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S. A. Jones and A. J. S. Summerlee


Experiments were carried out to establish whether infusion of relaxin prolongs gestation and labour in the rat by suppressing release of oxytocin, and whether the effects of relaxin on birth could be reversed by the opioid antagonist naloxone. Female rats were implanted with subcutaneous osmotic minipumps for the infusion of purified porcine relaxin into the jugular vein for 84 h from either day 19 or day 20 of gestation. Infusion of relaxin delayed the onset of labour and in those animals which delivered during relaxin infusion, delivery was longer by approximately 45 min. Plasma oxytocin levels 40 min after delivery of the first fetus were 45·25 ± 3·6 pmol/l (mean ± s.d.) in unoperated controls and significantly (P < 0·01) depressed (23·89 ± 3·9) in rats that delivered during infusion of relaxin. Rats that delivered after the infusion of relaxin had finished, gave birth significantly (P < 0·05) faster than controls and plasma oxytocin levels were significantly (P < 0·01) raised (77·87 ±15·9 pmol/l). Naloxone treatment (1 mg/kg; i.m.) given immediately after the delivery of the first fetus reversed the inhibitory effect of relaxin and the interval between successive deliveries was slightly faster than that of controls. Plasma oxytocin levels in relaxin-infused naloxone-treated rats were significantly (P < 0·01) higher than values in unoperated control rats. The results confirm that relaxin suppresses oxytocin release possibly through an opioid system and this may be important in the control of the timing of birth.

J. Endocr. (1986) 111, 99–102

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Blood oxytocin was assayed on the lactating guinea-pig mammary gland.

Transient release of oxytocin was detected in the range of 9–889 μu./ml. plasma in the external jugular vein blood of cows during machine milking. In 24 experiments oxytocin was detected in 17; in nine of these, hormone release by conditional reflex was obtained in response to auditory or visual stimuli associated with the milking routine. All stimuli associated with the milking routine caused oxytocin release. There was no difference in the plasma concentrations detected in response to conditional and nonconditional stimuli. In this and previous work no release of oxytocin was detected in 32% of experimental machine milkings although in most cases milk yields were normal.

Milk ejection was accompanied by increases in intramammary pressure from 12·5–24 mm. to 23–44 mm. Hg; the pressure after ejection was maintained virtually constant for over 1 hr. There was no initial drop in pressure, associated with a neural segmental reflex. Rise in intramammary pressure accompanied the detection of oxytocin in jugular blood in four out of seven experiments. The pressure changes and latency could be simulated by rapid intravenous injection of 100 m-u. synthetic oxytocin. The half-life of synthetic oxytocin in the circulation was 127 ± 9·3 sec. (range 85–164 sec.).

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L. Zhang, J. J. Dreifuss, M. Dubois-Dauphin and E. Tribollet


The discovery that oxytocin is synthesized and stored in corpora lutea of ruminants has fostered a renewed interest in the possible roles of oxytocin in ovarian function. In the present study we describe the distribution of binding sites for oxytocin in the guinea-pig ovary. Sections were reacted with a radioiodinated oxytocin antagonist (125I-labelled OTA) to yield autoradiograms on film. Specific binding sites for oxytocin were defined as those which bound 0·05 nmol 125I-labelled OTA/l and where 1 μmol non-radioactive oxytocin/1 displaced the radioactivity. 125I-Labelled OTA consistently labelled the ovarian stroma and the theca interna, but not the corpora lutea, the granulosa cells or the theca externa. The amount of 125I-labelled OTA bound to ovarian stroma and theca interna was high in animals killed during dioestrus, and low during and shortly after oestrus. These data suggest that the binding sites are regulated by steroid hormone levels and that in the guinea-pig ovary oxytocin could exert a role in follicular steroidogenesis, maturation or ovulation rather than in luteal function. Oxytocin-binding sites were also shown in the uterus but their numbers varied only slightly during the cycle.

