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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.
The effect of a corticosteroid (dexamethasone), progesterone, oestrogen and prostaglandin F2alpha on gestation length in normal and ovariectomized mares. Dexamethasone (100 mg/day) injected from Days 318 to 324 of pregnancy induced parturition. Progesterone (500 mg/day) administered daily from Day 318 of pregnancy also shortened gestation while oestrogen (50 mg/day) administered on the same schedule as progesterone had no such effect. Dexamethasone treatment superimposed on the oestrogen or progesterone treatments shortened gestation. Prostaglandin F2alpha alone did not induce parturition, unlike PGF2alpha with dexamethasone, or PGF2alpha given to ovariectomized mares.
The use of prostaglandin F2alpha in controlling the oestrous cycle of the mare and steroid changes in the peripheral blood. Prostaglandin F2alpha (PGF2alpha), administered by untrauterine infusion and intramuscular injection, was used to induce oestrus and ovulation in non-cyclic mares. A satisfactory response rate (80% or more) was obtained and the dose (2-5--7-5 mg) and the time taken for ovulation to occur (up to 9 days) was the same irrespective of the route of administration. Only about one-half of the mares conceived to mating at the induced oestrus but the low conception rate could be attributed to the infertile condition of the mares. Plasma progesterone remained at basal levels after PGF2alpha and oestroge...
Urinary oestrogen and plasma progesterone levels in non-pregnant mares. Eight mares were observed clinically for 14 months to detect oestrus and ovarian activity, and assays of urinary oestrogens and plasma progesterone were performed. Cyclical ovarian activity occurred during 60% of the experimental period. Persistent luteal activity was found over 20% of the period and for the rest of the time activity was absent. Cyclical activity was characterized by the alternation of periods of low and high progesterone levels corresponding to the follicular and luteal phases of the ovarian cycle. During the follicular phase, urinary oestrogen levels increased to a maximum o...
Plasma LH levels in the mare during the oestrous cycle. Plasma levels of LH were determined by heterologous radioimmunoassay utilizing highly purified equine LH as standard. Samples were taken regularly from eleven mares for twenty-six oestrous cycles over a period of 10 weeks. The mean cycle length was 20-5 +/- 3-1 (S.D.) days, and ovulation occurred on average 4-3 +/- 1-6 (S.D.) days from the time heat was first detected. Levels of LH were persistently low from Days 5 to 16 of the cycle (ovulation = Day 0). They then increased slowly over a number of days and continued to rise beyond the levels observed at any time during the immediate preovulato...
Postnatal changes in total and free thyroxine and triiodothyronine in foal serum. Total tyhroxine (TT4), free thyroxine (FT4), total triiodothyronine (TT3) and free triiodothyronine (FT3) concentrations in foal umbilical cord blood were respectively 14, 5 7 and 3 times the concentrations of these hormones in adult horse blood. The TT4 levels in foals declined rapidly to reach adult concentrations by Day 16 and FT4 levels declined steadily during the first 3 months of life. Foal TT3 levels rose during the first 10 hr after birth and thereafter declined, although they were still X2-5 higher than adult levels at 3 months of age. Levels of FT3 similarly increased after birth be...
Endocrine patterns of the mare at term. The levels of progesterone and total progestins increased during late pregnancy and then fell dramatically during the last few days before parturition, although birth occurred in the presence of significant amounts of plasma progesterone. Oestrogen levels decreased slowly but insignificantly until birth occurred, and then remained at a low level until the post-partum oestrus 10 to 12 days later. Total 11beta-hydroxycorticosteroids were unchanged from Day 45 before to Day 15 after birth. The endocrine pattern of one ovariectomized pregnant mare during the last month of gestation was similar to ...
Patterns of urinary oestrogen excretion in individual pregnant mares. Urine collections (24 hr) were made at weekly intervals from four Pony mares from the 3rd or 4th month of pregnancy until parturition. Separation of oestrogens on Celite columns was followed by Kober measurements of oestrone and equilin. Individual differences were noted in peak amounts of total oestrogens excreted (200 to 800 mg/day), when oestrone constituted 80 to 95% at the 6th to 7th months. Although equilin increased in later gestation, oestrone remained the major product. Total oestrogen values decreased rapidly from the peak and then more gradually towards the end of pregnancy. During ...
