Effect of dosage and frequency of injection of luteinizing hormone releasing hormone on release of luteinizing hormone and follicle stimulating hormone in estradiol-treated steers

The objective was to determine how estradiol (0 vs 1 mg) and changes in the dosage of luteinizing hormone releasing hormone (LHRH; 1,000 ng/steer vs 1 ng/kg body weight) and frequency of LHRH injection (25 vs 50 min) affect LH and follicle stimulating hormone (FSH) release in steers. In steers pretreated with estradiol peak concentrations of LH in serum after LHRH averaged 14.4 ng/ml, which was greater (P less than .001) than peak concentrations in steers given oil (7.4 ng/ml). Increasing the dosage of LHRH from 1 ng/Kg body weight (approximately or equal to 300 ng/steer) to 1,000 ng/steer increased (P less than .001) peak LH values from 7.5 to 14.4 ng/ml. Furthermore, increasing the frequency of LHRH injections from once every 50 min to once every 25 min increased (P less than .001) LH release, but only in steers given estradiol. Estradiol reduced basal concentrations of FSH by 65% and then increased LHRH-induced FSH release by 276% (P approximately .07) relative to values for steers given oil. Only when 1,000 ng LHRH was given every 25 min to steers pretreated with estradiol were LH and FSH release profiles similar to the preovulatory gonadotropin surges of cows in magnitude, duration and general shape. The results demonstrate that increases in the dosage or frequency of LHRH pulses increase LHRH-induced release of LH, but not of FSH. Furthermore, these results are consistent with the hypothesis that in cows, estradiol increases responsiveness of the gonadotrophs to LHRH and then increases the magnitude and frequency of pulses of LHRH secretion beyond basal levels, thereby causing the preovulatory gonadotropin surges.     

Effect of luteinising hormone releasing hormone and its analogues on plasma luteinising hormone concentrations in incubating bantam hens

The ability of synthetic vertebrate luteinising hormone releasing hormones (LHRHs) and their long-acting analogues to maintain elevated plasma luteinising hormone (LH) concentrations and to stimulate ovarian growth was investigated in incubating bantam hens. Chicken LHRH-II (pGlu1-His2-Trp3-Ser4-His5-Gly6-Trp7-Tyr8-Pro9-G ly10-NH2) was more effective than chicken LHRH-I (pGlu1-His2-Trp3-Ser4-Tyr5-Gly6-Leu7-Gln8-Pro9-Gly10-N H2) or porcine LHRH (pGlu1-His2-Trp3-Ser4-Tyr5-Gly6-Leu7-Arg8-Pro9-Gly10-N H2) in stimulating the release of LH. Long-acting analogues of chicken LHRHs (chLHRHs) were created by substituting D-amino acids in position 6. An intravenous injection (10 micrograms/bird) of D-Arg6-chLHRH-II or of a long-acting mammalian analogue of LHRH (buserelin) resulted in a sustained release of LH for up to 8 h. Less sustained releases of LH were observed after the same doses of D-Ala6-chLHRH-I or of D-Trp6-chLHRH-I. Repeated subcutaneous injections of D-Arg6-chLHRH-II or buserelin at 7 to 9 h intervals for 9 d resulted in loss of pituitary gland responsiveness to these analogues. For this reason, the treatment failed to maintain elevated plasma LH concentrations and did not stimulate the growth of the ovary or oviduct.     

Pituitary effects of steroid hormones on secretion of follicle-stimulating hormone and luteinizing hormone

Steroid hormones have a profound influence on the secretion of the gonadotropins, follicle-stimulating hormone (FSH) and luteinizing hormone (LH). These effects can occur as a result of steroid hormones modifying the secretion of gonadotropin-releasing hormone (GnRH) from the hypothalamus, or a direct effect of steroid hormones on gonadotropin secreting cells in the anterior pituitary gland. With respect to the latter, we have shown that estradiol increases pituitary sensitivity to GnRH by stimulating an increase in expression of the gene encoding the GnRH receptor. Since an estrogen response element (ERE) has not been identified in the GnRH receptor gene, this effect appears to be mediated by estradiol stimulating production of a yet to be identified factor that in turn enhances expression of the GnRH receptor gene. However, the importance of estradiol for enhancing pituitary sensitivity to GnRH during the periovulatory period is questioned because an increase in mRNA for the GnRH receptor precedes the pre-ovulatory rise in circulating concentrations of estradiol. In fact, it appears that the enhanced pituitary sensitivity during the periovulatory period may occur as a result of a decrease in concentrations of progesterone rather than due to an increase in concentrations of estradiol. Estradiol also is capable of altering secretion of FSH and LH in the absence of GnRH. In a recent study utilizing cultured pituitary cells from anestrous ewes, we demonstrated that estradiol induced a dose-dependent increase in secretion of LH, but resulted in a dose-dependent decrease in the secretion of FSH. We hypothesized that the discordant effects on secretion of LH and FSH might arise from estradiol altering the production of some of the intrapituitary factors involved in synthesis and secretion of FSH. To examine this hypothesis, we measured amounts of mRNA for activin B (a factor known to stimulate synthesis of FSH) and follistatin (an activin-binding protein). We found no change in the mRNA for follistatin after treatment of pituitary cells with estradiol, but noted a decrease in the amount of mRNA for activin B. Thus, the inhibitory effect of estradiol on secretion of FSH appears to be mediated by its ability to suppress the expression of the gene encoding activin.     

