Secretion of Luteinizing Hormone and Follicle Stimulating Hormone in Intact and Ovariectomized Mares in Summer and Winter

Abstract

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 determined peak hormone concentrations based on assay sensitivity, assay variability and repeatability of peaks in both independent assays. Peaks in LH and FSH were observed for mares in all five reproductive states, except for FSH concentrations in estrous mares. High frequency peaks of short duration were observed only in ovariectomized mares. Low frequency peaks of relatively long duration were observed in both intact and ovariectomized mares in both seasons. With the exception of estrous mares, there was variation among mares in the patterns of LH and(or) FSH within any one group; all estrous mares exhibited high, variable LH concentrations and low, constant FSH concentrations. In general, peaks in both gonadotropins occurred simultaneously. Ovariectomized mares exhibited more (P less than .05) peaks/24 h than intact mares for both gonadotropins. Ovariectomized mares also exhibited more (P less than .05) FSH peaks/24 h in summer than in winter.

Kinetic studies and production rate of equine (e) FSH in ovariectomized pony mares. Application to the determination of a dosage regimen for eFSH in a superovulation treatment

“…2·89 to 3·45 mg/day per mare). In 400-500 kg body weight cyclic mares, these rates were lower, ranging from 0·84 to 2·6 mg/day (Irvine 1979) which is consistent with the fact that plasma concentrations of FSH are from three to four times lower in cyclic than in ovariectomized mares (Freedman et al 1979, Thompson et al 1987.…”

“…This dosage regimen corresponded to the maximal daily production rate that we determined in ovariectomized pony mares. This dose was also expected to supply approximately 300% of the amount of eFSH produced in cyclic mares (Freedman et al 1979, Thompson et al 1987. As the results of this first trial gave satisfactory ovarian stimulation but undesirable side effects, a second trial (trial 2) was performed with a dose divided by 2: 1·72 mg eFSH per mare (2·2 µg/kg three times a day).…”

“…In addition, the linearity of eFSH disposition needs to be checked in order to allow interpolation between doses. The aims of the present study were: (1) to determine the kinetic parameters of eFSH; (2) to evaluate the physiological production rate of FSH during the breeding season in ovariectomized pony mares; and accordingly (3) to analyse in clinical trials the effects on follicular growth, ovulation and embryo production of dosage regimens expected to supply a quantity of eFSH equal to approximately 100 and 50% of the FSH secreted in ovariectomized mares which also corresponds to 300 and 150% of the FSH secreted in cyclic mares (Freedman et al 1979, Thompson et al 1987.…”

The Nonpregnant Mare: A Review of Some Current Research and of the Last 25 Years of Endocrinology

“…How day length and its melatonin signal are translated into a change in GnRH pulse generator activity is unknown. In the seasonally anestrous mare, investigators have shown that LH and FSH pulse frequency is very low [Fitzgerald et al, 1987;Thompson et al, 1987], but that it gradually increases together with the LH:FSH ratio as the mare makes the transition into the breeding season . On the other hand, the termination of the breeding season is marked by failure of preovulatory follicles to ovulate; this implies inadequate LH, possibly due to failure to increase GnRH pulse generator activity following luteolysis [Snyder et al, 1979].…”

“…2) and 1-4 times daily during diestrus (Fig. 3) [Thompson et al, 1987;; and in these states, daily samples or brief windows of peripheral blood sampling are inadequate to describe the cyclic secretory patterns of both gonadotropins.…”