Plasma hormone concentrations after administration of dexamethasone during the middle of the luteal phase in cows

The effect of glucocorticoids on gonadal steroid and gonadotropic hormone concentrations and subsequent follicular activity in cows undergoing normal estrous cycles was evaluated by administration of dexamethasone (DXM) during the middle of the luteal phase. Seven cows were given physiologic saline solution twice daily from day 13 to day 17 of the estrous cycle (control experiment). During the next estrous cycle, cows were administered DXM (2 mg, IM) twice daily on days 13 through 17. Plasma specimens obtained twice daily throughout the control and DXM-treatment cycles were assayed for progesterone and estradiol concentrations. The appearance of estrus after DXM treatment was delayed until days 23 to 25 in 3 cows and was not seen by day 35 in the other 4 cows, compared with mean (+/- SD) cycle length of 22.4 +/- 3.2 in cows during the control experiment. Progesterone concentration remained significantly (P less than 0.01) high on days 19 to 23, whereas estradiol values failed to increase (P less than 0.05) on days 19 and 20 after treatment with DXM. Blood samples were obtained at 15-minute intervals for 12 hours to compare (by analysis of covariance) the effect of DXM treatment on plasma hormone concentrations on day 15 of each cycle with those of day 10. Compared with values during the control experiment, a significant (P less than 0.05) decrease was observed in the size of the pulses of luteinizing hormone (LH) and estradiol, although the number of pulses of each hormone per 12 hours was not affected when cows were given DXM. Baseline concentrations of LH and estradiol were not altered by type of treatment.(ABSTRACT TRUNCATED AT 250 WORDS)     

Circulating concentrations of 17-estradiol influence pattern of LH in circulation of cows

The objective of the research was to determine the relationship between circulating 17β-estradiol (E2) and secretion of luteinizing hormone (LH) in cows. A second objective was to determine if response to E₂ was influenced by interval between ovariectomy and the start of E₂ treatment. Thirty-one nulliparous cows 3 yr of age were randomly assigned to a 2 × 4 factorial arrangement of treatments. Sixteen cows were ovariectomized at 18 mo of age (long term), and the other 15 cows were ovariectomized at 36 mo of age (short term). At the time of ovariectomy of cows in the short term group, 11 cows in the short term group and 12 cows in the long term group were implanted subcutaneously with 1, 2 or 4 polydimethylsiloxane capsules containing E₂. The other eight cows served as non-implanted controls (n=4-short term, n=4-long term). All cows were fitted with jugular vein catheters on day 29 of treatment, and on day 30 blood samples were collected at 12-min intervals for 6 hr. At the end of 6 hr, luteinizing hormone-releasing hormone (LHRH) was administered and blood sampling continued at 12-min intervals for an additional hour. Serum was analyzed for LH and E₂. Variables of LH secretion analyzed were mean concentration, frequency of pulses, amplitude of pulses and maximum concentration after LHRH. There were no significant interactions for any of the variables of LH among cows ovariectomized for the long and short term. There was a significant linear increase in mean concentration of LH with increased circulating concentration of E₂. Frequency of LH pulses was not affected by circulating concentration of E₂. As circulating concentration of E₂ increased, amplitude of LH pulses increased and response to LHRH increased - resulting in an increase in mean LH. Interval from time of ovariectomy to the start of E₂ treatment only had a minor influence on mean concentration of LH and profile of LH concentrations in circulation.     

