The content of vasopressin, oxytocin and their related neurophysins was measured in the hypothalamus and pituitary gland of mid-trimester human fetuses. Vasopressin was present in both tissues approximately 3–4 weeks before oxytocin. The levels of the hormones in the pituitary gland increased 1000-fold over the next 3–4 months. During this time, the very high vasopressin/oxytocin ratio gradually decreased but did not reach unity in the period studied. In contrast, both the vasopressin-associated neurophysin and the oxytocin-associated neurophysin appeared in the pituitary gland at the same gestational age and showed the same exponential increase with fetal age. Lower levels of the neurophysins and the nonapeptides were found in the hypothalamus and the levels increased more slowly with fetal age. Our results suggest that the high vasopressin/oxytocin ratios observed in fetal life are due to differences in the rate of maturation of the hormone precursor, rather than to differences in the rate of de-novo synthesis.
G. D. Burford and I. C. A. F. Robinson
H. Mayer, D. Schams, H. Worstorff, and A. Prokopp
Milking cows with a 1-min manual stimulation (treatment 1) and without any udder preparation (treatment 2) was compared by application of an improved, highly sensitive radioimmunoassay for oxytocin and recordings of milk-flow curves. Both treatments caused the release of oxytocin, but treatment 2 generally seemed to be less efficient. Milking characteristics supported the advantage of manual stimulation; milk yield and milk flow were significantly higher, while 'machine-on' time was shorter. This clearly indicates the importance of the right timing of release of oxytocin before commencement of milking. Substitution of stimulation by an i.v. injection of 0·5 i.u. oxytocin (treatment 3) resulted in milking parameters very similar to those of treatment 2. This implies that manual stimulation has other effects besides the secretion of oxytocin which are also responsible for optimal milk removal.
J. Endocr. (1984) 103, 355–361
R A D Bathgate and C Sernia
In this study arginine vasopressin (AVP) and oxytocin (OT) receptors have been characterized in the brushtail possum. AVP receptors were characterized using [3H]AVP and the radioiodinated AVP V1a receptor antagonist 125I-labelled [(C6H5-CH2CO)-O-methyl-d-Tyr-Phe-Gln-Asn-Arg-Pro-Arg-Tyr- NH2] while OT receptors were characterized using the radioiodinated OT receptor antagonist 125I-labelled d(CH2)5[Tyr(Me)2,Thr4,Orn8, Tyr-NH2 9]-vasotocin. The receptor affinities and densities have been compared with the rat AVP and OT receptors. Low densities of OT receptors were present in the possum ovary and kidney. High densities of AVP-binding sites were found in the possum adrenal, testis, mesenteric artery, ovary and renal medulla and lower densities in the possum liver. The AVP-binding sites showed marked differences in ligand-binding characteristics from the rat AVP V1a and V2 receptors. Receptor affinities were similar between tissues, except for a distinctly lower value in the renal medulla. It is concluded that the brushtail possum expresses AVP receptors with distinct ligand specificities from those of the rat AVP V1a and V2 receptors.
Journal of Endocrinology (1995) 144, 19–29
H. D. Nicholson, R. T. S. Worley, S. E. F. Guldenaar, and B. T. Pickering
An oxytocin-like peptide is present in the interstitial cells of the testis, and testicular concentrations of oxytocin have been shown to increase seminiferous tubule movements in vitro. We have used the drug ethan-1,2-dimethanesulphonate (EDS), which depletes the Leydig cell population of the adult rat testis, to examine further the relationships between the Leydig cell, testicular oxytocin and tubular movements. Adult rats were injected i.p. with a single dose of EDS (75 mg/kg) or of vehicle (25% dimethyl sulphoxide). Histological study 3 and 10 days after treatment with EDS showed a reduction in the number of interstitial cells, and levels of oxytocin immunoreactivity were undetectable by radioimmunoassay. Immunostaining revealed very few oxytocin-reactive cells. Spontaneous contractile activity of the seminiferous tubules in vitro was also dramatically reduced, but could be restored by the addition of oxytocin to the medium. Four weeks after EDS treatment, the interstitial cells were similar to those in the control animals both in number and in immunostaining; immunoassayable oxytocin was present and tubular movements were normal. The EDS effect, seen at 3 and 10 days, was not altered by daily treatment with testosterone. However, repopulation of the testes with oxytocin-immunoreactive cells was not seen until 6 weeks in the testosterone-treated animals.
We suggest that the Leydig cells are the main source of oxytocin immunoreactivity in the testis and that this oxytocin is involved in modulating seminiferous tubule movements and the resultant sperm transport. The results also imply that testosterone does not play a major role in controlling tubular activity in the mature rat.
