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SUMMARY
The existence of a circadian rhythm in the sensitivity of the hypothalamus of the laying hen to stimulation by progesterone was investigated by injecting 0·5 mg progesterone subcutaneously during the proposed period of maximum insensitivity. Following this treatment increases in plasma concentrations of both LH and progesterone were observed which were comparable to the spontaneous preovulatory rises in the plasma levels of the hormones.
The ability of either progesterone or luteinizing hormone releasing hormone (LH-RH) to induce premature ovulation varied according to the stage of follicular development. Neither hormone was more than 28% effective when injected within 6·5 h of the previous ovulation, whereas both hormones were 100% effective approximately 27 h after the terminal ovulation of a clutch sequence. Failure to ovulate in response to LH-RH given 6·5 h after ovulation was associated with a lack of progesterone secretion.
Both LH and progesterone were secreted when ovulation was induced by injections of either LH-RH or progesterone, and LH was secreted in response to progesterone given 6·5 h after ovulation. These results demonstrate that progesterone stimulates the secretion of LH and LH stimulates the secretion of progesterone. The precise physiological role of these two hormones, however, was not established.
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ABSTRACT
The progesterone, androstenedione and oestradiol contents of the theca and granulosa tissues of the four largest follicles in the ovarian hierarchy of the hen were determined. The granulosa tissue contained significantly (P < 0·05) more progesterone and less androstenedione and oestradiol than the theca tissue. The content of progesterone was greatest in the granulosa tissue of the first three follicles in the hierarchy and in each of these follicles there was a peak in progesterone content of the granulosa 4 h before ovulation. The theca of the second, third and fourth follicles and the granulosa of the third and fourth follicles contained significantly (P < 0·05) more androstenedione than either tissue in the largest follicle. The content of androstenedione was maximal approximately 8 h before ovulation in both tissues of the second and third follicles. The content of oestradiol in the granulosa did not vary as follicles changed position within the hierarchy or during the ovulatory cycle. The oestradiol content of the theca tissue remained constant during the third and fourth positions in the hierarchy and declined throughout the second and first positions until a nadir was observed approximately 20 h before ovulation.
It was concluded that the synthesis of androstenedione and oestradiol ceases in both follicular tissues after the follicle is exposed to the penultimate preovulatory surge of LH and that progesterone production is stimulated in the granulosa of the three largest follicles at the time of the preovulatory release of LH.
J. Endocr. (1984) 103, 71–76
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The ovulation-inducing property of androgens in the laying hen was investigated. In a first experiment, four different androgens were injected subcutaneously into single-comb White Leghorn hens on the day of the last oviposition of a sequence. The hens were killed 10 h later and examined for the presence of an ovum in the oviduct. Testosterone induced ovulation in accordance to the dose injected (median effective dose, 966 ± 193 μg/hen) but the responses to 5α-dihydrotestosterone and 5α-androstane-3α, 17β-diol were not dose-related. The effect of 4-androstene-3,17-dione was more like that of progesterone since it induced ovulation 2 h earlier than the three other androgens.
The physiological significance of the ovulation response to an injection of testosterone was examined in more detail in experiment 2. Seven out of ten hens which were injected with 1 mg testosterone/kg body weight ovulated within 10 h after the injection. Blood samples were taken at hourly intervals and the concentrations of testosterone and progesterone were determined by radioimmunoassay. An injection of testosterone produced an increase in the concentration of testosterone in plasma which was considerably greater and occurred earlier than the preovulatory increase of testosterone in the control birds. The increase in the concentration of progesterone in the hens injected with testosterone was similar in magnitude but occurred earlier than the spontaneous preovulatory increase of progesterone in the control hens. The possible physiological role of testosterone in the ovulation cycle is discussed.