Journal of Endocrinology (1991) 131, 421–426

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The concentration of oxytocin in the jugular venous plasma of goats was studied by radioimmunoassay and the intramammary pressure measured upon manual stimulation of the udder and hand-milking. Before stimulation the concentrations of oxytocin were close to the limit of detection, about 3 pg/ml. Manual stimulation of the udder resulted in a shortlasting increase in plasma oxytocin in 11 out of 18 experiments carried out on nine goats. There were large variations between and within individuals in the magnitude of the oxytocin increase. Hand-milking was in general an efficient stimulus for oxytocin release. In experiments in which manual stimulation had led to a large increase in plasma oxytocin, the ensuing hand-milking did not lead to a further increase. In three experiments neither manual stimulation of the udder nor hand-milking resulted in any significant increase in plasma oxytocin.

The intramammary pressure increased upon manual stimulation of the udder and/or hand-milking in four out of six experiments. There was, in general, a close time-relationship between changes in intramammary pressure and plasma oxytocin.

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J Frayne and H D Nicholson


The aim of the present study was to determine whether LH stimulates oxytocin production by adult rat Leydig cells directly or indirectly via testosterone. Purified adult rat Leydig cells were cultured in the presence or absence of 0·1 ng/ml LH or 1, 10 or 100 ng/ml testosterone for 22 h. Culture medium was collected at 2-hourly intervals and assayed for oxytocin and testosterone. In the presence of LH, Leydig cells produced significantly higher levels of both testosterone (basal production 1·4± 0·13 ng, LH-stimulated 4·1 ±0·13 ng/106 cells per 2 h) and oxytocin (basal production 8·3± 1·2 pg, LH-stimulated 20·2± 1·3 pg/106 cells per 2 h). Testosterone also stimulated oxytocin secretion. However, the increase was smaller compared with that seen with LH and was not found to be dose-dependent. Furthermore, testosterone production was only significantly increased by LH during the first 10 h of the 22-h culture period whereas LH stimulated oxytocin production throughout the whole culture period.

To further determine the effect of LH on oxytocin production, cultures were performed in the presence of LH and/or 400 μm aminoglutethimide. In the presence of aminoglutethimide both the basal and LH-stimulated production of testosterone was significantly reduced. However, in the same cultures aminoglutethimide did not alter either the basal or LH-stimulated production of oxytocin.

These data show that LH does not act via testosterone to stimulate oxytocin production and therefore acts directly or by some alternative indirect mechanism.

In this study it was found that two other factors, cell density and lipoprotein, also influenced oxytocin production by isolated Leydig cells. Decreasing the density at which Leydig cells were cultured from 106 to 10 cells/well significantly increased both their basal and LH-stimulated production of oxytocin. Lipoproteins were also found to stimulate oxytocin production in a dose-dependent manner and to synergize with LH to further increase LH-stimulated oxytocin production. The results of this study show the production of oxytocin by Leydig cells to be regulated not only by the gonadotrophin LH but also by lipoproteins which are known to be present in interstitial fluid. These data add to the accumulating evidence that intratesticular oxytocin may be a paracrine factor.

Journal of Endocrinology (1994) 143, 325–332

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C. J. M. Poole, D. A. Carter, M. Vallejo and S. L. Lightman


The effect of an atrial natriuretic peptide on the secretion of the neurohypophysial peptides arginine vasopressin (AVP) and oxytocin has been studied in vivo and in vitro. Atriopeptin III was administered intracerebroventricularly to conscious rats and plasma concentrations of AVP and oxytocin were determined both in controls and in rats which had their drinking water replaced by 2% NaCl. The release of both AVP and oxytocin was inhibited when basal levels were increased by the saline treatment. The inhibition of AVP release lasted for 40 min whereas oxytocin release was inhibited for 10 min only. In a further experiment the stimulated release of AVP and oxytocin from the isolated neurointermediate lobe of the rat was also inhibited by atriopeptin III.