GnRH对人工孪生处理母牛下丘脑－垂体－性腺轴的调控

从农区黄牛群中选择２１头产后正常母牛，分比三组。组Ⅰ在产后发情周期第１７天注射孕马血清促性腺激素（ＰＭＳＧ），发情当天注射抗ＰＭＳＧ血清，配种时注射生理盐水。组Ⅱ和组Ⅲ的ＰＭＳＧ及其抗血清处理方法与组Ⅰ相同，但在配种时分别注射促性腺激素释放激素（ＧｎＲＨ）或其抗体。各组母牛在ＰＭＳＧ处理前安装颈静脉血管导管，每日间隔１５分钟收集血样，连续３小时。发情当天，间隔３０分钟收集血样，直至发情征兆消失。应用双重酶标免疫测定方法和酶联免疫吸咐测定方法，分别检测血清中ＧｎＲＨ、ＬＨ和Ｐ＿４（孕酮）水平。结果表明，（１）在结合应用ＰＭＳＧ及其抗体处理的母牛发情期间，外源ＧｎＲＨ可使外周血中ＧｎＲＨ和ＬＨ水平升高。用ＧｎＲＨ抗体中和内源ＧｎＲＨ，可使血中ＧｎＲＨ和ＬＨ的水平降低，并阻抑排卵。（２）在母牛排卵前，通过某种途径调控ＧｎＲＨ和ＬＨ的脉冲释放水平，可以提高母牛的超排效果，并有可能控制母牛的排卵数。（３）用ＰＭＳＧ及其抗体和ＧｎＲＨ超排处理的母牛，发情期间的ＧｎＲＨ在排卵前有多个脉冲释放峰，但ＬＨ只有一个脉冲释放峰，而且ＧｎＲＨ脉冲释放高峰出现的时间较ＬＨ峰早。（４）在配种后第８天检出的血清孕酮水平与排卵数呈强正相关?     

Influence of estradiol,progesterone, and nutrition on concentrations of gonadotropins and GnRH receptors,and abundance of mRNA for GnRH receptors and gonadotropin subunits in pituitary glands of beef cows

Nutritionally induced anovulatory cows (n = 28) were used to determine the effect of steroids on regulation of synthesis and secretion of gonadotropins. Anovulatory cows were ovariectomized and received intravaginal inserts containing estradiol (E2), progesterone (P4), E2 and P4 (E2P4), or a sham intravaginal insert (C) for 7 d. Concentrations of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) were quantified in serum and E2 and P4 were quantified in plasma. Cows were exsanguinated within 1 to 2 h after removal of intravaginal inserts and pituitary glands were collected and stored at -80 degrees C until messenger ribonucleic acid (mRNA) for gonadotropin-releasing hormone receptor (GnRH-R) and gonadotropin subunits, pituitary content of GnRH-R, and LH and FSH were quantified. Pituitary glands from five proestrous cows were harvested to compare gonadotropin characteristics between ovariectomized, anovulatory cows and intact cows. Plasma concentrations of E2 were greater (P < 0.05) in E2-treated cows than in sham-treated cows. Concentrations of P4 were greater (P < 0.05) in cows treated with P4 than in sham-treated cows. Mean serum concentrations of LH and FSH were not significantly influenced by steroid treatments. However, frequency of LH pulses of ovariectomized, nutritionally induced anovulatory cows was increased (P < 0.05) by treatment with E2 and amplitude of LH pulses was greater (P < 0.05) in cows treated with E2 or P4 than in cows treated with E2P4 or sham-treated. Quantity of mRNA for LHbeta in the pituitary gland was greater when cows were treated with P4. Concentrations of LH in the pituitary gland were not affected by steroid treatments; however, pituitary concentrations of FSH were less (P < 0.1) in E2 cows than in sham-treated cows. The number of GnRH-R was increased (P < 0.05) in cows treated with E2, but P4 treatment did not influence the number of GnRH-R. Abundance of mRNA for GnRH-R, common alpha-subunit, and FSHbeta were not affected by treatments. Pituitary concentrations of LH were greater (P < 0.05) and concentrations of FSH were less (P < 0.05) in proestrous cows than in ovariectomized, anovulatory cows treated with or without steroids. Abundance of mRNA for GnRH-R, common alpha-subunit, LHbeta and FSHbeta were similar for proestrous and anovulatory cows. We conclude that treatment of nutritionally induced anovulatory cows with progesterone and estradiol may cause pulsatile secretion of LH.     

Influence of estradiol on duration of anestrus and incidence of short estrous cycles in postpartum cows

