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ABSTRACT

The effect of decreasing oestrogen secretion on the oviducal migration of embryos was investigated in pregnant rats. The reduction of oestradiol production was achieved by administration of the aromatase inhibitor 4-hydroxy-4-androstene-3,17-dione (4-OH-A) at various times after coitus.

When 4-OH-A was administered from days 2 to 5, nearly half the embryos were retained in the oviducts at midday on day 5 of pregnancy, in contrast with control animals in which all embryos were transferred to the uterus. Shorter treatments were less effective. The rate of secretion of oestradiol from the ovary on days 2–5 of pregnancy in control rats was low in the morning and high in the afternoon. Treatment with 4-OH-A from days 2 to 5 reduced the secretory surges of oestradiol in the afternoon by 77% without significantly changing the progesterone output. Systemic testosterone levels were significantly increased by this treatment.

To assess whether changes in the transport of ova were due to an increase in testosterone concentrations the influence of exogenous testosterone on embryo transport and oestradiol production was tested. Testosterone administered by subdermal implants from days 2 or 3 to day 5 disturbed embryo transport in a manner similar to that of 4-OH-A. The longest period of testosterone administration decreased ovarian oestradiol production by 82% without changing the secretion of progesterone.

Since ovarian oestradiol production in pregnant rats was reduced similarly by treatment with 4-OH-A and testosterone implants, and since exogenous oestradiol given s.c. on days 2, 3 and 4 counteracted the blocking effect of 4-OH-A and testosterone on embryo transport, the delay in oviducal transport of the embryos can be accounted for by decreased availability of oestradiol rather than increased testosterone production. This interpretation supports a crucial role for endogenous oestradiol in timing the passage of embryos to the uterus in the pregnant rat.

J. Endocr. (1988) 118,93–100

Concentrations of progesterone and oestradiol were measured in peripheral plasma samples collected at the time when the uteri of rhesus monkeys with an intra-uterine device (IUD) and those without an IUD were flushed in attempts to recover uterine embryos. The proportion of successful attempts in IUD-bearing monkeys was much lower than in the non-IUD-bearing animals. Steroid measurements indicated that this reduced success rate was not due to an effect of the IUD on the timing of ovulation within the menstrual cycle or to a steroid-mediated disturbance in the rate of embryo transport to the uterine lumen. Successful embryo recoveries were associated with a higher progesterone concentration, suggesting that one reason for failure was that the attempt had been made too close to ovulation. There was no evidence of any asymmetry between the left or right ovaries in their ovulatory or steroidogenic activity.

SUMMARY

Superfoetation has been induced experimentally in the mouse by treating females with gonadotrophins twice within a short period. Successive ovulations resulted from the treatments. After each treatment with gonadotrophins the females were paired with males, or were artificially inseminated, the spermatozoa carrying distinctive gene markers so that the paternity of offspring could be identified. If females were mated after one treatment and artificially inseminated after the other, embryos differing in age by 2½-3 days could develop from conception to birth in the same female. No examples of superfoetation were found when females mated in response to both treatments or when the difference in age of embryos from the first and second treatments was 3½-4 days; few females, however, could be induced to mate in response to each treatment.

In the experiments in which the age difference of the embryos was 2½-3 days, the second treatment with gonadotrophins delayed the transit of the first set of embryos along the uterine tubes. This often resulted in the blastocysts from the first treatment and the recently ovulated pronucleate eggs from the second treatment being found together in the tubes. Both groups of embryos apparently then moved normally along the tubes, implanted in the uterus at or about the same time, and were probably born simultaneously. If the age difference was 3½-4 days, transit of the first set of embryos was not delayed and many of these embryos were destroyed in the uterus.

The incidence of successive ovulations and the response of the female tract to repeated injections of gonadotrophins, the duration of the fertilizing capacity of spermatozoa in the female tract, aspects of sperm transport, and the possibility of increasing the incidence of mixed litters are discussed.

SUMMARY

Single doses of up to 0·4 mg/kg oestrone injected s.c. were found to interfere with pregnancy in the rat up to the 11th day post coitum (p.c.). As little as 0·02 mg/kg was effective during the period of tubal transport, while 0·1 mg/kg was necessary to prevent implantation after the ovum was free in the uterus. After implantation had taken place, 0·4 mg/kg injected on days 8, 9 or 10 destroyed some or all embryos or, if injected on days 10 or 11, caused delay in parturition. Delay in parturition was no longer reliably produced by the injection of 0·4 mg/kg on the 16th day of gestation.

