4-day Cyclic Rats Research Articles

Age-Related Alterations in Follicular Development and Hormonal Profiles in Rats with 4-Day Estrous Cycles1

Two experiments were conducted to examine the hypothesis that an alteration in follicular development is associated with advancing maternal age in the absence of prolonged estrous cycles. In Experiment 1, serum and four follicles (from one ovary per rat) were collected from young and middle-aged, 4-day cycling rats on estrus or metestrus. Number and diameter of nonatretic antral follicles greater than 200 microns in diameter were determined from serial sections of the other ovary from each rat. In Experiment 2, serum and follicles (12 +/- 2) from both ovaries were collected from young and middle-aged rats on each day of a 4-day estrous cycle. All microdissected follicles were assayed for estradiol-17 beta (E2) and all sera were assayed for E2, follicle-stimulating hormone, and luteinizing hormone by radioimmunoassay. Numbers of follicles greater than 400 microns in diameter did not differ, while numbers of follicles 200-400 microns in diameter were reduced in middle-aged rats compared to young rats (Experiment 1). The mean diameter of follicles greater than 400 microns in diameter and the follicular content of E2 was greater in middle-aged than in young rats. In Experiment 2, a greater proportion of large follicles were observed in middle-aged rats than in young rats on all days, and a greater proportion of follicles with high concentrations of E2 were observed on diestrus. We interpreted these data as indicative of an early age-related change in the control of follicular recruitment, growth, and maturation.

Preprogramming of luteal progestational function by preovulatory luteinizing hormone surge in cycling rats.

Effects of hCG and anti-ovine LH serum (aLH) administration at the time of the preovulatory LH surge on both luteal progesterone secretion and subsequent vaginal cyclicity were investigated. In control 4-day cycling rats, blood progesterone (P) increased from 0400 hr on estrus (E) to 2300 hr on metestrus (M), then declined to a low level by 1800 hr on diestrus (D). A single iv injection of hCG (1.25 or 5 IU) at 1900 hr on proestrus (PE) suppressed the metestrous rise in blood P levels and prolonged the diestrous phase, in a dose-dependent manner, without affecting the numbers of oocytes released at ovulation. Attenuation of the LH surge by a single iv injection of aLH (0.3ml) at 1900 hr on day PE also had a similar effect. Injections of 0.1 ml aLH (ip, every 11-12 hr) from 0800 hr, but not from 0000 hr, on day E in the rats pretreated with hCG on day PE elevated blood P levels, from days E through D, above the values measured in control animals. The lutein cells from rats treated with hCG on day PE were found, by electron microscopy, to be in the process of structural luteolysis at as early as 2300 hr on day M, whereas luteinization appeared still insufficient in the lutein cells from the LH surge-attenuated animals even at this stage. These results confirm a previous finding that tonic levels of LH secreted during the postovulatory period cause luteolysis of a new crop of corpora lutea, and suggest that the magnitude of the ovulatory LH surge preprogrammes luteal function not only by potentiating luteal ability to secrete P but also by determinig the timing of the expression of luteal sensitivity to the suppressing effect of LH. The prolonged diestrous phase accompanied by suppressed luteal function observed in the animals treated with hCG alone on day PE suggests that the magnitude of the LH surge is probably involved also in regulation of the subsequent follicular growth and estrogen secretion.

Prolactin messenger ribonucleic acid concentrations in 4-day cycling rats and during the prolactin surge.

