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T. CHARD, N. R. H. BOYD, M. L. FORSLING, A. S. McNEILLY, and J. LANDON

SUMMARY

A method is described for the extraction and concentration of oxytocin from plasma which is simpler and more rapid than other available procedures. It has proved satisfactory for both radioimmunoassay and bioassay of circulating oxytocin. The recovery of added oxytocin was 60 ± 5·5% and showed no significant variation between plasma from pregnant and non-pregnant women, or plasma from other species. The sensitivity of the assay is related to the volume of plasma extracted. With a 10 ml plasma sample and the radioimmunoassay method described previously, the maximum sensitivity under optimal conditions is 0·75 μu. (1·5pg)/ml. In the second stage of human labour, oxytocin is not detectable in maternal plasma at this level of sensitivity. Of 36 cord venous plasma samples studied, 15 showed positive results in the range 1·5–20 μu. (3–40 pg)/ml; of 16 simultaneous cord arterial and venous plasmas, 12 showed positive results; the arterial samples showed a range of 8–145 μu. (7–290 pg)/ml with an average of 45 μu./ml; the venous samples showed a range of 0–100 μu. (0–200 pg)/ml with an average of 24 μu./ml. Plasma oxytocin levels during the second stage of labour in the goat averaged 120 μu. (240 pg)/ml by radioimmunoassay and 100 μu. (200 pg)/ml by bioassay. The half-life of infused oxytocin in the non-pregnant human subject as determined by radioimmunoassay was 5 min.

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T. J. Parkinson, H. J. Stewart, M. G. Hunter, D. S. C. Jones, D. C. Wathes, and A. P. F. Flint

ABSTRACT

Analysis of ovine conceptus RNA by slot blotting, Northern analysis and nested polymerase chain reaction failed to detect oxytocin–neurophysin prohormone mRNA. Probes used hybridized with both the 3' end of the prohormone mRNA and the oxytocin-coding sequence. Northern analysis of bovine and porcine conceptus RNA was also negative, and polymerase chain reaction demonstrated oxytocin–neurophysin mRNA in ovine corpus luteum, but not in human corpus luteum or decidua, or in ovine endometrium. Infusion of oxytocin into the uterine lumen in cyclic ewes between days 9 and 19 or 20 after oestrus failed to prolong the luteal phase of the cycle and had no effect on endometrial oxytocin receptor concentrations or uterine prostaglandin F secretion. Oxytocin administered systematically prevented luteolysis and reduced uterine prostaglandin F secretion. Taken together, these data suggest that blastocyst-derived oxytocin is unlikely to contribute to corpus luteum maintenance in early pregnancy. They are inconsistent with a previous report that the ovine blastocyst synthesizes and secretes oxytocin.

Journal of Endocrinology (1991) 130, 443–449

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J. R. Seckl and S. L. Lightman

ABSTRACT

We have investigated the secretion of oxytocin and arginine vasopressin (AVP) during vaginocervical stimulation in the conscious goat and examined the effect of the opioid antagonist naloxone on peptide release to this stimulus. Goats were implanted with guide tubes overlying the cisterna magna under anaesthesia and allowed to recover. Vaginocervical stimulation for 60 s resulted in a marked (P < 0·01) release of oxytocin into the plasma but neither plasma AVP nor cerebrospinal fluid (CSF) concentrations of oxytocin changed significantly.

In a second series of experiments, unoperated goats were infused with saline or naloxone (4 mg bolus + 12 mg/h) in random order on two separate occasions. Infusion of naloxone had no effect on basal plasma concentrations of oxytocin or AVP. There was a marked and significant (P < 0·01) potentiation of oxytocin secretion following vaginocervical stimulation in animals infused with naloxone. Naloxone-infused animals showed a significant (P < 0·01) rise in plasma AVP after stimulation but plasma AVP did not change in the saline-infused controls.

We conclude that vaginocervical stimulation leads to the selective release of oxytocin from the neurohypophysis without affecting concentrations of oxytocin in the CSF. Endogenous opioids inhibit the stimulated secretion of oxytocin and AVP in vivo in response to vaginocervical stimulation in the goat.

J. Endocr. (1987) 115, 317–322

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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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P. J. BENTLEY

SUMMARY

Analogues of oxytocin contract the rat uterus in vitro more strongly in the presence of 0·5 mm-Mg2+ than in the absence of the ion. This effect can be referred to an increased 'affinity' of the hormone for its receptors. Neither Ca2+ (1 mm), Sr2+ (0·5 mm) nor lower concentrations of Mg2+ (0·1 mm) have this effect. Manganese mm) was more effective than Mg2+ (0·5 mm); the former potentiated the action of 3-valine oxytocin 9·9 times; that of 8-arginine oxytocin 3·3 times and that of 8-isoleucine oxytocin 1·3 times while 0·5 mm-Mg2+ potentiated the same analogues 5·7, 1·9 and 1·1 times, respectively. The results are discussed in relation to the mechanism of action of oxytocin.

