Equine endocrinology : studies on gonadotrophic, releasing, sex steroid and thyroid hormones in horses

Abstract

A competitive protein binding assay was developed for the measurement of serum total thyroxine (T4) concentrations in horses. This method corrected for the presence of ethanol extractable substances, present in sera, shown to spuriously interfere in such assays. Total T4 and total triiodothyronine (T3) concentrations in 13 cycling and early pregnant mares (determined every two to three days for four weeks during spring) were 2.97 ± 0.7 µg/100 ml, and 73.3 ± 12.7 ng/100 ml respectively. Total and free T4 concentrations were measured in horses in various physiological states, with the concentrations in untrained mares in spring being 3.19 ± 1.3 µg/100 ml and 5.68 ± 2.4 ng/100 ml respectively. Results suggested that in horses in training, total T4 decreases with intensity of training and there is a concomitant alteration in the T4 binding proteins so that free T4 remains at a comparable level to that in the untrained horse. Total and free T4 concentrations are significantly higher in untrained horses during the autumn/winter period, than in the spring. Total and free T4, and total and free T3 concentrations were measured in foals from birth to three months of age. Both total and free T4 concentrations were high at birth (12.1 and 3.1 times the adult concentration), and decreased to be at adult levels by approximately 15 and 10 days of age respectively. Total T3 and free T3 concentrations in cord blood were 6.8 times and 2.3 times the adult level, and began increasing immediately to reach peak concentrations of 15 and 8.7 times the adult level at 2 to 3 days, thereafter declining, but still 2.7 and 2.6 times the adult concentration in 32 day old foals. These very high levels in the newborn foal are related to the maturity of its neuromuscular system and thermogenic capacity. A radioimmunoassay (RIA) method for the measurement of serum FSH in horses was developed. The assay met the requirements for a valid RIA, and is the first reported for the measurement of this hormone. RIA methods for measurement of serum LH in horses were also developed, and modifications of methods for measurement of serum progesterone and oestradiol-17β described. Serum concentrations of FSH, LH, progesterone and oestradiol-17β were measured in cycling mares. Two surges of FSH occur at approximately 10 day intervals during the oestrous cycle, one in the late oestrous/early dioestrous period, and another during mid-dioestrus. Such surges are thought to prepare and prime follicles to respond to the increased LH concentrations during oestrus which result in final maturation processes, ovulation and corpus luteum establishment. In pregnant mares, continued surges of FSH appear responsible for the follicular proliferation which occurs, with PMSG having an "LH-like" action on these FSH prepared and primed follicles and inducing the peri-ovulatory changes and secondary corpus luteum establishment. FSH surges occur during the two to three weeks prior to parturition, with a marked surge within four days of foaling, and a further surge one to five days after foaling. These FSH surges probably prepare and prime follicles for the ovulation 9 to 15 days post-partum. Mares in various depths of acyclicity were treated during a 22 day period with exogenous gonadotrophin releasing hormone (GnRH) and progesterone on a regime to reproduce the gonadotrophin surges, and luteal phase progesterone concentrations occurring in the normal ovulatory cycle. Typical pre-ovulatory follicles were induced by this treatment, but ovulation resulted in corpus luteum maintenance only in mares in shallow acyclicity in which a peri-ovulatory LH surge typical of the normal cycle was also induced. The results suggested that additional FSH surges might be necessary to adequately prepare and prime follicles before the final follicular maturation processes, including steroidogenesis, can occur.