A single dose of 4·0 mg/kg testosterone prevented implantation in 75% of rats if injected s.c. during days 5–8 p.c., and produced either foetal loss or delay in parturition in 69% of rats when injected on days 9–11 p.c. A single dose of 20 mg/kg testosterone was effective in all rats tested in preventing implantation if injected on days 1 and 5, producing foetal loss if injected on day 9 or delaying parturition if injected on days 11 and 16.

A single dose of 4·0 mg/kg androst-5-ene-3β,17β-diol prevented implantation or caused foetal loss in 93% of rats injected on days 5–11 p.c. This hormone was considerably more potent in preventing implantation and causing foetal loss than its androgenic potency as compared with testosterone would suggest. In contrast to testosterone, androst-5-ene-3β,17β-diol did not cause delay in parturition at doses up to 20 mg/kg.

The effects of two other steroids tested, androst-4-ene-3,17-dione and 17α-methyl-androst-5-ene-3β,17β-diol, on pregnant rats were equivalent to or less than their androgenic potency would suggest.

It has previously been reported that high nutrient intakes which promote rapid maternal growth throughout pregnancy are associated with poor pregnancy outcome when compared with normally growing adolescent animals. The present study examined the maternal plasma concentrations of a number of putative endocrine regulators of nutrient partitioning between the maternal and fetal compartments in relation to placental and fetal growth in this novel experimental paradigm. Embryos were recovered on day 4 after oestrus from superovulated adult ewes that had been inseminated using semen from a single sire and synchronously transferred, in singleton, to the uterus of peripubertal adolescent recipients (n = 38), which had been induced to ovulate at 32 weeks of age (live weight 47.4 +/- 0.4 kg). Post-transfer, the adolescent recipients were offered a high (n = 21) or moderate (n = 17) level of a complete diet calculated to achieve rapid (RMG) or normal (NMG) maternal growth rates. After day 100 of gestation, the feed intake of the NMG group was adjusted weekly to meet the increasing nutrient demands of the gravid uterus. Pregnancy rate following embryo transfer was higher (P < 0.05) in the RMG (90%) than in the NMG (59%) group. For ewes delivering live young at term, liveweight gain during the first 100 days of gestation was 294 +/- 12.9 and 84 +/- 4.7 g/day for the RMG (n = 16) and NMG (n = 10) groups respectively, and body condition score immediately prior to parturition was higher in RMG than in NMG ewes (2.9 +/- 0.04 vs 1.9 +/- 0.15 score units respectively, P < 0.001). For the RMG and NMG groups respectively, mean placental weight was 327 +/- 18.1 and 485 +/- 16.6 g with lamb birth weights of 3.49 +/- 0.13 and 4.82 +/- 0.21 kg (P < 0.001). The reduction in placental mass in the RMG group reflected a decrease in the number (P < 0.001) and size (P < 0.01) of the fetal cotyledons. The duration of gestation was shorter (P < 0.001) and colostrum yield at parturition lower (P < 0.001) in the RMG group. Maternal insulin concentrations, determined three times weekly, were higher (P < 0.001) throughout gestation in the RMG group and irrespective of treatment group were negatively correlated (P < 0.01) with placental weight and lamb birth weight. High glucose levels throughout gestation and a decreased response to an exogenous insulin challenge on day 95 of gestation implied a degree of insulin resistance in the RMG group but, in spite of these high maternal glucose concentrations, the reduced size of the placenta probably constrained fetal growth. Maternal IGF-I levels determined weekly, were elevated (P < 0.001) during the second and third trimester in RMG versus NMG groups and a sustained elevation in maternal tri-iodothyronine and thyroxine concentrations was evident in the RMG group from mid-gestation. In contrast, GH pulse frequency and mean GH concentrations, determined on day 68 and 122 of gestation, were lower (P < 0.05) in the RMG group, and irrespective of treatment group, were correlated negatively with feed intake and positively with placental weight and colostrum yield at parturition. Progesterone concentrations were lower in the RMG group during the second and third trimesters (P < 0.001) and, irrespective of treatment group, were positively associated (P < 0.001) with placental weight, gestation length and colostrum yield. These results suggest that in pregnant adolescent sheep on high dietary intakes, elevated insulin and IGF-I levels ensure that the anabolic drive to maternal tissue synthesis is established during early gestation at the expense of placental growth. The consequent restriction in placental transport capacity is the primary limitation to fetal growth and reduced GH and placental progesterone secretion may impair colostrum production.