Changes in patterns of luteinizing hormone secretion before and after the first ovulation in the postpartum mare

To determine whether luteinizing hormone (LH) secretion during the first estrous cycle postpartum is characterized by pulsatile release, circulating LH concentrations were measured in 8 postpartum mares, 4 of which had been treated with 150 mg progesterone and 10 mg estradiol daily for 20 days after foaling to delay ovulation. Blood samples were collected every 15 min for 8 h on 4 occasions: 3 times during the follicular phase (Days 2-4, 5-7, and 8-11 after either foaling or end of steroid treatment), and once during the luteal phase (Days 5-8 after ovulation). Ovulation occurred in 4 mares 13.2 +/- 0.6 days postpartum and in 3 of 4 mares 12.0 +/- 1.1 days post-treatment. Before ovulation, low-amplitude LH pulses (approximately 1 ng/ml) were observed in 3 mares; such LH pulses occurred irregularly (1-2/8 h) and were unrelated to mean circulating LH levels, which gradually increased from less than 1 ng/ml at foaling or end of steroid treatment to maximum levels (12.3 ng/ml) within 48 h after ovulation. In contrast, 1-3 high-amplitude LH pulses (3.7 +/- 0.7 ng/ml) were observed in 6 of 7 mares during an 8-h period of the luteal phase. The results suggest that in postpartum mares LH release is pulsatile during the luteal phase of the estrous cycle, whereas before ovulation LH pulses cannot be readily identified.     

Effect of continuous infusion of oxytocin on length of the oestrous cycle and luteolysis in cattle

In Exp. I oxytocin (60 micrograms/100 kg/day) was infused into the jugular vein of 3 heifers on Days 14-22, 15-18 and 16-19 of the oestrous cycle respectively. In Exp. II 5 heifers were infused with 12 micrograms oxytocin/100 kg/day from Day 15 of the oestrous cycle until clear signs of oestrus. Blood samples were taken from the contralateral jugular vein at 2-h intervals from the start of the infusion. The oestrous cycle before and after treatment served as the controls for each animal. Blood samples were taken less frequently during the control cycles. In Exp. III 3 heifers were infused with 12 micrograms oxytocin/100 kg/day for 50 h before expected oestrus and slaughtered 30-40 min after the end of infusion for determination of oxytocin receptor amounts in the endometrium. Three other heifers slaughtered at the same days of the cycle served as controls. Peripheral concentrations of oxytocin during infusion ranged between 155 and 641 pg/ml in Exp. I and 18 and 25 pg/ml in Exp. II. In 4 our of 8 heifers of Exps I and II, one high pulse of 15-keto-13,14-dihydro-prostaglandin F-2 alpha (PGFM) appeared soon after the start of oxytocin infusion followed by some irregular pulses. The first PGFM pulse was accompanied by a transient (10-14 h) decrease of blood progesterone concentration. High regular pulses of PGFM in all heifers examined were measured between Days 17 and 19 during spontaneous luteolysis. No change in length of the oestrous cycle or secretion patterns of progesterone, PGFM and LH was observed.(ABSTRACT TRUNCATED AT 250 WORDS)     

Pulsatile secretion of LH during the periovulatory and luteal phases of the oestrous cycle in the Père David's deer hind (Elaphurus davidianus)

Changes in the secretion of LH during the oestrous cycle were studied in 5 tame Père David's deer in which ovulation was synchronized with progesterone implants and prostaglandin injections. Plasma LH concentrations were measured in samples collected at 15-min intervals for a 36-h period, starting 16 h after the removal of the progesterone implants (follicular phase), and for a further 10-h period 10 days after the removal of the progesterone implants (luteal phase). In all animals, there was a preovulatory surge of LH and behavioural oestrus which occurred at a mean time of 59.6 h (+/- 3.25) and 69 h respectively following implant removal. LH pulse frequency was significantly higher during the follicular phase (0.59 +/- 0.03 pulses/h) than the luteal phase (0.24 +/- 0.2 pulses/h), thus confirming in deer findings from research on domesticated ruminants. There were no significant differences between the follicular and luteal phases in mean plasma LH concentrations (0.57 +/- 0.09 and 0.74 +/- 0.13 ng/ml) or mean pulse amplitude (0.99 +/- 0.14 and 1.05 +/- 0.21 ng/ml) for the follicular and luteal phase respectively. The long interval from the removal of progesterone to the onset of the LH surge and the absence of a significant difference in mean LH concentration or pulse amplitude in the follicular and luteal phases resemble published data for cattle but differ from sheep in which there is a short interval from luteal regression to the onset of the surge and a marked increase in LH pulse amplitude during the luteal phase.     