Pituitary PRL mRNA concentrations were measured during the 4-day rat estrous cycle. Adult female Sprague-Dawley rats were killed at 3-h intervals throughout the cycle and hourly between 1000 and 2400 h on proestrus (n = 5-12). Serum PRL was increased on the afternoons of proestrus (P) and estrus (E), with peak concentrations at 1700 h (P, 624 +/- 126; E, 261 +/- 107 ng/ml). PRL mRNA concentrations were elevated during the evening on P and E (2300 h: P, 14.4 +/- 1.5; E, 16.1 +/- 1.3 ng cDNA bound/100 micrograms pituitary DNA) to values 2-fold higher than those at 0800 h on each respective day. On diestrus (D) PRL mRNA levels decreased abruptly during the morning (1100 h, 1.7 +/- 0.3 ng cDNA bound), followed by a 6- to 7-fold increase between 1700 and 2000 h on the same evening. In contrast, PRL mRNA levels were elevated at 0800 h on metestrus (M). The changes in PRL mRNA concentrations obtained on M and D were not associated with increased PRL secretion. A more detailed examination of P revealed that PRL mRNA levels increased during the morning (1000 h, 9.9 +/- 2.6 ng cDNA bound), then decreased abruptly at 1100 h (4.9 +/- 1.2). The morning rise in mRNA concentrations was followed by a 2-fold rise in pituitary PRL content. As serum PRL rose during the afternoon surge, a coincident decrease in pituitary PRL content and an increase in PRL mRNA levels were observed. The relationship between PRL secretion and gene expression was further examined in ovariectomized estradiol-replaced rats receiving either bromocriptine (1.2 mg/day, sc) or vehicle control sc. The vehicle-treated group expressed a characteristic afternoon PRL surge between 1500 and 2100 h. Pituitary PRL decreased during the surge to 10% of morning values, and PRL mRNA levels increased 2-fold beginning 2 h after initiation of the surge. These changes in serum PRL, pituitary PRL, and PRL mRNA levels were abolished by bromocriptine administration. These data reveal that alterations in PRL mRNA concentrations occur on a daily basis during the rat estrous cycle. Increases occur during the evenings of P and E at the time of the increase in PRL secretory activity. The effect of blocking the PRL surge in ovariectomized estradiol-replaced rats suggests a regulatory interaction between secretion and gene expression.(ABSTRACT TRUNCATED AT 250 WORDS)

Human Chorionic Gonadotropin Affects Tissue Levels of Progesterone and Cyclic Adenosine 3′,5′,-Monophosphate in the Metestrus Rat Uterus in Vitro1

The effect of human chorionic gonadotropin (hCG) on in vitro progesterone (P) level in the uterus was investigated by using short-term incubations of uterine tissue taken from 4-day cyclic rats at different stages of the estrous cycle. Control incubations resulted in a decrease of endogenous P content in uteri removed from rats in proestrus (PRO), estrus (EST), and metestrus (MET): -25% (n = 6), -60% (n = 6), and -45% (n = 8), respectively. The amount of P found after incubation of MET tissue in the presence of hCG was significantly higher (p less than 0.001) than that found after control incubations. The hCG effect was dose-dependent and was not observed with PRO or EST tissue. Although the mean P level found in MET tissue after incubation with 10 IU/ml hCG was not significantly different from the mean level found in unincubated tissue (1562 +/- 341 vs. 1470 +/- 174 pg/mg protein, n = 8), an obvious synthesis was observed in two experiments. It thus seems likely that observed hCG effect would involve a de novo P synthesis rather than a decrease of P catabolism. Furthermore, hCG induced a dose-dependent increase of cyclic adenosine 3', 5'-monophosphate (cAMP) uterine levels in MET tissue. N,O'-dibutyryl cyclic AMP [Bu)2 cAMP) at 5 mM induced a significative increase (p less than 0.01) of P uterine level in EST and MET tissue compared to control incubations, but had no effect on PRO tissue. Our results suggest the progressive maturation throughout the rat estrous cycle of a luteinizing hormone/hCG- and cAMP-dependent process able to regulate uterine P content.(ABSTRACT TRUNCATED AT 250 WORDS)

Rat ovarian renin: characterization and changes during the estrous cycle.

To demonstrate the existence and help clarify the function of renin in the rat ovary, we have characterized rat ovarian renin and examined ovarian renin levels during different stages of the rat estrous cycle. We show that high concentrations of active renin are present in the rat ovary (2.9 ng angiotensin I/h/mg). Ovarian renin activity has a pH optimum of about 7.0 and is due to a glycosylated aspartyl protease with an apparent mol wt of 39,000. These properties of rat ovarian renin are identical to previously characterized rat kidney renin. In PMSG-treated immature and adult 5-day cycling rats, ovarian renin was increased about 2-fold at estrus. At all stages of the estrous cycle in the 5-day cycling rat, the ratio of active to inactive ovarian renin was about 3:1, whereas about 90% of the renin in plasma was inactive. In the hypophysectomized diethylstilbestrol-treated rat ovary, over 90% of active renin remained in the residual ovary after the granulosa cells had been expressed, suggesting a theca-interstitial localization for renin. These studies indicate that active renin exists in the rat ovary, that its levels are cyclically increased at estrus, and that this increase may be due to enhanced local production and activation of the renin precursor. These findings greatly strengthen the concept of a functional renin-angiotensin system in the rat ovary.