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The changes in the binding of FSH during follicular maturation were examined in the hen using 125I-labelled bovine FSH (bFSH) and unlabelled bFSH. The binding of 125I-labelled bFSH was not inhibited by bovine LH or chicken LH but was inhibited by extracts of chicken pituitary glands. The ovarian stroma, which contained both interstitial tissue and small follicles, bound the greatest amount of FSH. As the follicles progressed through the yolk-filled hierarchy of maturation, they bound decreasing amounts of FSH. In the two largest follicles of the hierarchy, there was a significant increase in the binding of FSH 12–16 h before ovulation. There were two peaks in the concentrations of LH; a preovulatory peak occurred 4–6 h before ovulation and a second peak occurred 14–16 h before ovulation. Plasma concentrations of testosterone, oestradiol and progesterone began to rise 9, 8 and 6 h, respectively, before ovulation. These data are consistent with the hypothesis that changes in the gonadotrophin concentration and binding regulate the order of the follicular hierarchy and the onset of preovulatory steroidogenesis in the hen.
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ABSTRACT
The production of shell-less eggs was induced in hens to measure the effects of the high demands made by shell formation on the blood minerals and hormones whose concentrations change during egg formation. In control hens laying hard-shelled eggs, the concentration of ionized calcium in plasma decreased at the onset of shell formation, but no change was found in hens laying shell-less eggs. Total calcium concentrations in plasma decreased slightly throughout the ovulation cycle in both groups. Concentrations of inorganic phosphorus in the plasma were increased in the control group during the period of shell formation and decreased when calcification was suppressed. Finally, the concentrations of 1,25-dihydroxycholecalciferol (1,25-(OH)2D3) in plasma were significantly increased 16 and 20 h following an ovulation compared with 4 h after ovulation, or compared with the concentrations observed in hens laying shell-less eggs. The variations in the plasma concentrations of ionized calcium, inorganic phosphorus and 1,25-(OH)2D3 associated with egg formation were therefore absent in hens laying shell-less eggs demonstrating their direct link with shell calcification. On the other hand, suppression of shell production had no influence on the changes in the plasma concentrations of progesterone, oestradiol and testosterone which are associated with the normal ovulatory cycle. It is concluded that the increases in intestinal and uterine calcium transport and in 1,25-(OH)2D3 production which occur at the onset of egg production in hens are mainly controlled by factors involved in maintaining calcium homeostasis rather than by gonadal hormones.
J. Endocr. (1986) 111, 151–157
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A comparative assessment was made of the hormonal control of calcium homeostasis in eight dairy cows which developed parturient paresis and in seven normal animals from the same herd. Plasma levels of calcium, phosphorus, magnesium, free hydroxyproline, 25-hydroxycholecalciferol (25-OHD), 1,25-dihydroxycholecalciferol (1,25-(OH)2D), parathyroid hormone, calcitonin, prolactin and oestrogen were monitored from 30 days prepartum to 15 days post partum.
Prepartum levels of plasma calcium, hydroxyproline and calcitonin were depressed in the paretic animals, and plasma levels of phosphorus and oestrogen were elevated. Plasma levels of 25-OHD remained stable in both groups, whereas levels of 1,25-(OH)2D, parathyroid hormone and prolactin rose sharply at parturition.
Plasma hydroxyproline, an index of bone resorption, began to rise 2 days prepartum in the control cows but not until 2 days post partum in the paretic cows. The data indicate that bone resorption was inhibited in the paretic group at the onset of lactation, and that a decreased capacity for bone resorption is a major factor in the susceptibility of some cows to this disease.
The failure of the paretic animals to resorb bone was not associated with an inability to synthesize the calcium-mobilizing hormones parathyroid hormone or 1,25-(OH)2D, or to regulate the production of calcitonin. However, hypocalcaemia in the affected animals was associated with a significantly higher plasma level of oestrogen (a known inhibitor of bone resorption) in the immediate prepartum period. Following parturition, plasma levels of oestrogen fell rapidly and active bone resorption ensued in the paretic animals.