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G E Mann, J H Payne, and G E Lamming

Abstract

In intact cyclic ewes intrauterine infusion of conceptus secretory proteins results in the suppression of both endometrial oxytocin receptor concentrations and oxytocin-induced prostaglandin F release. However, similar infusion in progesterone-treated ovariectomized ewes, while suppressing endometrial oxytocin receptors, does not fully inhibit oxytocin-induced prostaglandin F release. To examine whether this anomaly resulted from an inadequate simulation of the luteal phase in the ovariectomized ewe treated with progesterone alone, the effects of additional treatment with two other ovarian hormones, oestradiol-17β and oxytocin, was investigated. Rather than permitting conceptus secretory protein to successfully inhibit oxytocin-induced prostaglandin F release, treatment with oestradiol-17β in addition to progesterone actually resulted in an advancement in the timing of release. However, treatment with oxytocin, alone or in combination with oestradiol, permitted the full inhibition of oxytocin-induced prostaglandin F release. To confirm that this effect did not result from the action of oxytocin alone, independently of the action of conceptus secretory protein, a second experiment was undertaken using a similar protocol but without the infusion of conceptus secretory protein. In this situation, oxytocin-induced prostaglandin F release was only partially inhibited suggesting that both luteal oxytocin and conceptus secretory proteins are necessary to facilitate the full inhibition of luteolysis during early pregnancy in the ewe.

Journal of Endocrinology (1996) 150, 473–478

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H. Vilhardt, T. Krarup, J. J. Holst, and P. Bie

ABSTRACT

Injections and infusions of oxytocin into conscious dogs caused an increase in plasma concentrations of glucose, insulin and glucagon. When blood glucose was clamped at a raised level the injection of oxytocin still increased insulin and glucagon concentrations in plasma. Infusion of somatostatin suppressed plasma concentrations of glucagon and insulin but did not prevent oxytocin-induced increments in blood glucose. Injection of oxytocin still caused a marked release of glucagon, whereas the insulin response was greatly diminished. When endogenous insulin and glucagon secretion was suppressed by infusion of somatostatin and glucose levels were stabilized by concomitant infusions of glucagon and insulin, injections of oxytocin did not alter blood glucose concentrations. It is concluded that the increase in blood glucose following the administration of oxytocin is secondary to the release of glucagon and that oxytocin exerts a direct stimulatory effect on glucagon and possibly insulin secretion.

J. Endocr. (1986) 108, 293–298

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R A D Bathgate and C Sernia

Abstract

In this study arginine vasopressin (AVP) and oxytocin (OT) receptors have been characterized in the brushtail possum. AVP receptors were characterized using [3H]AVP and the radioiodinated AVP V1a receptor antagonist 125I-labelled [(C6H5-CH2CO)-O-methyl-d-Tyr-Phe-Gln-Asn-Arg-Pro-Arg-Tyr- NH2] while OT receptors were characterized using the radioiodinated OT receptor antagonist 125I-labelled d(CH2)5[Tyr(Me)2,Thr4,Orn8, Tyr-NH2 9]-vasotocin. The receptor affinities and densities have been compared with the rat AVP and OT receptors. Low densities of OT receptors were present in the possum ovary and kidney. High densities of AVP-binding sites were found in the possum adrenal, testis, mesenteric artery, ovary and renal medulla and lower densities in the possum liver. The AVP-binding sites showed marked differences in ligand-binding characteristics from the rat AVP V1a and V2 receptors. Receptor affinities were similar between tissues, except for a distinctly lower value in the renal medulla. It is concluded that the brushtail possum expresses AVP receptors with distinct ligand specificities from those of the rat AVP V1a and V2 receptors.

Journal of Endocrinology (1995) 144, 19–29

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A. S. McNEILLY and I. C. HART

It has been suggested that in certain circumstances oxytocin may stimulate the release of prolactin in the rat (Benson & Folley, 1956) and goat (Bryant, Greenwood & Linzell, 1968). In order to test this hypothesis in a physiological situation the changes in blood levels of oxytocin and prolactin were determined in a series of samples taken during parturition in the goat, where oxytocin levels are known to be high (Folley & Knaggs, 1965; McNeilly, Martin, Chard & Hart, 1972).

The cannulation and blood sampling technique has been described previously (McNeilly et al. 1972). Jugular blood samples were taken continuously during the whole of labour in six pedigree British Saanen goats and all plasma samples were stored at −20 °C until assay. Oxytocin was

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J. RUSSELL and R. R. CHAUDHURY

Although the release of oxytocin from the neurohypophysis has been demonstrated both after physiological stimuli such as suckling (Fitzpatrick, 1961) and after administration of pharmacological agents such as nicotine (Bisset & Walker, 1953), very little work appears to have been carried out to investigate how quickly oxytocin is repleted at the posterior pituitary gland after release. In the present investigation the oxytocin content of the pituitary gland was determined at 2, 10, 30 and 120 min after intravenous administration of nicotine to rats. The effect of previous administration of pheniramine, an antihistamine, on the oxytocin-releasing action of nicotine was studied in a further series of experiments.