The influence of exposure to exogenous estradiol on the interval from parturition to first ovulation, luteinizing hormone (LH) secretion and luteal function was examined in cows. Cows were assigned at parturition to one of three treatments. Cows received either a 3.0 (1-E; n = 30) or .75 cm (1/4-E; n = 28) implant containing 17 beta-estradiol or served as untreated control animals (C; n = 33). Implants were administered within 2 days following parturition and removed on day 40 postpartum (day 0 = day of parturition). Single blood samples were collected twice weekly and analyzed for progesterone to determine length of postpartum anestrus and duration of the initial increase in progesterone. Sequential blood samples were collected on day 35 +/- .1 postpartum (15 min intervals for 18 hrs) from 5 cows in each treatment and analyzed for LH. Concentrations of estradiol were higher (P less than .01) in the 1-E (5.3 +/- .24) than in C (3.9 +/- .23) or 1/4 E (3.9 +/- .25) cows on day 35 postpartum. The interval from parturition to the first estrous cycle of normal duration was similar for cows in the C and 1-E treatment (53 +/- 2.4 and 56 +/- 2.4 days, respectively). Cows in the 1/4-E treatment had a longer (P less than .05) interval (68 +/- 2.5 days). Secretion of LH was similar among treatments on day 35 postpartum. The first normal luteal phase after parturition was preceded by a transient rise in progesterone in 81, 64 and 85% of the cows in the C, 1-E and 1/4-E treatments, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)     

Plasma concentrations of luteinizing hormone and adrenocorticotropic hormone in blood collected during the luteal and follicular phases of the estrous cycle in cows.

Luteinizing hormone (LH) and ACTH concentrations were measured in plasma from 7 cows to determine whether ACTH secretion changes with the phase of the estrous cycle, and to determine whether any ACTH peaks are associated with LH peaks. Blood was collected every 5 minutes for 190 minutes during the luteal and follicular phases of the estrous cycle. Radioimmunoassays were used to measure ACTH and LH in plasma. Mean concentration of ACTH in all cows did not differ significantly between luteal (35.1 +/- 8.0 pg/ml) and follicular (37.5 +/- 9.4 pg/ml) phases of the estrous cycle. Mean concentration of luteal-phase LH of all cows (2.0 +/- 1.1 ng/ml) was significantly (P less than 0.01) lower than mean concentration of follicular-phase LH (5.4 +/- 1.6 ng/ml). Frequency of peaks in ACTH concentration was low during the sampling period. Mean number of luteal-phase ACTH peaks (0.29 +/- 0.49) was not significantly different from that of follicular-phase samples (0.43 +/- 0.530). Unlike ACTH, mean frequency of LH peaks was significantly (P less than 0.05) higher in plasma from cows in the follicular phase of the estrous cycle (2.9 +/- 0.7), compared with that from cows in the luteal phase (0.29 +/- 0.49).     

Influence of dose, frequency, and duration of infused gonadotropin-releasing hormone on secretion of luteinizing hormone and follicle-stimulating hormone in nutritionally anestrous beef cows.

Nutritionally induced anovulatory cows were ovariectomized and used to determine the relationships between dose, frequency, and duration of exogenous gonadotropin-releasing hormone (GnRH) pulses and amplitude, frequency, and concentrations of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) in serum. In Experiment 1, cows were given pulses of saline (control) or 2 micrograms of GnRH infused i.v. during a 0.1-, 1.25-, 5-, 10-, or 20-min period. Concentrations of LH and FSH during 35 min after GnRH infusion were greater than in control cows (P < 0.01), and FSH concentrations were greater when GnRH infusions were for 10 min or less compared with 20 min. In Experiment 2, the effect of GnRH pulse frequency and dose on LH and FSH concentrations, pulse frequency, and pulse amplitude were determined. Exogenous GnRH (0, 2, or 4 micrograms) was infused in 5 min at frequencies of once every hour or once every 4th hr for 3 d. There was a dose of GnRH x frequency x day effect on LH and FSH concentrations (P < 0.01), indicating that gonadotropes are sensitive to changes in pulse frequency, dose, and time of exposure to GnRH. There were more LH pulses when GnRH was infused every hour, compared with an infusion every 4th hr (P < 0.04). Amplitudes of LH pulses were greater with increased GnRH dose (P < 0.05), and there was a frequency x dose x day effect on FSH pulse amplitude (P < 0.0006). We conclude that LH and FSH secretion in the bovine is differentially regulated by frequency and dose of GnRH infusions.     

