Ovarian steroids modulate the self-priming effect of luteinizing hormone-releasing hormone on bovine pituitary cells in vitro

Influence of inducing luteal regression before a modified fixed-time artificial insemination protocol in postpartum beef cows on pregnancy success

Most fixed-time insemination protocols utilize an injection of GnRH at the beginning of the protocol to initiate a new follicular wave. However, the ability of GnRH to initiate a new follicular wave is dependent on the stage of the estrous cycle. We hypothesized that administering PGF(2α) 3 d before initiating a fixed-time AI protocol would improve synchrony of follicular waves and result in greater pregnancy success. Therefore, our objective was to determine whether inducing luteal regression 3 d before a fixed-time AI protocol would improve control of follicular turnover and pregnancy success to fixed-time AI. Multiparous crossbred cows at 3 locations (n = 108, 296, and 97) were randomly assigned to 1 of 2 treatments: 1) PGF(2α) [25 mg; intramuscularly (i.m.)] on d -9, GnRH (100 μg; i.m.) and insertion of a controlled internal drug-releasing device (CIDR) on d -6, PGF(2α) (25 mg; i.m.) and CIDR removal with PGF(2α) (25 mg; i.m.) at CIDR removal on d 0 (PG-CIDR) or 2) GnRH (100 μg; i.m.) and insertion of a CIDR on d -5 and CIDR removal with PGF(2α) (25 mg; i.m.) at CIDR removal and 4 to 6 h after CIDR removal (5-d CIDR). Cows were time-inseminated between 66 and 72 h (PG-CIDR) or 70 to 74 h (5-d CIDR) after CIDR removal, and GnRH was administered at the time of fixed-time AI. At location 1, ovulatory response to the first injection of GnRH was determined by ultrasonography at the time of GnRH and 48 h after GnRH administration. Among cows with follicles ≥10 mm in diameter, more (P = 0.03) PG-CIDR-treated cows ovulated after the initial GnRH injection (88%, 43/49) compared with the 5-d CIDR-treated cows (68%, 34/50). Pregnancy outcome was not influenced by location (P = 0.96), age of the animal (P = 1.0), cycling status (P = 0.99), BCS (P = 1.0), or any 2-way interactions (P ≥ 0.13). However, pregnancy success was influenced by synchronization protocol (P = 0.04). Pregnancy outcome was greater (P = 0.04) for the PG-CIDR protocol (64%) compared with the 5-d CIDR protocol (55%). In summary, control of follicular turnover was improved by inducing luteal regression 3 d before initiation of a fixed-time AI protocol, and pregnancy success was improved with the PG-CIDR protocol compared with the 5-d protocol.

Influence of inducing luteal regression before a modified controlled internal drug-releasing device treatment on control of follicular development

At the initiation of most controlled internal drug-releasing (CIDR) device protocols, GnRH has been used to induce ovulation and reset follicular waves; however, its ability to initiate a new follicular wave is variable and dependent on stage of the estrous cycle. The objectives of the current studies were to determine 1) if inducing luteal regression before the injection of GnRH at time of insertion of a CIDR resulted in increased control of follicular development, and 2) if removing endogenous progesterone by inducing luteal regression before insertion of the CIDR decreased variation in LH pulse frequency. In Exp. 1 and 2, Angus-cross cycling beef heifers (n = 22 and 38, respectively) were allotted to 1 of 2 treatments: 1) heifers received an injection of PGF(2α) on d -3, an injection of GnRH and insertion of a CIDR on d 0, and a PGF(2α) injection and CIDR removal on d 6 (PG-CIDR) or 2) an injection of GnRH and insertion of a CIDR on d 0 and on d 7 an injection of PGF(2α) and removal of CIDR (Select Synch + CIDR). In Exp. 3, Angus-cross beef heifers (n = 15) were assigned to 1 of 3 treatments: 1) PG-CIDR; 2) PGF(2α) on d -3, GnRH on d 0, and PGF(2α) on d 6 (PG-No CIDR); or 3) Select Synch + CIDR. Follicular development and ovulatory response were determined by transrectal ultrasonography. Across all experiments, more (P = 0.02) heifers treated with PG before GnRH initiated a new follicular wave after the injection of GnRH compared with Select Synch + CIDR-treated heifers. In Exp. 1, after CIDR removal, interval to estrus did not differ (P = 0.18) between treatments; however, the variance for the interval to estrus was reduced (P < 0.01) in PG-CIDR heifers compared with Select Synch + CIDR heifers. In Exp. 3, there was a tendency (P = 0.09) for LH pulse frequency to be greater among PG-CIDR and PG-No CIDR compared with the Select Synch + CIDR, but area under the curve, mean LH concentrations, and mean amplitude did not differ (P > 0.76). In summary, induction of luteal regression before an injection of GnRH increased the percentage of heifers initiating a new follicular wave. Removal of endogenous progesterone tended to increase LH pulse frequency, and the modified treatment increased the synchrony of estrus after CIDR removal.