Changes in the ovarian dynamics and endocrine profiles in goats treated with a progesterone antagonist during the early luteal phase of the estrous cycle

The aim of the present study was to determine the physiological role of endogenous progesterone in the regulation of ovarian dynamics, gonadotropin and progesterone secretion during the early luteal phase in the goat. Cycling Shiba goats received subcutaneously a vehicle (control group, n=5) or 50 mg of RU486 (RU486 group, n=4) daily from 1 to 7 days after ovulation (day 0) determined by transrectal ultrasonography. Ovarian dynamics were monitored by the ultrasonography and blood samples were collected daily until the subsequent ovulation for analysis of progesterone, luteinizing hormone (LH) and follicle stimulating hormone (FSH) secretion. Blood samples were also collected at 10 min intervals for 6 h on day 3 and day 7 for the analysis of pulsatile patterns of LH and FSH. The LH pulse frequency was significantly (P<0.05) higher in the RU486 group than in the control group on day 7 (4.8+/-1.1 pulses/6 h versus 1.2+/-0.4 pulses/6 h). The shape of the FSH pulses was unclear on day 3 and day 7 in both groups and the overall means of FSH concentration for 6 h on day 3 and day 7 were not significantly different between the RU486 and the control groups. The pattern of daily FSH concentrations showed a wave-like fluctuation in both groups. There was no significant difference in the inter-peak intervals of the wave-like pattern of daily FSH secretion between the RU486 and the control groups (4.1+/-0.6 days versus 4.5+/-0.6 days). The maximum diameter of the largest follicle that grew from day 1 to day 7 in the RU486 group tended to be greater than that in control goats (6.4+/-0.8 mm versus 5.0+/-0.8 mm, P=0.050), whereas no significant difference was detected in the size of the corpus luteum and progesterone concentrations between the control and RU486 groups on almost all days during the treatment period. These results indicate that the rise of the progesterone concentration suppresses the pulsatile LH secretion and follicular growth, whereas progesterone has no physiological role in the regulation of FSH secretion and luteal function during the early luteal phase of the estrous cycle in goats.     

Temporal associations among pulses of 13,14-dihydro-15-keto-PGF2alpha, luteal blood flow, and luteolysis in cattle

Luteal blood flow was studied in heifers by transrectal color-Doppler ultrasound. Data were normalized to the decrease in plasma progesterone to <1 ng/ml (Day 0 or Hour 0). Blood flow in the corpus luteum (CL) was estimated by the percentage of CL area with color flow signals. Systemic prostaglandin F2alpha (PGF) treatment (25 mg; n=4) resulted in a transient increase in CL blood flow during the initial portion of the induced decrease in progesterone. Intrauterine treatment (1 or 2 mg) was done to preclude hypothetical secondary effects of systemic treatment. Heifers were grouped into responders (luteolysis; n=3) and nonresponders (n=5). Blood flow increased transiently in both groups; induction of increased blood flow did not assure the occurrence of luteolysis. A transient increase in CL blood flow was not detected in association with spontaneous luteolysis when examinations were done every 12 h (n=6) or 24 h (n=10). The role of PGF pulses was studied by examinations every hour during a 12-h window each day during expected spontaneous luteolysis. At least one pulse of 13,14-dihydro-15-keto-PGF2alpha (PGFM) was identified in each of six heifers during the luteolytic period (Hours -48 to -1). Blood flow increased (P<0.02) during the 3-h ascending portion of the PGFM pulse, remained elevated for 2 h after the PGFM peak, and then decreased (P<0.03) to baseline. Results supported the hypothesis that CL blood flow increased and decreased with individual PGFM pulses during spontaneous luteolysis.     

