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  • Author: K. P. McNatty x
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Diurnal variation of cortisol in the peripheral plasma of sheep has been demonstrated after the animals had been allowed to adapt to the environment in which they were tested. This variation was not seen in sheep tested after only 7 days in their new environment (McNatty, Cashmore & Young, 1972). Although adapted sheep have higher plasma levels of cortisol in the morning daylight hours than during the remainder of the 24-h day (McNatty et al. 1972), it is not known whether this diurnal variation is re-established while sheep are habituating to their new environment. We have limited evidence to suggest that sheep require 14–28 days to become adapted to a new environment (D. C. Thurley & K. P. McNatty, unpublished observation).

We wish to describe the diurnal changes in the peripheral plasma cortisol levels of sheep while they are adapting to a new environment and exposed to diffuse light.


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The newly formed corpus luteum of many species is refractory to the lytic action of prostaglandin F (PGF). This phenomenon was studied utilizing porcine, bovine and human granulosa-luteal cells in tissue culture. The steroidogenic potential of the granulosa-luteal cells was critical in determining whether PGF could inhibit progesterone production. Since the steroidogenic potential of the granulosa-luteal cell is related to the amount of LH bound to the cell, the bound LH may protect the granulosa-luteal cells from the lytic action of PGF. Finally, a 'see-saw' type of interaction between LH and PGF is postulated to account for the resistance of the newly formed corpus luteum to PGF

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Reports on diurnal variation in plasma corticosteroids in man, monkey, dog, rat, mouse and other species have been reviewed by Yates, Russell & Maran (1971) and by Yates & Urquhart (1962). We wish to report evidence of diurnal variation in the peripheral plasma cortisol levels of sheep, demonstrable after they had been allowed adequate time to adapt to the environment in which they were tested.

Three anoestrous New Zealand Romney ewes were placed indoors in crates with minimal restraints. They were placed immediately adjacent to each other and were free to sit or stand. They were given hay, lucerne pellets and water ad libitum. No attempt was made to regulate feeding and an excess of feed was ensured at all times. Each animal had an indwelling jugular cannula, inserted at least 3 h before the first sampling, which remained in situ throughout the experiment. On each occasion 10 ml blood

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The steroidogenic potential of granulosa cells harvested from human Graafian follicles containing varying concentrations of pituitary and steroid hormones was examined. The mitotic activity and production of progesterone by granulosa cells in vitro was found to be correlated with their hormonal environment at the time of harvesting. Only cells from follicles containing some FSH and high concentrations of oestradiol underwent spontaneous mitosis in vitro. However, mitosis could be induced by adding FSH and high concentrations of oestradiol to the culture, provided that the concentration of LH was low. Cells harvested from follicles containing LH, FSH and high concentrations of oestradiol secreted significantly more progesterone than cells from follicles which did not contain all three hormones.

It is suggested that after the initiation of follicular development by FSH, a long period of exposure (8–10 days) to both FSH and oestradiol is necessary before the maximum biosynthetic capacity of granulosa cells is achieved; this synthetic potential is then only realized under the influence of LH and prolactin. Premature exposure to LH inhibits both the mitotic activity and the steroidogenic potential of these cells.

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Large and rapid variations were found in the plasma cortisol levels of housed and cannulated sheep. Adrenaline injected i.v. caused increased plasma levels of cortisol that were proportionate to the dose. This response of cortisol to adrenaline was larger when sheep were newly housed, than when the sheep had been housed and sampled for 2 weeks. Response to adrenocorticotrophin also diminished over 2 weeks. Dexamethasone abolished the response to adrenaline. Tyrosine and DOPA had little effect on cortisol levels, dopamine and noradrenaline had some effect, but none had as great an effect as adrenaline.

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The concentrations of FSH, oestradiol and androstenedione in the follicular fluid of normal and cystic human follicles were measured at different stages of the menstrual cycle. In addition, the number of granulosa cells in the follicles was determined.

In follicles in which FSH was detectable, the concentration of oestradiol was greater than that of androstenedione, irrespective of the stage of the cycle. In contrast, in those follicles in which FSH was undetectable and in all cystic follicles irrespective of the level of FSH, the concentration of androstenedione was greater than that of oestradiol. In follicles containing FSH there was a highly significant linear correlation between the number of granulosa cells and the concentration of follicular oestradiol (P < 0·001).

It is suggested that in human ovaries up to 90% of the oestradiol in follicular fluid may originate from the granulosa cells.

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Human granulosa cells with differing steroidogenic potentials were cultured in vitro. The effects of prostaglandin F (PGF) and PGE2 on the progesterone output and viability of these cells were investigated.