Plasma progestagens in the pregnant mare in the first and last 90 days of gestation. Plasma progestagens were measured in eighteen pregnant mares to establish normal levels in the first and last 90 days of gestation. Progestagens increased from 25 ng/ml at 90 days before birth to 60 ng/ml at 10 days before birth, decreased to 58 ng/ml at 5 days before, and were 3 ng/ml or less by 5 days after perturition. During the first 90 days of pregnancy, progestagens reflected luteal activity with an increase in plasma levels from Day 30 to Day 60.
The influence of intrauterine saline infusion on luteal function and cyclical activity in the mare. Intrauterine saline infusion in the dioestrous mare shortened the ovulatory interval by inducing premature luteolysis. Plasma progestagen levels began to decrease approximately 1 day after the infusion and had declined to less than 1-0 ng/ml in 4 days. The CL, including others formed from ovulations during dioestrus, must be 4 to 5 days old before intrauterine saline will induce luteolysis. Of 10 mares infused on Day 4 or 5 after ovulation, only six had a shortened ovulatory interval. Of 10 mares infused on Day 6 or 7 after ovulation, seven had a shortened ovulatory interval and three failed t...
Plasma progestagens in cyclic, pregnant and post-partum mares. The pattern of plasma progestagen levels during the oestrous cycle was similar to that previously reported except for lower peak levels. The lack of significant difference (p less than 0-01) between CPBA and RIA values suggests that progesterone itself is the major component during the oestrous cycle. Progesterone levels during the first and second post-parturient oestrous cycles were similar to those observed during the cycle of the non-pregnant mare. During pregnancy there were two peaks of plasma progestagens. The first, which occurred during the 3rd month, coincided with high levels of PMS...
Studies on mild composition and lactogenic hormones in the mare. Small samples of mammary secretion were collected from eight Thoroughbred mares during the last week of pregnancy, at foaling and after 1 week of lactation. Specific assays showed the presence of both lactose and triglyceride in all samples before birth and progressive increases in their concentrations in colostrum and in milk. Levels of 6-0 g lactose/100 ml and 1-8 g triglyceride/100 ml were present in milk at 7 to 9 days post partum. The secretory capacity of the udder is thus normally established well before parturition in the mare. Attempts to demonstrate the occurrence of a placental lact...
Development of pituitary and adrenal glands in the fetal horse. The various cell types in the adult anterior pituitary may be distinguished at the ultrastructural level of the sizes of the secretory granules within the cytoplasm. In the fetal adenohypophysis, with the exception of prolactin-producing cells, all types may be identified after Day 100 of gestation. Morphological evidence suggests that they are producing and secreting trophic hormones. The three constituent layers of the adrenal cortex are also discernible by Day 100 of pregnancy. The cells of the zona glomerulosa have large numbers of darkly stained inclusions within the cytoplasm. The zona f...
The effect of HCG on duration of oestrus, ovulation time and fertility in mares. Two experiments were conducted to determine the effects of HCG on duration of oestrus, dioestrus, the length of the oestrous cycle, the time of ovulation and fertility in non-lactating mares. In the first experiment, the injection of HCG was repeated for three successive cycles. Mares injected with 2000 i.u. HCG on Day 2 of oestrus during their first cycle had a shorter oestrus and ovulated sooner than untreated control mares, but in the third cycle, treated mares had a longer oestrus and ovulated longer after the onset of oestrus than controls. In the second experiment, one intramuscular inje...
The distribution of delta5-3beta-hydroxysteroid dehydrogenase in the graafian follicle of the mare. Graafian follicles of various sizes obtained from mares at different stages of the oestrous cycle were examined histologically and histochemically for delta5-3beta-hydroxysteroid dehydrogenase (3beta-HSD) activity and related enzymes. The 3beta-HSD activity was not found in the theca interna of any follicles but was present in the membrana granulosa of well-vascularized large follicles in the late luteal phase of the cycle and at oestrus. These findings indicate that pregnenolone cannot be converted into progesterone in the theca interna. It is suggested that this conversion occurs in the memb...