Analogs of growth hormone-releasing hormone induce release of growth hormone in the bovine

Biological potencies of three 29 amino acid growth hormone-releasing hormone analogs (GHRH[1-29]) were determined in the bovine and compared to synthetic human GHRH (44 amino acids; hGHRH[1-44]NH2) for their ability to increase serum growth hormone (GH) concentrations. Four prepubertal Holstein heifers (179 +/- 10 kg) received hGHRH(1-44)NH2 or analogs (D-Ala2, Nle27, Agm29 GHRH[1-29], [JG-73]; D-N-MeAla2, Nle27, Agm29 GHRH[1-29], [JG-75]; and desamino-Tyr1, D-Ala2, Nle27, Agm29 GHRH[1-29], [JG-77]) at the following doses: 0, 6.25, 25, 100 and 400 micrograms/animal. All treatment-dose combinations were administered to each heifer with at least a 1-d interval between treatments. Sixteen blood samples were collected via jugular cannulas 20 min before and up to 6 h after treatment injection. There was a linear dose-dependent GH release in response to hGHRH(1-44)NH2 and the three analogs. Growth hormone peak amplitudes for the three analogs were similar to those observed after administration of the hGHRH(1-44)NH2 (P greater than .05). However, when total area under the GH response curves for each treatment was averaged over all the doses, JG-73 stimulated greater GH release than hGHRH(1-44)NH2 (P less than .05) Heifers injected with the 400-microgram dose of hGHRH(1-44)NH2 or the three analogs showed a primary release of GH followed by a secondary release 1 h later. At all other doses, only a primary GH release was observed.(ABSTRACT TRUNCATED AT 250 WORDS)     

Neuroendocrine regulation of growth hormone secretion in sheep. V. Growth hormone releasing factor and thyrotrophin releasing hormone.

The effects of intravenous (IV) and intracerebroventricular (ICV) administration of either bovine growth hormone releasing hormone (GRF) or thyrotrophin releasing hormone (TRH) on plasma growth hormone (GH) and glucose levels have been examined in sheep. Intravenous GRF 1-29NH2 at 3 and 30 micrograms stimulated an increase in GH levels in a dose-dependent fashion; administration of GRF into a lateral cerebral ventricle, however, produced a smaller GH response which was similar at these two doses. Evaluation of somatostatin levels in petrosal sinus blood (which collects pituitary effluent blood) showed that ICV administration of GRF stimulated a release of somatostatin into the blood. Furthermore, concurrent administration of GRF and a potent anti-somatostatin serum ICV resulted in a much enhanced release of GH which was similar to that obtained with a comparable dose of GRF given IV. TRH (as another putative GH-secretagogue) was also administered both IV and ICV. When given IV, 200 micrograms (but not 100 micrograms) TRH produced an elevation in GH levels. By contrast, when 5 micrograms TRH was given ICV there was a decrease in circulating GH levels, but no change in plasma somatostatin concentrations. These results indicate that the smaller GH response to ICV- compared with IV-administered GRF is due to the release of somatostatin within the brain. In addition, it would seem that TRH is not a physiological GH-secretagogue in sheep.     

