Luteal expression of cytochrome P450 side-chain cleavage, steroidogenic acute regulatory protein, 3beta-hydroxysteroid dehydrogenase, and 20alpha-hydroxysteroid dehydrogenase genes in late pregnant rats: effect of luteinizing hormone and RU486

A decrease in serum progesterone at the end of pregnancy is essential for the induction of parturition in rats. We have previously demonstrated that LH participates in this process through: 1) inhibiting 3beta-hydroxysteroid dehydrogenase (3beta-HSD) activity and 2) stimulating progesterone catabolism by inducing 20alpha-hydroxysteroid dehydrogenase (20alpha-HSD) activity. The objective of this investigation was to determine the effect of LH and progesterone on the luteal expression of the steroidogenic acute regulatory protein (StAR), cytochrome P450 side-chain cleavage (P450(scc)), 3beta-HSD, and 20alpha-HSD genes. Gene expression was analyzed by Northern blot analysis 24 and 48 h after administration of LH or vehicle on Day 19 of pregnancy. StAR and 3beta-HSD mRNA levels were lower in LH-treated rats than in rats administered with vehicle at both time points studied. P450(scc) mRNA levels were unaffected by LH. The 20alpha-HSD mRNA levels were not different between LH and control rats 24 h after treatment; however, greater expression of 20alpha-HSD, with respect to controls, was observed in LH-treated rats 48 h after treatment. Luteal progesterone content dropped in LH-treated rats at both time points studied, whereas serum progesterone decreased after 48 h only. In a second set of experiments, the anti-progesterone RU486 was injected intrabursally on Day 20 of pregnancy. RU486 had no effect on 3beta-HSD or P450(scc) expression but increased 20alpha-HSD mRNA levels after 8 h treatment. In conclusion, the luteolytic effect of LH is mediated by a drop in StAR and 3beta-HSD expression without effect on P450(scc) expression. We also provide the first in vivo evidence indicating that a decrease in luteal progesterone content may be an essential step toward the induction of 20alpha-HSD expression at the end of pregnancy in rats.     

The suppressive effect of the maternal pituitary on relaxin secretion during the second half of pregnancy in rats does not require the presence of the nonluteal ovarian tissue

This study was conducted to determine whether the suppressive effect of the maternal pituitary on relaxin secretion, luteal growth, and progesterone secretion during the second half of pregnancy in rats (Golos and Sherwood, 1984) requires the presence of the nonluteal ovary. At proestrus, rats received unilateral follicular autotransplants to the kidney capsule. These developed into ectopic corpora lutea (eCL) which maintained pregnancy following removal of the in situ ovary on Day 13 of pregnancy. On Day 8, the conceptus number was left unchanged (eCL-5C) or adjusted to 2 (eCL-2C). Serum relaxin levels, eCL relaxin levels, and eCL weights were significantly lower in eCL-2C rats than in eCL-5C rats from Day 12 through Day 20. Following hypophysectomy on Day 13, serum relaxin levels increased significantly in both groups. Mean luteal weights also increased following hypophysectomy of eCL-2C rats. Although not statistically significant, serum progesterone levels tended to be higher in eCL-5C rats than in eCL-2C rats from Day 12 until Day 18; serum progesterone levels tended to increase following hypophysectomy regardless of conceptus number. It is concluded that the presence of the non-luteal components of the ovary are not required for the suppressive effect of the maternal pituitary on relaxin secretion and luteal growth during the second half of pregnancy.     