Studies on fetal, neonatal and maternal cortisol metabolism in the mare. Fetal plasma cortisol concentrations (mean +/- S.E.M.) in ten animals with indwelling umbilical catheters ranged from 13-9 +/- 1-5 ng/ml (227 to 244 days) to 18-1 +/- 2-2 ng/ml (290 to 310 days). Maternal values did not change over this period (15-9 +/- 1-7 ng/ml). Fetal cortisol production rates in two fetuses were 3-6 and 3-8 mg/kg/day at 291 days; there was little placental transfer of cortisol. In three fetuses (319 to 321 days) plasma cortisol values 1 to 10 days before birth were higher (30 to 50 ng/ml) than in any other group, and at birth the values were comparable with those found in ...
Relationship between luteal function and metabolic clearance and production rates of progesterone in the mare. The functional activity of the equine CL was measured by its ability to convert [4-14C]3beta-hydroxy-5-en-20-one to [4-14C]pregn-4-en-3,20-dione. The capacity of homogenates of CL of different ages followed the temporal pattern of plasma progestagen concentrations. In ten mares which ovulated twice at intervals of 0 to 9 days in the same cycle, the conversion capacity of the CL from the second ovulation was similar to that from the first ovulation. After adjusting for age, the CL from the first of two ovulations had a similar conversion capacity as that resulting from a single ovulation. Plasm...
Oestrus, ovulation and plasma hormones after prostaglandin F2alpha in mares. The luteolytic effect of PGF2alpha, administered by intrauterine infusion or subcutaneous injection during early dioestrus, was observed in mares of mixed breeds. An infusion of 10 mg on Days 7 to 9 after ovulation caused a sharp fall in plasma progesterone levels and induced oestrus and ovulation. Oestrus was significantly longer than in the natural cycle but the time of ovulation in relation to the end of oestrus was normal. The time of return to oestrus following luteolysis was not dependent on the amount of PGF2alpha within the range of doses given. Luteolysis could be induced as early as ...
Effect of photoperiod on reproductive activity and hair in mares. The effects of photoperiod on reproductive activity and hair changes in pony mares were studied in 2 experiments. In experiment I, the effect of a fixed daily photoperiod on the onset of the breeding season was studied in 36 mares from Nov 13, 1973, to June 13, 1974. The 4 treatment groups were as follows: daily photoperiod equivalent to the normal day length (control group); constant light 24 hours a day with no dark (L24:D0 group); 16-hour daily photoperiod with 8 hours of dark (L16:D8 group); and 9-hour daily photoperiod with 15 hours of dark (L9:D15 group). The intervals from beginning of ...
The use of prostaglandin F2alpha-tham salt in mares in clinical anoestrus. Seventy-three Standardbred and Thoroughbred mares in clinical anoestrus during the breeding season were treated with PGF2alpha. The mares were divided into four categories; foaling, barren, maiden and unknown. The response was consistent in all groups. Mares at Days 2 to 4 of dioestrus showed no decrease in plasma progesterone levels, and those at Days 6 to 8 showed a return of progesterone to baseline levels (less than 1 ng/ml) in 24 to 48 hr after treatment. Plasma progesterone levels returned to basal levels in 24 to 48 hr in 93% of mares; progesterone levels had not changed by 96 hr in 7% ...
Levels of LH, prolactin and oestrogens in the serum of post-partum mares. The levels of LH, prolactin and oestrogen were measured by radioimmunoassay in serum obtained from six mares during the post-partum period, and LH was also assayed in blood samples collected at 6-hr intervals from four normal cyclic mares during oestrus and at daily intervals during dioestrus. The concentrations of oestrogen and LH suggested that the mares were having ovulatory cycles during the post-partum period irrespective of behavioural oestrus, and the levels of prolactin seemed to rise in association with probable ovulatory LH peaks. In these mares levels of LH had increased significant...
