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Topic:Hormones
Hormones in horses are chemical messengers produced by various glands and tissues, regulating numerous physiological processes essential for maintaining homeostasis. These hormones influence a wide range of functions, including growth, metabolism, reproduction, and stress responses. Key hormones in equine physiology include cortisol, estrogen, testosterone, and insulin, among others. The levels and effects of these hormones can vary based on factors such as age, sex, and environmental conditions, impacting overall health and performance. This page compiles peer-reviewed research studies and scholarly articles that explore the production, regulation, and physiological roles of hormones in equine biology.
Androgen and estradiol effects on gonadotropin secretion and response to GnRH in ovariectomized pony mares. In Exp. 1, 16 long-term ovariectomized pony mares were used to determine the effects of treatment with estradiol benzoate (EB) and dihydrotestosterone (DHT) benzoate alone, and in combination, on secretion of follicle stimulating hormone (FSH) and luteinizing hormone (LH) in daily blood samples and after three consecutive injections of gonadotropin releasing hormone (GnRH). Administration of EB alone, or in combination with DHT, every other day for 11 d reduced (P less than .05) concentrations of FSH and increased (P less than .05) concentrations of LH in daily blood samples, and increased (P ...
Comparison of the measurement of plasma testosterone and plasma oestrogens for the diagnosis of cryptorchidism in the horse. The results of performing 1720 blood tests for equine cryptorchidism are described. Using the paired sample human chorionic gonadotrophin (hCG) stimulation test and measuring testosterone, 6.7 per cent of tests did not give a clear result. If only the testosterone concentration in the pre-hCG blood sample was used, this percentage rose to 14 per cent. The paired sample hCG stimulation test was 94.6 per cent accurate. A comparison was made between the paired hCG stimulation test and the measurement of conjugated oestrogen in a single sample. The latter did not give as many doubtfuls but gave fa...
Induction of ovulation in anestrous mares with pulsatile administration of gonadotropin-releasing hormone. Four seasonally anestrous mares (Standardbred), housed under a nonstimulatory photoperiod of 8 hours light:16 hours dark, were administered gonadotropin-releasing hormone (GnRH) in a pulsatile pattern (50 or 250 micrograms of GnRH/hour) for 8 to 18 days during February and March 1985. Treatment with GnRH, irrespective of dose or month, induced an increase in serum luteinizing hormone from a mean pretreatment value typical of anestrus (0.58 +/- 0.02 ng/ml +/- SE) to 10.84 +/- 1.27 ng/ml on day 8 of GnRH treatment. Ovulation in the 4 mares occurred 8.8 +/- 0.7 days after the initiation of pulsat...
Equine thyroid function assessment with the thyrotropin-releasing hormone response test. The effect of thyrotropin-releasing hormone (TRH) on equine thyroid function was determined by quantifying serum thyroxine (T4) and 3,5,3'-triiodothyronine (T3) before and after TRH administration. Thyrotropin-releasing hormone was administered IV to adult horses (n = 5) and ponies (n = 6) at a dose of 1 mg or 0.5 mg, respectively. Serum T4 and T3 concentrations were determined before and 0.25, 0.5, 1, 2, 4, 6, 8, 12, and 24 hours after TRH administration. Serum T4 increased from a basal concentration of 24.4 +/- 8.7 ng/ml (mean +/- SD) to a maximum value of 48.2 +/- 10.2 by 4 hours after TRH ...
Spontaneous and stimulated adrenocorticotropin and vasopressin pulsatile secretion in the pituitary venous effluent of the horse. Plasma ACTH, arginine vasopressin (AVP), and catecholamines were measured at 5-min intervals in the pituitary venous effluent of the unanesthetized horse. Pulses of ACTH and AVP were found to be surprisingly brief (usually of less than 10-min duration) and frequent (averaging between 15-25 min). A highly significant relationship in the changes in concentration of these two hormones was demonstrated (P less than 0.0002) both at rest and after a mild hypoglycemic stimulus. Although there was also a significant correlation (P less than 0.005) between simultaneous plasma ACTH and AVP values the pu...
Isolation and characterization of three forms of luteinizing hormone from the pituitary gland of the horse. Three isoforms of equine luteinizing hormone (eLH-A, eLH-B and eLH-C) have been isolated from horse pituitary glands. Separation was achieved on the basis of charge heterogeneity by ion-exchange chromatography. These charge differences were apparent after final purification, as determined by electrophoretic mobility on polyacrylamide disc gels (RF = 0.14, 0.19 and 0.26 for eLH-A, -B and -C, respectively). Apparent size differences were also noted between the isohormones by gel filtration on Sephadex G-100. Ve/Vo ratios for eLH-A, -B and -C were 1.72, 1.54 and 1.47, respectively. All 3 isoforms...
