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SANDRA M. EGAN and A. LIVINGSTON

SUMMARY

In the presence of 62 μu. or more of [3H]oxytocin/ml there was specific uptake of oxytocin by lactating rat mammary glands in vitro within 400 s under conditions similar to those used in the biossay of oxytocin with rat mammary strips in vitro. This uptake was blocked by pre-incubation with non-radioactive oxytocin. A similar, rapid, specific uptake of oxytocin by uterine tissue in vitro was observed. There was no specific uptake of oxytocin by non-target tissues such as heart and skeletal muscle. Measurements of inulin and water spaces of the tissues showed that, over these short periods of time, diffusion into mammary tissue was much less than into the other tissues. The ratios of uptake of [3H]oxytocin: [3H]inulin and [3H]oxytocin: [3H]water were much higher for mammary tissue than those for other tissues used, indicating a preferential (tissue-specific) uptake. Uterine tissue from stilboestrol-primed rats also showed a preferential uptake of oxytocin, though not as great as that for mammary tissue. It is suggested that the specific uptake of oxytocin by mammary and uterine tissue is due to binding to specific receptors.

There was a variation in the specific uptake of oxytocin with the day of lactation of the mammary tissue, and specific uptake was only observed after the 8th day. This could indicate synthesis of receptors during lactation. In a similar way, synthesis of receptors may occur in the non-pregnant uterus due to the influence of exogenous oestrogens, leading to the increase in specific uptake by non-pregnant uterine tissue for oestrogen-primed rats. There is some evidence of more than one type of binding site for oxytocin. Biological action may only be associated with one of these sites.

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M. D. MITCHELL, L. A. MOUNTFORD, R. NATALE and J. S. ROBINSON

A specific radioimmunoassay for oxytocin has been established with a sensitivity of 0·8 pg/tube. This assay has been applied to the measurement of oxytocin in serial samples of peripheral plasma and amniotic fluid from pregnant rhesus monkeys, collected at weekly intervals by venepuncture and amniocentesis. Concentrations of oxytocin in both fluids were generally low and showed no trends throughout the latter half of gestation.

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B. K. FOLLETT and P. J. BENTLEY

SUMMARY

(1) Three daily injections of stilboestrol resulted in a thirteenfold increase in uterine sensitivity to oxytocin compared with that of uteri from rats in normal oestrus. These sensitive uteri have been used extensively for routine bioassay of oxytocin. The index of precision was 0·055 in six 'trial' assays.

(2) The results are discussed in relation to other sensitive assay procedures for oxytocin. The present method is as sensitive and accurate but more simple and economical.

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F. Ellendorff and D. Schams

ABSTRACT

The neuroendocrine reflex theory of milk ejection was investigated in the horse under natural suckling conditions. To this end 12 lactating mares were provided with acute jugular catheters and with intramammary pressure (IMP) recording catheters. The foal had free access to the contralateral mammary complex. Intramammary pressure could thus be recorded while blood was drawn simultaneously for oxytocin analysis from the undisturbed animal. Suckling periods associated with a characteristic increase in IMP lasted significantly longer than unsuccessful nursing attempts. Elements of successful sucklings involved physical stimulation of the mammary gland, a quiet phase and a sudden increase in IMP. Successful suckling took place at about 20-min intervals with a wide range from < 5 min to > 100 min. Between 5 and 10 mU oxytocin i.v. were sufficient to evoke an increase in IMP identical in shape and duration to a naturally induced increase in IMP. Mean peak oxytocin levels reached 15·8 pmol/l plasma, with a maximal release of 39 pmol/l. In the majority of cases (> 80%) peak oxytocin release did not occur until after the increase in IMP; in some cases an oxytocin surge was not detectable at all, despite a milk ejection-associated increase in IMP. In three cases increase in IMP could be observed while the foals were away from the mother with no signs of any intention to suckle. The data indicate that in the horse some elements of the neuroendocrine reflex, such as tactile stimulation of the teat and a surge of oxytocin before an increase in IMP, are facultative and not essential for normal milk ejection.

J. Endocr. (1988) 119, 219–227

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R J Windle and M L Forsling

Abstract

Oxytocin was administered to virgin female rats at doses of 25–200 pmol/min during 0·077 mol NaCl/l infusion at 150 μl/min on each day of the oestrous cycle. The resultant rates of urine flow, glomerular filtration (GFR) and electrolyte excretion were determined. Oxytocin caused significant increases in urine flow (P<0·001) and sodium excretion (P<0·001); both responses being dose-dependent (P<0·02 and P<0·01 respectively). Significant variations in the renal responsiveness to the hormone occurred over the 4 days of the oestrous cycle. On oestrus the lowest dose of 25 pmol oxytocin/min produced a significant increase in urine flow (from 139·5 ± 4·3 to 165·6 ± 7·1 μl/min, P<0·005) and a dose of 50 pmol/min produced a significant increase in sodium excretion (from 10·6 ± 0·1 to 14·5 ± 0·7 μmol/min, P<0·005). Significant increases in urine flow and sodium excretion were seen on pro-oestrus with hormone administration rates of 50 and 100 pmol/min respectively and on dioestrus day 2 with a rate of 100 pmol/min. On dioestrus day 1 no increase in urine flow or sodium excretion was seen over the dose range of oxytocin administration. A dose of 100 pmol oxytocin/min significantly increased GFR on pro-oestrus and dioestrus day 2, but not on the other 2 days of the cycle. The circulating hormone concentrations produced by oxytocin infusion were similar on each day of the cycle and so could not account for the differences seen. Therefore, these results suggest varying renal responsiveness to oxytocin during the reproductive cycle of the female rat.