Effect of alpha-melanotropin hormone on serum levels of luteinizing hormone and progesterone in experimental rat autoimmune oophoritis

We studied the effect of alpha-melanotropin hormone (alpha-MSH) on experimental autoimmune oophoritis (EAO), an inflammatory process induced in female rats. During proestrus, serum levels of LH and progesterone in rats with EAO were higher than those of control rats. However, administration of alpha-MSH to these rats decreased the levels of LH. Similarly, in the following diestrus, rats with EAO had high levels of LH but treatment with alpha-MSH decreased the levels to diestrus 2 control values. Treatment with alpha-MSH also reduced the LH levels of control rats in diestrus 2 compared to untreated controls. However, alpha-MSH treatment had no effect on progesterone levels of either control or rats with EAO. Thus, although alpha-MSH induced notable changes in levels of LH, this decrease was unable to block the illness.     

Relationship between luteinizing hormone and decidual luteotropin in the maintenance of luteal steroidogenesis

Between Days 6-11 of pregnancy or pseudopregnancy, the decidual tissue of the rat produces a prolactin-like hormone, decidual luteotropin, which can sustain luteal progesterone production when prolactin is suppressed. However, this effect is dependent upon the presence of the pituitary. The present investigation was undertaken to determine whether decidual luteotropin and luteinizing hormone (LH) act together to sustain luteal steroidogenesis and if so, to find out whether the need for LH is due to the inability of the decidual tissue to produce LH-like material and/or whether LH affects decidual luteotropin production. Pseudopregnant rats with or without decidual tissue were hypophysectomized on Day 8 and treated with either 1.5 IU human chorionic gonadotropin (hCG)/day or with vehicle. Within 24 h, serum progesterone dropped in both vehicle-treated groups and decidual luteotropin levels declined by 80% in the decidual tissue. Human CG administration had no effect on progesterone production in the control group. Yet in rats with decidual tissue, hCG stimulated progesterone production for at least 48 h and maintained the decidual tissue content of decidual luteotropin. Progesterone, but not hCG treatment, maintained decidual luteotropin concentrations in ovariectomized rats. To compare the luteotropic activity of the decidual tissue with that of the placenta, pregnant or pseudopregnant rats with decidual tissue were hypophysectomized on Day 8 and treated with 1.5 IU hCG. Control groups had decidual tissue or placentas removed and were similarly treated. Human CG stimulated progesterone production only in rats with placental or decidual tissue.(ABSTRACT TRUNCATED AT 250 WORDS)     

Ovarian steroid production in rats treated with gonadotropin-releasing hormone during early pregnancy

In the pregnant rat, luteinizing hormone (LH) stimulates the ovarian production of testosterone (T) which is aromatized to estradiol (E2). E2 promotes progesterone (P) synthesis by the ovary. To determine if the administration of gonadotropin-releasing hormone (GnRH) disrupts pregnancy by suppressing ovarian steroid production, rats were treated on days 7-12 of pregnancy with 25, 50 or 100 micrograms/day of GnRH or 0.2, 1 or 5 micrograms/day of a GnRH agonist (GnRH-Ag). The higher two doses of GnRH or GnRH-Ag within 24 h suppressed peripheral levels of plasma P and terminated pregnancy within 48 h. By day 12, P levels in the ovarian vein in rats treated with GnRH or GnRH-Ag in respective doses were 2098 +/- 261, 732 +/- 437, 110 +/- 15, and 2575 +/- 463, 49 +/- 9, 43 +/- 8 compared to 1833 +/- 433 ng/ml in controls. Daily treatment of P (4 mg) and E2 (0.5 microgram) simultaneously with GnRH-Ag at its maximum dose reversed the abortifacient effect of GnRH-Ag and maintained pregnancy. Peripheral levels of Plasma LH in all groups were higher than controls on days 10 and 12. Ovarian vein levels of T on days 10 or 12 of pregnancy were either not significantly different from controls (at 2703 +/- 607 or 3249 +/- 690 pg/ml, respectively) or increased dramatically to 9547 +/- 1769 on day 10 and to 5985 +/- 1426 pg/ml on day 12 in rats treated with 0.2 microgram of GnRH-Ag. Similarly, ovarian vein levels of E2 on days 10 or 12 were either not significantly different from controls (at 2022 +/- 227 or 2793 +/- 184 pg/ml, respectively) or increased dramatically to 2980 +/- 58 pg/ml on day 10 in rats treated with 25 micrograms of GnRH or to 3296 +/- 241 on day 10 and to 3420 +/- 325 pg/ml on day 12 in rats treated with 0.2 microgram of GnRH-Ag. These results indicate that the abortifacient effect of GnRH administration in rats is not due to its effect on the uterus, but to its suppressive effects on ovarian P secretion. There was no evidence to show that a GnRH-induced fall in ovarian secretion of either T or E2 were involved in this process.     

