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Topic:Endocrine System
The endocrine system in horses comprises a network of glands and hormones that regulate various physiological processes, including metabolism, growth, reproduction, and stress response. Key components of the equine endocrine system include the pituitary gland, thyroid gland, adrenal glands, and pancreas. Hormones such as insulin, cortisol, and thyroid hormones are produced and released into the bloodstream to maintain homeostasis and respond to internal and external stimuli. Dysregulation of the endocrine system can lead to conditions such as Equine Cushing's Disease (Pituitary Pars Intermedia Dysfunction) and Equine Metabolic Syndrome. This page compiles peer-reviewed research studies and scholarly articles that explore the structure, function, and disorders of the endocrine system in horses, providing insights into its impact on equine health and management.
Secretion of luteinizing hormone and follicle stimulating hormone in intact and ovariectomized mares in summer and winter. Sequential samples of blood were drawn via jugular catheters every 15 min for 24 h from four mares in each of five reproductive states: intact anestrous mares in winter, intact diestrous mares in summer, intact estrous mares in summer, ovariectomized mares in winter and ovariectomized mares in summer. Estrous mares were sampled on d 4 or 5 of estrus and diestrous mares on d 10 or 11 of diestrus. Each sample of plasma was assessed for concentrations of luteinizing hormone (LH) and follicle stimulating hormone (FSH) in two independent radioimmunoassays. A computer program was developed that dete...
Effects of age, season and active immunization against estrogen on serum prolactin concentrations in stallions. Prolactin concentrations in stallion serum were measured by radioimmunoassay based on antiserum generated against equine prolactin and radioiodinated canine prolactin. Prolactin concentrations in serum collected from 152 stallions at a slaughterhouse were higher (P less than .01) in summer than in winter (5.7 +/- .15 vs 2.0 +/- .17 ng/ml). Moreover, there was an effect of age (P less than .02) in the analysis of variance; there was no interaction between age and season. In general, prolactin concentrations increased with age up to 3 to 5 years. Samples of serum collected from five control and ...
Changes in thecal and granulosa cell LH and FSH receptor content associated with follicular fluid and peripheral plasma gonadotrophin and steroid hormone concentrations in preovulatory follicles of mares. Individual antral follicles from 11 horse mares were studied at three stages of the oestrous cycle to determine the characteristics of the presumptive ovulatory follicle. Mares were ovariectomized (ovex) during the late luteal phase on Day 14 after ovulation (Group 1) and on the 1st (Group 2) or 4th (Group 3) day of oestrus. Every follicle greater than 5 mm in diameter was dissected from each ovary; follicles greater than or equal to 15 mm in diameter were analysed separately while others were pooled by size for subsequent analyses. The presumptive ovulatory follicle possessed the following ch...
Release of LH, FSH and GnRH into pituitary venous blood in mares treated with a PGF analogue, luprostiol, during the transition period. Nine mares received cannulae to collect blood from the pituitary venous outflow in the intercavernous sinus (ICS) and the jugular vein; in 4 mares, only jugular cannulae were used. Those 4 mares and 3 of the mares with cannulae in both positions received 7.5 mg luprostiol i.m. and 1 mare with both cannulae was treated with 3.75 mg uprostiol i.v. Blood samples were kept before and after treatment at 2-, 5- or 10-min intervals and concentrations of LH, FSH and GnRH were determined by RIA. Treatments resulted in an immediate sharp rise of LH and FSH in ICS and jugular blood samples within 2-10 mi...
Patterns of oxytocin secretion during the oestrous cycle of the mare. From a group of 11 cyclic mares, blood samples were collected at 3-min intervals for 2 h and at 15-min intervals for an additional 6 h during four stages of the oestrous cycle. Mean plasma oxytocin concentrations (pg/ml, LSM +/- se) were greater on Day 15 after ovulation (169.9 +/- 17.6) than on Day 0 (82.6 +/- 17.6; P less than 0.01), Day 3 (97.2 +/- 20.4, P less than 0.01) and Day 7 after ovulation (104.0 +/- 25.0, P less than 0.05). Oxytocin was secreted in a pulsatile manner throughout the oestrous cycle, with short (0-29 min), medium (30-89 min) and long (greater than 90 min) duration rhy...
