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.
Vervuert I, Coenen M, Wedemeyer U, Chrobok C, Harmeyer J, Sporleder HP.Physical exercise is known to affect calcium homeostasis in horses, but there is little information on the hormonal regulation of calcium metabolism during exercise. In order to evaluate the effects of exercise and training on calcium homeostasis and intact plasma parathyroid hormone, 7 untrained Standardbred horses were studied in a 6 week training programme. These horses were accustomed to running on the treadmill 3 weeks before onset of training and were exercised on a high-speed treadmill with an initial incremental standardised exercise test (SET 1: 6 incremental steps of 5 min duration e...
Perkins GA, Lamb S, Erb HN, Schanbacher B, Nydam DV, Divers TJ.Plasma ACTH levels have been variable in horses with a positive clinical response for therapy for equine Cushing's Disease (ECD). Therefore, our purpose was to determine the value of monitoring plasma adrenocorticotropin (ACTH) levels during treatment of equine Cushing's disease (ECD) with either cyproheptadine (n = 32) or pergolide (n = 10). First, we validated the chemiluminescent ACTH assay (specificity, precision, accuracy, intra-assay and interassay variations) and tested methods of handling the whole blood from the time of collection to when the ACTH was assayed. The sensitivity and spec...
The Journal of physiologyNovember 15, 2002
Volume 547, Issue Pt 1 67-76 doi: 10.1113/jphysiol.2002.027409
Giussani DA, Forhead AJ, Gardner DS, Fletcher AJ, Allen WR, Fowden AL.This study used between-breed embryo transfer in the horse to investigate the effects of maternal size and uterine capacity on fetal growth and postnatal cardiovascular and neuroendocrine functions. Equine embryos were transferred to establish eight Thoroughbred-in-Thoroughbred (TinT), seven Pony-in-Pony (PinP), five Thoroughbred-in-Pony (TinP) and eight Pony-in-Thoroughbred (PinT), pregnancies. Maternal and foal weights and placental microscopic area were measured at birth. At 6 days of postnatal life, arterial blood pressure and heart rate were monitored and blood samples were taken for horm...
Rathi R, Colenbrander B, Stout TA, Bevers MM, Gadella BM.Progesterone (P(4)) is a physiological inducer of the acrosome reaction (AR) in stallion spermatozoa. However, the capacitation-dependent changes that enable progesterone binding, and the nature of the signaling cascade that is triggered by progesterone and results in induction of the AR, are poorly understood. The aim of the current study was, therefore, to investigate the protein kinase dependent signaling cascades involved in progesterone-mediated induction of the AR in stallion spermatozoa. In addition, we aimed to determine whether bicarbonate, an inducer of sperm capacitation, acted via ...
Gentry LR, Thompson DL, Gentry GT, Davis KA, Godke RA, Cartmill JA.An experiment was conducted to determine the effects of high vs low body condition scores (BCS) produced by restricted feeding on reproductive characteristics, hormonal secretion, and leptin concentrations in mares during the autumnal transition and winter anovulatory period. Mares with BCS of 6.5 to 8.0 were maintained on pasture and/or grass hay, and starting in September, were full fed or restricted to produce BCS of 7.5 to 8.5 (high) or 3.0 to 3.5 (low) by December. All but one mare with high BCS continued to ovulate or have follicular activity during the winter, whereas mares with low BCS...
Johnson CA, Thompson DL, Cartmill JA.The present experiment characterized the pituitary responsiveness to exogenous GnRH in the first 10 d after ovulation following commercially available deslorelin acetate implantation at the normal dosage for hastening ovulation in mares. Twelve mature, cyclic mares were assessed daily for estrus and three times weekly for ovarian activity starting May 1. Mares achieving a follicle at least 25 mm in diameter or showing signs of estrus were checked daily thereafter for ovarian characteristics. When a follicle >30 mm was detected, mares were administered either a single deslorelin acetate impl...
Geor RJ, McCutcheon LJ, Hinchcliff KW, Sams RA.In several species, physical conditioning (training) provokes a large shift in substrate utilisation during submaximal exercise. Few studies in horses have quantitatively examined these effects. Therefore, the effects of exercise training on plasma glucose kinetics during submaximal exercise were examined in 7 horses (5 Thoroughbred, 2 Standardbred; age 3-9 years) that had been paddock-rested for at least 6 months. Two days after determination of maximum aerobic capacity (VO2max), horses ran on a treadmill (4 degree incline) at 55% of VO2max (UT) for 60 min or until fatigue and then completed ...