Role of the nongravid part of the uterus in the luteolytic effects of estrogen in pregnant rats

Effects of a low dose of estradiol on the luteal function were studied in intact pregnant rats. The pregnant rats received daily sc injections of 0.1 microgram estradiol or sesame oil from day 7 to 14 of pregnancy (day 1 = day of insemination). Serum progesterone levels on day 15 were significantly lower in the estrogen-treated group than in the oil-treated group. In order to study how estrogen induced luteolysis, the pregnant rats received each of the following treatments on day 7 of pregnancy: (1) The uterus except that containing a single conceptus was removed by hysterectomy (hysterectomy group); (2) All but a single conceptus were removed by aspiration, so that rats carried only a single conceptus with the whole part of the nongravid uterus (aspiration group). Each group of rats received also sc injections of 0.1 microgram estradiol or sesame oil from day 7 to 14 of pregnancy. Estradiol treatment caused a significant decline in serum progesterone levels in the aspiration group on day 15, but this was not the case in the hysterectomy group. There was no significant difference in serum LH levels among any of the groups on day 15 of pregnancy. These results indicated that estradiol induced luteolysis in the intact pregnant rats, which would, at least in part, be mediated through the uterus.     

The relationship of conceptus number and fetal sex to maternal serum testosterone concentration in the rat

On Day 8 of pregnancy, the number of implantation sites in pregnant rats was adjusted to 1, 2, 4, 6, or greater than 10. Blood was collected on Days 11, 12, 15, 18, and 20 for the determination of serum testosterone, progesterone, and androstenedione. Serum testosterone levels exhibited a direct linear relationship with implantation number, increasing from 1 through greater than 10 implants. This linear relationship was particularly evident at Days 12, 15, and 18 of pregnancy. Serum progesterone levels increased from one to four conceptuses and plateaued above this number. There was no apparent relationship between the number of conceptuses and serum androstenedione levels, which may reflect the multiple origins of this steroid in the pregnant rat. In a separate group of rats in which the number of conceptuses was adjusted to three on Day 8 of pregnancy, blood was collected on Days 11, 12, 15, 18, and 20. Fetal sex was determined between the last bleeding and the day of parturition. Serum testosterone was determined and results were examined with regard to the number of male/female fetuses in the litter of three. There was no relationship between maternal serum testosterone levels and the number of male fetuses, indicating that the fetal testis does not make a significant contribution to circulating maternal testosterone levels.     

The effects of luteinizing hormone releasing hormone (LH-RH) in pregnant rats. I. Postnidatory effects.

The effects of LH-RH on pregnancy in rats were investigated. A single 500 mcg injection of LH-RH on Days 9, 10, or 11 of pregnancy terminated pregnancy, whereas injection on Days 6-8 or 13-16 had little or no effect. The ED 50 on Day 10 for b.i.d. administration was 150 mcg and 550 mcg for a single injection. Administration on Day 9 was followed by a decrease in circulating progesterone levels on Days 10 and 11. The administration of large doses of progesterone reversed the effects of LH-RH administration on Days 7-12. Treatment with estradiol-17beta did not potentiate the effect of progesterone, but appeared to slightly retard fetal resorption when administered alone. The results suggest that the antifertility effect of LH-RH is mediated via functional luteolysis.     