Effect of the timing of controlled internal drug-releasing device insertion on the gonadotropin-releasing hormone-induced luteinizing hormone surge and ovulatory response

Concentrations of progesterone have been reported to influence GnRH-induced LH surges. At the beginning of many synchronization protocols, GnRH is used to synchronize follicular growth. Therefore, the objective of this study was to determine the effect of elevated concentrations of progesterone from a controlled internal drug-releasing device (CIDR) on the GnRH-induced LH surge and ovulatory response. Angus-cross beef heifers (n = 113; 41 pubertal and 72 prepubertal) were assigned to 1 of 3 treatments: 1) GnRH at CIDR insertion (CIDR-0), 2) GnRH 6 h before CIDR insertion (CIDR-6), or 3) GnRH 48 h after CIDR insertion (CIDR+48). Follicle size was determined before GnRH administration, and ovulatory response was determined 2 d later. Blood samples were collected from a subset of 60 heifers at -30, 0 (GnRH administration), 30, 60, 90, 120, 150, 180, 210, 240, 300, and 360 min after GnRH. Heifers receiving CIDR+48 had greater (P < 0.01) concentrations of progesterone compared with those receiving CIDR-0 and CIDR-6. There was no difference (P > 0.76) between treatments in concentrations of estradiol. There tended to be a cycling status x ovulation interaction on concentrations of progesterone (P = 0.11), and there was a cycling status x ovulation interaction on concentrations of estradiol (P = 0.02). The estradiol-to-progesterone ratio was significant because of treatment (P = 0.002), cycling status (P = 0.001), and a treatment x cycling status interaction (P = 0.02). Cycling status tended (P = 0.11) to have an influence on ovulation (29/41 and 42/72 for pubertal and prepubertal heifers). Ovulation was induced in more (P < 0.05) CIDR-0 (26/38) and CIDR-6 (28/37) heifers than CIDR+48 (17/38) heifers. There was no influence of treatment (P = 0.19), concentrations of estradiol (P = 0.90), or the estradiol-to-progesterone ratio (P = 0.21) on concentrations of LH, but there was an effect (P < 0.01) of progesterone on LH concentrations. Heifers with elevated progesterone at GnRH administration had a reduced LH surge compared with heifers with decreased concentrations of progesterone. Heifers that ovulated tended to have a greater (P = 0.11) magnitude of LH surge than heifers that did not ovulate. In summary, elevated concentrations of progesterone at GnRH administration decreased the GnRH-induced LH surge, and heifers in the CIDR+48 treatment had a decreased ovulatory response. However, there tended to be a difference in the magnitude of the LH surge only between heifers that did and did not ovulate.