Guillaume D, Bruneau B, Briant C.The effects of two GnRH antagonists were tested in order to delay and/or synchronise ovulation in mares. Five mares received Antarelix (0.01 mg.kg(-1)), 5 mares received Cetrorelix (the same dose), 5 mares (control mares) received the vehicle intravenously, twice daily, for 8 days from the day the largest follicle reached 22 mm following prostaglandin administration. Ovulation was postponed in all mares injected with Antarelix (19.4 +/- 1.2 days after the beginning of the treatment) and in 2/5 mares injected with Cetrorelix (20 +/- 1 days) vs. 6.2 +/- 0.4 days in control mares. During the trea...
Watson ED, Thomassen R, Steele M, Heald M, Leask R, Groome NP, Riley SC.Dominant and subordinate follicles were collected from mares on the day after the dominant follicle reached 30 mm in diameter, to investigate regulation of folliculogenesis during spring transition and the breeding season. Concentrations of oestradiol-17beta, progesterone and inhibin A, but not inhibin isoforms with pro- and alpha C-immunoreactivity, were significantly higher in preovulatory follicles than in dominant anovulatory transitional follicles. Steroidogenic activity was regained gradually in the dominant follicles of successive anovulatory waves through spring transition. The dominan...
Williams RJ, Marlin DJ, Smith N, Harris RC, Haresign W, Davies Morel MC.To determine the effects of exercise, high heat and humidity and acclimation on plasma adrenaline, noradrenaline, beta-endorphin and cortisol concentrations, five horses performed a competition exercise test (CET; designed to simulate the speed and endurance test of a three-day event) in cool dry (CD) (20 degrees C/40% RH) and hot humid (30 degrees C/80% RH) conditions before (pre-acclimation) and after (post-acclimation) a 15 day period of humid heat acclimation. Plasma adrenaline and noradrenaline concentrations pre-acclimation were significantly increased compared with exercise in the CD tr...
Lackey BR, Gray SL, Henricks DM.The multi-functional proteins, insulin-like growth factor-I (IGF-I) and leptin were present in seminal plasma from different species. Concentrations of IGF-I in equine and porcine semen were 20 and 17.5 ng/ml, respectively. Seminal plasma concentrations of leptin were 1 ng/ml in human and 11 ng/ml in porcine samples.
Evans MJ, Kitson NE, Alexander SL, Irvine CH, Turner JE, Perkins NR, Livesey JH.We wish to use a gonadotrophin-releasing hormone (GnRH) antagonist in the mare as a tool for investigating the control of the oestrous cycle. The aim of this study was to test the effectiveness of the antagonist cetrorelix by testing both in vitro, using perifused equine anterior pituitary cells, and in vivo in seasonally acyclic mares. Pituitary cells were prepared and after 3-4 days incubation, loaded onto columns and given four pulses of GnRH (at 0, 30, 60 and 90 min; dose-response study). After the second GnRH pulse, infusion of cetrorelix began (0, 100, 1000 and 2000 pmol/l) and continued...
McCue PM, Farquhar VJ, Carnevale EM, Squires EL.Deslorelin implants, approved for use in inducing ovulation in mares, have been associated with prolonged interovulatory intervals in some mares. Administration of prostaglandins in the diestrous period, following a deslorelin-induced ovulation, has been reported to increase the incidence of delayed ovulations. The goals of the present study were: (1) to determine the percentage of mares given deslorelin that experience delayed ovulations with or without subsequent prostaglandin treatment, and (2) to determine if removal of the implant 48 h after administration would effect the interval to sub...
Porter RH, Duchamp G, Nowak R, Daels PF.An attempt was made to elicit maternal behavior in non-parturient Welsh pony mares through a combination of hormonal treatment and vaginal-cervical stimulation (VCS). Lactation was induced in 16 nonpregnant, non-parturient mares via a combination of estradiol, progesterone and a dopamine antagonist (sulpiride). During the adoption trials, each lactating mare was confined behind a padded bar and a newborn foal was held near her head. Eight of the mares received two 3-min periods of VCS when the foster foal was introduced. Following VCS, the foal was released and its interactions with the adopti...
Pinto CR, Paccamonti DL, Eilts BE, Short CR, Godke RA.Recent studies suggest that nitric oxide (NO) may have a role in regulating ovarian physiology. To investigate the role of NO during ovulation in mares, inhibitors of the nitric oxide synthase (NOS) were administered to estrous mares. Forty cycling mares (20 horses and 20 pony mares) were allotted to one of the three treatment groups. Once a follicle was at least 27 mm in diameter, but smaller than 35 mm, mares were given one of the following treatments: saline solution 0.9% (n = 20, w/v, i.v., every 12 h), Nomega-nitro-L-arginine methyl ester hydrochloride (L-NAME; n = 10, 148 micromol/kg, i....