Progesterone is not responsible for the blood pressure fall in late-pregnant New Zealand genetically hypertensive rats

In various models of experimental and genetic hypertension in rats, blood pressure is markedly reduced during late pregnancy. The period during which the blood pressure reduction occurs is also the period when plasma progesterone is maximally elevated, and administration of progesterone to renal hypertensive rats has been reported to reduce blood pressure (J. Armstrong, 1959, Proc. Soc. Exp. Biol. Med. 102:452-455). To test the possibility that elevated plasma progesterone is responsible for the blood pressure reduction in late pregnancy, on Day 14 of pregnancy a group of New Zealand genetically hypertensive (NZGH) rats was ovariectomized and implanted with progesterone-filled capsules, to maintain plasma progesterone at low levels just sufficient to maintain pregnancy, and compared with intact, pregnant NZGH. Ovariectomy did not alter the characteristic course of blood pressure reduction seen in late-pregnant intact NZGH rats. In addition, daily administration of progesterone (15 mg/kg, sc) for 14 days did not alter blood pressure of either nonpregnant NZGH rats or New Zealand normotensive rats with chronic 1-kidney, 1-clip hypertension. It is concluded that blood pressure of NZGH rats is reduced to near normotensive levels in late pregnancy, as reported for other models of rat hypertension, but that elevated plasma progesterone levels are not requisite for that reduction and do not reduce blood pressure of renal hypertensive rats.     

Porcine follicular fluid treatment at proestrus diminishes the serum progesterone level of rats in early pregnancy

Proestrous rats were treated with porcine follicular fluid (pFF) or porcine serum (pS) extract, afterwards they were put with males together. Next morning, the sperm positive females were considered as day 1 pregnant animals. On days 2, 8 and 14 of pregnancy serum progesterone level was determined by RIA. On days 2 and 8 of pregnancy serum progesterone level of pFF treated animals was significantly lower than that of pS treated ones, but it was not different from the controls on day 14 of pregnancy. The decreased progesterone level indicates that there are biologically active endogenous substances in the pFF (presumably inhibin or granulosa cell luteinization inhibitor) which may responsible for some forms of corpus luteum insufficiency and for some unexplained infertility cases.     

Role of luteinizing hormone-releasing factor (LH-RF) and LH on maintenance of early gestation in rats.

The role of luteinizing hormone (LH) and LH-releasing hormone (LH-RH) in the maintenance of early pregnancy in rats was studied. Serum levels of progesterone (P) and LH were measured daily in untreated pregnant rats from Day 4 through parturition. Serum levels of P and LH were determined on Days 11 and 15 of pregnancy in animals treated with antisera to LH (LH-A/S) and to LH-RH (LH-RH-A/S) on Days 8-10. Serum levels of P peaked on Days 7 and 16 in untreated animals, after which they declined sharply just before delivery. Serum LH fluctuated between 30-160 ng/ml during pregnancy but did not exhibit any distinctive peaks. Treatment with .2 ml LH-A/S on Days 8-10 reduced serum P to virtually undetectable levels on Day 11, and only a slight recovery was evident on Day 15. Lower doses of LH-A/S had no effect. Administration of 1.3 ml LH-RH-A/S had no effect on serum levels of P or LH, and did not impede fetal development. The results indicate that LH is essential to the luteotropic complex of early pregnancy in the rat, and also that LH-RH-A/S can maintain to some extent basal levels of P and LH during early pregnancy.     