Regulation of luteal function by post-ovulatory luteinizing hormone secretion in 4-day and 5-day cycling rats.

Regulation of luteal function by post-ovulatory luteinizing hormone secretion in 4-day and 5-day cycling rats.

Pituitary responsiveness to LHRH during pregnancy in the rat: effect of progesterone

The LH and FSH responses to a standard infusion of LHRH were studied on days 8, 12, 15, 17, 19, 21 and 22 after conception as well as on day 23, i.e. after parturition. Groups of rats were also killed on days 8, 15, 19, 22 and 23 and on the day of pro-oestrus of the 4-day cycle for the assay of progesterone, 20 alpha-dihydroprogesterone (DHP), oestradiol-17 beta, LH and FSH. Finally, the post-partum surges of LH and FSH were compared with those at pro-oestrus in 4-day cyclic rats. The LH and FSH responses to LHRH were relatively low on days 8 and 12, twice as high on days 15, 17 and 19, had increased further on day 21 and reached maximal values on day 22. The gonadotrophin responses were low on day 23. The post-partum surges of LH and FSH were much higher than the pro-oestrous surges. Pituitary contents of LH and FSH were higher than on pro-oestrus of the 4-day cycle. On day 23, however, the pituitary contents had declined by 60-80%. No apparent relationship was found between plasma concentrations of LH and FSH and LHRH responsiveness during pregnancy. Concentrations of oestradiol-17 beta on day 22 were higher than on all other days of pregnancy, but lower than on pro-oestrus of the 4-day cycle. Concentrations of progesterone were high until day 19 and low on days 22 and 23; the concentration of DHP was low until day 19 and very high on days 22 and 23.(ABSTRACT TRUNCATED AT 250 WORDS)

Retardation of preovulatory desensitization to negative oestrogen feedback: mechanism of the effect of progesterone in 5-day cyclic rats.

Recent studies have shown that oestrogen can induce desensitization to its own gonadotrophin-inhibiting effect in female rats by an action on the medial preoptic area (MPOA). Probably as a consequence of this action, sensitivity to the negative oestrogen feedback declines markedly between metoestrus and dioestrus of the 4-day ovarian cycle. To study this desensitization process in 5-day cyclic rats, females exhibiting regular 5-day vaginal cyclicity were ovariectomized on consecutive days of the cycle, injected with oestradiol benzoate (OB) or oil on the day of ovariectomy and autopsied 24 h after the injection. Estimation of the serum concentration of LH revealed that desensitization to negative oestrogen feedback occurred only between day 2 of dioestrus and pro-oestrus, i.e. 2 days later than in females with a 4-day cycle. In the latter animals, an injection of progesterone in metoestrus or early dioestrus, which induced lengthening of the ovarian cycle for 1 day, delayed the onset of desensitization to a degree similar to that found in spontaneously 5-day cyclic rats. In acutely ovariectomized females, progesterone implants placed in the MPOA, but not those located in the mediobasal hypothalamus, increased the LH-inhibiting effect of low doses of OB. The results suggest that the prolonged secretion of progesterone recorded in 5-day cyclic rats retards follicle maturation and delays the forthcoming ovulation by acting, at least partly, on the MPOA and antagonizing the desensitizing effect of oestrogen. In this way, inhibition of gonadotrophin secretion by oestrogen is enhanced and the increase in tonic LH secretion necessary for the completion of follicle maturation is retarded.