J. Endocr. (1987) 112, 97–102

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C. A. FOX and G. S. KNAGGS

The direct estimation of oxytocin in blood during suckling in women has only been reported by two groups of workers, who obtained conflicting results. Hawker & Robertson (1957) and Hawker, Walmsley, Roberts, Blackshaw & Downes (1961) failed to detect a change in the oxytocin content of peripheral venous blood during suckling. However, Coch, Fielitz, Brovetto, Cabot, Coda & Fraga (1968) demonstrated the presence of oxytocin (12–25 μu./ml. plasma) in internal jugular venous blood collected from women during suckling. The present study investigates the possible release of oxytocin during suckling and coitus in man by direct estimation of the hormone in the blood. No previous estimation during human coitus has been reported, but in domestic animals an increase in the oxytocin content of the blood has been claimed after mating (Walmsley, 1963) and after artificial stimulation of the genitalia (Fitzpatrick, 1957; Roberts & Share, 1968).

Three suckling experiments, with the same

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R. B. Heap, I. R. Fleet, A. J. Davis, J. A. Goode, M. H. Hamon, D. E. Walters and A. P. F. Flint


The mechanisms of lymphatic-vascular transfer across the ovarian vascular pedicle were studied in anaesthetized sheep 8–15 days after ovulation. [3H]Prostaglandin F (PGF), [14C]mannitol and [36Cl]Na were infused continuously into either a uterine lymphatic or a uterine vein and the kinetics of transfer into the adjacent utero-ovarian vein or ovarian plasma were studied. Transfer occurred according to the sequence [36Cl] > [14C] > [3H] indicating that PGF is not transferred by rapid diffusion, as with [36Cl]Na, nor by a paracellular route, as with [14C]mannitol, but by a slower process probably involving facilitated diffusion.

Transfer into the adjacent utero-ovarian vein or ovarian blood was greater when compounds were infused into a uterine lymphatic than into a uterine vein. Substantially more [3H]PGF occurred in the adjacent corpus luteum than either of the other compounds after a lymphatic infusion. Intra-lymphatic infusion of PGF stimulated the release of ovarian oxytocin but the effect was not confined to the adjacent ovary. Intravenous (jugular) infusion of PGF failed to stimulate ovarian oxytocin secretion whereas close-arterial infusion into the ovaries was effective, and the possibility was investigated that any systemic effect of PGF was mediated through neural mechanisms. Noradrenaline and acetylcholine were both effective in causing the release of ovarian oxytocin when infused close-arterially into the ovary. With infusions of acetylcholine, ovarian oxytocin secretion rate was increased over fivefold without any change in posterior pituitary release. Noradrenaline and acetylcholine produced a concomitant fall in ovarian blood flow, and neurotransmitter-induced ischaemia may have played a role in ovarian oxytocin release. The finding that PGF infused into a uterine lymphatic stimulates ovarian secretion of oxytocin, and that the effect is bilateral whereas PGF accumulation in ovarian tissue is unilateral, implies that its mechanism of action may not be solely directed at the luteal cell.

Journal of Endocrinology (1989) 122, 147–159

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In the present study a very simple test has been developed to demonstrate the time of onset of lactation in pregnant rats. Pro-oestrous female albino rats were placed with a male rat overnight. The next morning, if spermatozoa were found in the vaginal smear, was counted as the first day of pregnancy. In our colony, rats usually deliver on day 22. Starting on day 20 of pregnancy the rats were tested as follows: under ether anaesthesia, 50 m-u. oxytocin were given i.p. and a nipple was cut superficially. If the mammary gland is ready for lactation milk will flow out 1 or 2 min. after oxytocin administration. The whole procedure takes less than 5 min. and the rat recovers quickly. Using this test 47 pregnant rats have been studied. The time when the first foetus was delivered was taken as the time of parturition. In none of the rats used

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Analogues of oxytocin and vasopressin modified through O-alkylation (methyl, ethyl and butyl) at position 2 of the peptide chain were synthesized and tested for effects on the rat uterus in vivo and on isolated rat and human myometrial preparations. None of the analogues showed any appreciable oxytocic activity. When tested together with oxytocin or vasopressin the analogues inhibited the uterine response to the hormones in a dose-dependent and reversible way. Ethyl-analogues were more powerful antagonists than methyl-analogues but further prolongation of the alkyl-chain (butyl) diminished the antagonistic properties of the compounds. The effect on antagonistic potency of deamination at position 1 of the analogues varied among the analogues and depended on the test system used. The possible clinical value of the antagonists is discussed.