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G. K. BENSON and S. J. FOLLEY

SUMMARY

The involution of the mammary glands of lactating rats, which normally follows the cessation of suckling, was greatly retarded over a period of 9 days by administering oxytocin to the mothers, following removal of the litters on the 4th day of lactation. This effect was obtained with a commercial extract of the natural hormone, the same extract without preservative (benzethonium chloride), and with synthetic oxytocin. Vasopressin administered under the same conditions had a less well-marked effect. No retardation of mammary involution could be obtained with oxytocin in the absence of the anterior pituitary gland.

Similar results were obtained by administering prolactin to the mothers, but growth hormone (GH) had only a slight effect in maintaining the mammary glands. When both prolactin and GH were given, the maintenance of gland structure was particularly marked.

A majority of the animals receiving synthetic oxytocin showed vaginal mucification which is taken to be indicative of the presence of a luteotrophic hormone (prolactin).

These results are discussed in relation to the possible role of oxytocin in the release of prolactin and other lactogenic and galactopoietic hormones from the anterior lobe.

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R. Claus and D. Schams

ABSTRACT

Oxytocin concentrations were measured radioimmunologically in sows on the day of standing oestrus over a 6-h period (controls, n=6) or 1 h before and 5 h after mating (n=5) or transcervical infusion of either 100 ml saline (0·9% (w/v) NaCl, n=7) or saline plus 10 μg oestradiol (simulation of seminal oestrogens, n=5).

In the controls, oxytocin was low, at around 1·0 pmol/l, throughout the investigation period. Similarly, saline infusion did not lead to a noticeable change in oxytocin concentrations in six out of seven sows. In one sow, however, infusion led to a maximum of 86 pmol/l at 1 min after infusion. Oestradiol led to no immediate increase in oxytocin concentrations. Later in the post-treatment period (2–5 h) they were only slightly increased (1 pmol/l vs 3 pmol/l). All mated sows reacted with a rapid and clear increase in oxytocin. Maximal concentrations (42·0±5·1 pmol/l; mean ± s.e.m.) appeared 2 min after the onset of ejaculation. Clearly increased concentrations were found for 40 min. It was concluded that mating specifically leads to a rise in oxytocin, probably due to both mechanical and pheromonal stimuli provided by the boar.

Journal of Endocrinology (1990) 126, 361–365

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D. S. C. Jones and A. P. F. Flint

ABSTRACT

Concentrations of oxytocin in corpora lutea from pregnant and non-pregnant sheep rose during luteinization to peak between days 5 and 9 after oestrus before falling to low levels on days 12 and 14. Concentrations of the 608-base mRNA encoding the oxytocin-neurophysin prohormone, measured by Northern blotting using a cRNA probe, were highest before day 5, and fell to low levels by day 9. The decline in oxytocin during the second half of the oestrous cycle and in pregnancy could therefore be accounted for by reduced luteal concentrations of oxytocin-neurophysin mRNA. The rate of decline in prohormone mRNA concentration between days 3 and 16 followed first order kinetics, suggesting that gene expression ceased early in the cycle. The lag between peak oxytocin-neurophysin mRNA and peak oxytocin concentrations may represent the time taken for post-translational processing of the prohormone.

J. Endocr. (1988) 117, 409–414

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W. E. ALLEN, T. CHARD and MARY L. FORSLING

In most species there is a release of oxytocin during the expulsive phase of labour with little or no release during the earlier stages (see Chard, 1972). However, information is lacking on the release of arginine-vasopressin (AVP) during parturition. It appears to be released in the rat (Fuchs & Saito, 1971) but not in the goat (McNeilly, Martin, Chard & Hart, 1972). The present studies demonstrate the release of both oxytocin and AVP during parturition in the horse.

Thirty-one jugular blood samples (20 ml) were collected during labour in three Welsh ponies which produced live foals, 20 samples from three mares in early pregnancy and 25 samples from three mares after elective Caesarean operation. Plasma was separated immediately at 4 °C and stored at -20 °C to obviate enzymic destruction. Oxytocin was determined by radioimmunoassay (Chard, Boyd, Forsling, McNeilly & Landon, 1970) and AVP by bioassay (Forsling, Jones & Lee,

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ANNA-RIITTA FUCHS

SUMMARY

The effect of ethanol on parturition was studied in conscious unrestrained rabbits fitted with intra-uterine balloons. The onset of labour was predicted by an oxytocin sensitivity test.

With blood ethanol concentrations of 0·25–0·35 g./100 ml., the onset of labour was inhibited, typical contractions were absent, delivery was prolonged, abnormal and postponed for about 30 hr. With a blood concentration of 0·2 g./100 ml., labour contractions were less intense than normal and delivery was prolonged. At a lower concentration, 0·14 g./100 ml., parturition was apparently normal.