Stimulation of estradiol-17β secretion by 7,12-dimethylbenz (a) anthracene during mammary tumor induction in sprague-dawley rats

Testosterone, androstenedione, progesterone, 17-hydroxyprogesterone, estrone and estradiol-17β serum levels were measured at given times after dimethylbenz (a) anthracene (DMBA) treatment of a sensitive rat strain Sprague-Dawley (S-D) and a resistant strain Wistar (W). Tumors appeared with a 100% incidence around the 14th to 15th estrous cycle after DMBA treatment in Sprague-Dawley rats. Hormonal determinations were made, during the 5th or 6th estrous cycle after DMBA treatment, in groups of 4-day cycling rats of both strains which were given DMBA or the carrier solution (sesame oil) when they were about 55-days old. In Sprague-Dawley female rats, DMBA treatment significantly stimulated estradiol-17β and estrone preovulatory surge on proestrous days. No such stimulation was found for any other steroid at any time of the estrous cycle. On the other hand, the resistant Wistar rats did not show any disturbed preovulatory or basal steroid hormone release after the carcinogen treatment. These results complete and explain previous findings concerning the hypothalamo-pituitary activity after DMBA treatment of S-D rats: an early and persistent alteration in the centers involved in the hormonal cyclicity of the hypothalamo-pituitary-ovarian axis must be a result of the DMBA treatment. This deregulation could probably account for the distant and selective production of tumors in the mammary gland induced by a single gastric administration of DMBA.

Preovulatory secretion of progesterone, luteinizing hormone, and prolactin in 4-day and 5-day cycling rats.

Timing of ovulation and changes in plasma progesterone, luteinizing hormone (LH), and prolactin (PRL) during periovulatory stages were determined in Holtzman rats exhibiting regular 4- or 5-day cycles under a daily artificial illumination from 0500 to 1900 h. The 5-day cycling rats ovulated between 0130 and 0930 h on estrus, whereas some of the 4-day cycling animals ovulated as early as about 0130 h and others as late as 1130 h on estrus. Onset time of preovulatory LH and progesterone surges was about 1500 h on proestrus in both the 4- and the 5-day cycling rats. Peak levels of plasma LH and progesterone were measured at 1700 to 1900 h on proestrus, while the first rises and peak values of plasma PRL were evident a few hours earlier than those of plasma LH in the rats with two cycle lengths. Plasma LH levels at 1900 h on proestrus as well as plasma progesterone levels at 1600 and 2300 h on proestrus and at 0130 and 0330 h on estrus were significantly lower in the 5-day cycling rats than in the 4-day cycling animals (p less than 0.05). In contrast, PRL levels from 1500 through 2300 h on proestrus remained consistently higher in 5-day cycling rats than in 4-day cycling rats, and significant differences in PRL levels between these rats were apparent at 1500, 1600, and 2100 h (p less than 0.05-0.01). Thus, these results demonstrate that the 5-day cycling rats exhibit the attenuated magnitude of LH surge accompanied by the augmented preovulatory PRL release, and that plasma progesterone levels reflect the magnitude of LH surge. A tentative working hypothesis concerning the etiology of the 5-day cycle has been proposed.

ALPHA AND LUTEINIZING HORMONE BETA SUBUNIT MESSENGER RIBONUCLEIC ACIDS DURING THE RAT ESTROUS CYCLE

Alpha and LH beta subunit mRNAs were measured in pituitaries of 4-day cycling rats during the estrous cycle. A two-fold increase in alpha mRNA occurred between 0800-2000 h on diestrus, but alpha mRNA concentrations were stable during other days of the cycle. LH beta mRNA concentrations were low during estrus and metestrus (11-16 pg cDNA bound/100 micrograms pituitary DNA), but were elevated (27-30 pg) between 0800-2000 h on diestrus. A second increase in LH beta mRNA was observed on the afternoon of proestrus, prior to the onset of the LH surge with maximum values (45 pg) coincident with peak LH secretion. LH beta mRNA concentrations declined rapidly and had fallen to basal values by midnight on proestrus. These data show that alpha and LH beta mRNAs change in a similar manner during metestrus, diestrus and estrus, suggesting coordinate regulation of alpha and LH beta gene expression at these times. During the LH surge, however, LH beta mRNA alone is increased, suggesting that the LH beta gene can be differentially expressed at times when maximum LH secretion is occurring.

Pituitary regulation of corpus luteum progesterone secretion in cyclic rats.