Serum concentrations of prolactin, thyroxine and triiodothyronine relative to season and the estrous cycle in the mare. Studies were conducted to characterize circulating concentrations of prolactin, thyroxine (T4) and triiodothyronine (T3) relative to season and the estrous cycle, and to evaluate the ability of thyrotropin-releasing hormone (TRH) to induce prolactin secretion in the horse mare. The increase in serum prolactin following the iv injection of 0, 5, 50 and 500 micrograms TRH was dose-related, while all three doses of TRH induced a significant and comparable increase in serum T4. Seasonal variations in serum prolactin were directly correlated (P less than .001) with changes in photoperiod (r = .80) ...
Direct demonstration of intrinsic follicle-stimulating hormone receptor-binding activity in acid-treated equine luteinizing hormone. After dissociating equine gonadotropins as a function of time at pH 3, we examined them by radioligand assay and sodium dodecyl sulfate polyacrylamide gel electrophoresis under nondissociating conditions (low, 0.1% SDS). Equine follicle-stimulating hormone (FSH) rapidly lost its receptor-binding activity, and low SDS-polyacrylamide gels demonstrated dissociation into subunits. Maximum dissociation occurred after 20-30 min of pH 3 incubation. Equine luteinizing hormone (LH), however, retained most biologic activity and was largely intact after 72 h of pH 3 incubation. Dose-response curves of ac...
Role of conceptus secretory products in establishment of pregnancy. Conceptuses produce steroids, prostaglandins, proteins and possibly other unidentified agents which may play a role in the establishment and maintenance of pregnancy. A key event in this process is protection of the corpus luteum (CL) from the luteolytic activity of prostaglandin (PG) F-2 alpha of uterine origin. Oestrogens produced by the pig conceptuses between Days 11 and 16 appear to exert an antiluteolytic effect resulting in the sequestering of PGF-2 alpha within the uterine lumen. Failure of the pregnant uterus to release PGF-2 alpha in an endocrine fashion, therefore, allows for mainte...
Impaired estrogen production by Leydig cells of the naturally retained testis in unilaterally cryptorchid boars and stallions. Estrogen production in vitro was compared for Leydig cells from cryptorchid and scrotal testes in boars and stallions. Animals with natural and experimental cryptorchidism were used. Purified Leydig cells were prepared from testes of mature animals by collagenase treatment and Percoll density gradients. After incubation for 3 hours (1 X 10(6) cells), estrone sulfate and estrone in the media were measured by direct radioimmunoassay. Androstenedione and testosterone in media extracts also were determined. Cells from the abdominal testis of unilateral cryptorchid boars and stallions showed impair...
The sympatho-adrenal system and plasma levels of adrenocorticotropic hormone, cortisol and catecholamines in equine grass sickness. Plasma levels of adrenocorticotropic hormone (ACTH), cortisol and catecholamines were used to study the role of the sympatho-adrenal system in equine grass sickness. Statistical evaluation determined differences of hormone levels between seven horses with grass sickness (one acute, five subacute and one chronic), six horses with colic (one with laminitis) and 16 control horses before and after mild stress. Plasma levels of the hormones were higher in horses with acute and subacute grass sickness than in the other groups. No differences were detected between horses with colic and stressed contr...
Androgen and progesterone effects on follicle-stimulating hormone and luteinizing hormone secretion in anestrous mares. Anestrous lighthorse mares were treated in December with dihydrotestosterone (DHT; 150 micrograms/kg of body weight), progesterone (P; 164 micrograms/kg), both DHT and P (DHT+P), testosterone (T; 150 micrograms/kg), or vehicle (n = 4/group). Daily blood sampling was started on Day 1, and on Day 4 all mares were administered a pretreatment injection of gonadotropin-releasing hormone (GnRH) and were bled frequently to characterize the responses of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) concentrations. Treatment injections were given on Day 4 and then daily through Day 17...
