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J. Malecki, G. Jenkin, and G. D. Thorburn

ABSTRACT

Groups of three goats at 50, 90 and 130 days of gestation were passively immunized against ovine LH (oLH) by i.v. infusion of 8 ml serum equivalent of the immunoglobulin fraction of rabbit anti-oLH serum (LHAS). Goats at the same stages of gestation as above served as controls and received 8 ml serum equivalent of the immunoglobulin fraction of normal rabbit serum (NRS). Plasma concentrations of progesterone were determined by specific radioimmunoassay of blood collected at 20-min intervals from 6 h before infusion of LHAS or NRS to 12 h after infusion. Less frequent sampling was performed from 2 days before to 6 days after infusion. Plasma from all LHAS-immunized goats exhibited binding of oLH. Twelve hours after immunization, titres ranged from 1:135 to 1:215. All LHAS-treated goats had titres of less than 1:10 by 5 days after immunization, but a low level of oLH binding was still detectable. Treatment with LHAS or NRS did not shorten the length of gestation, with all goats delivering live offspring between 142 and 147 days after conception. Plasma concentrations of LH ranged from less than 0·15 μg/l to 4·8 μg/l and were greater than 0·15 μg/l in 181 of 255 samples (71%) for both the NRS-treated group, throughout the experiment, and the LHAS-treated groups before infusion of antiserum. Luteinizing hormone was not detectable in plasma samples obtained after LHAS infusion in goats at 50 or 130 days of pregnancy. Plasma concentrations of LH exceeded 0·15 μg/l in only five of 51 (10%) samples in 90-day-pregnant goats treated with LHAS, the maximum value reached being 0·80 μg/l.

Plasma concentrations of progesterone did not alter significantly after infusion of LHAS or NRS at 50, 90 or 130 days of gestation, nor were they significantly different between the groups treated with LHAS and NRS.

The role of LH in the maintenance of pregnancy in the goat after 50 days of gestation is questioned.

J. Endocr. (1987) 114, 413–436

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G. E. Rice, G. Jenkin, and G. D. Thorburn

ABSTRACT

The subcellular distribution of progesterone and oxytocin within the ovine corpus luteum was investigated using differential and density gradient centrifugation. Progesterone and oxytocin were associated with particles which sedimented to a density of 1·049–1·054 g/ml and 1·054–1·061 g/ml respectively. Particle-associated progesterone did not, however, display physical or biochemical characteristics consistent with its storage within secretory granules. When particle-associated progesterone was incubated in HEPES buffer at 37 °C, 70% of the total progesterone was recovered in the incubation medium. The remaining stable particle-associated progesterone was not affected by treatments which stimulated oxytocin release and which have been shown to cause the release of peptides and biogenic amines from secretory granules. These results suggest that particle-associated progesterone represents the intercalation of progesterone into cell membranes and they do not support the hypothesis that progesterone is stored, in a protein-bound form, in luteal secretory granules.

J. Endocr. (1986) 108, 109–116

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P. SINGH-ASA, G. JENKIN, and G. D. THORBURN

The effectiveness of trilostane and azastene as inhibitors of adrenal steroidogenesis was compared by in-vitro and in-vivo methods. A radioimmunoassay was developed for the measurement of cortisol in ovine plasma, incubation medium and tissue extract using a specific antiserum raised against cortisol 21-acetate,3-carboxymethyloxime : bovine serum albu

Trilostane (20 μmol/l) decreased cortisol synthesis and release both in unstimulated and in ACTH-stimulated adrenal tissues in vitro. The same concentration of azastene had a lesser effect on unstimulated adrenals and was completely ineffective in blocking the stimulatory action of ACTH. In vivo, trilostane suppressed adrenal steroidogenesis in pregnant and cyclic ewes but the suppression in pregnant ewes was over a longer period, and after lower doses.

It is concluded that trilostane had an inhibitory effect on ovine adrenal steroidogenesis both in vitro and in vivo.