Pituitary LH and PRL secretion during the early postovulatory period of the rat estrous cycle seem to affect the corpus luteum (CL) autonomy to secrete progesterone. Thus, while PRL would act luteotropically, LH would be luteolytic. To further investigate these facts, 4-day cyclic rats, treated with either 1 mg bromocriptine (CB) or 0.25 ml 70% ethanol (ETOH) at 1600 h on estrus, were injected with 0.5 ml of either an anti-LH serum (LHAS) or normal horse serum (NHS) at 0800 h on metestrus. Rats treated at 0800 h on metestrus with both, CB and LHAS, were also used. To verify through a different procedure the effect of LH and/or PRL deprivation in estrous cycle CL progesterone secretion, hypophysectomy (HYPOX) and sham HYPOX (SHAM) were done at 0800 h on metestrus in either CB- or ETOH-injected rats at 1600 h on estrus. Hypophysectomized rats at 1600 h on estrus were also used. Progesterone secretion was prolonged up to 0800 h on diestrus in those rats deprived of LH from 0800 h on metestrus (ETOH/LHAS, -/CB + LHAS, ETOH/HYPOX) compared with controls (ETOH/NHS, ETOH/SHAM). This luteotropic effect was absent in those rats lacking estrous afternoon PRL (CB/LHAS, CB/HYPOX, HYPOX/-). No effect on CL progesterone secretion was detected in those rats exclusively deprived of PRL on the afternoon of estrus (CB/NHS, CB/SHAM). These results suggest that in the absence of the protective effects of PRL secretion on the afternoon of estrus, rat CL become extremely sensitive to the luteolytic effects of early diestrous LH levels, and this results in 4-day estrous cycles.

Changes in plasma progesterone, estradiol, follicle-stimulating hormone and luteinizing hormone during diestrus and ovulation in rats with 5-day estrous cycles: effect of antibody against progesterone.

Progesterone secretion remained significantly higher during diestrus in the 5-day cyclic rat than in the 4-day cyclic animal. Injection of a sufficient amount of antiprogesterone serum (APS) at 2300 h on metestrus in a 5-day cycle advances ovulation and completion of the cycle by 1 day in the majority of animals (75 and 80%, respectively). Progesterone (250 micrograms) administered with APS eliminated the effect of the antiserum. Within 2 h after administration of APS, levels of both follicle-stimulating hormone (FSH) and luteinizing hormone (LH) elevated significantly, while a significant elevation of plasma estradiol above the control value followed as late as 36 h after the treatment. None of the 5-day cyclic rats treated with APS showed ovulatory increases of FSH and LH at 1700 h on the second day of diestrus, although 3 of the 4 animals receiving the same treatment ovulated by 1100 h on the following day. The onset of ovulatory release of gonadotropins might have been delayed for several hours in these animals. These results indicate that recurrence of the 5-day cycle is due to an elevated progesterone secretion on the morning of diestrus, and suggest that a prolongation of luteal progesterone secretion in an estrous cycle suppresses gonadotropin secretion. Rather than directly blocking the estrogen triggering of ovulatory LH surge, the prolonged secretion of luteal progesterone may delay the estrogen secretion itself, which decreases the threshold of the neural and/or hypophyseal structures for ovulatory LH release.

Administration of human chorionic gonadotropin but not of luteinizing hormone-releasing hormone at pro-oestrus or late di-oestrus has a deleterious effect on pregnancy in the rat.

Embryonic and fetal mortality is studied, which is induced by human chorionic gonadotropin (hCG) and luteinizing hormone-releasing hormone (LHRH) given prior to ovulation. 5-day cyclic rats were injected with 20 IU hCG or with 1 microgram LHRH on day 3 of di-oestrus or on the day of pro-oestrus, and mated 4 h later. Autopsy was performed on day 3 or between days 13 and 18 of pregnancy. Advancement of ovulation by LHRH did not induce embryonic or fetal mortality. Administration of hCG at pro-oestrus or day 3 of di-oestrus induced a considerable mortality, which for the greater part occurred after day 3 of pregnancy but before implantation. This embryonic mortality could be prevented by anti-hCG serum given 21 h after hCG. It is speculated that embryonic mortality induced by hCG is caused by a relatively long-lasting disturbance of steroid metabolism, due to a long metabolic half-life of hCG. The disturbance of steroid metabolism as a possible cause of implantation failure is discussed.