J. Endocr. (1987) 112, 311–316
R A D Bathgate and C Sernia
In this study oxytocin (OT) receptors have been characterized and localized in the testis of the rat using the radioiodinated OT receptor antagonist 125I-labelled d(CH2)5 [Tyr(Me)2,Thr4,Tyr9-NH2]-vasotocin (OTA). Receptor density and localization have been compared with the rat testis arginine vasopressin (AVP) receptor using the radioiodinated AVP V1a receptor antagonist 125I-labelled d(CH2)5Sar7-AVP and the radioiodinated linear AVP V1a antagonist 125I-labelled [(C6H5-CH2CO)-O-methyl-d-Tyr-Phe-Gln-Asn-Arg-Pro-Arg-Tyr-NH2]. 125I-labelled OTA bound with high affinity to membrane fractions of the rat testis (K a = 13·8 ± 1·25 litres/nmol), mammary tissue (K a=20·3± 4·36 litres/nmol) and uterus (K a=27·8±0·74 litres/nmol). Competition studies with various OT and AVP receptor agonists and antagonists confirmed that the binding was to OT receptors. AVP receptors in the testis were found to be identical to AVP V1a receptors in the liver. The AVP receptor density in the testis was much higher than the OT receptor density (109 ±12·3 vs 5·2 ±0·79 (mean ± s.e.m.) fmol/mg protein). Autoradiographical localization showed that both OT and AVP receptors were present in the interstitial spaces in the testis consistent with binding to Leydig cells. AVP receptors were also localized on the epithelial surfaces of the seminiferous tubules and on testicular blood vessels. This study has, for the first time, found OT receptors in the testis of the rat which have similar ligand-binding characteristics to mammary and uterine OT receptors. The receptor localizations are consistent with binding to Leydig cells.
Journal of Endocrinology (1994) 141, 343–352
K. M. Burgess, G. Jenkin, M. M. Ralph, and G. D. Thorburn
The effect of RU486, a synthetic progesterone receptor antagonist, on basal uterine prostaglandin (PG) release and release in response to oxytocin injection has been investigated in late-pregnant sheep (days 135–140 of gestation). Fifteen hours after i.m. injection of RU486 (50 mg; n = 5) or vehicle alone (n = 4), bolus injections of oxytocin (50, 500 and 5000 mU) were administered via a uterine artery ipsilateral to the pregnant uterine horn at 2-hourly intervals. Uteroovarian vein concentrations of 13,14-dihydro-15-keto PGF2α (PGFM) and PGE2 were determined before and during oxytocin stimulation. Basal concentrations of both PGFM and PGE2 were significantly (P < 0·001) increased in ewes 15 h after RU486 administration compared with ewes receiving vehicle alone. Concentrations of PGFM, but not PGE2, increased significantly (P < 0·001) following injection of each dose of oxytocin in both treated and untreated animals. The response to oxytocin, measured both as the area under the curve and as the peak height of PGFM release, was significantly (P <0·05) greater in RU486-treated ewes. There was no significant effect of oxytocin on the area or peak height of PGE2 response in either RU486-treated or control animals. These results demonstrate that treatment of late-pregnant ewes with RU486 results in an increase in basal uterine PGFM and PGE2 as well as oxytocin-stimulated PGFM release.
Journal of Endocrinology (1992) 134, 353–360
S. A. Way and G. Leng
In urethane-anaesthetized ovariectomized rats, injection of porcine relaxin (7·5 and 15 μg/kg, i.v.) caused a sustained increase in circulating plasma oxytocin and vasopressin concentrations; 10 μg relaxin/rat i.v. produced a smaller but significant increase in plasma oxytocin concentration in conscious ovariectomized rats. A significant increase in oxytocin concentration and inhibition of the spontaneous milk-ejection reflex was also seen in anaesthetized (ovary intact) lactating rats following injection of relaxin (7·5 μg/kg, i.v.). To investigate whether relaxin acts by increasing the electrical activity of oxytocin neurones or by facilitating stimulus-secretion coupling in the pituitary, the electrical activity of neurones in the supraoptic nucleus was recorded in urethane-anaesthetized lactating rats and in ovariectomized rats. Porcine relaxin (10 μg/rat, i.v.) increased the firing rate of both oxytocin and vasopressin neurones in the supraoptic nucleus in lactating rats. The response to relaxin was unaffected by subsequent injection of naloxone (1 mg/kg, i.v.). Oxytocin neurones were also activated by injection of relaxin (10 μg/rat) into ovariectomized rats. Combining the electrophysiological data, the neuronal activation following relaxin was significantly correlated with the level of spontaneous activity prior to relaxin injection. The results show that relaxin acts centrally to increase circulating plasma oxytocin and vasopressin concentrations by an opioid-independent mechanism.
