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O. P. SHARMA and R. L. HAYS

SUMMARY

Administration of methallibure (50 mg/kg body weight, daily) to male rabbits resulted in a 45% reduction in sperm number in ejaculates obtained during the treatment period. Recovery occurred within 48 h after the last dose of methallibure. This decrease in sperm number did not occur when oxytocin (0·2 i.u./kg body weight) was administered simultaneously with methallibure. This suggests that methallibure prevents the release of oxytocin during ejaculation.

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JJ Evans, AH Youssef, MM Abbas, and J Schwartz

For full fertility in the female ovulation is necessary, which is dependent on the production of a surge of LH. An understanding of the processes which result in the high levels of LH requires delineation of the activities of individual component cells. In this study the responses of gonadotrophs to two signalling hypothalamic peptides, GnRH and oxytocin, were investigated. A cell immunoblot method was used to identify and distinguish between cells which secrete LH and those which contain LH but do not secrete the glycohormone. Rats were killed on the morning of pro-oestrus, the pituitary collected and the cells dispersed onto a protein-binding membrane for study. Cells were then incubated with GnRH and oxytocin, after which the membranes including the attached cells were stained by immunocytochemistry for LH. GnRH increased the total number of immunopositive cells which were present in a concentration-dependent manner. The most prominent change after 2 h incubation was in the number of secreting cells, whereas after 4 h there was also a marked increase in numbers of nonsecreting cells. Oxytocin also increased the total number of immunopositive cells in a concentration-responsive manner, however the profile of action of oxytocin was different from that observed for GnRH. Oxytocin had a relatively greater effect on numbers of immunopositive nonsecreting cells. Thus, the results reveal the potential for gonadotrophs to be flexibly and appropriately modulated by selected hypothalamic peptides. When cells were preincubated with oxytocin prior to GnRH there was not an additive increase in the numbers of immunopositive cells, suggesting that the two agonists act, in a nonidentical manner, on similar cells. The increase in the total number of immunopositive cells implies that there was a production of LH or post-translational processing, induced by exposure to GnRH or oxytocin. The results confirmed the heterogeneity of gonadotrophs and the existence of functionally distinguishable subpopulations, and revealed a difference between the effects of GnRH and oxytocin on expression and secretion of LH.

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M. L. FORSLING, M. A. M. TAVERNE, N. PARVIZI, F. ELSAESSER, D. SMIDT, and F. ELLENDORFF

SUMMARY

Plasma oxytocin concentrations were measured during late pregnancy, parturition and lactation in the miniature pig. Measurements were made of plasma oestradiol, oestrone and progesterone to determine whether there was any relationship between the concentrations of oxytocin and these steroids in the circulation.

Plasma oxytocin concentrations were low or undetectable in late pregnancy. Rises of up to 68·8 μu./ml were seen at the time of delivery of the foetuses and at the expulsion of the placenta. The only steroid that seemed to relate to oxytocin release was progesterone. Oxytocin release was consistently seen when progesterone concentrations had fallen to below 10 ng/ml but no increase in concentration was observed while oestrone and oestradiol increased to their maximum concentrations of 3·86–11·6 and 0·43–0·70 ng/ml respectively.

During lactation, when both oestrogen and progesterone concentrations were low, suckling caused the levels of oxytocin to increase to 7·4 μu./ml. These increases were greater during the first 2 weeks of lactation than later.

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T. D. M. Williams, D. C. Abel, C. M. P. King, R. Y. Jelley, and S. L. Lightman

ABSTRACT

The regulation of both arginine vasopressin (AVP) and oxytocin secretion was studied during rapid and prolonged osmotic stimuli in normal adult volunteers. In five subjects given an intravenous infusion of 0·85 mol NaCl/l at 0·05 ml/kg per min over 2 h there was a significant (P<0·05) rise only in plasma AVP, with no significant change in plasma levels of oxytocin. In six further subjects 5 days of restriction to 500 ml fluid daily resulted in significant increases of both plasma and 24-h urinary AVP, whereas there was no change in corresponding oxytocin levels. During another 5-day period in which the same subjects were given an additional 200 mmol sodium as well as having their fluid intake restricted to 1000 ml daily, there were again significant rises in plasma and 24 h urinary AVP with no change in corresponding oxytocin levels. We conclude that, in man, AVP is selectively secreted in response to both dehydration and high sodium intake, whilst even after the stimulus of rapidly increasing plasma osmolality during intravenous infusion of hypertonic saline the rise in oxytocin is not statistically significant. It therefore appears unlikely that oxytocin has a significant role in the physiological control of fluid balance in man.