Ginther OJ, Meira C, Beg MA, Bergfelt DR.Deviation during a follicular wave in mares begins when the largest follicle (F1) reaches a mean diameter of 22.5 mm and is characterized by continued growth of F1 to become the dominant follicle and regression of F2 to become the largest subordinate follicle. In the present study, F1 was ablated at the expected beginning of deviation (Hour 0) to provide a reference point for characterizing the intrafollicular changes preceding experimental deviation between F2 and F3. Diameters and concentrations of follicular fluid factors in F2 and F3 were determined in F1-ablated mares at Hours 0, 12, 24, ...
Thway TM, Wolfe MW.Equids and primates are the only species known to express the placental hormone chorionic gonadotropin (CG). CG is a member of the heterodimeric glycoprotein family and is composed of an alpha subunit linked to a hormone-specific beta subunit. Previously, we have reported that epidermal growth factor (EGF) regulates the equine glycoprotein hormone alpha subunit promoter through a protein kinase C (PKC)/mitogen-activated protein kinase (MAPK) signal transduction pathway in trophoblasts. The current study investigates the regulatory element/factors involved in the induction of equine glycoprotei...
Blanchar TL, Brinsko SP, Rigby SL.A tendency for deslorelin implants to suppress subsequent follicular growth and delay return to estrus following induced ovulation has been documented in nonlactating mares. To investigate this phenomenon in lactating mares, 22 broodmares in southeast Texas were administered either deslorelin or hCG to induce ovulation in the first postpartum estrus during February and March 2001. Mares were teased daily and examined twice weekly (Tuesdays and Thursdays) by transrectal ultrasonography. When a follicle >35 mm diameter was detected on Tuesday, mares were treated with either 2,500 U hCG admini...
Scoggin CF, Meira C, McCue PM, Carnevale EM, Nett TM, Squires EL.Equine pituitary extract (EPE) has been reported to induce heightened follicular development in mares, but the response is inconsistent and lower than results obtained in ruminants undergoing standard superovulatory protocols. Three separate experiments were conducted to improve the ovarian response to EPE by evaluating: (1) effect of increasing the frequency or dose of EPE treatment; (2) use of a potent gonadotropin-releasing hormone agonist (GnRH-a) prior to EPE stimulation; (3) administration of EPE twice daily in successively decreasing doses. In the first experiment, 50 mares were randoml...
Chopin JB, Chopin LK, Knott LM, de Kretser DM, Dowsett KF.An 8-year-old mare, with a foal at foot, was inseminated on foal heat with frozen semen, with the resultant pregnancy lost between days 34 and 41. The right ovary developed a large anovulatory follicle that was non-responsive to multiple doses of ovulating agents. The follicle eventually appeared to luteinise, although plasma progesterone concentrations did not reflect this. Another follicle developed, responded to GnRH and resulted in a pregnancy from frozen semen that went to term with a healthy foal. When the mare was examined after foaling, the structure on the right ovary appeared to be a...
Watson ED, Al-Zi'abi MO.The mare is a seasonal breeder and undergoes a period of ovarian transition in spring between winter anoestrus and cyclicity. During spring transition LH concentrations are low and many mares have successive large anovulatory follicular waves which reach the size of preovulatory follicles. Follicular angiogenesis is essential for growth and health of preovulatory follicles. The aim of the present study was to investigate the morphology and vascularity of transitional anovulatory follicles. On gross inspection, the wall of transitional follicles was visibly less well vascularized than that of p...
Chavatte-Palmer P, Arnaud G, Duvaux-Ponter C, Brosse L, Bougel S, Daels P, Guillaume D, Clément F, Palmer E.The induction of lactation is performed in ruminants by steroidogenic impregnation, followed by drugs intended to increase prolactin secretion. The aim of this study was to induce lactation in barren mares and to evaluate milk production. Five treated and 5 control mares were used in June and September in year 1, and 12 mares were used in year 2. Mares were administered a vaginal pessary (500 mg altrenogest and 50 mg estradiol benzoate) for 1 week. The 2nd week, another sponge with 100 mg estradiol benzoate was administered, together with 50 mg/100 kg body weight (BW) sulpiride in oil (IM q12h...