Effect of preovulatory concentrations of estradiol and initiation of standing estrus on uterine pH in beef cows

Previous studies have indicated that initiation of standing estrus within 24h of fixed-time AI influenced pregnancy rates. Furthermore, uterine environment at time of insemination can influence sperm transport. We hypothesized that preovulatory concentrations of estradiol would influence uterine pH at time of insemination. The objective of this study was to determine the influence of elevated preovulatory concentrations of estradiol on uterine pH following a fixed-time AI protocol. Cows were synchronized with the CO-Synch (n=57) protocol, and 29 cows were treated with an injection of estradiol cypionate (ECP; 1mg) 36h before the second injection of GnRH. Cows that exhibited standing estrus or were treated with ECP had increased (P<0.05) concentrations of estradiol compared to cows not in estrus and not administered ECP, respectively. There was an ECP by standing estrus interaction on uterine pH (P=0.01). Control cows that exhibited estrus had a reduced uterine pH (6.72+/-0.10; P=0.05) compared to control cows not exhibiting estrus (7.0+/-0.06). Cows treated with ECP and detected in standing estrus had a greater uterine pH (7.0+/-0.07) compared to control cows in estrus (P=0.02) and ECP cows not in estrus (6.81+/-0.09; P=0.06). The interval between the initiation of standing estrus and when pH was determined also influenced uterine pH. Cows that initiated standing estrus within 4h of pH determination had a lower uterine pH (6.74+/-0.12) compared to cows that initiated estrus 4-8h (7.09+/-0.08; P=0.07) or 8-12h (7.10+/-0.15; P=0.03) after pH determination. In summary, elevated concentrations of estradiol influenced standing estrus but only influenced uterine pH when pH was determined within 4h of the initiation of standing estrus.

Insight into the neuroendocrine site and cellular mechanism by which cortisol suppresses pituitary responsiveness to gonadotropin-releasing hormone

Stress-like elevations in plasma glucocorticoids rapidly inhibit pulsatile LH secretion in ovariectomized sheep by reducing pituitary responsiveness to GnRH. This effect can be blocked by a nonspecific antagonist of the type II glucocorticoid receptor (GR) RU486. A series of experiments was conducted to strengthen the evidence for a mediatory role of the type II GR and to investigate the neuroendocrine site and cellular mechanism underlying this inhibitory effect of cortisol. First, we demonstrated that a specific agonist of the type II GR, dexamethasone, mimics the suppressive action of cortisol on pituitary responsiveness to GnRH pulses in ovariectomized ewes. This effect, which became evident within 30 min, documents mediation via the type II GR. We next determined that exposure of cultured ovine pituitary cells to cortisol reduced the LH response to pulse-like delivery of GnRH by 50% within 30 min, indicating a pituitary site of action. Finally, we tested the hypothesis that suppression of pituitary responsiveness to GnRH in ovariectomized ewes is due to reduced tissue concentrations of GnRH receptor. Although cortisol blunted the amplitude of GnRH-induced LH pulses within 1-2 h, the amount of GnRH receptor mRNA or protein was not affected over this time frame. Collectively, these observations provide evidence that cortisol acts via the type II GR within the pituitary gland to elicit a rapid decrease in responsiveness to GnRH, independent of changes in expression of the GnRH receptor.

Insulin-like growth factor I enhances gonadotropin-releasing hormone-stimulated luteinizing hormone release from bovine anterior pituitary cells

The role of insulin-like growth factor I (IGF-I) in the release of luteinizing hormone (LH) is unclear in ruminants. In the present study, the effects of IGF-I on the release of LH stimulated by gonadotropin-releasing hormone (GnRH) were examined in primary cultures of bovine anterior pituitary (AP) cells, and the interaction between estradiol-17beta (E(2)) and IGF-I was characterized. GnRH(100nM)-stimulated LH release from the cultured cells was increased (P<0.05) 12, 24 and 36h after addition of IGF-I (250ng/ml), with a maximum at 12h (48.4ng/ml media versus 35.4ng/ml media in controls). IGF-I at concentrations of 25, 250 and 500ng/ml increased the release by 18.7, 24.2 and 28.9%, respectively (P<0.05), when compared with controls (37.2ng/ml media). E(2) (10nM), IGF-I (250ng/ml) and combined treatment of E(2) plus IGF-I also induced significant increases in LH release (P<0.05). The amounts of LH release after treatment with E(2) alone was 37.3% greater than with IGF-I alone (39.0ng/ml media versus 28.4ng/ml media) (P<0.05). When E(2) and IGF-I were added together (45.6ng/ml media), the release of LH was significantly greater than with either E(2) alone or IGF-I alone (P<0.05). E(2) (10nM) significantly (P<0.05) increased the amount of GnRH bound to the cells by 51.6% when compared with controls, however, IGF-I (250ng/ml) failed to increase GnRH binding. These results show that IGF-I enhances GnRH-stimulated LH release without changing the number of GnRH receptors in cattle, and IGF-I interacts with E(2) to increase the response to GnRH.

