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

ABSTRACT

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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G. MOSDØL, Ø. V. SJAASTAD, and ANNE KRISTINE BLOM

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

Abstract

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

ABSTRACT

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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MF Walter, ML Forsling, and DG Shirley

In order to determine the possible role of endogenous oxytocin in controlling electrolyte and water excretion in animals whose renal function is being assessed by invasive techniques, rats were anaesthetized and subjected to micropuncture surgery. Clearance measurements were made in the presence and absence of the potent oxytocin receptor antagonist d(CH(2))(5)[Tyr(Me)(2), Thr(4), Orn(8), Tyr(NH(2))(9)]-vasotocin. In rats infused with vehicle alone, glomerular filtration rate (GFR), sodium excretion and urine flow rate remained stable. In contrast, in antagonist-treated rats GFR was modestly reduced (P<0.05), and there were large falls in both absolute and fractional sodium excretion (P<0.01 in each case) and absolute and fractional water excretion (P<0.05 in each case), indicating effects on both filtered load and fractional tubular reabsorption. The antinatriuresis was not accompanied by a change in the fractional excretion of lithium, suggesting that proximal tubular function is unaffected by oxytocin receptor antagonism; nor was it accompanied by a change in the fractional excretion of potassium, suggesting that the tubular effect is located beyond the potassium secretory site, i.e. downstream of the cortical collecting tubule. We conclude that circulating plasma concentrations of oxytocin during anaesthesia and moderate surgery are sufficient to enhance GFR and reduce fractional tubular sodium and water reabsorption. This has important implications for the interpretation of invasive studies such as micropuncture.

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D. Schams, R. Koll, and C. H. Li

ABSTRACT

The effect of insulin-like growth factor-I (IGF-I), epidermal growth factor (EGF), fibroblast growth factor (FGF) and nerve growth factor (NGF) on production of oxytocin and progesterone by cultured bovine granulosa and luteal cells was studied. Secretion of oxytocin was stimulated, in a dose-dependent manner, by IGF-I at 48 and 120 h of culture to levels much higher than those after stimulation with LH, FSH, EGF, FGF or NGF. A similar effect of IGF-I was observed for progesterone but, in contrast to oxytocin, secretion of progesterone was not increased by EGF, NGF or FGF. During primary culture, for 4 h, of dispersed bovine luteal cells obtained from corpora lutea between days 4 and 10 of the oestrous cycle, all the growth factors tested failed to stimulate secretion of oxytocin or progesterone. The data suggest the relevance of growth factors (especially IGF-I) for ovarian physiology and their possible importance for differentiation of follicles and luteinization.

J. Endocr. (1988) 116, 97–100

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H Ungefroren, M Davidoff, and R Ivell

Abstract

Northern blot hybridization showed that bovine and sheep testis, unlike testes from other mammals, contain moderate levels of an apparently normal oxytocin gene transcript. In situ hybridization localized this mRNA to within the seminiferous tubules, possibly in the Sertoli cells. Conflicting with this result, immunohistochemistry showed that both oxytocin and the syngeneic neurophysin I epitopes are both clearly restricted to the Leydig cells, being expressed here at a low level. Since illegitimate transcription from spurious start sites can lead to a lack of translation product, the integrity of the major ruminant testicular transcripts of the oxytocin gene was checked using differential hybridization, RNase protection and multiple polymerase chain reaction assays. All tests showed the transcripts to have a normal, translatable composition and to be transcribed from the conventional 5' initiation site. Therefore, the block in oxytocin gene expression within the tubules is probably due to a lesion at the post-transcriptional level. The low level peptide expression in the Leydig cells can probably be attributed to the presence of functional transcripts in these cells, which are below the level of significant detection for the in situ hybridization assay.

Journal of Endocrinology (1994) 140, 63–72

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E. L. M. Bolwerk and H. H. Swanson

ABSTRACT

Oxytocin is released during parturition and may also play a role in maternal behaviour. Oxytocin, injected in the cerebral ventricles, has been reported to accelerate the onset of maternal behaviour in oestrogen-pretreated virgin Sprague—Dawley rats within 2 h of injection. This study was an attempt to replicate and extend these findings in Wistar rats. In the first experiment, 16 virgin females were ovariectomized and a cannula was placed into the cerebral ventricle. Forty-eight hours after a single injection of 24 μg oestradiol benzoate (OB), 400 ng oxytocin or saline were injected into the ventricle. In the second experiment three groups were observed: an untreated control group plus two ovariectomized and cannulated groups treated with OB in a regimen designed to mimic pregnancy. After 10 days of OB administration they received an injection of either saline or oxytocin (400 ng) into the ventricle. Immediately after this injection they were exposed to the pups and observations started. In both experiments no rat became maternal in the first 1·5 h after the intracerebroventricular injection. Oxytocin therefore did not induce a rapid onset of maternal responsiveness in oestrogen-pretreated Wistar rats.

J. Endocr. (1984) 101, 353–357

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I. C. A. F. ROBINSON and J. A. PARSONS

Lactating guinea-pigs were passively immunized with an antiserum to oxytocin of high titre, specificity and avidity. Single i.v. injections of 0·1–0·4 ml antiserum produced high titres which decayed slowly (half-life ≃7 days). Passively administered antiserum was effective in vivo; the clearance of exogenous oxytocin from plasma was greatly slowed in immunized animals. Passive immunization with 0·4 ml antiserum reduced milk transfer to the litter during suckling episodes of 10 min, and overall litter growth rates were significantly decreased. Non-immune serum was without effect. Plasma neurophysin levels showed the same large rises during suckling in immunized animals, indicating that neurohypophysial activation was unimpaired. Despite the presence of high titres of antiserum, some milk transfer still occurred at milk ejection. In-vitro experiments showed that more than 25% of oxytocin remained free 20 s after mixing with plasma taken from passively immunized animals. It is probable that the antiserum in the circulation was unable to bind all the oxytocin released from the posterior pituitary gland before it reached the mammary gland.