Thyrotropin releasing hormone interactions with growth hormone secretion in horses

Light horse mares, stallions, and geldings were used to 1) extend our observations on the thyrotropin releasing hormone (TRH) inhibition of GH secretion in response to physiologic stimuli and 2) test the hypothesis that stimulation of endogenous TRH would decrease the normal rate of GH secretion. In Exp. 1 and 2, pretreatment of mares with TRH (10 microg/kg BW) decreased (P < 0.001) the GH response to exercise and aspartate infusion. Time analysis in Exp. 3 indicated that the TRH inhibition lasted at least 60 min but was absent by 120 min. Administration of a single injection of TRH to stallions in Exp. 4 increased (P < 0.001) prolactin concentrations as expected but had no effect (P > 0.10) on GH concentrations. Similarly, 11 hourly injections of TRH administered to geldings in Exp. 5 did not alter (P > 0.10) GH concentrations either during the injections or for the next 14 h. In Exp. 5, it was noted that the prolactin and thyroid-stimulating hormone responses to TRH were great (P < 0.001) for the first injection, but subsequent injections had little to no stimulatory effect. Thus, Exp. 6 was designed to determine whether the inhibitory effect of TRH also waned after multiple injections. Geldings pretreated with five hourly injections of TRH had an exercise-induced GH response identical to that of control geldings, indicating that the inhibitory effect was absent after five TRH injections. Retrospective analysis of pooled, selected data from Exp. 4, 5, and 6 indicated that endogenous GH concentrations were in fact lower (P < 0.01) from 45 to 75 min after TRH injection but not thereafter. In Exp. 7, 6-n-propyl-2-thiouracil was fed to stallions to reduce thyroid activity and hence thyroid hormone feedback, potentially increasing endogenous TRH secretion. Treated stallions had decreased (P < 0.01) concentrations of thyroxine and elevated (P < 0.01) concentrations of thyroid-stimulating hormone by d 52 of feeding, but plasma concentrations of GH and prolactin were unaffected (P > 0.10). In contrast, the GH response to aspartate and the prolactin response to sulpiride were greater (P < 0.05) in treated stallions than in controls. In summary, TRH inhibited exercise- and aspartate-induced GH secretion. The duration of the inhibition was at least 1 h but less than 2 h, and it waned with multiple injections. There is likely a TRH inhibition of endogenous GH episodes as well. Reduced thyroid feedback on the hypothalamic-pituitary axis did not alter basal GH and prolactin secretion.     

Adrenocorticotropic hormone and cortisol in calves after corticotropin-releasing hormone.

The aim for this study was to analyze responsiveness of the hypothalamo-pituitary-adrenocortical axis to exogenous bovine corticotropin-releasing hormone (bCRH) in calves. Two dose-response studies were carried out, using either bCRH alone (dose rates of 0, .01, .03, and .1 microg bCRH/kg live weight) or in combination with arginine-vasopressin (bCRH:AVP, 0:0, .1:.05, .5:.25, and 1:.5 microg kg live weight). The bCRH was administered i.v. to calves (n = 5 to 7 per dose) housed individually or in groups. Serial blood samples were obtained from before to 300 min after injection and analyzed for plasma ACTH and cortisol concentrations. The lowest bCRH dose that produced a response in all calves was .1 microg/kg. In the experiment using bCRH with AVP, increasing the bCRH dose from .1 to 1 microg/kg resulted in an increase in peak ACTH concentration (321 vs. 2,003 pg/mL) but did not significantly affect the peak cortisol concentration (37 vs. 40 ng/mL). The time to reach the peak cortisol concentration increased with the dose of bCRH with AVP (from 38 to 111 min). The ACTH and cortisol concentrations determined at any time between 20 and 90 min after bCRH injection were correlated to the integrated responses calculated as areas under the ACTH and the cortisol curves (r between .61 and .99, P<.05). In comparison with results from studies in humans, pigs, and sheep, our data showed that the pituitary of calves seems less sensitive to CRH than that of other mammals, despite a greater capacity to produce ACTH. Moreover, the calf's adrenals seem to have a lower capacity to produce cortisol than adrenals of other mammals. As in other species, it seems that AVP enhances the release of ACTH and cortisol. For CRH challenge to be used in calves, we suggest injecting at least .1 microg of bCRH/kg live weight either with or without AVP and taking several blood samples before injection and between 20 and 90 min after injection.     