Changing frequency of pulsatile luteinizing hormone and progesterone secretion during the luteal phase of the menstrual cycle of rhesus monkeys

Experiments were conducted to examine the pulsatile nature of biologically active luteinizing hormone (LH) and progesterone secretion during the luteal phase of the menstrual cycle in rhesus monkeys. As the luteal phase progressed, the pulse frequency of LH release decreased dramatically from a high of one pulse every 90 min during the early luteal phase to a low of one pulse every 7-8 h during the late luteal phase. As the pulse frequency decreased, there was a corresponding increase in pulse amplitude. During the early luteal phase, progesterone secretion was not episodic and there were increments in LH that were not associated with elevations in progesterone. However, during the mid-late luteal phase, progesterone was secreted in a pulsatile fashion. During the midluteal phase (Days 6-7 post-LH surge), 67% of the LH pulses were associated with progesterone pulses, and by the late luteal phase (Days 10-11 post-LH surge), every LH pulse was accompanied by a dramatic and sustained release of progesterone. During the late luteal phase, when the LH profile was characterized by low-frequency, high-amplitude pulses, progesterone levels often rose from less than 1 ng/ml to greater than 9 ng/ml and returned to baseline within a 3-h period. Thus, a single daily progesterone determination is unlikely to be an accurate indicator of luteal function. These results suggest that the changing pattern of mean LH concentrations during the luteal phase occurs as a result of changes in frequency and amplitude of LH release. These changes in the pulsatile pattern of LH secretion appear to have profound effects on secretion of progesterone by the corpus luteum, especially during the mid-late luteal phase when the patterns of LH concentrations are correlated with those of progesterone.     

The effect of active immunization against progesterone on plasma concentrations of total and free progesterone, estradiol-17beta and LH in the cyclic ewe

Nine mature cyclic ewes were actively immunized against progesterone which was rendered immunogenic by conjugation to bovine serum albumin (BSA). Seven control ewes were immunized with BSA. In ewes immunized against progesterone, the concentration of total plasma progesterone increased to 24.3 ng/ml vs 2.8 ng/ml in control animals (P<0.001). However, immunization did not affect the plasma levels of free, unbound progesterone. The correlation coefficient between total plasma progesterone concentrations on Days 4 to 11 of the estrous cycle and antibody titer was r=0.983. Estradiol-17beta concentrations in immunized ewes were higher than in controls on Days 6 to 15 of the estrous cycle (P approximately 0.05). Frequent sampling for LH over a 6-h period on Days 2, 5, 8, 11 and 14 of the cycle revealed no significant differences in the frequency and amplitude of LH pulses between immunized and control ewes. The immunized ewes had estrous cycles of normal length and maintained normal pregnancies. It is suggested that the immunized cyclic ewe is capable of maintaining adequate levels of free progesterone by greatly increasing progesterone synthesis, thus neutralizing the effect of the antibodies.     

Changes in episodic luteinizing hormone secretion leading to puberty in the lamb

In this study, we monitored episodic luteinizing hormone (LH) secretion throughout development in eight April-born ewe lambs to determine if a change in LH pulse patterns preceded first ovulation at puberty. LH pulses were measured in samples collected every 12 min for 6 h once in July, twice a month from 22 August to 2 October, and then weekly until puberty. Progesterone concentrations, measured in samples taken 3/wk, were used as an index of first ovulation, which occurred at 29.3 +/- 0.7 wk of age. LH pulse frequencies throughout most of this period ranged from 0 to 2 pulses/6 h, with no change over time. However, during the week prior to the first progesterone rise, there was a significant increase in pulse frequency to a level seen during the follicular phase in post-pubertal lambs. This increase in pulse frequency was evident in 7 of 8 lambs; pulses were not analyzed in the last lamb because samples were taken during the LH surge. In contrast, LH pulse amplitude did not increase prior to puberty. In fact, pulse amplitude declined linearly during the 3 wk before first ovulation and then increased during the follicular phase in post-pubertal animals. These results support the hypothesis that an increase in the frequency of episodic LH secretion is a key event leading to the onset of ovarian cycles in the lamb. Whether an increase in pulse amplitude is also necessary remains unclear. If so, it must occur just before the LH surge, since it was not detected in any samples taken before puberty in this study.     