Effect of pulsatile gonadotrophin release on mean serum LH and FSH in peri-parturient mares. Changes in the pattern of LH and FSH in serum were studied in 6 mares foaling during the summer. Samples were collected frequently (every 15 min) for 24 h twice before foaling, -33 +/- 2 and -12 +/- 2 days, and for 12 h after foaling, on Day 0 and Day 4. Simultaneous pulses of FSH and LH were observed before foaling (r2 = 0.99). Before foaling, gonadotrophin pulses were infrequent (6 in 264 h of observation). On the day of foaling, LH and FSH pulse frequency increased (P less than 0.005) with 2-4 pulses per mare. The amplitudes of pulses of LH and FSH were higher before parturition than for th...
Comparison by three different radioimmunoassay systems of the polymorphism of plasma FSH in mares in various reproductive states. FSH was measured in the pituitary, and in pituitary venous and jugular blood collected at frequent intervals from mares in various reproductive states, using 3 validated and highly specific radioimmunoassay systems based on different antibodies, 'o', 'h' and 'e'. In the pituitary, 4 forms of FSH were found which differed in isoelectric point and relative potency in the 3 assays. In jugular blood, mean FSH concentrations and short-term patterns depended on the assay used and the reproductive state of the mare. In pituitary venous blood, although FSH concentrations were greatly elevated above ju...
Hormonal changes associated with induced late abortions in the mare. Two mares received PGF-2 alpha twice daily until abortion and 2 mares received a combined treatment with oestradiol benzoate and oxytocin. The mares were about 150 days pregnant. The PG-treated animals aborted after 37 and 61 h, respectively, and the fetuses were expelled in intact fetal membranes. The other 2 mares aborted 13 and 27 h after the first oxytocin injection, respectively, and showed strong uterine contractions and expelled the fetuses in disrupted fetal membranes. Concentrations of 15-ketodihydro-PGF-2 alpha increased both after PG and oxytocin injections and in association with t...
Plasma prolactin concentrations in non-pregnant mares at different times of the year and in relation to events in the cycle. Plasma prolactin concentrations were measured in mares using an homologous radioimmunoassay. An annual rhythm in plasma prolactin was found, with concentrations higher during the summer than during the winter. In addition to this seasonal pattern, occasional high concentrations of prolactin were seen when concentrations were otherwise basal. Blood samples taken from mares during an oestrous cycle in October-November showed that prolactin values were basal for most of the cycle, with a marked rise in prolactin shortly before the onset of oestrus. This prolactin peak was associated with an incre...
Steroid secretion by different cell types of the horse conceptus. Horse conceptuses were recovered non-surgically at Day 12-Day 15 and were dissociated with collagenase. Separation of the cells on a 31.8% Percoll gradient gave two bands of cells and indirect evidence suggests that the low density cells (LDC) are endoderm and the higher density cells (HDC) are trophectoderm. Each band was incubated for 24 h in Minimum Essential Medium and concentrations of oestradiol and progesterone in the medium were measured by radioimmunoassay (RIA). The LDC secreted predominately progesterone (log oestradiol/progesterone = -0.994 +/- 0.141; N = 15) whereas the HDC secret...
Sexual behavior of mares. Behavior during the estrous phase of the ovulatory cycle of the mare is analogous in most ways to that of estrous females of other species. Proceptive behaviors bring the mare into the proximity of the male and attract his attention. Positioning facilitates mounting, intromission, and ejaculation. Estrous signs appear to be more intense in the few days prior to ovulation than during the transition periods that separate the recurring estrous and diestrous phases. Sexual behavior is absent during diestrus. Detection of estrus in mares is problematic in that it requires the presence (or at least ...
Pulsatile administration of gonadotropin-releasing hormone advances ovulation in cycling mares. Cycling standardbred mares were infused with saline or 20 micrograms gonadotropin-releasing hormone (GnRH) in a pulsatile pattern (one 5-sec pulse/h, 2 h or 4 h) beginning on Day 16 of the estrous cycle. Although serum concentrations of luteinizing hormone (LH) increased significantly earlier in all three GnRH-treated groups (within one day of the initiation of infusion) compared to saline-infused controls, there were no differences in peak periovulatory LH concentrations among treatments (overall mean +/- SEM, 8.98 +/- 0.55 ng/ml). The number of days from the start of treatment to ovulation w...
