, from the umbilical vein after the placentae had been expelled and the piglets separated, and by cardiac puncture from two male and two female piglets (Fig. 5). The plasma progesterone concentration in the placental umbilical vein was threefold greater than that in plasma of newborn piglets and about 20-fold greater than that in the maternal jugular vein. The plasma oestrogens were more concentrated in the placental venous blood than in the plasma of neonatal piglets, or in the jugular plasma of the sow. Plasma corticosteroid concentrations of newborn piglets greatly exceeded those in the maternal jugular vein and in the placental vein. Hormone Lactation There were

changes during lactation and after weaning

cyclical changes in the plasma concentration of progesterone or oestrogens in lactating sows nursing 9-15 piglets. The mean plasma progesterone concentration was 0-2 + 0-16 ng/ml (number of observations, 28, Fig. 3). Total un¬ conjugated oestrogens were below the sensitivity of the assay (less than 10 pg/ml), while plasma corticosteroid concentrations (21 + 1-3 ng/ml) were slightly lower than those in pregnancy (Fig. 3; see also Figs 1 and 2). Weaning In three lactating sows (4424, 3457, 3214; Figs 2 and 3) the piglets were removed from the dam for an increasing number of hours each day over a period of up to 6 days so that weaning was completed 26,30 and 31 days after parturition respectively. During the period of weaning the plasma oestrogen concentration started to increase (Fig. 2), reaching a peak value of 10, 62 and 60 pg/ml respectively in the three sows studied on, or immediately before, the day of behavioural oestrus. Oestrus was observed in the three animals 12, 3 and 4 days after weaning was completed. Plasma corticosteroid concentrations reached high values in two of the sows at the time of behavioural oestrus (Figs 2 and 3). After oestrus and fertile mating there was a sharp increase in the plasma concentration of progesterone. Within 3 weeks after mating plasma concentrations of progesterone had reached values of 20-35 ng/ml, though no rise in plasma oestrogens could be detected (Fig. 3). no

Early weaning An experiment

designed to examine the effects of early weaning on the endocrine response of the sow. Fig. 6 shows that, during gestation, hormone con¬ centrations in sow 4448 were typical of those observed in the other sows. Immediately after delivery the piglets were removed and oestrus occurred 5-6 days later, with the normal signs of vulval oedema and a lordosis response, but the sow was not receptive to the boar. No increase in plasma oestrogens (< 10 pg/ml) was detected. The con¬ centrations of plasma progesterone increased to high values comparable to those associated with the secretory activity of normal corpora lutea. After 20 days, plasma progesterone values had decreased and remained undetectable for at least 4 days during which time the sow showed vulval enlargement but no sign of behavioural was

oestrus or elevation of plasma oestrogens. Subsequently, plasma progesterone concentrations were again raised for at least 14 days, and then declined rapidly to undetectable values when oestrus was observed. The sow was successfully mated and had a normal pregnancy. On the second and third day of oestrus there was a small rise in plasma oestrogens (16 pg/ml). Throughout the period from par¬ turition to fertile mating plasma corticosteroid concentrations were very variable

(26 + 2-2 ng/ml).

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Days Fig. 6. Early weaning and the plasma concentrations of progesterone, total unconjugated oestro¬ gens and corticosteroids in sow 4448 (2nd pregnancy). Parturition occurred on day 0, and the pig¬ lets were separated from the sow immediately after delivery. Oe (top arrow), oestrus and fertile mating; N.R. (lower arrows), non-receptive when tested with the boar. DISCUSSION

The maintenance of gestation in the sow depends on a continued secretion of ovarian progesterone. The placentae may contribute to progesterone production in late pregnancy, albeit in a small way, since the concentration in umbilical venous plasma taken from the cord at birth was appreciably higher than that in maternal or foetal plasma. Ovariectomy performed at any time during gestation rapidly terminates pregnancy thus indicating that placental progesterone secretion is in¬ adequate for the maintenance of pregnancy (du Mesnil du Buisson & Dauzier, 1953); progesterone replacement therapy will prevent abortion after ovariectomy (Day, Anderson, Emmerson, Hazel & Melampy, 1959; Fèvre et al. 1968). Hypophysectomy during pregnancy also causes abortion (du Mesnil du Buisson, Léglise, Anderson &