Gérard N, Loiseau S, Duchamp G, Seguin F.Follicular development and ovulatory processes in mammals involve local biochemical changes as a result of substantial modifications in cellular metabolism, the most well known of which is steroid variation. In the present study, the intrafollicular variation of several other components was studied using proton nuclear magnetic resonance ((1)H NMR). This approach made it possible to demonstrate that the intrafollicular biochemical content changes during follicular growth and maturation. Follicular fluid was aspirated by ovarian puncture of the dominant follicle at various physiological stages ...
Chandler KJ, Dixon RM.Urinary cortisol and creatinine concentrations, and the cortisol:creatinine ratio were compared between 12 healthy horses (group 1), 13 horses with Cushing's disease (group 2), and eight horses with dysautonomia syndrome (equine grass sickness) (group 3). The mean (sd) urinary cortisol concentrations were 112 (55.7), 250 (357) and 864 (526) nmol/litre in groups 1, 2 and 3, respectively; the mean (sd) urinary creatinine concentrations were 18.9 (7.3), 12.0 (6.7) and 45.2 (26.4) nmol/litre in groups 1, 2 and 3, respectively, and the mean (sd) ratios were 6.1 (2.6), 19.8 (23.8) and 21.3 (14.5) (x...
Macpherson ML, Simmen RC, Simmen FA, Hernandez J, Sheerin BR, Varner DD, Loomis P, Cadario ME, Miller CD, Brinsko SP, Rigby S, Blanchard TL.The objectives of this study were 1) to determine whether insulin-like growth factor-I (IGF-I) and insulin-like growth factor binding proteins (IGFBPs) were present in seminal plasma of stallions; 2) to compare semen parameters (IGF proteins, sperm numbers, morphology, and motility) from stallions at sexual rest (SR) and when sexually active (SA); 3) to compare semen parameters between stallions with high and low seminal plasma IGF-I concentrations; and 4) to examine the relationship between seminal plasma IGF-I concentrations and fertility parameters of stallions. Ejaculates were collected fr...
Alexander SL, Irvine CH.The primary aim of this study was to define the secretory dynamics of oxytocin and vasopressin in pituitary venous effluent from ambulatory horses during acute endotoxaemia, a stimulus that may release both hormones. Our secondary aim was to investigate the role of oxytocin in regulating adrenocorticotropic hormone (ACTH) secretion by comparing oxytocin, vasopressin, corticotropin-releasing hormone (CRH) and ACTH secretory profiles during endotoxaemia and by monitoring the ACTH response to oxytocin administration. Pituitary venous blood was collected nonsurgically continuously and divided into...
Farquhar VJ, McCue PM, Carnevale EM, Nett TM, Squires EL.Following induction of ovulation with deslorelin acetate (Ovuplant), gonadotrophin concentrations are reduced in the subsequent cycle, leading to increased interovulatory intervals in some mares. This study determined whether implant removal after 2 days prevented the decrease in gonadotrophin concentrations and follicular growth during the ensuing cycle. Twenty-four mares were randomised equally into 3 groups. Group 1 ovulated spontaneously, Groups 2 and 3 received the deslorelin implant to induce ovulation. Two days after treatment, the implant was removed from Group 3. On Day 10 postovulati...
Bollwei H, Weber F, Kolberg B, Stolla R.Uterine and ovarian blood flow was investigated in four mares during two consecutive estrous cycles using transrectal color Doppler sonography. The uterine and ovarian arteries of both sides were scanned to obtain waves of blood flow velocity. The pulsatility index (PI) reflected blood flow. There were significant time trends in PI values of all uterine and ovarian blood vessels during the estrous cycle (P < 0.05). PI values did not differ between the uterine arteries ipsi- and contralateral to the corpus luteum or the ovulatory follicle. PI values of the uterine arteries showed a wave shap...
Bollwein H, Mayer R, Weber F, Stolla R.Transrectal color Doppler ultrasound was used for the noninvasive investigation of luteal blood flow during the estrous cycle in six mares. Color was displayed in Power-Mode, in which the number of color pixels on the ultrasound image is related to the number of moving blood cells. Three pictures with a maximum number of color pixels of the corpus luteum (CL) during an examination period of about 20 min were selected and digitized on a laptop equipped with an external frame grabber card. The intra-class correlation coefficient for the number of color pixels was 0.90. In all estrous cycles simi...