Effects of prostaglandins on peripheral progesterone and uterine diamine oxidase in the pregnant hamster

Serum progesterone and uterine levels of diamine oxidase (DAO) activity were determined during pregnancy in hamsters. Progesterone was elevated on Day 1 of pregnancy, had a transient peak on Day 5, remained relatively constant on Days 6–10, and then increased on Days 13 and 14. Uterine DAO activity could not be detected until Day 7 of pregnancy, approximately 1 $\text{1}\text{2}$ days after the initiation of implantation. DAO activity was associated with placental tissue, and more than 90% of the activity was localized in the maternal placenta. The temporal relationship between changes in serum concentrations of progesterone and uterine levels of DAO activity following PG administration also was studied. Serum progesterone was significantly depressed by 6 hr after treatment with PGs on Day 7 of pregnancy. However, uterine levels of DAO activity at 6 hr in the treated animals were not different from those in control animals. In contrast, both the serum progesterone concentrations and uterine levels of DAO activity were significantly lower at 24 hr after PG treatment. The effects of PG treatment on uterine DAO activity were completely blocked by concomitant administration of progesterone. However, concomitant administration of Provera® only blocked the effect of one PG analog that was tested (9-deoxo-9-methylene-16,16-dimethyl0-PGE₂). The data indicate that changes in uterine DAO activity following treatment with the PGs used here are primarily a consequence of a decrease in peripheral progesterone (i.e. a luteolytic effect of the PG).     

Replacement of progesterone restores the nocturnal surge of prolactin following suppression with a gonadotropin-releasing hormone agonist during early pregnancy in the rat

Continuous administration of a GnRH agonist (GnRH-Ag) at a dose of 5 micrograms/day, commencing on day 7 of pregnancy resulted in the suppression of daily nocturnal surges of prolactin (PRL) on day 8, and serum progesterone (P4) levels with subsequent termination of pregnancy. Replacement with dydrogesterone, a synthetic analog of P4 at a dose of 4 mg/day s.c. restored the magnitude of nocturnal PRL surges. These data suggest that GnRH-Ag may act either at the level of the brain to suppress the nocturnal PRL surge, resulting in a fall in serum P4 levels or at the level of the corpus luteum itself or at both sites simultaneously to terminate pregnancy.     

Effect of RU 486 on luteal function in the early pregnant rat

A dose of 30 mg RU 486/kg, an antiprogesterone, was administered to pregnant rats on Day 2 (Group 1) or Day 4 (Group 2) of pregnancy. RU 486 significantly changed serum progesterone and oestradiol concentrations and luteal 3 beta-HSD and 20 alpha-HSD activities in Group 1, and implantation was significantly inhibited. The luteal 3 beta-HSD activity in Group 2 rats on Day 6 was significantly (P less than 0.01) lower than the control value (7.5 +/- 0.6 and 10.1 +/- 0.6 mU/mg protein respectively). This decline in the 3 beta-HSD activity was followed by a marked decrease in the serum progesterone concentration, resulting in a significant decrease of the progesterone/oestradiol ratio and implantation was completely inhibited. The 20 alpha-HSD activity, which could not be detected on Day 6 in the control rats, was twice as great in Group 2 than in Group 1 rats (17.5 +/- 1.2 and 7.4 +/- 3.1 mU/mg protein respectively). Ultrastructural examination of corpora lutea of Group 2 rats confirmed luteolysis. These results suggest that RU 486 has a luteolytic effect and its anti-implantation effect is concomitant with luteolysis of the corpora lutea of pregnancy.     

Effects of gonadotropin-releasing hormone treatment on ovarian steroid production during midpregnancy