Estradiol-17beta triggers luteinizing hormone release in the protandrous black porgy (Acanthopagrus schlegeli Bleeker) through multiple interactions with gonadotropin-releasing hormone control

We investigated the mechanism of estradiol-17beta (E2) action on stimulation of LH (=gonadotropin II) release in the black porgy fish (Acanthopagrus schlegeli Bleeker) using an in vivo approach and primary cultures of dispersed pituitary cells in vitro. In vivo, E2 but not androgens (testosterone [T] and 11-ketotestosterone [11-KT]) significantly stimulated plasma LH in a dose-dependent manner. Estradiol-17beta also increased brain content of seabream GnRH. GnRH antagonist prevented E2 stimulation of LH release in vivo, indicating that the effect of E2 on LH was mediated by GnRH. In vitro, sex steroids (E2, T, 11-KT) alone had no effect on basal LH release in the cultured pituitary cells, but GnRH significantly stimulated LH release. Estradiol-17beta potentiated GnRH stimulation of LH release, an effect that was inhibited by GnRH antagonist, and 11-KT, but not T, also potentiated GnRH stimulation of LH release. The potentiating effect of 11-KT on GnRH-induced LH release in vitro was stronger than that of E2. These data suggest that E2 triggers LH release in vivo by acting both on GnRH production at the hypothalamus and on GnRH action at the pituitary. In contrast, 11-KT may only stimulate GnRH action at the pituitary. The E2) induction of LH release, through multiple interactions with GnRH control, supports a possible central role of E2in the sex change observed in the protandrous black porgy.

Effect of adrenocorticotrophic hormone on gonadotrophin releasing hormone-induced luteinizing hormone secretion in vitro