Changes in serum concentrations of luteinizing hormone and follicle-stimulating hormone following injection of gonadotropin-releasing hormone during pregnancy and after parturition in mares

High concentrations of estrogens in the peripheral circulation during late gestation inhibit synthesis of LH and markedly reduce pituitary content of LH at the end of pregnancy in most domestic species. Because blood concentrations of estrogen peak shortly before mid-gestation in the mare and then gradually decrease until parturition, we hypothesized that pituitary content of LH may increase during late gestation. To test this hypothesis 10 horse mares were challenged with a maximally stimulatory dose (2 micrograms/kg) of GnRH on d 240 and 320 of gestation and d 3 after parturition. A separate group of four mares were treated with GnRH on d 2 or 3 estrus. Blood samples were collected at -2, -1, 0, .25, .5, .75, 1, 1.5, 2, 2.5, 3, 3.5, 4, 5, 6, 7 and 8 h relative to injection of GnRH and serum was analyzed for concentration of LH and FSH. Basal serum concentration and total quantity of LH released after GnRH stimulation (assessed by determining the area under the response curve) were not different on d 240 and 320 of gestation or on d 3 after parturition (12.5 +/- 3.5, 5.7 +/- 1.5 and 29.1 +/- 12.1 ng.min/ml, respectively) and were less (P less than .05) than on d 3 of estrus (311.0 +/- 54.0 ng.min/ml). There was little difference in the basal serum concentration of FSH at any of the time points examined. In contrast, GnRH-induced release of FSH continually decreased (P less than .05) from d 240 of gestation (559.8 +/- 88.9 ng.min/ml) to d 3 of estrus (51.8 +/- 6.2 ng.min/ml).(ABSTRACT TRUNCATED AT 250 WORDS)     

Plasma luteinizing hormone response to increasing doses of synthetic gonadotrophin-releasing hormone in heifers

The dosenresponse relationship for a synthetic gonadotrophin-releasing hormone (GnRH) was studied in normally cycling heifers using the area under the luteinizing hormone (LH) curve as a response parameter. Oestrus was synchronized by an injection of 0.5 mg cloprostenol before the experiment started and after the 3rd treatment with GnRH. Treatment with GnRH as assigned in a Latin square included 5 dose levels (0, 10, 50, 100, 250 μg) and 5 treatment days over a period of 22 days. GnRH was capable of inducing an increase of plasma LH within 30 min after injection. Plasma LH response increased with increasing doses of GnRH, the largest increase being observed when the dose was raised from 50 μg to 100 μg. One heifer did not respond to any of the doses applied. The existence of an individual treshold dose of GnRH is suggested.     

Effect of administration of adrenocorticotropic hormone on plasma concentrations of testosterone, luteinizing hormone, follicle stimulating hormone and cortisol in stallions

In boars and rabbits, administration of adrenocorticotropic hormone (ACTH) results in a testis-dependent, short-term increase in concentrations of testosterone in peripheral plasma. This experiment was designed to assess the short-term effects of a single ACTH injection on plasma concentrations of testosterone, luteinizing hormone (LH), follicle stimulating hormone (FSH) and cortisol in stallions. Eight light horse and two pony stallions were paired by age and weight and then one of each pair was randomly assigned to the treatment (ACTH, .2 IU/kg of body weight) or control (vehicle) group. Injection of ACTH increased (P<.01) plasma concentrations of cortisol by approximately twofold in the first 60 minutes; control stallions showed no change (P>.10) in concentrations of cortisol during the blood sampling period. Control stallions exhibited a midday increase (P>.05) in concentrations of testosterone similar to that reported previously; ACTH treatment prevented or delayed this increase such that concentrations of testosterone in treated stallions were lower (P<.05) than in controls 4 to 5 hours after injection of ACTH. Treatment with ACTH had no effect (P<.10) on plasma concentrations of LH or FSH up to 12 hours after injection.     

Pulsatile administration of gonadotropin-releasing hormone agonist to gilts actively immunized against gonadotropin-releasing hormone

Sexually mature gilts (n = 20) were actively immunized against GnRH. Primary and booster immunizations of GnRH conjugated to bovine serum albumin induced production of antibodies in all gilts. Nineteen of the gilts became acyclic with suppressed concentrations of gonadotropins and estradiol. Intravenous challenges with 100 micrograms GnRH and 5 micrograms D-(Ala6, des-Gly-NH2(10)) ethylamide GnRH (a GnRH agonist that did not cross-react with antibodies produced by the gilts) caused release of LH and FSH, indicating maintenance of secretory capacity of pituitary gonadotropes in the immunized animals. Gilts were given 100 ng GnRH agonist at 2-h intervals for 72 h (n = 4) or 144 h (n = 10) or did not receive agonist (n = 5). Blood samples were taken every 6 h, and detectable concentrations of LH were observed in 42% and 52% of samples taken from gilts treated with or without agonist. In contrast, serum concentrations of FSH and estradiol were undetectable. Reproductive tracts and anterior pituitaries were taken from gilts at the conclusion of pulsatile administration of GnRH agonist or at 144 h for controls. Pituitary concentration of LH and FSH, uterine wet and dry weight, and size of the uterus were similar among groups. Paired ovarian weights for treated gilts pulsed with GnRH agonist for 72 h were heavier (P less than .05); however, ovaries from all immunized gilts were atrophied without follicular structures.(ABSTRACT TRUNCATED AT 250 WORDS)     