The role of the uterus in regulation of heat duration in cycling rats

These experiments examined the effects of hysterectomy on heat duration and on the reinduction of estrous behavior by progesterone (P) following the termination of spontaneous heat in 4-day cycling rats. Hysterectomy did not affect the onset of estrus but prolonged heat duration. The average duration of sexual receptivity for hysterectomized (H) and sham-hysterectomized (SH) rats was 18.2 and 13.0 hr, respectively. Furthermore, H animals injected with either 0.5 mg P within 2 hr, or 4.0 mg P 24 hr following the termination of natural estrus showed significantly higher lordosis and solicitation responses than SH rats similarly treated. These behavioral findings were correlated with the level of hypothalamic progestin receptors. That is, H animals had a significantly higher concentration of progestin receptors than SH rats immediately following the termination of spontaneous heat and also 24 hr later. Both in estrous-cycling rats and in gonadectomized animals treated with estradiol benzoate (EB), hysterectomy resulted in higher serum estradiol (E2) levels. The results of these experiments suggest that prolongation of the period of sexual receptivity and the facilitated behavioral responses to P following the cessation of estrus in hysterectomized animals may be due to a lowered clearance rate of circulating estradiol which presumably enhances the estrogen conditioning of the neural substrates.

Reduction of central epinephrine concentrations is consistent with the continued occurrence of ovulation in rats treated with an inhibitor (LY 134046) of phenylethanolamine N-methyltransferase.

Groups of 4-day cyclic rats were injected (i.p.) with LY 134046 (50 mg/kg), a central inhibitor of phenylethanolamine N-methyltransferase, or saline at 09.00, 13.00 and 19.00 h on the day of proestrus. The incidence of ovulation was examined the following estrous morning. There was no difference in the number of ova in drug-treated animals compared to saline-treated controls. In other groups of 4-day cyclic rats, LY 134046 or saline was injected daily at 10.00 h for 5 consecutive days from proestrus to proestrus inclusive. The animals were decapitated the following day and ova were counted. Epinephrine concentrations were determined by radioenzymatic assay in the mediobasal hypothalamus (MBH) and the medial preoptic area (MPOA). All saline-treated controls and 10/14 of the drug-treated animals had ovulated, while epinephrine concentrations in the MBH and MPOA had been reduced by 95.8 and 94.7%, respectively, compared to saline-treated controls. These experiments suggest that a significant surge of luteinizing hormone occurs to initiate ovulation even after a severe reduction in central epinephrine concentration has taken place.

Inhibin-like activity in media from cultured rat granulosa cells collected throughout the oestrous cycle

Granulosa cells of antral ovarian follicles from adult 5-day cyclic rats were cultured on each day of the cycle. The rat granulosa cell conditioned medium (rGCCM) was harvested and renewed on each day of a 4-day culture period. Inhibin-like activity and progesterone were estimated in rGCCM using an in-vitro bioassay system with dispersed rat anterior pituitary cells and radioimmunoassay respectively. Removal of steroids from rGCCM with dextran-coated charcoal was effective and did not significantly change the inhibin-like activity of the treated samples. On day 1 of culture the inhibin-like activity of rGCCM for each day of the oestrous cycle was 20-90% higher than on days 2, 3 and 4 of culture when low and constant levels were observed. Media collected after culture on days 1 and 2 from pro-oestrous cells contained larger amounts of inhibin-like activity than media collected on the other days of the cycle. On day 1 of culture, rGCCM from pro-oestrous cells contained higher concentrations of progesterone than that from cells collected on the other days of the cycle. On days 2, 3 and 4 progesterone levels in rGCCM were undetectable (less than 320 pmol/l) except in media from pro-oestrous cultures on day 2. Addition of FSH (62 micrograms/l) to granulosa cell cultures in medium with or without 10% fetal calf serum (FCS) did not alter the inhibin-like activity of rGCCM from pro-oestrous cells. The presence of FCS maintained the production of inhibin-like activity since rGCCM from cells cultured without FCS was devoid of FSH-suppressing activity after 3 days of culture.(ABSTRACT TRUNCATED AT 250 WORDS)