Rombauts, 1964; du Mesnil du Buisson & Denamur, 1969; Kraeling & Davis, 1974),

indicating that a continuous secretion of pituitary luteotrophin is necessary to main¬ tain gestation, and that the possible secretion of a placental luteotrophin is inadequate to replace the function of the pituitary. The present study confirms the finding of other workers that the progesterone concentration in peripheral plasma is high during early pregnancy (Guthrie et al. 1972 ) but somewhat lower during late gesta¬ tion (Short, 1960; Tillson et al. 1970), and is consistent with the finding that the ovarian vein concentration of progesterone declines gradually throughout this period (Masuda, Anderson, Henricks & Melampy, 1967). A notable feature of the endocrine pattern of pregnancy in the sow is the high concentration of total unconjugated oestrogens in peripheral plasma. Plasma oestrogen values increased in all animals during the last week of pregnancy to values approximately 30-fold greater than those at oestrus, and in some sows the con¬ centration exceeded 3 ng/ml. Similar results have been briefly reported by Sasser, Hegge, Christian & Falk (1973). As judged by the Chromatographie separation technique, the unconjugated oestrogen found in highest concentration during late pregnancy was oestrone and the pattern of oestrogen values during late gestation was comparable with that of their urinary excretion (Rombauts, 1962; Raeside, 1963; Fèvre et al. 1968). Further, oestrone has been identified in urine as the major oestrogen (Velie, 1959; Lunaas, 1962; Rombauts, 1962; Raeside, 1963; Fèvre et al. 1968).

The source of circulating oestrogens in late pregnancy in the sow is presumed to be the gravid uterus since after ovariectomy or hypophysectomy the urinary ex¬ cretion in sows maintained on progesterone follows a pattern similar to that of intact animals and declines to undetectable values after parturition (Fèvre et al. 1968). The high concentration of total unconjugated oestrogens in umbilical venous plasma obtained at parturition, which was about twice that found in maternal jugular and foetal plasma, is also consistent with the view that oestrogen secretion is placental in origin. This view is further supported by an earlier demonstration that the pig placenta will synthesize oestrone and oestradiol-17/? from labelled dehydroepiandrosterone and androstenedione in vitro (Ainsworth & Ryan, 1966). However, the parti¬ cipation of the foetal piglet in oestrogen metabolism should not be overlooked since the foetal liver is a rich source of enzymes that hydrolyse steroid sulphates such as oestrone sulphate (Ainsworth, 1972). Both the parity of the sow and the number of foetuses probably contributed to the individual differences in plasma oestrogen values noted in our animals. A similar finding was described by Fèvre et al. (1968) and Rombauts, Fèvre & Terqui (1971) who reported that oestrone excretion was positively correlated with the number of foetuses. The gradual increase in the concentration of total unconjugated oestrogens in plasma during late pregnancy in the sow contrasts with the sharp increase seen in the sheep which occurs only during the last 48 h of gestation and attains values of up to 1 ng/ml (Challis, 1971; Challis, Harrison & Heap, 1971a; Bedford et al. 1972 ; Thorburn et al. 1972). The pattern of change in plasma oestrogen concentration in the sow resembles more closely that which has been reported in the cow (Robinson, Baker, Anastassiadis & Common, 1970; Henricks, Dickey, Hill & Johnston, 1972; Robertson, 1974) and the goat (Challis & Linzell, 1971), which are species, like the