Davies Morel MC, Newcombe JR.Despite the widespread use of hCG to advance ovulation in the mare there is little information on efficacy of dose rates and any contraindications of its use. This study aims to investigate the effect of dose of hCG on ovulation within 48h and the effect of hCG on: ovulation, multiple ovulation (MO), pregnancy, multiple pregnancy (MP) rates and synchrony of MO; additionally whether any seasonal effect is evident. Sequential ultrasonic scanning was used to monitor the occurrence of ovulation, within 48h of treatment, in 1291 Thoroughbred mares treated with either 750iu hCG or 1500iu hCG s.c. Ov...
Thompson DL, Pickett BW, Berndtson WE, Voss JL, Mett TM.Stallions were subjected to a gradually increasing photoperiod beginning on October 15, 1973. The maximum artificial daylength (16 hr) was imposed on February 8, 1974, and maintained until October 6, 1974. Two ejaculates were collected from each of five treated and four control stallions weekly, with an interval of 24 hr between die paired ejaculates on the first and alternate weeks, and an interval of 1 hr on the second and alternate weeks. During summer, stallions subjected to the artificial photoperiod produced less (P<.05) gel-free semen and gel per ejaculate and had more (P<.05) spermatoz...
Held F, Ekstrand C, Cvijovic M, Gabrielsson J, Jirstrand M.Cortisol is a steroid hormone relevant to immune function in horses and other species and shows a circadian rhythm. The glucocorticoid dexamethasone suppresses cortisol in horses. Pituitary pars intermedia dysfunction (PPID) is a disease in which the cortisol suppression mechanism through dexamethasone is challenged. Overnight dexamethasone suppression test (DST) protocols are used to test the functioning of this mechanism and to establish a diagnosis for PPID. However, existing DST protocols have been recognized to perform poorly in previous experimental studies, often indicating presence of ...
Stewart F, Thomson JA, Leigh SE, Warwick JM.Several cDNA clones corresponding to mRNA for the alpha-subunit of the horse (Equus caballus) pituitary and placental (chorionic) gonadotrophic hormones have been isolated and sequenced. Polyadenylated mRNA was purified from horse pituitary glands (the source of FSH and LH) and horse placental tissues (the source of chorionic gonadotrophin; CG). The mRNA preparations were characterized by in-vitro translation and Northern hybridization techniques using human and ovine gonadotrophin cDNA clones as probes. Complementary DNA libraries were created from the pituitary and placental mRNAs and a huma...
Holdstock NB, Allen VL, Fowden AL.During the switch from parenteral to enteral nutrition at birth, endocrine glands such as the pancreas must assume a glucoregulatory role for the first time if the neonate is to survive the transition to extrauterine life. Objective: To determine the adaptations in pancreatic endocrine function during the neonatal period in term pony foals delivered by different methods. Methods: By measuring insulin and glucagon concentrations, pancreatic alpha and beta cell responses to exogenous glucose (0.5 g/kg bwt) and arginine (100 mg/kg bwt) and to endogenous muzzling for 90 min were determined periodi...
Douglas RH, Umphenour N.Routine measurement of estrogens, testosterone, T4, insulin, FSH, and LH at least four times per year (e.g., during each of the four seasons) may improve the efficiency of stallion management. Benefits may not be realized in the short term but will provide valuable historical data on individual stallions that, when added to other data, will improve ability of management personnel to initiate early treatment and delay or slow declining fertility. This ability will be greatly improved as more data and products become available. There appears to be a relationship between low total estrogen concen...
Tharasanit T, Colenbrander B, Bevers MM, Stout TA.The only gonadotrophin preparation shown to stimulate commercially useful multiple ovulation in mares is equine pituitary extract (EPE); even then, the low and inconsistent ovulatory response has been ascribed to the variable, but high, LH content. This study investigated the effects of an LH-free FSH preparation, recombinant human follicle stimulating hormone (rhFSH), on follicle development, ovulation and embryo production in mares. Five mares were treated twice-daily with 450 i.u. rhFSH starting on day 6 after ovulation, coincident with PGF(2alpha) analogue administration; five control mare...
Anderson ST, Kidd LJ, Barton AJ, Greer RM.Breeding mares typically foal yearly. Little is known about the dynamics of maternal bone stores during gestation and lactation, the timing of any maternal bone mobilisation, re-accretion post-foaling, or the dynamics of bone metabolism in foals. We measured serum osteocalcin (OC) and serum pyridinoline (PYD) concentrations in 18 mares monthly from 6months gestation to foaling, and in both mares and foals for 4months after birth. From 6 to 11months of gestation, there was no change in mean monthly OC. Serum PYD increased between 7months gestation and foaling. After foaling, mean serum OC was l...