Effect of copper status, supplementation, and source on pituitary responsiveness to exogenous gonadotropin-releasing hormone in ovariectomized beef cows

The effect of Cu status, supplementation, and source on pituitary responsiveness to exogenous GnRH was evaluated using nine multiparous, nonpregnant, nonsuckling, ovariectomized Angus cows (7.1 +/- 3.3 yr; 622.9 +/- 49.8 kg; BCS = 6.0 +/- 0.5). Cows were considered Cu-deficient based on liver Cu concentrations (< 30 mg of Cu/kg of DM) after receiving a low-Cu, forage-based diet supplemented (DM basis) with 5 mg of Mo/kg and 0.3% S for 216 d. Copper-deficient cows were stratified based on age, BW, BCS, and liver Cu concentration and assigned randomly to repletion-phase treatments. Treatments included 1) control (no supplemental Cu); 2) organic (ORG; 100% organic Cu); and 3) inorganic (ING; 100% inorganic CuSO4). Treatments were formulated to meet all NRC recommendations, except for Cu, which was supplemented to ORG and ING cows at 10 mg of Cu/kg of dietary DM. During the 159-d repletion phase, Cu status was monitored via liver biopsy samples, and all cows received exogenous progesterone. A controlled intravaginal drug-release device (replaced every 14 d) was used to maintain luteal phase progesterone as a means to provide negative feedback on the hypothalamic-pituitary axis. During the repletion phase, liver Cu concentrations did not differ between ORG and ING cows at any time. By d 77 of the repletion phase, all supplemented cows were considered adequate in Cu, and liver Cu concentrations were greater in supplemented than in nonsupplemented control cows on d 77 (P < 0.05) and throughout (P < 0.01) the repletion phase. Beginning on d 99, exogenous GnRH was administered to all cows at low (0, 3, and 9 microg; Exp. 1) and high doses (0, 27, and 81 microg; Exp. 2) at six different times. Cows were catheterized every fifth day, and blood samples were collected every 15 min for 1 h before and 4 h after GnRH administration and analyzed for LH concentration. In Exp. 1, Cu status and supplementation did not affect basal or peak LH concentrations, but total LH released tended (P < 0.07) to be greater in Cu-supplemented vs. control cows when 3 microg of GnRH was administered. In Exp. 2, there was no effect of Cu supplementation or source on basal, peak, or total LH released, regardless of GnRH dose. Pituitary LH concentrations did not differ across treatments. In conclusion, Cu status, supplementation, and source did not affect GnRH-induced LH secretion or pituitary LH stores in ovariectomized, progesterone-supplemented cows in this experiment.     

Injection of neuropeptide Y into the third cerebroventricle differentially influences pituitary secretion of luteinizing hormone and growth hormone in ovariectomized cows.

Hypothalamic neurons that control the luteinizing hormone (LH) and growth hormone (GH) axes are localized in regions that also express neuropeptide Y (NPY). Increased hypothalamic expression of NPY due to diet restriction has been associated with suppressed secretion of LH and enhanced secretion of GH in numerous species. However, these physiological relationships have not been described in cattle. Thus, two studies were conducted to characterize these relationships using ovariectomized (Experiment 1) or ovariectomized estrogen-implanted (Experiment 2) cows. In Experiment 1, four well-nourished, ovariectomized cows received third cerebroventricular (TCV) injections of 50 and 500 micrograms of NPY in a split-plot design. Venous blood was collected at 10-min intervals from -4 hr (pre-injection control period) to +4 hr (postinjection treatment period) relative to TCV injection. NPY suppressed (P < or = 0.04) tonic secretion of LH irrespective of dose and tended to stimulate (P < or = 0.10) an increase in tonic secretion of GH. In Experiment 2, six ovariectomized cows that were well nourished and implanted with estradiol received TCV injections of 0, 50, or 500 micrograms of NPY in a replicated 3 x 3 Latin Square. Both doses of NPY suppressed (P < 0.06) mean concentration of LH relative to the 0-microgram dose. The 50-microgram dose of NPY tended (P < 0.10) to increase the amplitude of GH pulses. In conclusion, TCV injection of NPY suppressed pituitary secretion of LH and simultaneously tended to increase pituitary secretion of GH.     