An in vitro perifusion study investigated the effect of different forms of adrenocorticotrophic hormone (ACTH) on gonadotrophin releasing hormone (GnRH)-induced luteinizing hormone (LH) secretion, particularly GnRH self-priming, and oestradiol sensitisation of the ovine pituitary. Fragments of pituitaries were obtained from mixed-breed adult nonpregnant female sheep (without corpora lutea, unless otherwise stated). The amount of LH released by different doses of GnRH (2.5 x 10(-10) M (n = 9 chambers), 1 x 10(-10) M (n = 9), or 5 x 10(-11) M (n = 6)) was evaluated by giving two GnRH pulses (5 min each) 2 h apart. In a duplicate set of chambers, ACTH1-24 (5 x 10(-7) M) was included in the perifusate 0.5 h before the first GnRH challenge. Potassium chloride (KCl; 100 mM) was administered 2 h after the second GnRH challenge to assess the viability of the tissue and the size of the releasable LH pool. Results were expressed as percentage of LH secretion. The influence of ACTH1-24 on oestradiol sensitisation was also examined using pituitaries obtained during the luteal phase. Pituitary tissues were perifused throughout with 1 x 10(-9) M or 6 x 10(-11) M oestradiol in the medium. The LH response to the second GnRH challenge (GnRH 2) was significantly greater (p < 0.01) than after the first (GnRH 1) at the highest dose of GnRH (2.5 x 10(-10) M; 2547 +/- 804 vs. 4547 +/- 1013%), but at the lower doses (1 x 10(-10) M or 5 x 10(-11) M), the self-priming effect of GnRH was not evident (3016 +/- 550 vs. 2932 +/- 490% and 841 +/- 205 vs. 711 +/- 87%). Treatment with ACTH1-24 (5 x 10(-7) M) did not affect tonic LH secretion nor the LH response to the first or second GnRH challenge at any of the GnRH doses tested. The LH released in response to KCl was also similar from control and ACTH1-24-treated tissue at all GnRH doses. Both lower doses of GnRH (1 x 10(-10) M or 5 x 10(-11) M) produced the self-priming effect when the pituitary tissue was sensitised with the higher dose of oestradiol (1 x 10(-9) M; 1711 +/- 239 vs. 5085 +/- 1307%, and 1502 +/- 376 vs. 2619 +/- 629%). In the presence of lower concentrations of oestradiol (6 x 10(-11) M), self-priming was observed only after the higher dose of GnRH (1 x 10(-10) M; 1293 +/- 214 vs. 2865 +/- 436%), not the lower dose (5 x 10(-11) M; 985 +/- 203 vs. 1271 +/- 436%). In spite of these differences, ACTH1-24 treatment did not affect LH secretion (neither basal nor potassium-induced). The effect of ACTH1-39 (1 x 10(-8) M or 5 x 10(-7) M; n = 6 chambers per combination) on GnRH-induced LH secretion was examined using the higher (2.5 x 10(-10) M) or lower dose of GnRH (1 x 10(-10) M), with or without oestradiol sensitisation (1 x 10(-9) M). At the lower dose (1 x 10(-8) M), ACTH1-39 influenced neither tonic nor GnRH-induced LH secretion. The LH released by KCl was also similar to the control and ACTH-treated tissue. In contrast, the higher dose of ACTH1-39 (5 x 10(-7) M) increased tonic LH secretion immediately after inclusion in the medium (104 +/- 3 vs. 161 +/- 20%), but suppressed the GnRH self-priming effect after 2.5 x 10(-10) M, i.e., the LH responses to GnRH 1 and 2 were similar (1786 +/- 294 vs. 1553 +/- 373%). However, the LH response to KCl was not significantly different (p > 0.05) between the control and ACTH-treated tissues (2333 +/- 286 vs. 2638 +/- 431%). When the effect of this higher dose of ACTH1-39 on oestradiol-priming was investigated, ACTH increased tonic LH secretion but suppressed the self-priming effect of GnRH (1 x 10(-10) M GnRH; 945 +/- 274 vs. 922 +/- 323%; p > 0.05), and decreased (p < 0.05) the LH released in response to KCl compared to the controls (1803 +/- 409 vs. 4302 +/- 1017%). In summary, in vitro, ACTH1-24 did not affect either tonic LH secretion, the GnRH self-priming effect, or oestradiol sensitisation. The entire ACTH1-39 increased tonic LH secretion, but reduced GnRH self-priming and oestradiol sensitisation. (ABSTRACT TRUNCATED)

Selectivity of juxtaposition between cup-shaped lactotrophs and gonadotrophs from rat anterior pituitary in culture

Semi-thin sections of three-dimensional reaggregates from adult female rat pituitary, cultured in serum-free defined medium, were stained for prolactin, gonadotropin, thyrotropin, growth hormone and S-100, using the double immunolabelling technique. The frequency of juxtaposition between lactotrophs and gonadotrophs was enumerated and compared with the expected frequency at random distribution of polygonal cell profiles in a hexagonal configuration. The proportions of lactotrophs and gonadotrophs in the aggregate sections were determined using stereometrical analysis. The observed frequency of juxtaposition did not differ significantly from the expected frequency. Hence, no reason was found to assume a selective adhesion between lactotrophs and gonadotrophs in adult female rat pituitary reaggregates. A constant proportion of lactotrophs was found to meet the criteria of a cup-shaped morphology, and 70% +/- 9% (mean +/- S.D.) of these so-called cup-shaped lactotrophs were found to be juxtaposed at their concave side to gonadotrophs. Administration of 0.01 nM 17 beta-oestradiol to the culture medium resulted in a significant reduction of the proportion of cup-shaped lactotrophs but did not affect the selectivity of juxtaposition to gonadotrophs. The selectivity of juxtaposition between cup-shaped lactotrophs and gonadotrophs may be the morphological correlate of the functional relationship between these cells, which are known to be involved in an intra-pituitary paracrine communication system.