A review of patterns of change in luteal function. Puberty: For the gilt and filly, the first corpus luteum (CL) appears to have a normal lifespan. For both species, first CL usually is associated with estrus but can form in the absence of estrus. For the ewe and cow, a transient (1 to 4 d, ewe; 3 to 10 d, cow) rise and fall of ovarian derived progesterone (P4) is detected in peripheral blood (80% of ewes; 50% of heifers) prior to first "normal" CL. The first CL of apparent normal lifespan is not accompanied by estrus in the ewe. The first CL in the cow may or may not be accompanied by estrus; first estrus in the cow can be anovulatory. Data a...
Seasonal variation in the feedback of sex steroid hormones on serum LH concentrations in the male horse. The possibility of seasonal variation in the feedback effect of testosterone or oestradiol was investigated by giving replacement treatment to geldings for 2-3 weeks during breeding and non-breeding seasons. In the non-breeding season, testosterone suppressed LH values (mean +/- s.e.m., ng/ml) in all geldings (before treatment, 7.5 +/- 2.3; final treatment week, 1.8 +/- 0.2; P less than 0.05), whereas early in the breeding season, testosterone caused a prolonged rise in LH (before, 6.8 +/- 2.3; final week, 18.9 +/- 6.4; P less than 0.05). In all testosterone experiments, LH returned to pretrea...
Hormonal responses to exercise and training. Current knowledge and understanding of the hormonal response to exercise are limited, whether in relation to horses, humans, or other species. The changes in plasma concentration of some hormones occur early in exercise, apparently owing to a neuronal stimulation, whereas others, being pituitary dependent, require hormonal stimulation. Also, although it is possible to observe changes in plasma concentrations of hormones, the mechanism by which this is achieved is not always understood, and unless the nonprotein-bound, or active, form of the hormone is also determined, changes in plasma concent...
Attempted conversion of twin to singleton pregnancy in two mares with associated changes in plasma oestrone sulphate concentrations. The removal of one of twin embryos was attempted by infusion of 24% (w/v) saline into the gestation sac in 2 mares by laparotomy. The treatment was successful in one mare (Case 1) and the untreated embryo remained viable. However, neither foetus survived in the second mare (Case 2). Plasma oestrone sulphate (E1S) concentrations fell immediately after treatment in both mares but recovered to approximately 50% of pretreatment levels in Case 1. In Case 2 plasma E1S concentrations declined steadily and were less than 1 ng/ml within 6 days of treatment. These preliminary results suggest that the me...
Identification of 3 beta-hydroxy-5,7-androstadien-17-one as a secretory product of the fetal horse gonad in vivo and in vitro. Isolation of 3 beta-hydroxy-5,7-androstadien-17-one, as a major component of steroids extracted from vein blood of the fetal gonads of the horse, supports the proposed role for the compound as a precursor for equilin formation in the placenta of the mare. The 5,7-diene was extracted from blood collected from gonadal veins of fetal ovaries and testes in situ, and from a fetal testis connected to an artery in the neck region of the mare. Perfusion of fetal gonads in the laboratory was carried out to allow longer periods of collection. In addition, isolated cell preparations from a fetal testis w...
Thyrotropin stimulation test–new perspective on value of monitoring triiodothyronine. Thyrotropin (thyroid stimulating hormone; TSH) stimulus to thyroid cells of horses and dogs resulted in increased serum triiodothyronine (T3) concentrations that were detected earlier than those of thyroxine (T4). Doubling of the base-line T3 values in horses was detected 0.5 hours after injection of 5 IU of TSH IV, with peak response of 5 times base-line value detected 2 hours after injection. Doubling of T4 values in horses was noticed between 2 and 3 hours, with the peak response of 2.4 times base-line value at 4 hours after injection of TSH. Doubling of base-line T3 values in dogs in respo...
Humoral, hormonal and behavioral correlates of feeding in ponies: the effects of meal frequency. The effect of meal frequency on body fluid, glucose, triiodothyronine (T3), heart rate and behavior was measured in 10 ponies. A simple reversal design was used in which each pony received one meal/day (1X) for 2 wk and six meals/day (6X) for 2 wk. The total intake/day was held constant. Feeding was followed by a rise in plasma levels of glucose, T3, protein and osmolality. One large meal was followed by significantly greater changes in all of the variables than was a meal one-sixth the size. Plasma T3 rose from 41 +/- 5 (SE) ng/liter before feeding to 43 +/- 5 ng/liter following a small meal,...