Response of the bovine corpus luteum to increased secretion of luteinizing hormone induced by exogenous gonadotropin releasing hormone

Two experiments were conducted to investigate the response of the bovine corpus luteum to surges of luteinizing hormone (LH) induced by natural gonadotropin-releasing hormone (GnRH) administered twice during the same estrous cycle. In experiment 1, eight mature beef cows, each cow serving as her own control, were injected intravenously (iv) with saline on days 2 and 8 of the cycle (day of estrus = day 0 of the cycle), then with 100 micrograms GnRH on days 2 and 8 of the subsequent cycle. Jugular blood samples were taken immediately prior to an injection and at 15, 30, 45, 60, 120 and 240 min postinjection, to quantitate changes in serum luteinizing hormone. Blood was also collected on alternate days after an injection until day 16 of the cycle, to characterize changes in serum progesterone concentrations. Although exogenous GnRH caused release of LH on days 2 and 8 of the cycle, the quantity of LH released was greater on day 8 (P less than .025). Serum levels of progesterone after treatment with GnRH on day 8 of the cycle did not differ significantly from those observed during the control cycles of the heifers. Because exposure of the bovine corpus luteum to excess LH, induced by GnRH early during the estrous cycle, causes attenuated progesterone secretion during the same cycle, these data suggest that a second surge of endogenous LH may ameliorate the suppressive effect of the initial release of LH on luteal function. Duration of the estrous cycle was not altered by treatment (control, 20.4 +/- .5 vs. treated, 20.4 +/- .4 days).(ABSTRACT TRUNCATED AT 250 WORDS)     

牛和绵羊在超数排卵情况下排卵受阻的卵巢组织学和内分泌研究

乏情母牛50头、有周期活动母绵羊38头,超排处理引起发情后的4(羊)或7～8(牛)天摘取卵巢进行组织学研究。观察到排卵阻断的5种类型:(1)典型黄体但包含未排的卵母细胞,(2)由未破卵泡黄体化而形成的非典型黄体,(3)纤维性结缔组织团块,其中有散在性黄体细胞(仅见于牛),(4)具备葛拉夫氏泡典型结构的滞留卵泡,(5)颗粒膜已黄体化的滞留卵泡。50头牛中共有滞留卵泡(≥10mm)127个、正常黄体29个、非正常黄体20个。严寒和饥饿可能是卵泡囊肿普遍发生的主要原因。试验牛曾用18甲基炔诺酮药管处理9天。在植入和取出药管时各注射1次(共2次)PMSG的母牛,具有很高的(28/30)早期反应率(出现≥3个卵泡的母牛数/接受处理的母牛数),而仅在去管时1次注射PMSG者,反应微弱(4/20)。试验羊中,只有单一用FSH处理者,在发情当天血清E_2—17β浓度形成峰值(168.1pg/ml),而且正常黄体数亦略高(P<0.1)于用FSH加LH处理者。发情后4天血清P_4浓度则以后者为高(P<0.05)。绵羊滞留卵泡表现为孕酮优势,卵泡液中E_2—17β对P_4的浓度比为1∶46。     

Relationship of pre- and post-ovulatory gonadotropin concentrations to subnormal luteal function in postpartum beef cattle

Early weaning of calves from anestrous cows results in formation of short-lived corpora lutea (CL) unless the animals are pretreated with a progestagen (norgestomet). This study was conducted to investigate the relationship between pre- and post-ovulatory gonadotropin secretion and luteal lifespan. Postpartum beef cows were assigned randomly into two groups, control (n = 5) and norgestomet (implant given at weaning for 9 d; n = 7). Calves from all cows were weaned 30 to 33 d postpartum. Coccygeal artery cannulas were placed into cows in the control group 1 d prior to weaning and 2 d before implant removal in cows in the norgestomet group. Plasma for determination of luteinizing hormone (LH), follicle stimulating hormone (FSH), estradiol-17 beta (E) and progesterone (P) was collected daily at 10-min intervals for 6 h from weaning (control) or the day prior to implant removal (norgestomet) to estrus (d 0) and on d 2, 4 and 6 following estrus. Average interval (X +/- SE; P less than .05) from weaning to estrus or implant removal was 4.2 +/- .8 and 2.3 +/- .2 d for the control and norgestomet groups, respectively. Estrous cycle length for the control group was 12.4 +/- 1.8 d compared with 20.4 +/- .3 d for the norgestomet group (P less than .05). Four of five control cows had an estrous cycle length of 7 to 14 d; all cows in the norgestomet group and the remaining control cow had an estrous cycle of normal length (16 to 21 d).2+ estrus.(ABSTRACT TRUNCATED AT 250 WORDS)     

Serum hormones in response to estradiol and (OR) progesterone in ovariectomized cows after thyroidectomy