Nature of gonadotropin-releasing hormone self-priming of luteinizing hormone secretion during the normal menstrual cycle

To investigate further the nature of the gonadotropin-releasing hormone self-priming effect on luteinizing hormone release, we administered two submaximal doses of gonadotropin-releasing hormone 2 hours apart to women at three stages of the menstrual cycle and analyzed the resultant luteinizing hormone secretory episodes with deconvolution analysis. When the characteristics of the secretory episodes associated with the second gonadotropin-releasing hormone challenge were compared with those associated with the first, both an enhanced maximal secretory rate and mass of luteinizing hormone secreted was demonstrable at each phase of the cycle. No differences in the luteinizing hormone secretory event half-duration were detected when the responses to the first and second gonadotropin-releasing hormone doses were compared. These data confirm the gonadal hormone milieu-associated self-priming effect of gonadotropin-releasing hormone on luteinizing hormone release and indicate that it is the rate with which luteinizing hormone molecules are discharged from the pituitary gland, rather than the duration of the secretory episode itself, that provides for the self-priming effect.

Superfused pituitary cell cultures: comparative responsiveness of cells derived from various stages of the estrous cycle to LHRH stimulation administered as short duration pulses

We have reinvestigated the question of maintenance of differential LHRH sensitivity in culture and further investigated the role of pulsatile LHRH in the in vitro release of pulsatile LH and FSH at different stages of the estrous cycle. Pituitaries were collected on each day of the 4 day cycle at 0800. In addition, pituitaries were also collected at 1500 and 1900 on proestrous. The cells were dispersed and exposed 48 hrs later to short duration 4 ng LHRH pulses; this dose was optimized for LH release and was applied at a frequency of 1 pulse/60 min. In terms of absolute magnitude of LH response, observed responsiveness was ranked in the following order: proestrous 1900 greater than estrous 0800 greater than diestrous 1 0800 greater than proestrous 1500 greater than diestrous 2 0800. Responsiveness was significantly greater at proestrous 1900 (p greater than 0.01), estrous 0800 (p greater than 0.05) and diestrous 1 0800 (p greater than 0.05) when compared to either of the other stages tested. The heightened LHRH sensitivity of proestrous was therefore maintained in cell culture indicating that the system should be valid for conducting studies on the control of gonadotropin secretion during this period. FSH did not respond in pulsatile manner to the LHRH levels employed further substantiating recent evidence that LHRH seems to function somehow less directly in FSH as compared to LH secretion.

Gonadal steroid-mediated alteration of luteinizing hormone secretion by anterior pituitary cells of young turkeys

The magnitude of luteinizing hormone (LH) release during a 3-hr test incubation was diminished (P less than 0.05) when anterior pituitary cells from young turkeys were cultured for 24 to 120 hr. This trend was evident with basal LH release and with LH release induced by luteinizing hormone-releasing hormone (LH-RH) or hypothalamic extract. Anterior pituitary cells were cultured with various concentrations (10(-14) to 10(-6) M) of estradiol (E2), progesterone (P4), or testosterone (T) for 24 hr and then exposed to LH-RH or control medium for 3 hr, still in the presence of steroids. Basal LH release was potentiated (P less than 0.05) when cells were cultured with 10(-8) or 10(-6) M T, but not with E2 or P4. When cells were cultured with E2, LH release in the presence of 10(-8) M LH-RH was enhanced (P less than 0.05) in a dose-dependent fashion. LH-RH mediated LH release was also enhanced (P less than 0.05) when cells were cultured with 10(-8) M P4 or 10(-6) M T. Gonadal steroids can act directly on the anterior pituitary of the young domestic turkey to modulate LH release, with T enhancing basal LH release and E2 potentiating LH-RH-mediated LH release.