Journal of Endocrinology (1997) 154, 347–353

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T. Higuchi, Y. Tadokoro, K. Honda and H. Negoro

ABSTRACT

A detailed secretory profile of oxytocin during suckling and parturition was determined in unanaesthetized freely moving rats. Ten pups were reunited with their mothers after 12–15 h of separation. Unless the milk-ejection reflex occurred, there was no difference in serum oxytocin levels before separation and during the suckling of either four or five, or nine or ten pups. Serum oxytocin levels increased abruptly by 50·1 ± 4·2 (s.e.m.) pmol/l (n = 9) at milk ejection, and declined rapidly with a half-life of about 1·5 min. The peak concentration of blood oxytocin at each milk ejection was independent of the previous suckling period; values from the first three milk-ejection reflexes following the introduction of the pups and those observed 3–5 h after introduction were similar. The process of parturition was monitored by recording intra-uterine pressure with a balloon implanted in the uterus. On day 22 or 23 of pregnancy, continuous and rhythmical contractions of the uterus occurred (onset of parturition), but serum levels of oxytocin (21·1 ± 1·9 pmol/l; n = 13) did not alter until the expulsive phase. During the expulsive phase, fetuses were delivered after fetus-expulsion reflexes which were recorded as sudden large increases in intra-uterine pressure. Basal levels of oxytocin in the blood increased during this phase (32·5 ± 4·4 pmol/l; n = 13) and, in addition, rose by about 15 pmol/l and declined slowly after fetus-expulsion reflexes. The increase, however, was quite different from that seen at milk-ejection reflexes.

J. Endocr. (1986) 110, 251–256

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A. S. McNEILLY and H. ALISON DUCKER

SUMMARY

Blood oxytocin was assayed on the lactating guinea-pig mammary gland. Thirty-six series of serial jugular blood samples were collected from 33 oestrous female goats during normal mating and the transient appearance of oxytocin (2 to 190 μu./ml plasma) was detected in 28 of the series. Coitus did not appear to be a major stimulus for the release of oxytocin. However, oxytocin release occurred in many experiments shortly before coitus when the male was present and continued after coitus for varying times up to 16 min after the male had left the mating room. Oxytocin (9–150 μu./ml plasma) was also released during simulated mating in all six experiments carried out in six dioestrous female goats.

In a further 16 oestrous female goats, oxytocin release (7–73 μu./ml plasma) occurred in response to individual exteroceptive stimuli associated with mating and these stimuli could be classified in descending order of effectiveness in causing oxytocin release as follows: presence of another goat > smell of the male > sound of the male > sight of the male.

The results indicate that oxytocin release occurs in oestrous female goats in response to a complex of stimuli occurring during mating and that physical stimuli associated with coitus are only important in causing oxytocin release during simulated mating in dioestrous female goats.

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T. K. YOUNG and H. B. VAN DYKE

SUMMARY

Rats deprived of drinking water for 7 days showed a striking depletion of neurohypophysial hormones from the posterior lobe of the pituitary gland. The average daily depletion rate was estimated to be 93 m-u. for vasopressin and 97 m-u. for oxytocin. When rats were allowed free access to water, dehydration was rapidly corrected as shown by normal haematocrit values and plasma osmolarities. Repletion of neurohypophysial hormones, rapid in the first 24 hr., continued gradually thereafter. The mean calculated repletion rate was 41 m-u./day for vasopressin and 42 m-u./day for oxytocin. Repletion was completed about 14 days after rehydration.

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M. R. Luck and B. Jungclas

ABSTRACT

Bovine granulosa cells secrete oxytocin when cultured in a serum-supplemented medium. The time-course of secretion is similar to that in the early corpus luteum in vivo, with a delay of 1 to 2 days followed by a peak and decline over the first 5 days of culture. We have investigated the basis of this time-course in vitro and studied the temporal characteristics of the stimulatory actions of ascorbic acid and adrenaline on this process.

Cells cultured on stirred microcarriers showed a similar pattern of secretion of oxytocin to those cultured on conventional flat plates, despite continuing and rapid mitosis. This indicated that the secretion profile in conventional culture was not an artifact related to the cessation of mitosis. Furthermore, secretion of oxytocin and progesterone by cells on microcarriers was stimulated without a corresponding change in mitotic rate, showing that the secretion per cell had been increased.

In conventional culture, addition of ascorbic acid to culture media (0·5 mmol/l) increased the secretion of oxytocin (up to 4·5-fold) but only if ascorbic acid was present during the first day of culture. The cells showed a progressive refractoriness to stimulation after 12 h. Since the time-course of secretion was unaltered by treatment, this resulted in a delay of 1 to 2 days before the action of the ascorbate was seen. The secretion of progesterone was similarly affected but with less stimulation and less consistency. In contrast, cells treated with adrenaline (10 μmol/l) secreted more oxytocin on the day of treatment and did so at any time during culture provided that there was sufficient basal secretion of hormone. Adrenaline also failed to alter the time-course of secretion but treated cells showed a persistent response, maintaining enhanced secretion for up to 3 days after the adrenaline had been removed.

Ascorbate and adrenaline were highly synergistic in their effects, provided that the ascorbate was present from the start of culture; the response to adrenaline strongly reflected the degree of ascorbate stimulation. We conclude that granulosa cells secrete oxytocin according to an inherent time-schedule and that there is a limited period during which they can respond to ascorbate. Since ascorbate is required for the biosynthesis of oxytocin, this suggests that the availability of ascorbate during corpus luteum formation may determine the amount of oxytocin which can be released subsequently in response to catecholamines.

J. Endocr. (1988) 116, 247–258

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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