To evaluate the effect of gonadal steroid treatment and thyroidectomy on concentrations of gonadotropins and thyroid-stimulating hormone in the bovine, nonlactating Holstein cows were either thyroidectomized and ovariectomized (THYOVEX; n=6) or ovariectomized only (OVEX; n=4), and subsequently treated with no gonadal steroids (control), estradiol-17β (E₂), progesterone (P₄), or P₄+E₂ in a 2 × 4 factorial experiment. Averaged across steroid treatments, baseline concentrations of luteinizing hormone (LH; P < .05) and follicle-stimulating hormone (FSH; P <.10) were higher in THYOVEX cows than in OVEX cows. Pulse frequencies and amplitudes of LH and FSH did not differ between THYOVEX and OVEX cows. Secretion of TSH was pulsatile and all concentrations and pulsatile characteristics of TSH were increased (P < .05) in THYOVEX compared to OVEX cows. Treatment with E₂ and P₄ decreased (P < .05) baseline concentrations and magnitude of LH pulses, whereas P₄+E₂ increased (P < .01) pulse frequency of LH and FSH. Amplitude of LH and FSH pulses were not affected by treatment with either steroid. Treatment with P₄+E₂ decreased (P < .05) baseline concentrations of TSH, whereas pulse frequency, and magnitude and amplitude of TSH pulses were not altered by treatment with steroids. Mean concentrations of LH and FSH were similar during 48 hr after termination of E₂ and P₄+E₂ treatments, but concentrations of TSH were higher (P = .06) after P₄+E₂ than after E₂. Secretion of TSH showed a diurnal variation, with the lowest concentrations in the morning and highest in the afternoon. These results indicate that thyroidectomy influenced secretion of gonadotropins in OVEX cows.     

Changes of very low-density lipoprotein concentration in hepatic blood from cows with fasting-induced hepatic lipidosis

The purpose of this study was to evaluate changes of very low-density lipoprotein (VLDL) components in hepatic blood (HB) from 5 nonlactating nonpregnant cows fasted from days 0 to 3 and subsequently refed to day 10 and, in addition, to assess those of other lipoproteins. Increased phospholipid concentrations in each lipoprotein after the start of fasting suggested their availability for the surface lipids of lipoproteins. Although the VLDL-triglyceride (TG) concentration in HB from all cows increased on day 1, the value on day 4 became similar to that on day 0. However, the concentration on day 10 was significantly increased. In all cows, the decreased ratio of the VLDL-TG concentration in HB to the non-esterified fatty acids (NEFA) concentration in portal blood (PB) on day 4 appeared to reflect relatively decreased secretion of TG as VLDL by NEFA excessively mobilized to the liver via PB. The markedly increased ratio on day 10 was considered to contribute to the improvement of hepatic lipidosis.     

Mammary secretion of oestrogens in the cow

Two experiments in vivo and one experiment in vitro were conduced to examine the mechanisms involved, which lead to mammary secretion of oestrogens and its importance for milk production and udder health in cows. In experiment 1 in six cows of the White-Black breed on day 268 of pregnancy catheters were inserted into uterine vein of pregnant horn, the abdominal aorta and the caudal superficial epigastric (milk) vein. Blood samples for estimation of oestrone, oestrone sulphate, oestradiol-17alpha and -17beta by RIA were obtained daily from day 7 pre-partum until day 1 post-partum. Only the concentration of oestradiol-17beta was statistically higher (P< or =0.01) in mammary venous plasma than in aortal and uterine plasma. In experiment 2 forty late-pregnant cows were divided into two groups according to their milk production in the previous lactation: group 1 (n=20) high-yielding cows (>6500kg milk per lactation), and group 2 (n=20) low-yielding cows (<3700kg milk per lactation). Blood samples for measurement of oestradiol-17beta by RIA were collected from milk and tail veins every fourth day during a period from day 20 prior to parturition to day 4 post-partum. The concentration of oestradiol-17beta was significantly higher (P< or =0.01) in the milk vein than in the peripheral plasma from day 12 pre-partum to parturition. In high-yielding cows the level of oestradiol-17beta in mammary venous blood was significantly higher (P< or =0.01) than in low-yielding cows. In six cows with pathological udder oedema ante-partum the concentration of oestradiol-17beta in milk vein was significantly higher (P< or =0.05) than in control cows. There were no statistically significant differences in the levels of oestradiol-17beta in cows with clinical mastitis (n=10) during 2 weeks after parturition and without it (P> or =0.05). In an in vitro experiment, homogenates of mammary tissue collected on day 7 pre-partum from two cows were incubated with 3H-androstendione. After incubation the samples were extracted and 3H-oestradiol-17beta was separated by HPLC. 3H-oestradiol-17beta was formed in a total yield of 37%. These results indicate that oestrone, oestrone sulphate and oestradiol-17alpha are not secreted by bovine mammary gland. Furthermore, the secretion of oestradiol-17beta starts about day 12 pre-partum and is associated with milk yield and udder oedema. Preliminary in vitro study suggests the synthesis of oestradiol-17beta by mammary tissue.     