The role of LH pulse frequency in ACTH-induced ovarian follicular cysts in heifers

The aim of the present study was to induce ovarian cysts experimentally in cattle using ACTH and to closely examine the role of LH pulse frequency in ovarian cyst formation. Five regularly cycling Holstein-Friesian heifers (15-18-month-old) were used. Ovaries were scanned daily using an ultrasound scanner with a 7.5 MHz rectal transducer. Daily blood samples were obtained via tail venepuncture for hormone analyses. Additional blood samples (for FSH and LH pulses) were obtained through an indwelling jugular vein catheters every 15 min for 8 h on Days 2 (early luteal phase; ELP), 12 (mid-luteal phase; MLP) and 19 (follicular phase; FP) of control estrous cycle and on alternate days during follicular cyst (FC) formation and persistence. Cysts were induced using subcutaneous injections of ACTH (Cortrosyn) Z; 1 mg) every 12 h for 7 days beginning on Day 15 of the subsequent estrous cycle. Plasma concentrations of progesterone (P4), estradiol-17beta, FSH and LH were determined by double antibody radioimmunoassay while cortisol concentration was determined by enzyme immunoassay (EIA). Ovarian follicular and endocrine dynamics were normal during the control estrous cycles. Ovarian follicular cysts were induced in four of the five heifers. Mean maximum size of cysts was larger (P<0.05) than that of ovulatory follicles (26.78+/-3.65 versus 14.1+/-0.90 mm), respectively. Cortisol levels were increased during ACTH treatment. High concentrations of estradiol and low progesterone were observed after cyst formation. LH pulse frequency was significantly reduced (P<0.05) during cyst formation and persistence compared to ELP (7.5+/-0.75) and FP (6.5+/-0.58), but was not significantly (P=0.23) different from MLP (2.8+/-0.29) pulses. Mean LH pulse amplitude and concentrations were not different. Similarly, the mean pulse frequency, amplitude and concentration of FSH were not different between control study and cystic heifers. These results suggest that the LH pulse frequency observed following ACTH treatment may interact with high estradiol concentration to induce ovarian cyst formation in heifers.     

Effects of indomethacin and aminoglutethimide phosphate in vivo on luteinizing-hormone-induced alterations of cyclic adenosine monophosphate, prostaglandin F, and steroid levels in preovulatory rat ovaries.

Changes in levels of cyclic adenosine monophosphate (cAMP), prostaglandin F (PGF), progesterone, testosterone, and estradiol-17beta, in preovulatory rat ovaries induced by exogenous luteinizing hormone (LH) have been measured. Ovarian cAMP reached maximal levels 15 min and 1h after LH administration by intravenous and intraperitoneal routes, respectively, and then declined to pre-LH levels by 8 h. Progesterone levels in ovaries and serum rose approximately in parallel with cAMP, but remained elevated throughout the 8-h sampling period. Ovarian testosterone increased to maximal levels 1 h after LH injection, followed by a rapid decline to below pre-LH levels. Ovarian estradiol-17beat concentrations declined steadily throughout the sampling period, reaching almost undetectable levels 8 h after LH treatment. Elevated ovarian PGF levels were observed only at the 4- and 8-h sampling times. Indomethacin treatment, 1 h before LH, prevented the LH-induced increase in ovarian PGF levels, depressed PGF values considerably in saline-injected controls but produced no significant inhibition of ovarian cAMP and progesterone levels. Aminoglutethimide phosphate depressed ovarian concentrations of all three steroids (progesterone, testosterone, and estradiol-17beta) to essentially undetectable levels, both in control and LH-injected rats, but did not alter the LH-induced changes in ovarian cAMP and PGF levels. These observations support the concept of cAMP as a mediator of the LH-induced alterations of ovarian steroidogenesis in vivo during the preovulatory period, but argue against an obligatory role of PGF in this process.     