a continuous ovarian secretion of progesterone is necessary to maintain It is notable that in the cow, goat and sow, oestrone is a more abundant pregnancy. than its more active metabolite, oestradiol-17/?. The ready interconversion oestrogen of these two steroids implies that a relatively large amount of oestrone is available which can be rapidly converted to an oestrogen of greater potency in tissues where it acts, such as the gravid uterus and, possibly, the mammary glands. Experiments in pregnant sheep show that the transfer constant, which is derived from the interconversions and clearance rates of these two steroids, favours the overall conversion of oestrone to oestradiol-17/? (Challis et al. 19736). It is difficult to define in precise clinical terms the time of onset of parturition in the sow, but in this study it has been taken as the time of expulsion of the first foetus. Parturition was preceded consistently by a decrease in the progesterone concentration of peripheral plasma when the oestrogen concentration had reached some of its highest values. There was some variation between animals in the time when the progesterone values started to decrease, though signs of impending delivery such as restlessness, nest-building, vulval oedema and mammary develop¬ ment, were frequently related to the period of high oestrogen and low progesterone values. The reason for the sudden withdrawal of progesterone secretion just before parturition may be related either to the removal of a pituitary luteotrophic stimulus (e.g. prolactin, see du Mesnil du Buisson & Denamur, 1969), or the production of a uterine luteolytic stimulus, such as prostaglandin F2a. A potent analogue of prosta¬ glandin F2ct (I.C.I. 79,939) has been found to be highly effective in terminating gestation in sows after intramuscular injection (Ash & Heap, 1973). However, this does not preclude foetal participation in the onset of parturition (Dzuik & Stryker, 1973), and a high corticosteroid concentration was found in foetal plasma at birth. Large amounts of cortisol in foetal blood have been previously implicated in the trigger mechanism responsible for the onset of parturition (Liggins, 1969) and in the stimulation of uterine prostaglandin production in the sheep (Liggins et al. 1972) and goat (Thorburn et al. 1972). High concentrations of plasma corticosteroids in foetuses near term have been reported in several species (Bassett & Thorburn, 1969; Liggins et al. 1972; Nathanielsz, Comline, Silver & Paisey, 1972). During parturition, plasma oestrogen concentrations declined progressively, reaching a low value by the time all the placentae had been delivered, and being undetectable one day later. In the post-partum and lactating sow, plasma oestrogens and progesterone were consistently low. There were no overt signs of a post-partum oestrus (Warnick, Casida & Grummer, 1950; Burger, 1952) in nursing sows, though in our studies these sows were not tested regularly for oestrus. The low concentration of plasma oestrogens during lactation, however, is consistent with the absence of overt oestrus (Fig. 6), and differs from the results of Shearer, Purvis, Jenkin & Haynes (1972) who reported a transient rise in oestradiol-17/? about 6 days after parturition. When piglets were weaned 25 days after parturition, the sow showed overt oestrus 9 days later by which time plasma oestrogen concentration had increased; pregnancy was established after mating. This finding contrasts with that in another sow who was not allowed to suckle her piglets. Although repeated signs of oestrus were observed, such as reddening and enlargement of the vulva and a lordosis response to back pressure, the sow refused service by the boar until 48 days after parturition

sow, where