Pituitary self-priming actions of gonadotropin-releasing hormone. Kinetics of estradiol's potentiating effects on gonadotropin-releasing hormone-facilitated luteinizing hormone and follicle-stimulating hormone release in healthy postmenopausal women

We examined the kinetically distinct characteristics of estradiol's effects upon pituitary luteinizing hormone (LH) and follicle-stimulating hormone (FSH) release in response to pulses of exogenous gonadotropin-releasing hormone (GnRH) in healthy postmenopausal individuals. The putative self-priming actions of GnRH on LH and FSH release were tested by intravenous injections of equal paired doses of GnRH (10 micrograms) before and after 1, 5, 10, and 30 d of pure estradiol-17 beta delivery via an intravaginal silastic ring. Self-priming actions of GnRH, as defined by heightened gonadotropin release in response to the second pulse of GnRH compared with the first, were completely absent in the hypoestrogenemic state. However, estradiol administration unmasked GnRH self-priming in a time-dependent fashion, with maximal expression after 5 and 10 d of steroid replacement, followed by attenuation by 30 d. Since estradiol's modulation of GnRH action was expressed differentially on LH and FSH release, we suggest that such facilitation of GnRH-stimulated pituitary LH and FSH release may provide an additional mechanism for dissociated secretion of gonadotropic hormones in health or disease.

Functional changes in luteinizing hormone-secreting cells from pre- and postpartum ewes

Luteinizing hormone (LH)-containing cells from ovine pituitaries obtained during gestation and at various times after parturition were examined to determine whether the ability to store and secrete LH in vitro was correlated with morphological changes. Pituitaries collected on days 50 and 140 of gestation and on days 2, 13, 22, and 35 after parturition were enzymatically dissociated and the resulting cells cultured in media containing estradiol (12 pg/ml), cortisol (12 ng/ml), or no steroid. After 4, 7, or 10 days of culture, cells were washed and basal LH, gonadotropin-releasing hormone (GnRH)-stimulated release of LH, and cellular content of LH were determined. The content of LH (ng/10(6) cells) was lowest on day 140 of gestation (2.7 +/- 0.3) and day 2 postpartum (2.2 +/- 0.6) and then increased (P less than 0.05) on days 13 (36.6 +/- 8.3), 22 (59.9 +/- 14.4), and 35 (54.6 +/- 19.3) postpartum. The percentage of pituitary cells containing immunoreactive LH nearly doubled (P less than 0.05) between days 2 (5.6 +/- 0.2%) and 35 (10.6 +/- 1.1%) postpartum. Moreover, LH-containing cells were smaller, and the percent total cellular volume occupied by secretory granules was less on day 2 than on days 22 and 35 after parturition. Secretion of LH after 4, 7, or 10 days of culture reflected the cellular content of LH and was not influenced by the presence of steroids in the media. These data indicate that decreased synthesis of LH during gestation is associated with hypoplasia of the LH-secreting cells. These cells are reactivated during the postpartum period and their capacity to synthesize LH gradually returns to normal.

Luteinizing hormone releasing hormone of fixed pulse frequency and duration. A simplified system for studying the effect of varying pulse concentration on LH release from cytodex I attached anterior pituitary cells

The literature indicates agreement concerning basic differences in the behavior of the pituitary toward pulsatile and continuous luteinizing hormone releasing hormone (LHRH); however, conflicting results seem to exist concerning pituitary behavior toward pulsatile LHRH (Hopkins, 1977; Smith and Vale, 1981). Most superfusion studies have utilized pulses of 15-30 minutes during which the cells were exposed to pharmacological quantities of LHRH. Differences in results may have arisen because of the varying methodologies utilized to administer pulse frequency, pulse duration, and pulse concentration; therefore, the present studies utilized standardized methodology in which the LHRH pulse frequency and pulse duration were maintained constant while the pulse concentration was varied. Pulsatile LHRH of fixed concentration was associated with a relatively rapid loss of responsiveness, while small increases in each subsequent pulse served to prolong the period of responsiveness. The results indicated that seemingly small changes in the methological pattern of LHRH stimulation are capable of exerting an influence on the response to subsequent LHRH stimulation. Caution should therefore be exerted in comparing the results from different experiments utilizing different methodological designs for applying LHRH stimulation. In practical terms, these studies indicate that results must be interpreted carefully from experiments in which a fixed pool of pituitary cells has been repeatedly stimulated by LHRH. This is especially true with dose-response curves generated by this method and with experiments designed to study LHRH self-priming and desensitization.