Journal of Endocrinology (1992) 132, 149–158
S. J. Downing and M. Hollingsworth
The influence of treatment with oestradiol on the effects of the uterine relaxants, relaxin, salbutamol (an agonist at β2-adrenoceptors) and cromakalim (a potassium channel opener) and their interactions with the uterine stimulant oxytocin were investigated in vivo in the ovariectomized rat. Oestradiol benzoate (0·4 μg/kg per day) significantly increased sensitivity to cromakalim as an inhibitor of spontaneous uterine contractions compared with vehicle-treated rats by approximately threefold. The same dose of oestradiol benzoate had no effect on uterine sensitivity to salbutamol. Previous studies have shown that this dose of oestradiol benzoate produces a twofold increase in uterine sensitivity to relaxin as an inhibitor of spontaneous contractions. Oestradiol influenced the ability of relaxin to inhibit oxytocin-stimulated uterine contractions. In corn oil-treated rats, uterine responses to relaxin were markedly reduced during oxytocin infusion compared with responses to relaxin before oxytocin; the maximum obtainable response to relaxin was less than 50% inhibition. In oestradiol-treated rats, uterine sensitivity to relaxin during oxytocin infusion was similar to that observed against spontaneous contractions. Cromakalim was able to inhibit uterine contractions during oxytocin infusion in both corn oil- and oestradiol-treated rats, uterine sensitivity to cromakalim being similar in the absence and presence of oxytocin for both hormone treatment groups. Salbutamol was also able to inhibit uterine contractions during oxytocin infusion in both corn oil- and oestradiol-treated rats. Oestradiol treatment increased the potency of salbutamol as an inhibitor of oxytocin-stimulated uterine contractions compared with corn oil treatment by 3·5-fold. The interaction of oestradiol and relaxin during late pregnancy may be important for attenuation of the myometrial response to stimulants.
Journal of Endocrinology (1992) 135, 29–36
S Mukaddam-Daher, M Jankowski, D Wang, A Menaouar, and J Gutkowska
We have recently uncovered the presence of an oxytocin system in the heart and found that oxytocin is a physiological regulator of atrial natriuretic peptide (ANP), a diuretic, natriuretic and vasodilator cardiac hormone. However, dynamic changes in these systems during gestation, when mechanisms of volume and pressure homeostasis are altered, are not clear. Accordingly, ANP, oxytocin and oxytocin receptors were evaluated in rat hearts and plasma at three stages of gestation (7, 14 and 21 days) and at 2 and 5 days postpartum. Compared with non-pregnant controls, plasma ANP was elevated in mid-gestation, but significantly decreased at term (21 days), to increase again postpartum. Right and left atrial ANP mRNA levels were not altered throughout gestation but increased by 1.5- to 2-fold postpartum (P<0.01). At term, ANP content in right (8.7+/-1.2 vs 12.7+/-1.1 micro g/mg protein, P<0.04) and left (3.5+/-0.6 vs 8.5+/-2.0 micro g/mg protein, P<0.01) atria increased. These findings imply that decreased plasma ANP at term results from inhibition of release rather than decreased synthesis. In parallel, oxytocin, a stimulator of ANP release, decreased in left atria at day 7 to 50% of non-pregnant levels and remained low throughout gestation. Oxytocin receptor mRNA increased in left atria at 7 and 14 days of gestation by 2- and 5-fold respectively, but decreased at 21 days to lower than non-pregnant levels to increase again (3-fold) postpartum. The changes in oxytocin receptor expression at term and postpartum paralleled oxytocin receptor protein determined by Western blot. These results imply that pregnancy is associated with dynamic changes in the cardiac oxytocin system (peptide and/or receptors), which may influence natriuretic peptide release. Together, these peptides would act on their receptors in the heart, vasculature and kidneys to maintain vascular tone and renal function throughout gestation and postpartum.
J. S. Tindal and L. A. Blake
Transection of the ventral central grey and surrounding midbrain tegmentum in anaesthetized lactating rabbits caused repeated milk ejections which, on comparison with the effects of i.v. infusions of synthetic oxytocin, synthetic arginine-vasopressin or a mixture of the two, were attributed to continuous release of 1·25–2·5 mu. oxytocin/min, although it is not known whether lesser amounts of vasopressin might also have been released. It is suggested that the ascending midbrain reticular formation, which is known to project rostrally through this region, controls the central inhibition of oxytocin release via the previously described septo-hippocampo-subicular route to the hypothalamus.
J. Endocr. (1986) 109, 405–409