Male albino rats (150–200 g) were used. Nicotine bitartarate at a dose of 1 mg/kg was injected slowly into the tail vein of the rat. Groups of rats were killed by decapitation at 2, 10, 30 and 120 min after the administration of nicotine.

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G. S. KNAGGS

Oxytocin disappears rapidly from the circulation; half-lives in the circulation for the intravenously injected hormone of 40 sec. to 4 min. have been reported in a variety of species. In connexion with the interpretation of blood levels of oxytocin during suckling (Folley & Knaggs, 1966) the following experiments were carried out to determine the half-life of exogenous oxytocin in the circulation of the sow. Although the results are limited they may be worthy of report since no previous information for the sow is available.

Two experiments were performed on an adult sow (no. 2) weighing 102 kg. which had just lost a premature litter. In the first, the whole experiment was carried out while the sow was maintained under the anaesthetic (cyclopropane/oxygen) given for the insertion of a jugular cannula (for cannulation technique see Folley & Knaggs, 1966). One i.u. oxytocin (Pitocin, Parke, Davis and Co.) in 1 ml. 0·9%

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D. F. SWAAB, F. NIJVELDT, and C. W. POOL

Netherlands Central Institute for Brain Research, Ijdijk 28, Amsterdam-0, The Netherlands

(Received 3 July 1975)

The 'classical' view on the distribution of oxytocin and vasopressin producing cells suggests that the paraventricular nucleus (PVN) is predominantly or entirely responsible for oxytocin production and the supraoptic nucleus (SON) synthesizes mainly vasopressin. However, recent hormone assays and electrophysiological studies indicate the presence of each hormone in both nuclei (for references see Burford, Dyball, Moss & Pickering, 1974). We report an immunofluorescence study in the SON and the magnocellular part of the PVN (Bodian & Maren, 1951) using antibodies to oxytocin (produced in our laboratory) and to vasopressin (produced by Drs Hollemans, Schellekens and Touber), purified by absorption with arginine-vasopressin and oxytocin respectively (Swaab & Pool, 1975).

Frontal cryostat serial sections of glyoxal-prefixed hypothalami from five male Wistar rats weighing 200 g were studied. Out of each group of six sections, the first was

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A. S. McNEILLY, M. J. MARTIN, T. CHARD, and I. C. HART

Neurophysin and the octapeptide hormones oxytocin and vasopressin are synthesized in the hypothalamus and stored in the posterior lobe of the pituitary gland. It has recently been shown that the release of both oxytocin and vasopressin or of vasopressin alone, in response to potent stimuli, is accompanied by a simultaneous release of neurophysin into the circulation (Burton, Forsling & Martin, 1971; McNeilly, Legros & Forsling, 1972). However, it has yet to be shown that neurophysin can be released at the same time as a specific release of oxytocin. This situation occurs in animals during both parturition (Folley & Knaggs, 1965) and lactation (Folley & Knaggs, 1966; McNeilly, 1972). The present report describes the simultaneous release of oxytocin and neurophysin during parturition in the goat.

Serial blood samples (approx. 10 ml each) were taken from an indwelling jugular cannula during the whole of labour in two pedigree British Saanen goats. Samples

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A. M. BURTON, D. V. ILLINGWORTH, J. R. G. CHALLIS, and A. S. McNEILLY

SUMMARY

Oxytocin levels in pregnant and parturient guinea-pigs were studied by means of a sensitive and specific radioimmunoassay. Oxytocin was released from the maternal pituitary in substantial amount only during the expulsive phase of labour, when the mean concentration in carotid arterial blood in five animals was 503 pg/ml plasma (range 96–2900 pg/ml). Oxytocin was not found in the plasma of the first born at the moment of birth, but was usually detected in amounts ranging from 96 to 455 pg/ml in those born subsequently. The mean half-time of oxytocin in the maternal circulation during late pregnancy was 62 ± 7·5 (s.e.m., n = 5) s. In-vivo experiments showed that the placenta was permeable to oxytocin in both directions.

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I. C. A. F. ROBINSON and J. M. WALKER

The use of agarose-bound neurophysin for the extraction of oxytocin from biological fluids is described. Oxytocin can be extracted from plasma, urine and cerebrospinal fluid with a high rate of recovery and samples varying widely in volume and oxytocin concentration can be tested by the method.

Columns can be used to extract and concentrate dilute samples, or to help identify small amounts of neurohypophysial hormones by affinity chromatography. The oxytocin can be eluted from the column directly into the buffer used for subsequent bioassay. The composition of the final extract is constant and independent of the composition of the sample. The specificity of the binding is high. It is suggested that the method has many advantages over others in current use.