Prolactin and the control of cycle length in the female rat

Bromocriptine (BRC) was injected on oestrus afternoon at 16.00 h into 5-day cyclic female rats thus causing reduced cycle length to 4-days. When injected on dioestrus 1 at 13.00 h the drug remained without effects. Blood progesterone concentration on dioestrus 1 afternoon and on dioestrus 2 morning was shown to be lower in 5-day cyclic rats given BRC than in their untreated counterparts. No changes in the rate of follicular growth were observed on dioestrus 2 afternoon in BRC injected 5-day cyclic rats. The pattern of prolactin secretion on oestrus afternoon was compared in 4- and 5-day cyclic female rats. High prolactin values were observed on early afternoon in the former and on late afternoon in the latter. The decline in progesterone secretion during the dioestrous period of the cycle was supposed to be responsible for cycle shortening in 5-day cyclic rats given BRC. It was concluded that prolactin might play a critical role in the establishment of a 5-day rhythm in the rat by controlling progesterone secretion during the dioestrous period of the cycle.

Progesterone secretion by luteinized unruptured follicles in mature female rats

The ability of luteinized unruptured follicles (LUF) to display luteal activity was investigated in mature female rats. Previous findings in our laboratory showed that increasing doses of LH, when injected on late diestrus in 4-day cyclic rats, were capable of inducing the formation of either LUF or postovulatory corpora lutea (POCL) in a dose dependent manner. Four-day cyclers were injected on diestrus 2 at 4.30 p.m. (day 0) with 2.7 μg or 5.4 μg/100 g of an ovine LH preparation ( × 2.94 NIH LH S 3) and were killed at different times during the three successive days following injection. Natural 4-day cyclers were killed at corresponding times following spontaneous LH release on proestrus afternoon (day 0). Both LUF and POCL were observed in LH-treated females. LUF appeared more numerous in females given 2.7 μg LH than in both natural cyclers and in females injected with 5.4 μg LH. On day 1 during the rising phase of luteal activity serum progesterone (P) level did not differ in the three groups despite the high number of LUF in females given 2.7 μg LH. On day 2 at 11 a.m. lower P values were observed in both groups of LH-treated females than in natural cyclers, this corresponding to a greater proportion of LUF in the former than in the latter. On day 2, at 5 p.m. by the time of full activity of POCL in natural cyclers, P did not differ in the three groups irrespective of the relative number of ruptured or unruptured follicles. On day 3, P sharply declined in LH-treated and natural cyclers. These results suggest that LUF are capable of secreting P during a period corresponding to the duration of corpus luteum's life span in cyclic female rats.

Rat ovarian epidermal growth factor receptors: Characterization and hormonal regulation

Rat ovarian epidermal growth factor (EGF) receptors were characterized in total ovarian homogenate obtained from animals at random stages of the estrous cycle. A single class of specific and high-affinity binding sites with an apparent dissociation constant ( K D) of 1.1±0.2 nM and a total site number of 78±4 fmoles/mg protein (652±17 fmoles/100 mg wet tissue) was observed. In 4-day cycling rats, ovarian EGF receptor levels are high (200 fmoles/100 mg ovary) on the morning of estrus as well as in the afternoon of proestrus and are lower by 30, 60 and 55% on the morning of diestrus I, diestrus II and proestrus, respectively. Chronic administration of an LHRH agonist is accompanied by low ovarian EGF receptor levels comparable to those observed on diestrus II. In intact or hypophysectomized female rats pretreated with pregnant mare's serum gonadotropins, administration of ovine follicle-stimulating hormone or human chorionic gonadotropin increases ovarian EGF receptor concentration and content while the administration of ovine prolactin, 17β-estradiol or progesterone has no effect. Treatment with dihydrotestosterone or testosterone slightly reduces ovarian EGF binding in intact but not hypophysectomized animals. The variation of ovarian EGF receptor levels observed during the estrous cycle as well as the modulation of these receptors by the administration of exogenous gonadotropins suggest that EGF could well play a physiological role in the regulation of ovarian functions.