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E. O. HÖHN

Chaudhury & Chaudhury (1962) reported that both local and systemic injection of oxytocin caused a significant increase in weight of the pigeon's crop sac and suggested that a direct prolactin-like effect of oxytocin on the crop was indicated by their results. Since the crop sac response is regarded as specific and is of great value in the assay of prolactin it was felt imperative to investigate the suggested prolactin-like action of oxytocin. The results to be reported below show no true prolactin-like effect from local oxytocin but demonstrate that the preparations used caused local irritation. The increased weight of the crop sacs observed by Chaudhury & Chaudhury can therefore readily be explained as due to inflammatory thickening. The results obtained with systemic oxytocin also failed to show clear-cut evidence of a prolactin-like crop sac response.

EXPERIMENTAL METHODS AND RESULTS

Common domestic pigeons (Columba livia) of both sexes weighing 180–293 g.

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H-L Ang, R Ivell, N Walther, H Nicholson, H Ungefroren, M Millar, D Carter, and D Murphy

Abstract

The bovine oxytocin gene has been expressed in the testes of two independent transgenic mouse lines. Hybridization and RNase protection analysis showed that the oxytocin transgene was transcribed from the normal functional promoter in the Sertoli cells of the seminiferous tubules in a developmentally regulated manner. Immunohistochemistry indicated that both oxytocin and neurophysin epitopes were expressed together in the Sertoli cells at stages I–V and X–XII of the cycle of the seminiferous epithelium. Furthermore, analysis with high- performance liquid chromatography showed that there was a tenfold increase in the amount of amidated oxytocin present in testicular extracts from the transgenic mice. However, there appeared to be no detectable effect of this overproduction of hormone on testicular morphology or fertility parameters. A significant decrease by 50% was detected only in the levels of intratesticular testosterone and dihydrotestosterone. The results point to a local paracrine role for oxytocin in the modulation of Leydig cell function.

Journal of Endocrinology (1994) 140, 53–62

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D. W. LINCOLN

Negative feedback, in which a hormone directly or indirectly restricts its own release, provides the servo-control observed in many endocrine situations. Such control could apply to oxytocin, for many studies claim that exogenous oxytocin reduces milk production (and ejection) in rats and other species (Carrol, Jacobsen, Kassouny, Smith & Armstrong, 1969; Mena & Beyer, 1969; Kuhn & McCann, 1970; Deis, 1971). In these studies oxytocin has always been given in amounts or in a form which we must now regard as somewhat unphysiological. One milliunit is a physiological dose for a rat and during the prolonged nursing of the rat pulses of this size are released at regular intervals of 5–15 min (Wakerley & Lincoln, 1971; Lincoln, Hill & Wakerley, 1973).

Experiments were conducted to determine whether oxytocin, in low doses, would inhibit or re-modulate the pulsatile release of endogenous oxytocin during suckling. Rats were taken at day 9–10 of

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R. C. JONES

Since the effects of injecting oxytocin into ewes just after artificial insemination were to be tested, the experiment described was designed to determine a range of doses of synthetic oxytocin which would produce 'milk let-down' in ewes. Intramuscular injection was chosen as the most practical route of administration since it was planned to use a large number of sheep in the subsequent experiments.

Martinet & Denamur (1960) have shown that both massage of the udder and i.v. injection of 10–50 m-u. oxytocin cause similar increases in intramammary pressure in goats and sheep and Debackere & Peeters (1960) and Debackere, Peeters & Tuyttens (1961) have shown that intramammary pressure in ewes was increased by distension of the vagina; a similar response was obtained by i.v. injection of 20–50 m-u. oxytocin. Since Fitzpatrick (1960) had found that oxytocin injected i.v. was 2·25 and 2·5 times more potent than when injected i.m., the

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M. J. Brimble, R. J. Balment, C. P. Smith, R. J. Windle, and M. L. Forsling

ABSTRACT

The contribution of oxytocin to the maintenance of renal Na+ excretion in the Brattleboro rat has been examined in animals infused with hypotonic saline. Brattleboro rats exhibited hypernatraemia and hyperosmolality associated with greatly increased plasma concentrations of oxytocin by comparison with Long–Evans control rats. Neurohypophysectomy to remove the secretion of the remaining posterior pituitary peptide, oxytocin, led to greatly diminished rates of Na+ excretion in the Brattleboro rat. Oxytocin replacement to achieve plasma levels equivalent to those in intact Brattleboro rats produced a substantial and sustained natriuresis in the neurohypophysectomized animal. Oxytocin secretion evoked in response to saline infusion would thus appear to be effective in promoting renal Na+ excretion in the absence of vasopressin in the Brattleboro rat.

Journal of Endocrinology (1991) 129, 49–54