Changes in the hypothalamic-hypophyseal axis of mares associated with seasonal reproductive recrudescence. Four groups of mares, representing anestrus (AN; n = 8), early transition (ET; n = 7), late transition (LT; n = 8) and estrus (EST; n = 12) were used to examine changes in the hypothalamus and anterior pituitary during the period of transition from winter anestrus into the breeding season. Mares were of mixed breeding, between the ages of 3 and 20 years, and had shown normal patterns of estrous behavior and ovulation during the breeding season previous to this experiment. Hypothalamic content of gonadotropin-releasing hormone (GnRH) and anterior pituitary content of luteinizing hormone (LH) an...
Active immunization of intact mares against gonadotropin-releasing hormone: differential effects on secretion of luteinizing hormone and follicle-stimulating hormone. Five lighthorse mares were actively immunized against gonadotropin releasing hormone (GnRH) to determine the relative importance of this hypothalamic hormone in the secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH). Five mares immunized against the conjugation protein served as controls. Mares were initially immunized in November and received secondary immunizations 4 wk later, and then at 6-wk intervals until ovariectomy in June. All mares immunized against GnRH exhibited an increase (p less than 0.01) in the binding of tritiated GnRH by plasma, an indication that a...
Endocrinologic, hematologic, and heart rate changes in swimming horses. Two identical experiments, using Standardbred and Thoroughbred horses (experiment A, n = 31; experiment B, n = 17) on a swimming regimen, were performed 1 week apart to evaluate short-term heart rate, hematologic, and endocrinologic changes. Horses were placed in 4 categories based on duration of swimming (1 to 5, greater than 5 to 10, greater than 10 to 15, and greater than 15 minutes). Heart rate, PCV, and plasma concentrations of total protein, cortisol triiodothyronine, thyroxine, insulin, and glucose of each horse were evaluated before, immediately after, and 1 hour after swimming. For ex...
Daily rhythm of cortisol, and evidence for a photo-inducible phase for prolactin secretion in nonpregnant mares housed under non-interrupted and skeleton photoperiods. Studies were conducted in anestrous mares to characterize daily rhythms of cortisol in non-interrupted [ambient and 16 h light (L): 8 h dark (D)] and skeleton (10L:4D:2L:8D, 10L:6D:2L:6D and 10L:8D:2L:4D) photoperiods, and to determine if there exists a photosensitive phase for the secretion of prolactin. Neither peak or nadir concentrations of cortisol, nor the time of peak or nadir concentrations differed among photoperiod treatments. Highest concentrations (66 +/- 4.4 ng/ml, mean +/- SE) occurred between 0700 and 0900, whereas lowest concentrations (31 +/- 3.6 ng/ml) were found from 1900 to...
Effect of graded doses of gonadotrophin-releasing hormone on serum LH concentrations in mares in various reproductive states: comparison with endogenously generated LH pulses. Luteinizing hormone release induced by a range of small (3.3-33 micrograms) and large (300-500 micrograms) i.v. doses of gonadotrophin-releasing hormone (GnRH) was measured in acyclic (n = 4), luteal phase (n = 3) and follicular phase (n = 5) mares and compared with endogenously generated LH pulses in the same reproductive states. Extrapolation from log-linear dose-response curves showed that an LH pulse comparable to an endogenous one would be simulated by i.v. injection of 7.0 (n = 4) and 4.1 (n = 6) micrograms GnRH in luteal and follicular phase mares respectively; a much smaller dose than ...
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 ...
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...
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) ...
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...
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...
Interrelationships between uterus and conceptus to maintain corpus luteum function in early pregnancy: sheep, cattle, pigs and horses. Processes associated with "Maternal Recognition of Pregnancy" are reviewed extensively from the ovine, bovine, porcine and equine species. Comparisons among these species indicate that CL maintenance is achieved primarily by a predominant antiluteolytic-anti PGF effect, and there is strong evidence for antiluteolytic-luteoprotective and luteotropic controls that complement this basic system. The nature of the chemical signals (steroids, prostaglandins and proteins) to regulate these processes among the species are described. Common to all of the species reviewed is a change in vascular dynamic...
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...
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...