Loy RG, Swan SM.Progesterone in oil solution injected into mares intramuscularly in midcycle blocked estrus and ovulation at levels of 100 mg. per day or higher. Fifty milligrams per day prevented estrus but not ovulation. The interval from end of treatment to estrus appeared to depend upon dosage, and the effects on subsequent cyclic patterns were confounded by season of the year. Neither 50 nor 100 mg. per day stopped estrus or blocked ovulation when treatment was started on day 1 of estrus. Injection of 100 mg. per day of progesterone in oil delayed postpartum estrus and ovulation, but 50 mg. per day did n...
Dowsett KF, Tshewang U, Knott LM, Jackson AE, Trigg TE.A series of experiments using an ovalbumin conjugated gonadotrophin releasing hormone was used to stimulate antibody production, suppress testosterone secretion and depress testicular function in yearling and 2 year old colts and fillies. In the preliminary experiment, an injectable oil-based formulation was administered to yearling colts. Testicular development and testosterone secretion were retarded for a period of approximately 28-32 weeks while antibody titres were greater than 1:1000. An implant and water-soluble vaccine (200 and 400 mg) is presently being tested in 2 year old colts. Tes...
Conley AJ, Scholtz EL, Legacki EL, Corbin CJ, Knych HK, Dujovne GD, Ball BA, Moeller BC, Stanley SD. and evidence indicates that the bioactive, 5α-reduced progesterone metabolite, 5α-dihydroprogesterone (DHP) is synthesized in the placenta, supporting equine pregnancy, but its appearance in early pregnancy argues for other sites of synthesis also. It remains unknown if DHP circulates at relevant concentrations in cyclic mares and, if so, does synthesis involve the non-pregnant uterus? Jugular blood was drawn daily from cyclic mares ( = 5). Additionally, ovariectomized mares (OVX) and geldings were administered progesterone (300 mg) intramuscularly. Blood was drawn before and after t...
McCue PM, LeBlanc MM, Squires EL.Equine follicle stimulating hormone (eFSH) has been used to induce follicular development in transitional mares and problem acyclic mares, as well as superovulate cycling mares. The most efficacious protocol is to administer 12.5 mg eFSH, intramuscularly, twice daily beginning 5 to 7 days after ovulation when the diameter of the largest follicle is 20 to 25 mm. Prostaglandins are to be administered on the second day of eFSH therapy. Treatment with eFSH is continued for 3 to 5 days until follicle(s) are >or=35 mm in diameter. The mare is subsequently allowed to 'coast' for 36 h, after which hum...
Ginther OJ, Castro T, Baldrighi JM, Wolf CA, Santos VG.Five mares that developed idiopathic persistent corpus luteum (PCL) were compared with 5 mares with apparently normal interovulatory intervals (IOIs). Progesterone (P4) and a metabolite of prostaglandin F2α (PGFM) were assayed daily beginning on the day of ovulation (Day 0). Transition between the end of an initial progressive P4 increase and the beginning of a gradual decrease in P4 occurred on mean Day 6. The gradual decrease in P4 between Days 6 and 12 was less (approached significance, P < 0.06) in the PCL group than in the IOI group. The P4 concentration on Day 12 (before luteolysis i...
Chavatte PM, Rossdale PD, Tait AD.The purpose of this study was to measure 3beta-HSD activity in the equine placenta and to assess the effect of fetal and maternal blood plasma progestagens on 3beta-HSD activity was measured in 8 late gestation (collected by caesarian section at 250 to 320 days) and 7 term (collected by caesarian section at 250 to 320 days) and 7 term (collected at birth) equine placentae using a tritium release assay with [3alpha-3H] pregnenolone as substrate. Mean +/- s.d. Km(app) and Vmax for term placentae were in general higher than for late gestation placentae (0.129 +/- 0.217 micromol/l and 23.85 +/- 9....
Holtan DW, Nett TM, Estergreen VL.Jugular vein plasma from 13 mares was extracted with diethyl ether and chromatographed on Sephadex LH-20 columns (.5 × 9 cm) after which progesterone and 17α-hydroxyprogesterone (170HP) were quantified by a competitive protein binding radioassay. During pregnancy, progesterone increased (P < .05) from 1.1 ± .4 ng/ml (mean ± SE) on day 0 to 7.5 ± 1.2 ng/ml on day 8 followed by a transient (nonsignificant) decrease to 4.8 ± .4 ng/ml on day 28. From days 28 to 44 progesterone again increased (P < .05) attaining a maximum concentration of 15.2 ± 1.4 ng/ml on day 64. Thereafter progesterone ...