J. Endocr. (1986) 108, 163–168

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I.C.A.F. ROBINSON, C. N. WOOLF, and J. A. PARSONS

The release of oxytocin and neurophysin during suckling has been studied in conscious unrestrained guinea-pigs. After prior separation, mothers and litters were allowed to suckle for a period of 10 min, and the weight gain of the litter recorded as an index of milk transfer. Maternal blood samples were obtained without disturbance through previously implanted intravenous cannulae and neurophysin and oxytocin determined on unextracted plasma by specific and sensitive radioimmunoassays. In 82 out of 118 experiments the young gained weight during suckling (1·6±0·1 (s.e.m.) g/pup) and this was associated with large rises in both oxytocin and neurophysin concentrations in plasma (mean concentrations: oxytocin 65·4fmol/ml, neurophysin 360fmol/ml). Where serial samples were taken, oxytocin and neurophysin showed a rapid rise and fall in concentration closely associated with the occurrence of milk ejection as judged by the behaviour of the litter. The present results provide the first direct evidence of a spurt release of both oxytocin and neurophysin measured simultaneously during milk ejection. The conscious lactating guinea-pig thus provides a useful laboratory model in which to study hormone release during milk ejection.

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B. Jungclas and M.R. Luck

ABSTRACT

We have examined the ability of granulosa cells, from carefully selected preovulatory bovine follicles, to secrete oxytocin in vitro. Although cells from 83% of follicles underwent functional luteinisation (greatly increased progesterone secretion) in serum-supplemented culture, only 69% had cells capable of oxytocin secretion. Secretion followed a similar time course in all cultures, with the peak appearing on day 3. Oxytocin, but not progesterone, output could be consistently increased by addition of pieces of theca interna tissue, or theca conditioned medium, to the cultures. The effect could be achieved by exposure to theca tissue at any time prior to peak output without altering the time course of secretion. Oxytocin could not be detected in follicular fluid from any of the selected follicles, nor in medium from theca cultured alone. We conclude that the potential for oxytocin secretion is a feature of follicular maturation which is lost during atresia and that the stimulus to secretion is associated with luteinisation but not with progesterone output. Finally, the intermixing of follicular cells during corpus luteum formation may provide a mechanism for the enhancement of oxytocin secretion within a predetermined time frame.

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S. A. Way, A. J. Douglas, S. Dye, R. J. Bicknell, G. Leng, and J. A. Russell

ABSTRACT

Pregnant rats were ovariectomized (or shamovariectomized) on days 17, 18 or 21 of pregnancy and oestradiol-17β and progesterone were replaced. Prepartum oxytocin concentrations were significantly lower in ovariectomized steroid-treated rats than in intact controls, and on day 21 of pregnancy injection of relaxin into acutely ovariectomized rats significantly increased plasma oxytocin concentrations. During parturition, injection of the opioid antagonist naloxone induced significant increases in plasma oxytocin concentration compared with salineinjected rats. The naloxone-induced increase was significantly less in ovariectomized steroid-treated rats than in rats with intact ovaries, indicating that endogenous opioid activity is less in ovariectomized rats than in intact rats. The progress of parturition in the ovariectomized steroid-treated rats was severely disrupted compared with sham-ovariectomized rats despite similar plasma oxytocin levels at the birth of pup number 2; this disruption was not overcome by injection of naloxone or by the consequent increase in oxytocin secretion, indicating deficient preparation of the uterus and birth canal in the absence of relaxin. We conclude that the decreased oxytocin concentrations prepartum, the prolongation of parturition and the decrease in opioid tone in ovariectomized steroid-treated rats may be partly due to a lack of relaxin produced by the ovary.

Journal of Endocrinology (1993) 138, 13–22

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G E Mann, J H Payne, and G E Lamming

Abstract

In intact cyclic ewes intrauterine infusion of conceptus secretory proteins results in the suppression of both endometrial oxytocin receptor concentrations and oxytocin-induced prostaglandin F release. However, similar infusion in progesterone-treated ovariectomized ewes, while suppressing endometrial oxytocin receptors, does not fully inhibit oxytocin-induced prostaglandin F release. To examine whether this anomaly resulted from an inadequate simulation of the luteal phase in the ovariectomized ewe treated with progesterone alone, the effects of additional treatment with two other ovarian hormones, oestradiol-17β and oxytocin, was investigated. Rather than permitting conceptus secretory protein to successfully inhibit oxytocin-induced prostaglandin F release, treatment with oestradiol-17β in addition to progesterone actually resulted in an advancement in the timing of release. However, treatment with oxytocin, alone or in combination with oestradiol, permitted the full inhibition of oxytocin-induced prostaglandin F release. To confirm that this effect did not result from the action of oxytocin alone, independently of the action of conceptus secretory protein, a second experiment was undertaken using a similar protocol but without the infusion of conceptus secretory protein. In this situation, oxytocin-induced prostaglandin F release was only partially inhibited suggesting that both luteal oxytocin and conceptus secretory proteins are necessary to facilitate the full inhibition of luteolysis during early pregnancy in the ewe.