During the second half of pregnancy, ovarian testosterone (T) through its conversion to estradiol (E) promotes progesterone (P) synthesis by the ovary which maintains the pregnancy. To determine if the administration of gonadotropin-releasing hormone (GnRH) disrupts pregnancy by suppressing ovarian production of T or its conversion to E, rats were treated from Day 11 through Day 18 of pregnancy with 50 or 100 micrograms/day of GnRH or 1, 5, or 10 micrograms/day of a GnRH agonist (GnRH-Ag; WY-40972) using an osmotic minipump. Rats were bled daily from the jugular vein under light ether anesthesia and on Days 14 or 18 of pregnancy both jugular and ovarian blood samples were obtained. While the GnRH-Ag treatment at the dose of 5 or 10 micrograms/day terminated pregnancy within 48 hr as indicated by vaginal bleeding, 1 microgram/day terminated pregnancy more slowly. Neither dose of GnRH was effective in terminating pregnancy through Day 18. By Day 14, peripheral levels of plasma P in rats treated with 0, 1, 5, or 10 micrograms of GnRH-Ag were 97 +/- 9, 24 +/- 1, 13 +/- 3, and 8 +/- 1, respectively. In the same groups, levels in the ovarian vein were 3205 +/- 633, 1317 +/- 273, 360 +/- 113, and 228 +/- 73 ng/ml. By Day 18, serum P levels in the peripheral circulation and in the ovarian vein were declining even more dramatically. Daily administration of P (4 mg) and E (0.5 micrograms) simultaneously with GnRH-Ag at the dose of 5 micrograms/day from Days 11 through 14 reversed the abortifacient effect of GnRH-Ag and maintained pregnancy indicating that the GnRH-Ag effect is not directly on the uterus. Ovarian vein levels of T on Days 14 or 18 of pregnancy were either not different from controls at 1407 +/- 163 or 1476 +/- 122 pg/ml, respectively, or increased dramatically in certain groups. Ovarian vein levels of E were either not different from controls at 292 +/- 13 pg/ml on Day 14 or increased significantly in rats treated at the dose of 1 microgram/day of GnRH-Ag. However by Day 18, treatment with GnRH-Ag at all doses suppressed ovarian secretion of E. These results suggest that while the GnRH-Ag induces abortion in rats by suppressing ovarian production of P, this abortifacient effect is not due to a fall in ovarian T levels nor to its aromatization to E in the ovary.     

Suckling-induced prolactin release potentiates mifepristone-induced lactogenesis in pregnant rats

Suckling, starting at 19:00 h on Day 18 of pregnancy, induced a significant increase in serum prolactin concentration at 20:00 h on Day 19 of pregnancy, but no increase in mammary gland casein or lactose content. Mifepristone (2 mg/kg) injection at 08:00 h on Day 19 of pregnancy induced significant increases in casein, but not in lactose, 24 h after administration. Mifepristone alone did not induce prolactin secretion, indicating that lactogenesis was induced by placental lactogen in the absence of progesterone action. When mifepristone was injected into suckling rats, serum prolactin concentrations were higher than in the untreated suckling rats. Casein in these rats increased significantly 12 h after mifepristone administration and lactose at 24 h after. If the suckling mifepristone-treated rats were given two injections of bromocriptine (1.5 mg/kg) at 12:00 h on Days 18 and 19 of pregnancy, serum prolactin concentrations were not increased by suckling, but casein and lactose concentrations in the mammary gland showed values similar to those obtained in the mifepristone-treated non-suckling rats. Mifepristone can therefore potentiate suckling-induced prolactin release in pregnant rats, demonstrating a direct central inhibitory action of progesterone on prolactin secretion. This suckling-induced prolactin secretion, unable to induce casein or lactose synthesis in the presence of progesterone, enhanced significantly synthesis of these milk components in the absence of progesterone action (rats treated with mifepristone). Fatty acid synthase, which is stimulated by the suckling stimulus in lactating rats, was not modified by mifepristone or suckling in pregnant rats.     

Induction of apoptosis by a gonadotropin-releasing hormone agonist during early pregnancy in the rat

We have demonstrated that continuous administration of a GnRH-agonist (GnRH-Ag) in the rat during early pregnancy suppressed plasma progesterone levels within 8 h after the commencement of treatment. The purpose of the present study is to evaluate the hypothesis that in vivo GnRH-Ag treatment induces apoptotic cell death during early pregnancy. Rats were treated individually on day 8 of pregnancy with 5 g/day of GnRH-Ag via an osmotic minipump. Sham-controlled rats received no treatment. Rats were killed at 4, 8 or 24 h after the treatment. At autopsy, blood samples were obtained and ovaries were removed. The corpora lutea (CL) from one ovary were removed for DNA laddering and RNA studies. As early as 8 h after the commencement of treatment, GnRH-Ag suppressed serum progesterone levels as expected, and increased the degree of DNA degradation in the CL that was time-dependent. At 24 h after the treatment, GnRH-Ag increased the Bax, a cell death gene, expression in the CL. Collectively, these data suggest that GnRH-Ag treatment induces apoptosis during early pregnancy in the rat.     