GnRH的调控机制

ＧｎＲＨ由丘脑下部合成［１］,并以脉冲方式分泌进入丘脑下部垂体门脉血系统,到达垂体前叶发挥诱导促性腺激素释放的作用。ＧｎＲＨ除对垂体ＬＨ／ＦＳＨ分泌有强大的调控作用外,还与性腺和胎盘的生殖功能有关［２,３］。１ＧｎＲＨ信号转导途径ＧｎＲＨ调节垂体ＬＨ...     

Regulation of the growth hormone and luteinizing hormone response to endotoxin in sheep

Infectious disease processes cause physiological adaptations in animals to reorder nutrient partitioning and other functions to support host survival. Endocrine, immune and nervous systems largely mediate this process. Using endotoxin injection as a model for catabolic disease processes (such as bacterial septicemia), we have focused our attention on regulation of growth hormone (GH) and luteinizing hormone (LH) secretion in sheep. Endotoxin produces an increase in plasma GH and a decrease in plasma LH concentrations. This pattern can be reproduced, in part, by administration of various cytokines. Antagonists to both interleukin-1 (IL-1) and tumor necrosis factor (TNF) given intravenously (IV) prevented the endotoxin-stimulated increase in GH. Since endotoxin will directly stimulate GH and LH release from cultured pituitary cells, the data suggest a pituitary site of action of the endotoxin to regulate GH. Studies with portal vein cannulated sheep indicated that gonadotropin releasing hormone was inhibited by endotoxin, suggesting a central site of action of endotoxin to regulate LH. However, other studies suggest that endotoxin may also regulate LH secretion at the pituitary. Thus, IL-1 and TNF regulate GH release from the pituitary gland while endotoxin induces a central inhibition of LH release.     

Detection of urinary luteinizing hormone in Japanese black cows after administration of gonadotropin-releasing hormone

The blood luteinizing hormone (LH) surge in cows is well studied. However, little is known about urinary LH in cows. This study examined urinary LH concentrations after administration of gonadotropin-releasing hormone (GnRH) in six Japanese black cows to induce LH secretion from the pituitary gland into the bloodstream. Abrupt rises in plasma and urinary LH were observed after GnRH administration. Plasma and urinary LH peaked at 2 and 5 hr, respectively. A positive correlation was observed between plasma LH concentrations and urinary LH amounts. Ovulation was confirmed in the cows after 48 hr of GnRH administration. These data strongly suggest that urinary LH is derived from plasma LH, which triggers ovulation in cows.     

Luteinizing hormone in the bovine pars tuberalis: secretion in response to luteinizing hormone releasing hormone and intracellular isoforms

The objective of this study was to examine the physiological characteristics of gonadotropes in the bovine (b) pars tuberalis as assessed by their ability to release Luteinizing Hormone (LH) in response to LH-Releasing Hormone (LHRH) and the intracellular distribution of LH isoforms. At slaughter, the stalk median eminence and associated pars tuberalis as well as the anterior pituitary gland were collected from each of 7 castrate males. Each stalk median eminence and pituitary gland was mid-sagitally sectioned and weighed. One half of each tissue was immediately frozen and subsequently homogenized to determine the intracellular distribution of bLH isoforms. Tissue extracts were desalted by flow dialysis against water and chromatofocused on pH 10.5-7.0 gradients. The remaining half of the pituitary was sliced with a Staddie-Riggs slicer. The pituitary slices and the remaining half of the stalk median eminence were perifused (0.1 ml/min) for a total of 360 min with effluent samples (1.0 ml) collected every 10 min. At 130 min tissues were stimulated with 5 x 10(-8) M LHRH. Concentrations of LH in the effluent samples and the fractions collected from chromatofocusing were determined by radioimmunoassay. The release of LH in response to LHRH was 43.9% and 47.0% above basal secretion for the pars tuberalis and pituitary, respectively, suggesting similar degrees of responsiveness. Pars tuberalis and pituitary extracts resolved into nine LH isoforms during chromatofocusing and were coded with letters beginning with the most basic form. No differences (P greater than .05) were observed in distribution of LH isoforms between the pars tuberalis and the pituitary gland.(ABSTRACT TRUNCATED AT 250 WORDS)