when a fertile mating occurred. During the episodes of repeated sub-oestrus, no increase could be detected in plasma oestrogen concentrations. However, since blood samples were taken only once daily at this stage of the experiment, it is possible that transient increases in oestrogen concentration may have been missed. A sur¬ prising finding was the apparently cyclic formation of corpora lutea as judged by peripheral plasma progesterone values. The life-span and secretory activity of these corpora lutea were comparable to those of a typical oestrous cycle.

We are indebted to Mrs Susan Broad, Mr P. Banks and Mr G. Bailes for skilled technical assistance and to Mr J. Smith for help with the care of animals. references

Abraham, G. F., Swerdloff, R., Tulchinsky, D. & Odell, N. D. (1971). Radioimmunoassay of plasma progesterone. Journal of clinical Endocrinology and Metabolism 32, 619-624. Ainsworth, L. (1972). The cleavage of steroid sulfates by sheep and pig fetal liver, fetal kidney and placental preparations in vitro. Steroids 19, 741-750. Ainsworth, L. & Ryan, K. J. (1966). Steroid hormone transformations by endocrine organs from pregnant mammals. I. Estrogen biosynthesis by mammalian placental preparations in vitro. Endocrinology 79, 875-883.

Ash, R. W., Banks, P., Bailes, G., Broad, S. & Heap, R. B. (1973). Plasma oestrogen, progesterone and corticoid concentrations in the pregnant, parturient and lactating sow. Journal of Reproduction and Fertility 33, 359-360. Ash, R. W., Banks, P., Broad, S. & Heap, R. B. (1972). Techniques for studying changes in some peri¬ pheral blood components and steroid hormones in pregnant and lactating sows. Journal of Physiology 226, 40P-41P. Ash, R. W. & Heap, R. B. (1973). The induction and synchronization of parturition in sows treated with I.C.I. 79,939, an analogue of prostaglandin F2a. Journal of agricultural Science 81, 365-368. Bassett, J. M. & Hinks, N. T. (1969). Micro-determination of corticosteroids in ovine peripheral plasma: effects of veni-puncture, corticotrophin, insulin and glucose. Journal of Endocrinology 44, 387-403. Bassett, J. M. & Thorburn, G. D. (1969). Foetal plasma corticosteroids and the initiation of parturition in sheep. Journal of Endocrinology 44, 385-386. Bedford, C. ., Challis, J. R. G., Harrison, F. A. & Heap, R. . (1972 a). The rôle of oestrogens and progesterone in the onset of parturition in various species. Journal of Reproduction and Fertility Suppl. 16, 1-23. Bedford, C. ., Harrison, F. A. & Heap, R. B. (19726). The metabolic clearance rate and production rate of progesterone and the conversion of progesterone to 20a-hydroxypregn-4-en-3-one in the sheep. Journal of Endocrinology 55, 105-118. Burger, J. F. (1952). Sex physiology of pigs. Onderstepoort Journal of veterinary Research Suppl. 2, 3-218. Challis, J. R. G. (1971). A sharp increase in free circulating oestrogens immediately before parturition in sheep. Nature 229, 208. Challis, J. R. G., Harrison, F. A. & Heap, R. . (1971a). Uterine production of oestrogens and pro¬ gesterone at parturition in the sheep. Journal of Reproduction and Fertility 25, 306—307. Challis, J. R. G., Harrison, F. A. & Heap, R. . (1973a). The kinetics of oestradiol-17/? metabolism in the sheep. Journal of Endocrinology 57, 97-110. Challis, J. R. G., Harrison, F. A. & Heap, R. B. (19736). The metabolic clearance rate, production rate and conversion ratios of oestrone in the sheep. Journal of Endocrinology 58, 435-446. Challis, J. R. G., Heap, R. B. & Illingworth, D. V. (19716). Concentrations of oestrogen and proges¬ terone in the plasma of non-pregnant, pregnant and lactating guinea-pigs. Journal of Endocrinology 51, 333-345. Challis, J. R. G. & Linzell, J. L. (1971). The concentration of total unconjugated oestrogens in the plasma of pregnant goats. Journal of Reproduction and Fertility 26, 401-404. Csapo, A. I. (1969). The four direct regulatory factors of myometrial function. In Progesterone: its regulatory effect on the myometrium. Ciba Foundation Study Group 34, 13-42. Day, B. N., Anderson, L. L., Emmerson, . ., Hazel, L. N. & Melampy, R. M. (1959). Effect of oestrogen and progesterone on early embryonic mortality in ovariectomized gilts. Journal of Animal Science 18, 607-613. du Mesnil du