Secretion of gonadotrophins change during the luteal phase of the bovine oestrous cycle in the absence of corresponding changes in progesterone or 17β-oestradiol

The hypothesis in the present study was that changes in circulating luteinizing hormone (LH) and follicle stimulating hormone (FSH) would occur during the luteal phase of the oestrous cycle (Days 4–19; Day 0, day of behaviourial oestrus) that were not related to corresponding changes in concentrations of progesterone and 17β-oestradiol. The stage of the oestrous cycle of cows (n = 18) was synchronised to obtain cows that were on alternate days of the cycle. Blood samples were collected every other day at 15 min intervals for 12 h from all cows: Days 4, 6, 8, 10, 12, 14, 16, 18 (n = 9) and Days 5, 7, 9, 11, 13, 15, 17, 19 (n = 9). Concentrations of LH, FSH, 17β-oestradiol and progesterone were determined in these samples. Data were compared across days to determine when significant changes occurred in concentrations or patterns of secretion of the gonadotrophins and ovarian steroid hormones during the oestrous cycle. There were significant changes in mean concentrations of FSH in circulation between Days 6 and 12. The most striking changes in secretion of gonadotrophins that could not be explained by changes in gonadal steroids were the fluctuations in amplitude of LH pulses between Days 7 and 12. Amplitude of LH pulses increased between Days 7 and 11 and subsequently decreased between Days 11 and 12 of the oestrous cycle. Some changes in gonadotrophin secretion that occurred in the present study can be explained by fluctuations in concentrations of progesterone and 17β-oestradiol in circulation. Other changes cannot be explained by fluctuations in circulating concentrations of these steroids. We accept our hypothesis because the concomitant changes in mean concentration of FSH between Days 6 and 11 and amplitude of LH pulses between Days 7 and 12 of the bovine oestrous cycle cannot be explained by changes in circulating concentrations of progesterone and 17β-oestradiol.     

Secretion patterns of luteinizing hormone in growing mithuns (Bos frontalis)

To characterize the luteinizing hormone (LH) secretion patterns in growing mithun (Bos frontalis), a semi-wild ruminant, six female mithuns (1 year old; BW: 145.5 kg) were maintained in a semi-intensive system. Plasma progesterone (P(4)) level was measured in twice-a-week samples collected for six weeks to assess ovarian status. This was followed by a frequent sampling period. Blood samples collected at 15 min intervals for 9 h were assayed for plasma LH. Luteinizing hormone patterns consisted of pulses of varying amplitudes. Luteinizing hormone pulses occurred at an average rate of 0.54/h ( approximately 5 pulses/9 h). The rate did not differ among mithuns. The mean plasma LH levels was correlated with body weight (r=0.82; p<0.05) and pulse amplitude (r=0.87; p<0.01). Neither the LH amplitude nor the frequency was affected by time (p>0.05). The mean plasma P(4) concentration was 0.37 ng/ml. In conclusion, we demonstrated a pulsatile nature of LH secretion in growing mithuns. In addition, the mean plasma LH level and LH amplitude were positively correlated with body weight. It appears that in contrast to cattle, five LH pulses per nine hours recorded in mithuns were not an indication of approaching puberty.     

Reproductive hormones associated with the ovarian cyst response to GnRH

Eighteen cows with ovarian cysts were administered 100 mug of GnRH and bled prior to treatment, at half hour intervals for 4 hours posttreatment and on days 1, 5 and 9 posttreatment. Blood plasma was analyzed for estradiol-17beta, progesterone and LH by radioimmunoassay. Response to treatment was recorded as positive if ovulation was detected within 30 days posttreatment. Fourteen cows (78%) initiated ovarian cycles by 30 days posttreatment. Mean pretreatment concentrations of estradiol-17beta, progesterone and LH and the GnRH induced LH release were not different for positive or no response cows. However, all seven cows that had pretreatment progesterone concentrations greater than 1.0 ng/ml had a positive response to treatment. Eight of the remaining eleven cows had a progesterone response (mean progesterone concentrations on days 5 and 9 posttreatment) greater than 1.0 ng/ml; seven had a positive response to treatment. In summary, most cows with ovarian cysts administered GnRH will initiate ovarian cycles within 30 days if: 1) pretreatment progesterone concentrations are greater than 1.0 ng/ml or 2) if progesterone response is greater than 1.0 ng/ml.     