Uterine sensitivity to exogenous oxytocin was unimpaired by ethanol treatment. The interference with parturition by ethanol is attributed to central inhibition of the abrupt reflex release of oxytocin.

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G. E. Rice, G. Jenkin and G. D. Thorburn

ABSTRACT

The subcellular distribution of progesterone and oxytocin within the ovine corpus luteum was investigated using differential and density gradient centrifugation. Progesterone and oxytocin were associated with particles which sedimented to a density of 1·049–1·054 g/ml and 1·054–1·061 g/ml respectively. Particle-associated progesterone did not, however, display physical or biochemical characteristics consistent with its storage within secretory granules. When particle-associated progesterone was incubated in HEPES buffer at 37 °C, 70% of the total progesterone was recovered in the incubation medium. The remaining stable particle-associated progesterone was not affected by treatments which stimulated oxytocin release and which have been shown to cause the release of peptides and biogenic amines from secretory granules. These results suggest that particle-associated progesterone represents the intercalation of progesterone into cell membranes and they do not support the hypothesis that progesterone is stored, in a protein-bound form, in luteal secretory granules.

J. Endocr. (1986) 108, 109–116

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J. MEITES and T. F. HOPKINS

SUMMARY

On the 4th day after parturition lactating rats were hypophysectomized and their litters removed. Control rats were injected only with physiological saline for the next 10 days, and showed pronounced regression of the mammary parenchyma and no secretion. When oxytocin was injected together with prolactin and adrenocorticotrophic hormone (ACTH) for 10 days, mammary secretion, often with duct engorgement, was observed in twenty-two out of twenty-four rats, whereas only three out of fourteen rats given prolactin and ACTH showed slight secretory activity. Lobule-alveolar structure was significantly better preserved in the former rats. The combination of oxytocin, prolactin and cortisol acetate was also more effective than prolactin and cortisol acetate in maintaining secretory function and in retarding mammary involution. These results in hypophysectomized rats, as well as similar findings previously reported by us in intact rats, demonstrate that the favourable effects of oxytocin in maintaining secretory activity and retarding mammary involution are exerted directly on the mammary gland.

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M. LIS, J. BÍLEK, I. SKALA and J. SLABA

Measurements of the contractions of mammary gland tissue are widely used for the assay of oxytocin both in vivo (Tindal & Yokoyama, 1962; Bisset, Clark, Haldar, Harris, Lewis & Silva, 1967) and in vitro (Mendez-Bauer, Cabot & Caldeyro-Barcia, 1960; Rydén & Sjöholm, 1962; Moore & Zarrow, 1965). It is generally accepted that this tissue is more selective than any other mammalian tissue currently used. The recently developed method of van Dongen & Hays (1966) for assaying oxytocin on rat mammary gland tissue in vitro is the most sensitive method so far devised. In this method the time of the beginning of milk expulsion is measured by means of microscopical observation of minute pieces of rat mammary gland. In our experience, however, the precision of this method is not suitable for quantitative measurements of relatively small changes of the oxytocin level in blood. In an attempt to measure the microscopically observed

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D. A. Poulain and J. G. Tasker

ABSTRACT

In urethane-anaesthetized lactating rats, intramammary pressure occasionally displayed recurrent variations or oscillations having a slow rise time, low amplitude, long duration and a periodicity of 1–4 min. These oscillations differed from changes in intramammary pressure characteristic of reflex milk ejections induced by suckling, and were also observed in unsuckled rats. They were suppressed by lesions of the pituitary stalk or by stimulating the septum, a structure that inhibits the activity of the magnocellular system. They could be induced by long-term low frequency stimulation of the pituitary stalk, lesions of the septum or long-term infusions of oxytocin at a low rate of 0·05–0·3 mu./min. We suggest that the recurrent oscillations in intramammary pressure constitute a particular mode of response of the mammary gland to a tonic release of oxytocin resulting from a moderate but sustained increase in the basal level of electrical activity of the oxytocin-secreting neurones.

J. Endocr. (1985) 107, 89–96

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J. A. COCH, C. FIELITZ, J. BROVETTO, H. M. CABOT, H. CODA and A. FRAGA

SUMMARY

The presence of a substance in the jugular blood of lactating women with the chromatographic, electrophoretic, and pharmacological properties of oxytocin has been demonstrated.

The concentration of this substance in the jugular plasma, during suckling, was equivalent to 12–25 μ-u./ml. synthetic oxytocin.

These figures are in good agreement with those calculated from the rate of infusion of oxytocin necessary to elicit milk ejections similar to that produced during suckling.