Prostaglandin F either alone or in combination with LH and FSH inhibited the production of progesterone over a wide range of concentrations (1–8000 ng/ml). However, the inhibitory effect of PGF was 200 times less effective when the cells were exposed to LH and FSH for 6 days before the addition of the prostaglandin. By contrast PGE2, at concentrations from 1 to 500 ng/ml, markedly stimulated the production of progesterone by granulosa cells, and this was not prevented by the addition of PGF. The degree of inhibition by PGF or stimulation by PGE2 was related to the biosynthetic capacity of the cells.

These studies suggest that PGF may act directly on the adenylate cyclase system of human granulosa cells by blocking its activation by LH, and they demonstrate that functional regression of the luteal cell can be induced independently of the blood vascular system.

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Progesterone concentrations in peripheral plasma were measured once daily during one oestrous cycle in each of eight sheep. In addition, on days 4–5, 8–9, 12–13 and 15–16 of the oestrous cycle, blood samples were collected at 30-min intervals throughout each 24-h period. A total of three ewes was sampled in each 24-h period and the same three animals were not bled again for at least 1 week. Plasma progesterone concentrations in all the ewes fluctuated considerably throughout each 24-h period. The within-sheep within-day variations observed in peripheral progesterone concentrations were compared with the between-sheep within-day variations and the within-sheep between-day variations previously reported. It is concluded that these previously reported variations in peripheral plasma progesterone concentration could be attributed to within-day variations in each animal.

On days 8–9 and 12–13 of the oestrous cycle there were significantly higher concentrations of progesterone in plasma during the hours of daylight than during the hours of darkness. In contrast, progesterone concentrations on days 4–5 and 15–16 were not found to be significantly different between the hours of daylight and darkness. These results suggest that diurnal changes in peripheral plasma progesterone concentration occur during the luteal phase of the ovine oestrous cycle.

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The concentration of the common α-subunit of the glycoprotein hormones and of the β-subunit of luteinizing hormone (LHβ) in peripheral blood and follicular fluid was measured throughout the menstrual cycle, and the effects of these subunits, either alone or in combination, on the production of progesterone by human granulosa cells in tissue culture were investigated.

Changes in the serum concentration of α-subunit and immunoreactive `LHβ-like' material throughout the menstrual cycle were similar to those of LH. The concentrations of the subunits before the mid-cycle gonadotrophin peak were not significantly different from those during the luteal phase of the menstrual cycle. Gel filtration of a pooled serum sample obtained at mid-cycle confirmed the presence of immunoreactive α-subunit together with intact LH; however, because of the cross-reactivity of LH in the LHβ assay a distinct peak of LHβ-subunit could not be demonstrated.

In follicular fluid, α-subunit was detectable in large follicles ( ≥ 8 mm) throughout the menstrual cycle at concentrations similar to those found in serum. By contrast α-subunit in small follicles ( < 8 mm) and `LHβ-like' material in all follicles were only detectable during or just after peak concentrations in peripheral plasma.

The LH subunits did not increase the rate of progesterone secretion by human granulosa cells when each was added alone, even at concentrations five times higher than those in plasma. However, when both subunits were added simultaneously there was an increased rate of progesterone secretion comparable to that achieved with intact LH.

It is concluded that the common α-subunit circulates in blood independently of the intact hormones, and that it is present in a proportion of developing Graafian follicles without affecting either the viability or biosynthetic potential of their granulosa cells. During the late follicular phase however, when both the α- and LHβ-like subunits are present in follicular fluid, they may recombine and enhance steroid production by granulosa cells which are undergoing luteinization at this time.

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The aim of the present study was to gain evidence that the level of LH secretion preceding the preovulatory LH surge is an important determinant of follicular maturation and corpus luteum function in the ewe. In addition it was hoped to establish whether the pattern of LH delivery to the ovary (pulsatile v. constant) is a critical factor in the maturation of a preovulatory follicle.

To accomplish this, progesterone-primed anoestrous ewes were repeatedly injected i.v. with LH or luteinizing hormone-releasing hormone (LH-RH), or given an i.v. infusion of LH, over a 72 h period. These animals, together with the appropriate controls, were exposed to a sexually active ram so that oestrous activity could be recorded. All ewes were subjected to intensive blood sampling regimes so that the plasma levels of LH and progesterone could be determined and compared to those which occurred in the same breed of sheep during the oestrous cycle.

Collectively the data suggest that the plasma levels of LH preceding the preovulatory LH surge are an important determinant of follicular maturation as judged by subsequent corpus luteum function. Moreover, they show that follicular maturation can be achieved with widely differing patterns of LH delivery to the ovary during the preovulatory period and that a strict pulsatile delivery of LH may not be an absolute requirement.