Journal of Endocrinology (1996) 150, 473–478

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E L Matthews and V J Ayad

Abstract

The purpose of the present study was to investigate the presence of high-affinity oxytocin-binding sites (putative oxytocin receptors) in the cervix of the non-pregnant ewe. [3H]Oxytocin binding to the peripheral layers of cervical tissue (comprising the serosal layer and the least dense collagenous and muscular layers) and the remaining dense collagenous cervical tissue were studied separately. [3H] Oxytocin-binding sites were detected in membrane fractions prepared from both of these regions, but binding to the peripheral cervix was variable and binding site concentrations were low, hence these were not characterized further. A high-affinity oxytocin-binding site, having a dissociation constant of 1·8 nmol/l, was characterized in the dense collagenous regions of the cervix of ewes killed during the oestrous period. Similar dissociation constants were determined for [Arg8]-vasopressin and the oxytocin-specific agonist [Thr4, Gly7]-oxytocin in competition studies.

[3H] Oxytocin binding to peripheral cervical tissue and to the dense collagenous cervix was generally low or undetectable during the luteal phase, but increased in both tissues around the time of luteolysis. Although specific binding to both tissues was variable during the oestrous period, it was higher at this time than during the luteal phase. [3H] Oxytocin-binding site concentrations were also found to be higher within the inner dense collagenous cervix of oestrous ewes than of pregnant, ovariectomized or anoestrous animals. During the oestrous cycle, oxytocin-binding site concentrations reached a maximum in the dense collagenous cervical tissue on the day of oestrus (141·8 ±44 (s.e.m.) fmol/mg protein), showing a general decline during the following days back to luteal phase values. This compared with concentrations of 513·3 ±132·1 and 216·1 ± 13·9 fmol/mg protein, measured for comparative purposes in endometrial and myometrial membrane preparations, respectively, on the day of oestrus in the same group of ewes. However, in membrane preparations of peripheral cervical tissue higher concentrations were measured on day 14 than on the following 2 days and maximal concentrations were not reached until the day after oestrus (52·7 ± 4·2 fmol/mg protein). Concentrations were maintained at similar values during the subsequent 2 days and significant specific binding was still measurable in both regions of the cervix on day 5.

The localization of oxytocin-binding sites within dense collagenous cervical tissue was investigated autoradiographically using the 125I-labelled specific oxytocin receptor antagonist [1(β-mercapto-β,β-cyclopentamethylene propionic acid), 2-(ortho-methyl)-Tyr2, Thr4, Orn8, Tyr9 -NH2]-vasotocin. The only clear specific labelling was localized to the luminal epithelium of the uterine section of the cervix of oestrous ewes, with labelling in ewes in the luteal phase clearly reduced or absent.

The results demonstrate the presence of a high-affinity oxytocin-binding site within the cervix of the oestrous ewe which is associated with secretory cells and which undergoes similar changes in concentration during the oestrous cycle to uterine oxytocin receptor sites. The significance of this novel putative site of oxytocin action remains to be established.

Journal of Endocrinology (1994) 142, 397–405

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H. Vilhardt, T. Krarup, J. J. Holst, and P. Bie

ABSTRACT

Injections and infusions of oxytocin into conscious dogs caused an increase in plasma concentrations of glucose, insulin and glucagon. When blood glucose was clamped at a raised level the injection of oxytocin still increased insulin and glucagon concentrations in plasma. Infusion of somatostatin suppressed plasma concentrations of glucagon and insulin but did not prevent oxytocin-induced increments in blood glucose. Injection of oxytocin still caused a marked release of glucagon, whereas the insulin response was greatly diminished. When endogenous insulin and glucagon secretion was suppressed by infusion of somatostatin and glucose levels were stabilized by concomitant infusions of glucagon and insulin, injections of oxytocin did not alter blood glucose concentrations. It is concluded that the increase in blood glucose following the administration of oxytocin is secondary to the release of glucagon and that oxytocin exerts a direct stimulatory effect on glucagon and possibly insulin secretion.

J. Endocr. (1986) 108, 293–298