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S. A. McKay, G. Jenkin, and G. D. Thorburn

ABSTRACT

Pregnenolone sulphate, pregnenolone, progesterone and 20α-hydroxy-4-pregnen-3-one concentrations in peripheral plasma of normal cyclic ewes were measured by radioimmunoassay. The concentrations of these steroids were correlated with that of progesterone. The concentrations of all the steroids measured in peripheral plasma varied in a cyclic manner and showed a significant (P <0·05) positive correlation with the concentration of progesterone.

Peripheral plasma concentrations of these steroids in ovariectomized and ovariectomized, dexamethasone-treated ewes were also determined. The plasma concentration of progesterone in ovariectomized ewes was undetectable but the concentrations of pregnenolone sulphate, pregnenolone and 20α-hydroxy-4-pregnen-3-one remained similar to those observed at oestrus. Administration of dexamethasone to ovariectomized ewes had no effect on pregnenolone sulphate or pregnenolone concentrations but 20α-hydroxy-4-pregnen-3-one concentrations, which were already very low, decreased further.

It is proposed that the ovary, probably the corpus luteum, secretes pregnenolone sulphate, pregnenolone and 20α-hydroxy-4-pregnen-3-one; however, pregnenolone sulphate and 20α-hydroxy-4-pregnen-3-one may also arise from the metabolism of circulating pregnenolone and progesterone.

J. Endocr. (1987) 113, 231–237.

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G. Jenkin, R. T. Gemmell, and G. D. Thorburn

ABSTRACT

The mechanism by which prostaglandin F terminates luteal function in the sheep is unclear even though it is used extensively in animal husbandry. At the time of luteal regression, a decrease in 3β-hydroxysteroid dehydrogenase (3β-HSD) activity is apparent in the corpus luteum, but it is not known whether the decrease in enzyme activity is the primary cause of structural luteolysis. The effect of trilostane, a 3β-HSD inhibitor, on luteal function and morphology has therefore been investigated. Intravenous injection of trilostane in the mid-luteal phase of the oestrous cycle caused a decrease in ovarian tissue progesterone content. A transient decrease in peripheral and utero-ovarian vein plasma progesterone was observed but there was no significant effect on the length of the luteal phase of the cycle. There was no significant change in plasma 13,14-dihydro-15-oxo-prostaglandin F during the period when plasma progesterone was depressed. Morphological examination of the corpora lutea revealed a decrease in the concentration of electron-dense granules without any other features of impending luteal regression. When plasma progesterone was reduced for more than 10 h by two injections of trilostane 4 h apart, there was again no subsequent effect on the length of the oestrous cycle or on the return to oestrus. Plasma progesterone returned to preinjection levels within 24 h of injection. This evidence suggests that competitive inhibition of 3β-HSD activity, per se, is ineffective in bringing about structural luteolysis.

J. Endocr. (1984) 100, 61–66

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J. J. Hirst, G. E. Rice, G. Jenkin, and G. D. Thorburn

ABSTRACT

The effect of protein kinase C activation and dibutyryl cyclic AMP on oxytocin secretion by ovine luteal tissue slices was investigated. Several putative regulators of luteal oxytocin secretion were also examined. Oxytocin was secreted by luteal tissue slices at a basal rate of 234·4 ± 32·8 pmol/g per h (n = 24) during 60-min incubations.Activators of protein kinase C: phorbol 12,13-dibutyrate (n = 8), phorbol 12-myristate,13-acetate (n = 4) and 1,2-didecanoylglycerol (n = 5), caused a dose-dependent stimulation of oxytocin secretion in the presence of a calcium ionophore (A23187; 0·2 μmol/l). Phospholipase C (PLC; 50–250 units/l) also caused a dose-dependent stimulation of oxytocin secretion by luteal slices. Phospholipase C-stimulated oxytocin secretion was potentiated by the addition of an inhibitor of diacylglycerol kinase (R59 022; n = 4).

These data suggest that the activation of protein kinase C has a role in the stimulation of luteal oxytocin secretion. The results are also consistent with the involvement of protein kinase C in PLC-stimulated oxytocin secretion. The cyclic AMP second messenger system does not appear to be involved in the control of oxytocin secretion by the corpus luteum.