[The effect of pregnant mare’s serum on the level of K+ ions in the immature rat ovary]. Pregnant Mare Serum Gonadotrophin enhanced potassium level in immature rat ovaries in comparison with controls, three hours after intravenous injection. In vivo 42K uptake was also 36% higher (p less than or equal to 0,01) in PMSG primed rat ovaries. This response was specific to the ovary. Experiments are carried out to determine correlation between K+ level and macromolecule biosynthesis.
Plasma immunoreactive calcitonin levels in pregnant mares and newborn foals. Plasma calcium and calcitonin levels were measured periodically during the two last months of pregnancy and at the time of parturition in 9 pregnant mares and their foals. In pregnant animals, there was an increase in plasma calcitonin levels in the days before parturition, which was not due to any change in plasma calcium. This result indicates that in the mare, as in the cow, in the days before parturition CT secretion escapes from its control by plasma calcium. In 0-day and 7-day-old foals plasma calcium levels were significantly higher than in their mothers, but plasma calcitonin levels we...
Periovulatory changes in peripheral plasma progesterone and estrogen concentrations in the mare. Concentrations of progesterone and estrogen were measured in peripheral blood plasma samples from mares around the time of ovulation. Samples were collected every 2 hours from 36 hours before, to 26 hours after, ovulation and assayed by radioimmunoassay. Progesterone concentrations were between 60 and 100 pg/ml for the period 24 hours before ovulation through 8 hours after ovulation. By 10 hours after ovulation, concentrations increased to 140 pg/ml and, by 26 hours after ovulation, reached 346 pg/ml. Plasma estrogen concentrations did not change significantly throughout the same period.
Sweat gland function in isolated perfused skin. 1. A technique for perfusion of skin has been used to investigate a possible neurochemical basis for the different patterns of sweating in domestic animals. Evaporative water loss was measured from excised trunk skin, ears or tails perfused with a nutrient Krebs solution, to which drugs were added as required. Perfused skin was observed to sweat in response to administration of sudorific drugs, and some features of the patterns of sweating were similar to those which could be induced by heating or by drugs in conscious animals. 2. In sheep and goat skin, injections of adrenaline, and to a less...
Control of estrus with prostaglandin F2alpha in mares: minimal effective dose and stage of estrous cycle. To determine the minimal effective dose of prostagiandin (PGF2alpha; tromethamine salt) given subcutaneously (SC), mares of mixed breeding (400 kg av body weight) were given 2-, 3-, 5-, and 10-mg doses from 7 to 9 days after ovulation. In some but not all mares given doses of 2 and 3 mg of PGF2alpha, luteolysis occurred, but doses of 5 or 10 mg of PGF2alpha were luteolytic in all mares. The 10-mg dose of PGF2alpha did not cause luteolysis in mares 1 day after ovulation, and caused luteolysis in only 2 of 5 mares on day 3 after ovulation. The same dose of PGF2alpha, however, caused luteolysis i...
Pregnant mare serum gonadotrophin: rate of clearance from the circulation of sheep. The process involved in the disappearance of PMSG from the blood of sheep, following a single intravenous injection, has been separated into two exponential components. Values (mean plus or minus S.E.) calculated from experiments on five animals were: metabolic clearance rate (37.8 plus or minus 1.6 ml hr-minus 1); rate constant of disposal (0.0315 plus or minus 0.0016 hr-minus 1); half-time of disposal (21.2 plus or minus 1.1 hr). The stage of the oestrous cycle, ovariectomy and the dose of PMSG used had no apparent effect on these values.
[Indications for prostaglandins in the field of reproduction in farm animals (author’s transl)]. Parenteral administration of 2.5-5 mg. of prostaglandin F2alpha to horses, 15 mg. to heifers or 25-30 mg to lactating cows and 15 mg. to sheep will induce regression of the corpus luteum (luteolysis) and a fertile oestrus within 48-72 hours. Because of their luteolytic effect prostaglandins may be used in various indications in the field of reproduction. An exception is the pig in which administration of prostaglandins does not induce luteolysis before D12 and therefore fails to induce oestrus. In horses, cattle and sheep, administration of prostaglandins during the first four days of the cycl...