Feedback of 17 beta-estradiol on secretion of luteinizing hormone during different seasons of the year

The working hypothesis was that the amount of increase in secretion of luteinizing hormone (LH) that results from positive feedback of 17 beta-estradiol (E2) is dependent on season of the year in mature bovine females. Seven beef cows, ovariectomized approximately 2 mo before the initiation of the experiment, were used in the initial year (1983) of the study. Three of the ovariectomized cows (OVX-E2) received an sc E2 implant, which provided low circulating levels of E2. The remaining four cows (OVX) were not implanted. Blood samples were collected serially (at 10-min intervals for 6 h) at each spring and fall equinox and at each summer and winter solstice. This protocol was replicated with a different group of cows in 1985 (OVX-E2, n = 4; OVX, n = 6). Concentration of LH in blood serum was quantified in all samples. Concentration of E2 in blood serum was measured in pools of samples from each serial blood collection. Concentrations of E2 were higher (P less than .05) in the implanted cows. Mean concentration of LH and amplitude of pulses of LH were higher (P less than .05) at each season of the year in cows that were ovariectomized and implanted with E2 than in cows that were ovariectomized and did not receive E2. An effect of season of the year on mean concentration of LH was detected (P less than .01). No influence of season or E2 was detected for frequency of pulses of LH. There was no significant treatment X season interaction.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of metoclopramide on luteinizing hormone secretion in postpartum anestrous cows.

The effect of metoclopramide (MC), a dopamine antagonist on luteinizing hormone (LH), was examined in anestrous primaparous cows. Metoclopramide has been found to be beneficial in overcoming fescue toxicosis; increasing LH secretion stimulates return to ovulatory function after parturition. Consequently, if MC had negative effect on LH secretion, it would indicate that administration of MC to reproducing animals might be limited. Of 14 postpartum (47 to 66 days) cows, 7 were given MC (4 mg/kg of body weight, IV), and 7 served as controls. Blood was obtained via jugular cannulas at 15-minute intervals for 8 hours; MC was given at the end of the first hour, and gonadotropin-releasing hormone (GnRH, 7 mg/kg), was given IV at the end of hour 7 as a challenge stimulus for LH secretion. Prior to GnRH administration, MC did not have significant effect on LH secretion, as judged by mean serum LH concentration, LH pulse frequency, and LH pulse amplitude. Administration of MC resulted in greater (P less than 0.05) LH response to GnRH, indicating enhanced secretory ability when the pituitary gland was challenged. Serum prolactin concentration was increased (P less than 0.01) by MC administration. Therefore, MC did not have adverse effect on LH secretion in postpartum cows.     

Effects of progesterone and weaning on LH and FSH responses to naloxone in postpartum beef cows