Silvia PJ, Squires EL, Nett TM.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...
Buff PR, Morrison CD, Ganjam VK, Keisler DH.Leptin is a protein hormone produced by adipose tissue that influences hypothalamic mechanisms regulating appetite and energy balance. In species tested thus far, including horses, concentrations of leptin increase as animal fat mass increases. The variables and mechanisms that influence the secretion of leptin are not well known, nor is it known in equine species how the secretion of leptin is influenced by acute alterations in energy balance, circadian patterns, and/or reproductive competence. Our objectives were to determine in horses: 1) whether plasma concentrations of leptin are secreted...
Jackson BF, Dyson PK, Hattersley RD, Kelly HR, Pfeiffer DU, Price JS.To investigate the relationship between stage of estrous cycle and bone cell activity in Thoroughbreds. Methods: Blood samples collected from forty-seven 2-year-old Thoroughbred mares in training for racing. Methods: Blood samples were collected monthly (in April through September) from the mares. Stage of estrus was determined by assessing serum progesterone concentration. Bone cell activity was determined by measuring concentrations of 2 markers of bone formation (osteocalcin and the carboxy-terminal propeptide of type I collagen [PICP]) and a marker of bone resorption (the cross-linked carb...
Chopin JB, Chopin LK, Knott LM, de Kretser DM, Dowsett KF.An 8-year-old mare, with a foal at foot, was inseminated on foal heat with frozen semen, with the resultant pregnancy lost between days 34 and 41. The right ovary developed a large anovulatory follicle that was non-responsive to multiple doses of ovulating agents. The follicle eventually appeared to luteinise, although plasma progesterone concentrations did not reflect this. Another follicle developed, responded to GnRH and resulted in a pregnancy from frozen semen that went to term with a healthy foal. When the mare was examined after foaling, the structure on the right ovary appeared to be a...
Tischner M, Niezgoda J.The aim of this study was to determine the effect of transportation on the behaviour and stress responses of the sympatho-adrenal and hypothalamo-pituitary-adrenal systems in mares and their foals by measuring the peripheral plasma concentrations of adrenaline, noradrenaline and cortisol. Control blood samples were taken from mares and their foals (n=5 pairs) on day 8 after parturition to establish whether blood sampling caused any changes in adrenaline, noradrenaline or cortisol concentrations in blood plasma. The experiment was conducted during foal oestrus (on day 9 after parturition). Adre...
Care AD, Abbas SK, Ousey J, Johnson L.Once lactation is established in mares, there is little change in the ionised calcium concentration in their milk. In contrast, the concentration of PTHrP(1-34) in the milk increases to a maximum level by the end of the second week of lactation, near which it remains for the rest of the lactation. As found in other species, the concentration of PTHrP(1-34) in mare's milk is considerably higher than that in plasma, sampled at the same time. No significant correlation could be demonstrated between the concentrations of PTHrP(1-34) and ionised calcium in the milk except during the last 10 weeks o...
Farquhar VJ, McCue PM, Carnevale EM, Nett TM, Squires EL.Following induction of ovulation with deslorelin acetate (Ovuplant), gonadotrophin concentrations are reduced in the subsequent cycle, leading to increased interovulatory intervals in some mares. This study determined whether implant removal after 2 days prevented the decrease in gonadotrophin concentrations and follicular growth during the ensuing cycle. Twenty-four mares were randomised equally into 3 groups. Group 1 ovulated spontaneously, Groups 2 and 3 received the deslorelin implant to induce ovulation. Two days after treatment, the implant was removed from Group 3. On Day 10 postovulati...
Byambaragchaa M, Choi SH, Joo HE, Kim SG, Kim YJ, Park GE, Kang MH, Min KS.Equine chorionic gonadotropin (eCG), produced by the endometrial cups of the placenta after the first trimester, is a specific glycoprotein that displays dual luteinizing hormone (LH)-like and follicle-stimulating hormone (FSH)-like effects in non-equid species. However, in equidaes, eCG exhibits only LH-like activity. To identify the specific biological functions of glycosylated sites in eCG, we constructed the following site mutants of N- and O-linked glycosylation: eCGβ/αΔ56, substitution of α-subunit N-linked glycosylation site; eCGβ-D/α, deletion of the O-linked glycosylation sites ...