Journal of Endocrinology (1990) 124, 225–232

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G. JENKIN, M. D. MITCHELL, P. HOPKINS, C. D. MATTHEWS, and G. D. THORBURN

A radioimmunoassay for the determination of concentrations of melatonin in the plasma of rhesus monkeys has been developed. Antiserum for the assay was raised against N-acetyl serotonin and there was a 100% cross-reaction with melatonin. Cross-reactivity with closely related indoles, precursors and metabolites was less than or equal to 1%. The lower limit of sensitivity of the assay was 4 pg/tube.

The assay has been used for the investigation of diurnal variations and cyclical changes in melatonin concentrations in peripheral plasma of the rhesus monkey. The concentrations of melatonin ranged between 26·6 and 85·3 pg/ml during sampling for 24 h.There was a distinct diurnal variation in the concentration of melatonin in plasma. The concentration during darkness (61·0 ± 7·1 (s.e.m.) pg/ml) was greater (P <0·01) than that during illumination (40·1 ± 6·1 pg/ml). There were no significant differences in the concentration of melatonin in plasma at any time during the 28 day menstrual cycle of the rhesus monkey.

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K. M. Burgess, G. Jenkin, M. M. Ralph, and G. D. Thorburn

ABSTRACT

The effect of RU486, a synthetic progesterone receptor antagonist, on basal uterine prostaglandin (PG) release and release in response to oxytocin injection has been investigated in late-pregnant sheep (days 135–140 of gestation). Fifteen hours after i.m. injection of RU486 (50 mg; n = 5) or vehicle alone (n = 4), bolus injections of oxytocin (50, 500 and 5000 mU) were administered via a uterine artery ipsilateral to the pregnant uterine horn at 2-hourly intervals. Uteroovarian vein concentrations of 13,14-dihydro-15-keto PGF (PGFM) and PGE2 were determined before and during oxytocin stimulation. Basal concentrations of both PGFM and PGE2 were significantly (P < 0·001) increased in ewes 15 h after RU486 administration compared with ewes receiving vehicle alone. Concentrations of PGFM, but not PGE2, increased significantly (P < 0·001) following injection of each dose of oxytocin in both treated and untreated animals. The response to oxytocin, measured both as the area under the curve and as the peak height of PGFM release, was significantly (P <0·05) greater in RU486-treated ewes. There was no significant effect of oxytocin on the area or peak height of PGE2 response in either RU486-treated or control animals. These results demonstrate that treatment of late-pregnant ewes with RU486 results in an increase in basal uterine PGFM and PGE2 as well as oxytocin-stimulated PGFM release.

Journal of Endocrinology (1992) 134, 353–360

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M. J. Taylor, G. Jenkin, J. S. Robinson, and G. D. Thorburn

After administration of an inhibitor of 3β-hydroxysteroid dehydrogenase activity to late-pregnant sheep, a sharp and significant fall in progesterone concentrations in peripheral plasma was noted, and lowered levels were sustained for up to 24 h. However, no significant change in the secretion of ovine placental lactogen (oPL) was noted. These results indicate that progesterone secretion does not regulate secretion of oPL during pregnancy.

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I. C. McMILLEN, G. JENKIN, G. D. THORBURN, and J. S. ROBINSON

Nuffield Institute for Medical Research and Nuffield Department of Obstetrics and Gynaecology, John Radcliffe Hospital, Headington, Oxford, 0X3 9DU

(Received 6 April 1978)

Growth hormone (GH) has been located in the ovine foetal pituitary gland by day 50 of gestation (Stokes & Boda, 1968). The concentration of GH in the plasma of foetal sheep is ten times higher than the postnatal value, increasing from 40 ng/ml on day 100 of gestation to 100–120 ng/ml on day 140 (Bassett, Thorburn & Wallace, 1970). After foetal hypophysectomy, the concentration of GH falls to < 2 ng/ml, indicating that it originates in the foetal pituitary gland (Wallace, Stacey & Thorburn, 1973). Labelled GH does not cross the ovine placenta (Wallace et al. 1973). After sectioning the foetal pituitary stalk, the concentration of GH in the foetal plasma drops to approximately 5 ng/ml (Wallace et al. 1973), which implies that the secretion of GH