Variable progesterone response and estradiol secretion in prepubertal beef heifers following treatment with norgestomet implants

Two experiments were conducted to determine the effects of norgestomet ear implants on progesterone response and estradiol secretion in prepubertal beef heifers. In the first experiment, 47 beef heifers were treated with norgestomet. The implants were implanted subcutaneously for 9 d. After implant removal, blood samples were taken from heifers 2 to 4 d per week for 40 d. Following progesterone determination in jugular venous plasma, heifers were classified according to their progesterone response: 1) no response (Group 1); no rise in progesterone above 1 ng/ml throughout the sampling period; 2) one cycle (Group 2); one increase in progesterone above 1 ng/ml for at least 2 d followed by no further increase in progesterone during the sampling period; and 3) two cycles (Group 3); a rise in progesterone above 1 ng/ml for at least 2 d followed by another cycle of normal duration. Heifers treated with norgestomet were classified as 23 with no response, 9 with 1 cycle and 15 with 2 cycles. Concentrations of estradiol were measured in jugular venous samples on Day 2 after implant removal. Mean concentrations of estradiol were greater in Group 3 than in Group 1 (P < or = 0.01). In Experiment 2, 29 prepubertal beef heifers were assigned randomly to either a 9-d treatment with norgestomet (n = 14) or to serve as untreated controls (n = 15). Blood plasma samples were collected daily from Days 0 to 44 after implant removal. After progesterone determination, heifers were classified as 8 with no response, 4 with 1 cycle and 3 with 2 cycles in the control group, and 5 with no response, 3 with 1 cycle and 6 with 2 cycles in the norgestomet group (frequencies did not differ; P > 0.1). Jugular venous blood plasma was also collected at 4-h intervals from 0 h to 96 h after implant removal and concentrations of estradiol were measured. Patterns of estradiol secretion differed (P < or = 0.05) and overall mean concentrations of estradiol over the first 96 h following implant removal were greater (P < or = 0.01) in norgestomet-treated heifers versus the controls. We conclude that norgestomet can produce a variable progesterone response with heifers with 2 cycles secreting more estradiol. Implants of norgestomet also causes more acute secretion of estradiol in prepubertal beef heifers.     

Steroid secretion rates and plasma binding activity in androstenedione-immune ewes with an autotransplanted ovary

Mature Merino ewes in which the left ovary and its vascular pedicle had been autotransplanted to the neck were divided into control (N = 5) and immunized groups (N = 6). The immunized ewes were treated (2 ml s.c.) with Fecundin 1 and 4 weeks before the start of blood sampling. Ovarian and jugular venous blood was collected every 10 min at two stages of the follicular phase (21-27 h and 38-42 h after i.m. injection of 125 micrograms of a prostaglandin (PG) analogue) and during the mid-luteal phase (8 h at 15-min intervals). The ewes were monitored regularly for luteal function and preovulatory LH surges. Hormone concentrations and anti-androstenedione titres were assayed by RIA and ovarian secretion rates of oestradiol-17 beta, progesterone and androstenedione were determined. After the booster immunization, progesterone increased simultaneously with titre in immunized ewes, reaching 30 ng/ml at the time of PG injection when median titre was 1:10,000. All ewes responded to PG with LH surges 42-72 h later: 2 of the immunized ewes then had a second LH surge within 3-4 days at a time when peripheral progesterone values were 2-3 ng/ml. The frequency of steroid and LH pulses was greater in immunized ewes (P less than 0.05) during the luteal phase but not the follicular phase. The secretion rate of androstenedione was 6-10 times greater (19-37 ng/min; P less than 0.001) in immunized ewes at all sampling stages. Progesterone secretion rates were 3 times greater (16 micrograms/min; P less than 0.001) during the luteal phase in immunized ewes. The amplitude of oestradiol pulses was significantly reduced in immunized ewes (4.8 vs 2.1 ng/min at +24 h and 6.5 vs 2.8 ng/min at +40 h in control and immunized ewes, respectively: P less than 0.05) during the follicular phase. However, the mean secretion rate of oestradiol at each phase of the cycle was not significantly different between treatment groups. Analysis of bound and free steroid using polyethylene glycol showed that greater than 98% of peripheral and ovarian venous androstenedione and 86% of peripheral progesterone was bound in immunized ewes but there was no appreciable binding (less than 0.1%) in control ewes. Similarly, 50% of ovarian venous oestradiol was bound in immunized ewes compared to 15% in control ewes. We conclude that immunization against androstenedione increases the secretion rate of androstenedione and progesterone but not of oestradiol.(ABSTRACT TRUNCATED AT 400 WORDS)     