Synchronization of estrus in embryo transfer recipients receiving demi-embryos with Syncro-Mate B or estrumate

One hundred thirty-five crossbred heifers were used to compare the effectiveness of Estrumate and Syncro-Mate B in synchronizing estrus in recipient heifers for transfer of demi-embryos. A higher percentage (P<0.01) of Syncro-Mate B heifers showed estrus within 5 d following treatment than did Estrumate heifers (100.0 vs 85.1%). Demi-embryos were transferred to 41 and 26 heifers treated with Syncro-Mate B and Estrumate, respectively. The pregnancy rate of heifers synchronized with Estrumate was higher (P<0.01) than that of heifers synchronized with Syncro-Mate B (64.3 vs 27.8%). A single blood sample was collected on the day of embryo transfer from all heifers receiving demi-embryos, and progesterone concentration was determined. There were no significant differences in mean serum progesterone concentrations either between heifers synchronized with Syncro-Mate B and Estrumate or between pregnant and open heifers within synchronization treatment. There was no relationship between serum progesterone concentration and pregnancy rate in the Estrumate recipients (P>0.10). However, 25.0% of the Syncro-Mate B recipients had serum progesterone levels <1.00 ng/ml, and these heifers had a lower pregnancy rate (P<0.05) than Syncro-Mate B heifers with serum progesterone levels between 1.00 to 3.00 ng/ml. It appears that the Syncro-Mate B heifers with <1.00 ng/ml progesterone had either a delay in corpora lutea function or a continuously reduced concentration of serum progesterone which altered the uterine environment of these heifers and caused the lower pregnancy rate in the Syncro-Mate B group. Based on the large difference in pregnancy rates, Estrumate appears to be a more effective method to synchronize estrus in recipient heifers receiving demi-embryos.     

Effects of estradiol and progesterone on early embryonic development in aging rats

Regularly cyclic, middle-aged female rats exhibit a decreased incidence of fertility, and those females that are fertile produce small litters. These decreases in fertility and litter size are associated with reduced numbers of normal blastocysts formed and implanted, suggesting that pre- and/or peri-implantation failures may be the causes for these aging-related reproductive declines. The present study examined the relationships and influence of circulating estradiol (E2) and progesterone (P) levels on early embryonic development and implantation in middle-aged rats. Serial blood samples obtained from cannulated, middle-aged pregnant rats revealed minor decreases in plasma P and increases in E2 levels during Days 2-4 of pregnancy, compared to young pregnant rats, resulting in significantly (p less than 0.001) decreased plasma P/E2 ratios. These alterations in endogenous hormone secretion in middle-aged pregnant rats were associated with fewer normal blastocysts on Day 5 of pregnancy and reduced numbers of normally implanting embryos. Correlation analysis further revealed a significant (p less than 0.05) inverse relationship between mean circulating E2 levels and numbers of normal conceptuses on Day 12 of gestation. Moreover, s.c. administration of P implants (in Silastic) to middle-aged pregnant rats increased serum P levels by about 34-40 ng/ml, and significantly (p less than 0.05) reduced the incidence of abnormal embryos before implantation. In contrast, treatment with E2 minipumps produced a sustained rise in serum E2 (by about 7-15 pg/ml) and resulted in the complete absence of embryos in the reproductive tracts by Day 5 of pregnancy.(ABSTRACT TRUNCATED AT 250 WORDS)