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Search for other papers by DOREEN V. ILLINGWORTH in
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Search for other papers by RUTH DEANESLY in
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SUMMARY
In guinea-pigs ovariectomized before implantation, pregnancy can be maintained during the first 3 or 4 weeks, a critical period of early growth, by synthetic progestagens that do not metabolize to progesterone (norgestrel, medroxyprogesterone acetate and melengestrol acetate). It follows that peripheral plasma progesterone levels are related to placental progesterone secretion. There were indications that progesterone secretion by the placenta begins as early as day 16–18 of pregnancy. In the experimental animals the plasma progesterone levels, measured by a competitive protein-binding technique, were 9·4 ± 2·07 ng/ml (mean ± s.e.m.; n = 18) by days 20–22 of pregnancy; they continued to rise and reached 26·4 ± 3·88 ng/ml (n = 8) by days 27 to 29.
The metabolic clearance rate of progesterone measured in four pregnant ovariectomized animals with progesterone or progestagen implants, was < 10 l/day as in normal pregnant animals. The calculated production rates of progesterone in the experimental animals (0·1 to 0·35 mg/day) reflect the placental production rate of progesterone. A simple 'assay' is described for the high affinity progesterone-binding protein of pregnancy (progesteronebinding globulin).
Search for other papers by DOREEN V. ILLINGWORTH in
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Search for other papers by J. S. PERRY in
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SUMMARY
The effects of hypophysial stalk-section on the growth and function of the corpus luteum of the non-pregnant guinea-pig have been compared with the effects of hypophysectomy (as previously described) and with the effects of prolactin administered to hypophysectomized animals. Stalk-section soon after ovulation did not impair the growth of the corpora lutea nor their ability to secrete progesterone. Stalk-section before day 9 of the oestrous cycle prevented the normal regression of the corpora lutea; they continued to grow and 3 weeks after ovulation were as large as those of pregnant animals, or of non-pregnant hysterectomized guinea-pigs. The corpora lutea regressed irregularly during the following 2 weeks. When performed on, or later than day 9, stalk-section did not prevent luteal regression at the normal time.
Administration of prolactin (10 i.u./day) to hypophysectomized guinea-pigs restored the growth-rate of the corpora lutea, which reached sizes comparable to those of the normal cycle, and those of stalk-sectioned animals, by 10 days after ovulation.
Our results indicate that prolactin can have substantial luteotrophic activity in the guinea-pig.
Search for other papers by J. R. G. CHALLIS in
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Search for other papers by R. B. HEAP in
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Search for other papers by DOREEN V. ILLINGWORTH in
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SUMMARY
A competitive protein-binding assay of progesterone and a radioimmunoassay of total unconjugated oestrogens were evaluated and employed in the measurement of these steroids in the plasma of guinea-pigs at different times during the reproductive cycle. Production rates (PR) were calculated from the product of the endogenous steroid concentration and its metabolic clearance rate. During the oestrous cycle, plasma progesterone levels rose from undetectable levels at the time of ovulation to 2·8 ± 0·33 (s.e.m.) ng/ml 5 days later; PR was 0·26 mg/day. During pregnancy, plasma progesterone concentrations in post-partum mated guinea-pigs rose from 15 ng/ml at day 15 post coitum (p.c.) (PR, 0·65 mg/day) to a peak value of 329 ± 14 ng/ml by days 30 to 45. The concentration fell 51–55 days p.c. (160 ± 14·7 ng/ml), but rose before parturition to 258 ± 22 ng/ml. After day 20 p.c., PR was 1·24 ± 0·3 mg/day. After parturition, plasma progesterone levels fell rapidly and during lactation were about 10 ng/ml (PR, 0·4 mg/day).
Total unconjugated oestrogens in arterial plasma were not detected in early pregnancy, but rose from values of 12·8 ± 1·9 pg/ml (days 31–35) to 31·0 ± 5·2 pg/ml (days 56–60). There was a slight fall before, and a rapid one after parturition. The ratio of progesterone to oestrogen in plasma fell during pregnancy and reached its lowest values between days 55 and 60.
Search for other papers by DOREEN V. ILLINGWORTH in
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Search for other papers by R. B. HEAP in
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Search for other papers by J. S. PERRY in
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SUMMARY
The metabolic clearance rate (MCR) of progesterone was measured by continuous infusion of the tritium-labelled steroid in guinea-pigs under light sodium pentobarbitone anaesthesia. In normal animals during the oestrous cycle, the MCR was 112·8 ± 7·0 (s.e.) 1 plasma/day/kg. In pregnant animals there was a sharp decrease in MCR between days 15 and 20, to 8·3 ± 0·8 1/day/kg. This low clearance rate was observed throughout the remainder of gestation. After parturition MCR returned slowly to non-pregnant levels, reaching 85 1/day/kg by day 12 post partum.
The decrease in metabolic clearance rate occurred at the same time as the increase in plasma concentration of progesterone. This may be related to an increase in binding proteins with a high affinity for progesterone. The stimulus for the decrease in MCR in pregnancy is obscure but appears to depend on the presence of a viable conceptus. In the non-pregnant guinea-pig the MCR is not reduced by exogenous oestrogen, hysterectomy or hypophysectomy.
Search for other papers by DOREEN V. ILLINGWORTH in
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Search for other papers by NICOLA ACKLAND in
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Search for other papers by ANGELA M. BURTON in
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SUMMARY
Pregnancy was sustained to term (>63 days) in 15 out of a total of 46 guinea-pigs, hypophysectomized within 4 days after mating. The majority (22/29) of complete or partial abortions occurred between 20 and 35 days post coitum. Analysis of plasma samples showed that the pregnant hypophysectomized animals with live foetuses had high progesterone and progesterone-binding protein concentrations, equivalent to those found in intact pregnant guinea-pigs. Animals that received daily injections of cortisone acetate from day 55 (7/15) gave birth to live young at the normal time (67–68 days). Plasma samples from one animal taken during parturition showed no measurable oxytocin in maternal plasma, but oxytocin was present in foetal plasma at the moment of birth.