Steroid feedback inhibition of pulsatile secretion of gonadotropin-releasing hormone in the ewe

The long-term negative feedback effects of sustained elevations in circulating estradiol and progesterone on the pulsatile secretion of gonadotropin-releasing hormone (GnRH) and luteinizing hormone (LH) were evaluated in the ewe following ovariectomy during the mid-late anestrous and early breeding seasons. GnRH secretion was monitored in serial samples of hypophyseal portal blood. Steroids were administered from the time of ovariectomy by s.c. Silastic implants, which maintained plasma concentrations of estradiol and progesterone at levels resembling those that circulate during the mid-luteal phase of the estrous cycle; control ewes did not receive steroidal replacement. Analysis of hormonal pulse patterns in serial samples during 6-h periods on Days 8-10 after ovariectomy disclosed discrete, concurrent pulses of GnRH in hypothalamo-hypophyseal portal blood and LH in peripheral blood of untreated ovariectomized ewes. These pulses occurred every 97 min on the average. Treatment with either estradiol or progesterone greatly diminished or abolished detectable pulsatile secretion of GnRH and LH, infrequent pulses being evident in only 3 of 19 steroid-treated ewes. No major seasonal difference was observed in GnRH or LH pulse patterns in any group of ewes. Our findings in the ovariectomized ewe provide direct support for the conclusion that the negative-feedback effects of estradiol and progesterone on gonadotropin secretion in the ewe include an action on the brain and a consequent inhibition of pulsatile GnRH secretion.     

Plasma progesterone and gonadotrophin concentrations and ovarian activity in post-partum dairy cows

Plasma progesterone concentrations in jugular vein blood samples collected every other day after calving from 13 Friesian dairy cows indicated that ovarian cyclic activity was initiated by 16.6 +/- 1.1 (s.e.m.) days post partum, except for 1 cow which did not resume cyclic activity until Day 98 post partum. Rectal palpation of the ovaries indicated that a developing follicle was recognizable at a mean time of 15.7 +/- 2.0 days after calving. During the first oestrous cycle after parturition there was a significantly shorter period when plasma progesterone levels were elevated than during the next 2 cycles. Concentrations of progesterone, LH, FSH and prolactin were determined for 4 cows, in blood samples taken every 6 h from 2 to 36 days post partum. Tonic LH release was lower during the first 10 days than subsequently, but the lack of change in pattern for FSH suggests dissimilar control mechanisms for these hormones during this time. Three cows showed evidence of a resumption of ovarian cyclicity during the sampling period: in 2 there was an initial LH surge of a magnitude which would normally give rise to ovulation, followed 4 days later by an increase in plasma progesterone lasting only 5 and 9 days. This progesterone was considered to be of follicular origin. A second LH surge was followed by the presence of a corpus luteum.     

Gonadotropin-releasing hormone secretion during the postpartum anestrous period of the ewe

An increase in episodic release of LH is putatively the initial event leading to the onset of postpartum ovarian cyclicity in ewes. This experiment was conducted to determine the relationship between hypothalamic release of GnRH and onset of pulsatile secretion of LH during postpartum anestrus. Control ewes (n = 7) were monitored during the postpartum period to determine when normal estrous cycles resumed. In controls, the mean interval from parturition to the first postpartum estrus as indicated by a rise in serum progesterone greater than 1 ng/mg was 25.8 +/- 0.6 days. Additional ewes (n = 4-5) at 3, 7, 14, and 21 days postpartum (+/- 1 day) were surgically fitted with cannula for collection of hypophyseal-portal blood. Hypophyseal-portal and jugular blood samples were collected over a 6- to 7-h period at 10-min intervals. The number of GnRH pulses/6 h increased (p less than 0.05) from Day 3 postpartum (2.2 +/- 0.5) to Days 7 and 14 (3.6 +/- 0.2 and 3.9 +/- 0.4, respectively). A further increase (p less than 0.05) in GnRH pulse frequency was observed at Day 21 postpartum (6.4 +/- 0.4 pulses/6 h). Changes in pulsatile LH release paralleled changes observed in pulsatile GnRH release over Days 3, 7, 14, and 21 postpartum (0.83 +/- 0.3, 2.8 +/- 0.4, 2.9 +/- 0.6, and 4.0 +/- 1.1 pulses/6 h, respectively). GnRH pulse amplitude was higher at Day 21 than at Days 3, 7, or 14 postpartum. These findings suggest that an increase in the frequency of GnRH release promotes the onset of pulsatile LH release during postpartum anestrus in ewes.     