Effects of multiple intramuscular injections and doses of dexamethasone on plasma cortisol concentrations and adrenal responses to ACTH in horses. Adrenocortical function was assessed in horses given multiple IM doses of dexamethasone to determine the duration of adrenocortical suppression and insufficiency caused by 2 commonly used dosages of dexamethasone (0.044 and 0.088 mg/kg of body weight). Dexamethasone was administered at 5-day intervals for a total of 6 injections. Daily blood samples were collected. The plasma was frozen and later assayed for cortisol. An ACTH response test was determined 2 days before the first injection of dexamethasone and again 8 days after the last dexamethasone injection. Maximum suppression of plasma cor...
Gas chromatography-mass spectrometry of androgens in equine ovarian follicles at ultrastructurally defined stages of development. Identification of 19-nortestosterone in follicular fluid. Follicular fluid was obtained from equine follicles at different stages of development as determined by ultrastructural study. Gas chromatography-mass spectrometry associated with stable isotope dilution permitted the demonstration of high levels of 4-estrene-3,7-dione and 17 beta-hydroxy-4-estren-3-one, 17 beta-hydroxy-4-estren-3-one levels often being about 10 times higher than those of testosterone. These findings suggest that in the mare ovary, an aromatizing pathway may proceed using these 19-norsteroids as intermediates. As a consequence of this high level of 19-norsteroids, testosterone...
Effects of interrupted photoperiods on the induction of ovulation in anestrous mares. The ability of interrupted photoperiods to induce early estrus and ovulation was examined. Horse mares were exposed to long (16 h light) or short (10 h light), noninterrupted photoperiods, ambient light, or various interrupted photoperiod treatments from December 1 to April 15 (135 d). Follicular development was assessed by rectal palpation and estrous behavior was determined by teasing with a stallion. Serum concentrations of progesterone were used as an indicator of corpus luteum function. Differences among the light treatment groups were compared for the following behavioral and ovarian cha...
Effects of estrous cycle and season on ultrasonic uterine anatomy in mares. The morphological changes in ultrasound images of the uterus at various times of the year were characterized in nonbred mares, using a linear-array scanner. The uterus was recorded as having an ultrasonic morphology characteristic of diestrus (uterine score 1, endometrial folds not visible), estrus (score 3, distinct endometrial folds), or an intermediate stage (score 2). In Experiment I, uterine scores for the first ovulatory period of the year were compared to scores for the second period in 23 pony mares. More mares (P<0.05) showed endometrial folding prior to the second ovulation of the...
Hormonal response to thyrotropin-releasing hormone in healthy horses and in horses with pituitary adenoma. Cortisol, triiodothyronine (T3), thyroxine (T4), insulin, and glucose responses to thyrotropin-releasing hormone (TRH) were evaluated in 12 healthy, mature horses and in 7 horses and 4 ponies with clinical signs of pituitary adenoma (PA). Within 1 hour after TRH administration, the increase in T3 and T4 was similar in healthy horses and animals with PA. Plasma cortisol in the group with PA increased (P less than 0.05) within 0.25 hours after TRH administration, and remained increased for 1.5 hours. In the control group, a significant increase in plasma cortisol concentrations did not develop a...
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...
Demonstration of a COOH-terminal extension on equine lutropin by means of a common acid-labile bond in equine lutropin and equine chorionic gonadotropin. The beta subunits of equine lutropin and equine chorionic gonadotropin were incubated in 0.013 N HCl for 30 min at 110 degrees C and separated into two fragments by reverse-phase high performance liquid chromatography. The amino acid and carbohydrate compositions of both fragments from each subunit were analyzed. The results demonstrated that equine lutropin-beta has a glycosylated COOH-terminal extension that differs only in carbohydrate composition from the COOH-terminal portion of equine chorionic gonadotropin-beta. This is the first demonstration of a glycosylated COOH-terminal extension i...
Embryonic loss in mares: Nature of loss after experimental induction by ovariectomy or prostaglandin F(2alpha). Twenty-one pregnant pony mares were assigned to one of the following groups: 1) controls, 2) ovariectomy at Day 12, 3) ovariectomy at Day 12 plus daily progesterone treatment on Days 12 to 40, 4) PGF(2alpha) on Day 12, 5) PGF(2alpha) on Day 21, and 6) PGF(2alpha) on Day 30. Based on daily examinations by ultrasound, the embryonic vesicle was maintained to Day 40 in all control mares and in mares that were ovariectomized on Day 12 and given progesterone. The embryonic vesicle was lost in all mares of the other four groups. Administration of progesterone prevented the embryonic loss associated w...