Mehl ML, Sarkar DK, Schott HC, Brown JA, Sampson SN, Bayly WM.Eight mature mares were exercised for 20 min at 60% VO2max and to fatigue at 95% VO2max. Plasma beta-endorphin (EN) concentrations were determined before exercise, after a 10 min warm-up, and at the end of each exercise test. Mean +/- s.e. beta-endorphin concentrations were significantly (P < 0.05) higher following work at 95% VO2max when compared to that at 60% VO2max (420.0 +/- 102.7 vs. 269.9 +/- 30.69 pg/ml). Pre-exercise samples were collected between 1000 and 1500 h. In order to evaluate whether any cyclic changes in EN secretion effects may have influenced results, a subsequent study...
Chin WW, Maizel JV, Habener JF.The sizes of the human and subhuman alpha-subunits of the glycoprotein hormones differ by four amino acids (hCG alpha, 92 amino acids; murine, equine, bovine, and ovine alpha, 96 amino acids). The shortening of the human alpha-subunit has been attributed to posttranslational proteolysis. We have recently determined the nucleotide sequences of the mRNAs encoding the precursors of the alpha-subunit of mouse TSH and rat gonadotropins using recombinant DNA techniques. In this report, we have compared these nucleotide sequences and their deduced amino acid sequences with those of the pre- alpha-sub...
Safir JM, Loy RG, Fitzgerald BP.To investigate the hypothesis that the onset of the breeding season in the mare may be due to a daylength-induced seasonal increase in LHRH pulse frequency, 5 mares were immunized against LHRH. Beginning 1 December, 5 immunized and 5 untreated control mares were exposed to an abrupt, artificial increase in daylength (16L:8D) to advance the onset of the breeding season. In control mares ovulation occurred 49.6 +/- 3.5 (s.e.m.) days later (18 January), whereas in 3/5 immunized mares ovulation had not occurred by 1 April. In the remaining 2 mares, although ovulation occurred once (Mare 79) or twi...
Sissener TR, Squires EL, Clay CM.Prostaglandin F2alpha secretion by the uterine endometrium between Days 13 and 14 postovulation causes luteal regression in mares. A mechanism involving interruption or suppression of this secretion causes pregnancy to be maintained. The present study was designed to determine the age of the conceptus when maximal suppression of PGF2alpha secretion occurs. Mares were examined daily during estrus with ultrasonography (day 0 = day of ovulation). Conceptus tissues were recovered nonsurgically on Days 9 (n = 7), 12 (n = 5), 13 (n = 5), and 16 (n = 7) and uterine biopsies on Day 14. Both uterine an...
Barrier-Battut I, Le Poutre N, Trocherie E, Hecht S, Grandchamp des Raux A, Nicaise JL, Vérin X, Bertrand J, Fiéni F, Hoier R, Renault A, Egron L....Inducing ovulation in a cyclic mare is often necessary. For this purpose, hCG has been used commonly, but the response can be reduced after successive administrations. The aims of this study were to test the effectiveness of buserelin in hastening ovulation in estrus mares, and its influence on fertility; and to investigate the effect of treatment on LH secretion. Five crossover trials were designed to compare the effect of two treatments: buserelin (40 microg in 4 doses i.v. at 12 h intervals) vs placebo (Experiments 1 and 2); buserelin 40 microg (in 4 doses i.v.) vs 20 microg (Experiment 3);...
McNaughten J, Pozor M, Macpherson M, Kelleman A, Woodward E, Troedsson M.Mares who have not delivered a foal early in life may experience limitations in cervical relaxation, primarily during oestrus. A closed cervix prevents intrauterine deposition of semen during natural breeding, may delay uterine clearance after insemination leading to intrauterine fluid accumulation in, and subsequent infertility. Therefore, a reliable pharmacological method of dilating the equine cervix would have practical application in veterinary medicine. The goal of this study was to investigate the effectiveness of topically applied, synthetic prostaglandin E1 analogue (PGE1 ) for stimul...
Evans MJ, Kitson NE, Alexander SL, Irvine CH, Turner JE, Perkins NR, Livesey JH.We wish to use a gonadotrophin-releasing hormone (GnRH) antagonist in the mare as a tool for investigating the control of the oestrous cycle. The aim of this study was to test the effectiveness of the antagonist cetrorelix by testing both in vitro, using perifused equine anterior pituitary cells, and in vivo in seasonally acyclic mares. Pituitary cells were prepared and after 3-4 days incubation, loaded onto columns and given four pulses of GnRH (at 0, 30, 60 and 90 min; dose-response study). After the second GnRH pulse, infusion of cetrorelix began (0, 100, 1000 and 2000 pmol/l) and continued...