Effects of hyperadrenal states on luteinizing hormone in cattle

The effect of an induced hyperadrenal state on luteinizing hormone (LH) secretion and subsequent ovarian function was examined in both intact and adrenalectomized (ADRX) heifers. Treatments were begun on Day 2 or Day 16 of an estrous cycle in order to examine their effect on corpus luteum development or ovulation, respectively. In Experiment I, continuous intravenous infusion of ACTH (1.0 mg/24 h) to intact heifers decreased LH concentrations during the early phase of the cycle (Days 3-5). Treatment of ADRX heifers with hydrocortisone succinate (HS) (100 mg/24 h) did not appear to change mean LH concentrations, although da Rosa and Wagner (1981) have reported reduced plasma concentrations of progesterone at mid-cycle in these ACTH-treated intact heifers and HS-treated ADRX heifers. ACTH treatment of ADRX heifers had no effect on LH or progesterone. In the second study, there were similar frequencies of LH surges at the anticipated time of ovulation in all treatment groups. HS (100 mg/24 h) in ADRX heifers and ACTH (0.5 mg/24 h) in intact heifers was given continuously beginning on Day 16 of an estrous cycle. Although some animals in all groups exhibited LH surges, the ACTH-treated intact and HS-treated ADRX heifers failed to show a consistent subsequent increase in progesterone concentrations in plasma, suggesting a failure of luteal development. Although no difference was seen in baseline concentrations of LH, there was a greater difference between basal and overall mean LH concentrations in control groups than was observed in ACTH- or HS-treated animals. These induced hyperadrenal states resulted in depression of ovarian function as shown by decreased plasma progesterone during the luteal phase of the cycle. It is not known if other noncorticoid steroids from the adrenal cortex are necessary for a full expression of this effect.     

Differential regulation of gonadotropin subunit messenger ribonucleic acids by gonadotropin-releasing hormone pulse frequency in ewes

Changes in the frequency of GnRH and LH pulses have been shown to occur between the luteal and preovulatory periods in the ovine estrous cycle. We examined the effect of these different frequencies of GnRH pulses on pituitary concentrations of LH and FSH subunit mRNAs. Eighteen ovariectomized ewes were implanted with progesterone to eliminate endogenous GnRH release during the nonbreeding season. These animals then received 3 ng/kg body weight GnRH in frequencies of once every 4, 1, or 0.5 h for 4 days. These frequencies represent those observed during the luteal and follicular phases, and the preovulatory LH and FSH surge of the ovine estrous cycle, respectively. On day 4, the ewes were killed and their anterior pituitary glands were removed for measurements of pituitary LH, FSH, and their subunit mRNAs. Pituitary content of LH and FSH, as assessed by RIA, did not change (P greater than 0.10) in response to the three different GnRH pulse frequencies. However, subunit mRNA concentrations, assessed by solution hybridization assays and expressed as femtomoles per mg total RNA, did change as a result of different GnRH frequencies. alpha mRNA concentrations were higher (P less than 0.05) when the GnRH pulse frequency was 1/0.5 h and 1 h, whereas LH beta and FSH beta mRNA concentrations were maximal (P less than 0.05) only at a pulse frequency of 1/h. Additionally, pituitary LH and FSH secretory response to GnRH on day 4 was maximal (P = 0.05) when the pulse infusion was 1/h.(ABSTRACT TRUNCATED AT 250 WORDS)