Two experiments were conducted to evaluate the effects of naloxone, an endogenous opioid receptor antagonist, on LH and FSH secretion in postpartum beef cows. In Experiment 1, 24 cows were divided into three equal groups. On day 15 postpartum, all cows were bled for 8 hr at 10 min intervals to evaluate LH secretory parameters. On day 18 postpartum, three treatments were administered: (a) saline at 0730 and 1130 hr; (b) 275 mg naloxone at 0730 and 1130 hr; (c) naloxone as in (b) above, plus this group was also treated with 50 mg progesterone (P4) twice daily from day 16 to day 19. In each treatment, jugular vein samples were collected at 10 min intervals from 0800 to 1600 hr. On day 19 the same treatments were administered at the same times, however, all cows were given 25 micrograms GnRH at 1200 hr to evaluate the LH secretory response. Naloxone increased mean LH concentration (P less than .05) and tended to increase pulse amplitude and frequency compared to controls. However, the most dramatic difference was due to P4 treatment which suppressed mean LH, pulse amplitude and frequency. Treatments had no effect on LH secretion in response to a 25 micrograms dose of GnRH. In Experiment 2, the effects of suckling on the naloxone response were examined in 16 postpartum cows. On day 21 postpartum, blood was collected at 10 min intervals for 8 hr and then calves were removed from half the cows. After 3 days of calf removal, all cows were sampled at 10 min intervals for 4 hr; then naloxone was injected after each 10 min sample at a dose rate of 200 mg/hr (33 mg per injection). Naloxone treatment and sampling continued for an additional 8 hr. Calf removal alone had very little effect on LH pulsatility. However, naloxone resulted in increased pulse frequency and mean LH compared to the control period. We conclude that LH release in the early postpartum cow is partially regulated by endogenous opioid peptides. We were unable to detect any effects on FSH secretion nor on pituitary sensitivity to exogenous GnRH.     

Effects of Full-Length Kisspeptin Administration on Follicular Development in Japanese Black Beef Cows

Kisspeptin is a key molecule that stimulates gonadotropin secretion via release of gonadotropin-releasing hormone (GnRH). In the present study, our aim was to investigate whether kisspeptin has stimulatory effects on follicular development via GnRH/gonadotropin secretion in cows. Japanese Black beef cows were intravenously injected with full-length bovine kisspeptin [Kp-53 (0.2 or 2 nmol/kg)] or vehicle 5 days after they exhibited standing estrus (Day 0). In cows injected with Kp-53 at 2 nmol/kg, the follicular sizes of the first dominant follicles increased on Day 6 and thereafter. Ovulation of the first dominant follicle occurred in 1 out of 4 cows treated with Kp-53 at 2 nmol/kg. Injection of Kp-53 at 2 nmol/kg increased the concentration of plasma luteinizing hormone (LH) but not follicle-stimulating hormone, over a 4-h period following injection in all cows. The present study suggests that administration of full-length kisspeptin causes LH secretion, which is sustained for a few hours, and it is capable of stimulating follicular development and/or ovulation.     

Comparison of luteinizing hormone, leptin and progesterone levels in the systemic circulation (Vena jugularis) and near the ovarian circulation (Vena cava caudalis) during the oestrous cycle in Mangalica and Landrace gilts

The objective of this study was to evaluate luteinizing hormone (LH) and luteal progesterone (P4) secretion in systemic blood and blood near the ovaries in Mangalica (M) and Landrace (L) gilts by implanting catheters into the Vena jugularis and the Vena cava caudalis via the Vena saphena, respectively. Furthermore, leptin was analyzed in jugular vein blood. Blood was collected twice daily from day 7 to day 19 of the oestrous cycle and frequently (10-min intervals for 6 h) on day 9, day 12 and day 15 in M (n=3) and L gilts (n=4). L gilts had congruent pulsatile LH secretion in both veins, but the LH concentrations in M were always below the assay sensitivity during the luteal phase. In both breeds, episodic P4 secretion was found in the jugular and caval veins, and both sampling site and breed had an influence on P4 secretion (P<0.05). The mean concentration of P4 was higher (P<0.01) in utero-ovarian blood (75.8+/-5.3 in M; 49.6+/-4.2 ng/ml in L) than in the periphery (31.3+/-2.0 in M; 21.2+/-1.8 ng/ml in L). M pigs had a lower number of corpora lutea (9.7+/-2.3 vs. 20.5+/-4.4), and analysis of the P4 secretion ratio per corpus luteum revealed an influence of breed (P<0.01). This ratio was significantly higher in M (3.8+/-0.3 and 8.7+/-0.7 ng/ml) compared with the L gilts (1.4+/-0.1 and 2.8+/-0.3 ng/ml) in the jugular and caval veins, respectively. Blood sampling from the Vena cava caudalis is potentially more precise than from the Vena jugularis for evaluation of ovarian P4 secretion. Both the higher P4 concentration and increased leptin secretion (11.3+/-0.6 vs. 3.0+/-0.1 ng/ml, P<0.05) and consequently the altered LH secretion pattern in the Mangalica may contribute to the lower fecundity of this breed.