The concentrations of androstenedione, estradiol-17β, progesterone and PGF2α contained in the follicular fluid produced by the follicles in collected ovaries of mares that have had estrous phase during the breeding season were measured and analyzed the relation between the growth stage of follicles and the hormone levels in the follicular fluid. An ultrasonographic diagnostic instrument was used to measure the diameter of the follicles in order to categorize the follicles into three groups the following: 8 small follicles (from 1.0 to less than1.5 cm), 8 medium follicles (from 1.5 to less than 3.0 cm), and 8 large follicles (from 3.0 to 5.0 cm), respectively. The analysis of the follicular fluid in ovaries of estrous mares showed that the concentrations of androstenedione were significantly higher in the medium or large follicles than in the small follicles and the concentrations of estradiol-17β were significantly higher in larger follicles than in the small or medium follicles (P<0.05). The concentrations of progesterone and PGF2α, on the other hand, did not significantly vary regardless of follicluar size. In the follicles within the mare ovaries that have had estrous stage, the concentrations of the hormones related the ovulation, namely androstenedione and estradiol-17β, were higher with larger follicles.
-NOTE-The long estrus period in mares associated with a very variable ovulation time remains a challenge for horse breeders who would like to breed just before ovulation [5]. The fact that the best time to inseminate is the one closest to follicular rupture stands unquestioned. It has been shown earlier that insemination of mares should be done from 3 days before to 12 hr after ovulation, otherwise the pregnancy rate is lowered or embryonic loss is increased [11]. But no accurate method for predicting the exact ovulation time in mares is yet well established.Endocrinological studies revealed cyclic changes in circulating and urinary estrogen conjugate (EC) concentrations at different stages of the estrous cycle. In both cases, estrogen levels registered a gradual increase during the follicular phase and then a decrease a few days before ovulation [6][7][8]. The EC in the urine is ten times higher than in the plasma due to the kidney's concentrating capacity and the resistance of EC metabolites to rapid degradation [3]. There are reports that estrogens excreted out through the urine can be an index of its secretion rate because they are not stored in the body [2]. Because estrone has been reported to be the principal EC component in urine [8], urine contains materials adequate for hormonal measurements and to estimate ovulation time on the basis of urinary changes in E1S concentrations.A total of 16 cycling draft mares of breeds Percheron and Breton between 4 to 16 years old were used in this study. The stages of the estrous cycle of each mare were first determined by routine rectal palpation, vaginal examination and ultrasonography. Urine samples were collected daily with a sterilized urinary catheter. The collection started on D-5 (n=6), D-4 (n=10) and D-3 (n=16) until the ovulation was confirmed by ultrasonography. Samples were centrifuged at 1,500 rpm for 5 min and stored at 30°C until assayed for E1S.Urinary E1S concentrations were determined in 2 hr with the EIA kit (Bri-Equine Oestrone Sulphate EIA: Bioresearch Ireland) which was originally designed for plasma E1S measurement. A preliminary experiment has been conducted to validate the plasma EIA kit for urinary E1S measurement. Urine samples from 2 mares in the last trimester of pregnancy (with high E1S concentrations) were diluted, and then assayed for E1S. The E1S standard curve ranged from 0.25 to 10 ng/ml urine. The intra and inter coefficients of variation were 5.7 and 7.7%, respectively. The recovery rates of E1S
ABSTRACT. Microdialysis System (MDS) is a novel technique used for investigation of molecule secretion between different cell populations. Local hormonal secretion at follicular wall has been still unclear. This MDS study was used to determine progesterone (P 4 ), androstenedione (A 4 ), estradiol-17β (E 2 ) and Prostaglandin F 2α (PGF 2α ) release in mare pre-ovulatory follicles. Follicles larger than 30 mm were isolated from the ovary and follicular fluid aspirated for hormone assay. Follicular fluid collected from small, middle and large follicles were analyzed by EIA. The concentrations of P 4 and PGF 2α were similar among the different sizes of follicles. The release of A 4 was observed in middle and large follicles. E 2 concentration was observed in middle follicles and was higher in large follicles compared with middle follicles. Follicular wall was cut and incubated for MDS and when LH was infused, there was an increase in P4 and A 4 release. PGF 2α release was considerably high after LH infusion compared to the control group. Infusion of PGF 2α increased P 4 and A 4 release but there was no change in E 2 release. This results suggest that in pre-ovulatory follicles, LH stimulates theca interna cells and also PGF 2α seemed to have a mediator role to induce steroid hormone production and luteinization of follicular cells. The nature of the mechanisms involved in selection of large follicles is still a perplexing research problem in reproduction. KEY WORDS: follicle, mare, MDS, ovulation, steroid-hormone.J. Vet. Med. Sci. 64(2): 119-122, 2002 Mares are seasonal polyestrous animals and the estrous cycle have several unique characteristics, including a long period of estrus, a considerable larger size of preovulatory follicles and the absence of a typical LH surge compared with other species [2,7,9]. However, endocrinological changes that occur in the different follicular compartments (theca, granulosa cells and follicular fluid) during follicular development, maturation, and ovulation still remain unclear.Follicles isolated from mare ovaries maintained the ability to secrete steroid hormones and to respond to hormonal stimulation in vitro [18]. The same study demonstrated a variable effect of LH on progesterone (P 4 ), androstenedione (A 4 ), and estradiol-17β (E 2 ) production depending on the stage of the estrous cycle. A 4 is one of the main secretory products of the theca layer that serve as substrate for the synthesis of estradiol in the granulosa cells [4,19]. These secretory products (steroids and/or prostaglandins) are accumulated in the follicular fluid and may be used as indicators of the follicular development, atretic stage or the proximity of ovulation [21,22]. Moreover, other report suggests that progesterone can modulate ovarian activity by altering FSH and LH receptor in the follicular wall [11]. In addition, prostaglandins have been considered as an essential mediator for the LH action on the ovulatory processes in several species, including the mares [13].Prominent structural and secre...
In an attempt to detect the existence of histaminergic mechanisms in the regulation of prolactin secretion in rats, the effects of histidine and histamine-receptor antagonists (chlorpheniramine and metiamide) on plasma prolactin levels in urethan-anesthetized rats were investigated in relation to other aminergic mechanisms. Chlorpheniramine was used as an H1-receptor antagonist and metiamide as an H2-receptor antagonist. Wistar male rats were used under urethan anesthesia and blood was obtained by cardiac puncture. Rat plasma prolactin was measured by radioimmunoassay. As already reported, L-DOPA and diethyldithiocarbamate markedly decreased plasma prolactin levels in rats. Dihydroxyphenylserine increased prolactin levels. Brocresine phosphate, a histidine decarboxylase inhibitor, markedly stimulated prolactin secretion in urethan-anesthetized rats. This stimulation was not blocked by pretreatment with histidine or L-DOPA. Histidine alone did not affect plasma levels of prolactin or methyldopamine-induced increase in plasma prolactin levels. On the other hand, histidine significantly stimulated prolactin secretion in chlorpheniramine-treated rats. In contrast, histidine depressed plasma prolactin levels in metiamide-treated rats. These findings indicate that a dopaminergic component is inhibitory and a noradrenergic one is stimulatory in prolactin secretion in rats and further that there exist the histaminergic mechanisms, relatively independent from other aminergic mechanisms, which might also play an important role in the regulation of prolactin secretion in rats. Furthermore, it is suggested that H1-receptor is stimulatory and H2-receptor is inhibitory in prolactin secretion in rats.
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