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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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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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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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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

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G. Peeters, J. J. Legros, C. Piron-Bossuyt, R. Reynaert, R. Vanden Driessche and E. Vannieuwenhuyse

Oxytocin and bovine neurophysin I (bNpI) were estimated by radioimmunoassay in jugular vein plasma which was collected continuously from 18 bulls. No release of peptides was observed during successive matings with a cow in oestrus or during successive mountings on a cow with ejaculations into an artificial vagina. Stimulation with an electro-ejaculator or, to a smaller extent, massage of the seminal vesicles and ampullae per rectum caused an increase of oxytocin accompanied by a release of bNpI. It is speculated that the release of these peptides is due to stimulation of afferent pelvic nerves in the rectal wall. Basal molar ratios of bNpI/oxytocin in the plasma were highly variable, often showing a large excess of either bNpI or oxytocin. After the onset of peptide release induced by stimulation, molar ratios approached 1:1. This might indicate that hormone release is by exocytosis. Basal bNpI does not provide a good reflection of the oxytocin level.

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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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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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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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G. D. Burford and I. C. A. F. Robinson

The content of vasopressin, oxytocin and their related neurophysins was measured in the hypothalamus and pituitary gland of mid-trimester human fetuses. Vasopressin was present in both tissues approximately 3–4 weeks before oxytocin. The levels of the hormones in the pituitary gland increased 1000-fold over the next 3–4 months. During this time, the very high vasopressin/oxytocin ratio gradually decreased but did not reach unity in the period studied. In contrast, both the vasopressin-associated neurophysin and the oxytocin-associated neurophysin appeared in the pituitary gland at the same gestational age and showed the same exponential increase with fetal age. Lower levels of the neurophysins and the nonapeptides were found in the hypothalamus and the levels increased more slowly with fetal age. Our results suggest that the high vasopressin/oxytocin ratios observed in fetal life are due to differences in the rate of maturation of the hormone precursor, rather than to differences in the rate of de-novo synthesis.

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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.