Buisson,

F. &

Dauzier, L. (1953). Influence de l'ovariectomie chez la truie pendant la la Société de biologie 151, 311-313.

gestation. Comptes rendus des séances de

du Mesnil du Buisson, F. & Denamur, R. (1969). Mécanismes du contrôle de la fonction lutéale chez la truie, la brebis et la vache. In 3rd Int. Congr. Endocr., Mexico, I.C.S. No. 184, pp. 927-934. Eds C. Guai & F. J. Ebling. Amsterdam: Excerpta Medica Foundation, du Mesnil du Buisson, F., Léglise, P. C, Anderson, L. L. & Rombauts, P. (1964). Maintien des corps jaunes et de la gestation de la truie au course de la phase préimplantatoire après hypophysectomie. In Vth Int. Congr. Anim. Reprod. Artif. Insem., Trento, 3, 571. Dzuik, P. J. & Stryker, J. L. (1973). Development and birth of decapitated fetal pigs. Journal of Animal Science 37, 309. Edqvist, L. E. & Lamm, A. M. (1971). Progesterone levels in plasma during the oestrous cycle of the sow measured by a rapid competitive protein binding technique. Journal of Reproduction and Fertility 25, 447-448. Fèvre, J., Léglise, P. C. & Rombauts, P. (1968). Du rôle de l'hypophyse et des ovaires dans la bio¬ synthèse des oestrogènes au cours de la gestation chez la truie. Annales de biologie animale, biochimie et biophysique 8, 225-233. Guthrie, H. D., Henricks, D. M. & Handlin, D. L. (1972). Plasma oestrogen, progesterone and lutein¬ izing hormone prior to estrus and during early pregnancy in pigs. Endocrinology 91, 675—679. Henricks, D. M., Dickey, J. F., Hill, J. R. & Johnston, N. E. (1972). Plasma estrogen and progesterone levels after mating, and during late pregnancy and post-partum in cows. Endocrinology 90, 13361342. Henricks, D. M., Guthrie, H. D. & Handlin, D. L. (1972). Plasma oestrogen, progesterone and LH levels during the estrous cycle in pigs. Biology of Reproduction 6, 210-218. Kraeling, R. R. & Davis, B. J. (1974). Termination of pregnancy by hypophysectomy in the pig. Journal of Reproduction and Fertility 36, 215-217. Liggins, G. C. (1969). The foetal role in the initiation of parturition in the ewe. In Foetal autonomy. Ciba Foundation Symposium. Eds G. E. W. Wolstenholme & M. O'Connor. London: Churchill. Liggins, G. C, Fairclough, R. J., Grieves, S. ., Kendall, J. Z. & Knox, B. S. (1973). The mechanism of initiation of parturition in the ewe. Recent Progress in Hormone Research 29, 111—150. Liggins, G. C, Grieves, S. ., Kendall, J. Z. & Knox, B. S. (1972). The physiological rôles of progesterone, oestradiol-17/? and prostaglandin F2(J in the control of ovine parturition. Journal of Beproduction and Fertility Suppl. 16, 85-104. Lunaas, T. (1962). Urinary oestrogen levels in the sow during oestrous cycle and early pregnancy. Journal of Reproduction and Fertility 4, 13-20. Masuda, H., Anderson, L. L., Henricks, D. M. & Melampy, R. M. (1967). Progesterone in ovarian venous plasma and corpora lutea of the pig. Endocrinology 80, 240-246. Nathanielsz, P. W., Comline, R. S., Silver, M. & Paisey, R. B. (1972). Cortisol metabolism in the fetal and neonatal sheep. Journal of Reproduction and Fertility Suppl. 16, 39-60. Niswender, G. D., Reichert, L. E., Jr & Zimmerman, D. R. (1970). Radioimmunoassay of serum levels of LH throughout the estrous cycle of pigs. Endocrinology 87, 576-580. Polge, C. (1972a). Reproductive physiology in the pig with special reference to early weaning. Pro¬ ceedings of the British Society for Animal Production 1, 5—18. Polge, C. (19726). The improvement of sow productivity. In Proc. Symp. at The Rowett Research Institute, Aberdeen. March, 1972. Eds A. S. Jones, V. R. Fowler & J. C. R. Yeats. Occasional Publication of the Rowett Research Institute, No. 3. Raeside, J. I. (1963). Urinary oestrogen excretion in the pig during pregnancy and parturition. Journal of Reproduction and Fertility 6, 427-431. Rayford, P. L., Brinkley, H. J. & Young, E. P. (1971). Radioimmunoassay determination of LH con¬ centration in the serum of female pigs. Endocrinology 88, 707-713. Robertson, H. A. (1974). Changes in the concentration of unconjugated oestrone, oestradiol-17a and oestradiol-17/? in the maternal plasma of the pregnant cow in relation to the initiation of parturition and lactation. Journal of Reproduction and Fertility 36, 1—8. Robinson, R., Baker, R. D., Anastassiadis, P. A. & Common, R. H. (1970). Estrone concentrations in the peripheral blood of pregnant cows. Journal of Dairy Science 53, 1592—1595. Rombauts, P. (1962). Excrétion urinaire d'oestrogènes chez la truie pendant la gestation. Annales de biologie animale, biochimie et biophysique 2, 151-156. Rombauts, P., Fèvre, J. & Terqui, M. (1971). Oestrogène et progestogènes au cours du cycle reproduction de la truie. Journal de recherches Porc en France (Paris) pp. 173-178. Sasser, R. G., Hegge, F. N., Christian, R. E. & Falk, D. E. (1973). Plasma estrogens in pregnant gilts. Journal of Animal Science 37, 327. Shearer, I. J., Purvis, K., Jenkin, G. & Haynes, N. B. (1972). Peripheral plasma progesterone and oestradiol-17/? levels before and after puberty in gilts. Journal of Reproduction and Fertility 30, 347-360. Short, R. V. (1960). Blood progesterone levels in relation to parturition. Journal of Reproduction and Fertility 1, 61-70. Stabenfeldt, G. H., Akins, E. L., Ewing, L. L. & Morrisette, M. C. (1969). Peripheral plasma progesterone levels in pigs during the oestrous cycle. Journal of Reproduction and Fertility 20, 443-449.

Thorburn, G. D., Bassett, J. M. & Smith, I. D. (1969). Progesterone concentration in the peripheral

plasma

of sheep

during

the oestrous

cycle. Journal of Endocrinology 45,

459-469.

Thorburn, G. D., Nicol, D. H., Bassett, J. M., Shutt, D. A. & Cox, R. I. (1972). Parturition in the goat

sheep: changes in corticosteroids, progesterone, oestrogens and prostaglandin F. Journal of Reproduction and Fertility Suppl. 16, 61-84. Tillson, S. A. & Erb, R. E. (1967). Progesterone concentration in peripheral blood plasma of the domestic sow prior to and during early pregnancy. Journal of Animal Science 26, 1366-1368. Tillson, S. ., Erb, R. E. & Niswender, G. D. (1970). Comparisons of luteinizing hormone and progesterone in blood and metabolites of progesterone in urine of domestic sows during the estrous cycle and early pregnancy. Journal of Animal Science 30, 795-805. Velie, W. (1959). Isolation of oestrone from the urine of the pregnant sow. Acta veterinaria scandinavica and

1, 19-26. Warnick, A. C, Casida, L. E. & Grummer, R. H. (1950). The occurrence of estrus and ovulation in post partum sows. Journal of Animal Science 9, 66-72.