mU Oxytocin Research Articles

The Role of Oxytocin in Primary Cesarean Birth Among Low-Risk Women.

To examine whether there is a threshold of oxytocin exposure at which the risk for primary cesarean increases among women who are nulliparous with a term, singleton, vertex fetus (NTSV) and how oxytocin interacts with other risk factors to contribute to this outcome. This was a secondary analysis of the Consortium on Safe Labor data set that used a retrospective cohort study design. Women who met the criteria for NTSV who were not admitted for a prelabor cesarean and for whom oxytocin data were available, were included in the sample. Robust logistic regression was used to examine the association of oxytocin exposure with primary cesarean birth, while controlling for demographic and clinical risk factors and clustering by provider. The sample comprised 17,331 women who were exposed to oxytocin during labor. The women were predominantly white non-Hispanic (59.2%) with an average (SD) gestational age of 39.4 (1.1) weeks and an 18.5% primary cesarean rate. Exposure to greater than 11,400-milliunits (mU) of oxytocin resulted in 1.6 times increased odds of primary cesarean birth compared with less than 11,400 mU (95% CI 1.01-2.6). Exposure to greater than 11,400 mU of oxytocin in labor was associated with an increased odds of primary cesarean birth in NTSV women.

High-dose versus low-dose oxytocin infusion regimens for induction of labour at term.

When women require induction of labour, oxytocin is the most common agent used, delivered by an intravenous infusion titrated to uterine contraction strength and frequency. There is debate over the optimum dose regimen and how it impacts on maternal and fetal outcomes, particularly induction to birth interval, mode of birth, and rates of hyperstimulation. Current induction of labour regimens include both high- and low-dose regimens and are delivered by either continuous or pulsed infusions, with both linear and non-linear incremental increases in oxytocin dose. Whilst low-dose protocols bring on contractions safely, their potentially slow induction to birth interval may increase the chance of fetal infection and chorioamnionitis. Conversely, high-dose protocols may cause undue uterine hyperstimulation and fetal distress. To determine the effectiveness and safety of high- versus low-dose oxytocin for induction of labour at term We searched the Cochrane Pregnancy and Childbirth Group's Trials Register (31 August 2014) and the reference lists of relevant papers. Randomised controlled trials and quasi-randomised controlled trials that compared oxytocin protocol for induction of labour for women at term, where high-dose oxytocin is at least 100 mU oxytocin in the first 40 minutes, with increments delivering at least 600 mU in the first two hours, compared with low-dose oxytocin, defined as less than 100 mU oxytocin in the first 40 minutes, and increments delivering less than 600 mU total in the first two hours. Two review authors independently assessed study eligibility, extracted data and assessed the risk of bias of included studies. Data were checked for accuracy. We have included nine trials, involving 2391 women and their babies in this review. Trials were at a moderate to high risk of bias overall.Results of primary outcomes revealed no significant differences in rates of vaginal delivery not achieved within 24 hours (risk ratio (RR) 0.94, 95% confidence interval (CI) 0.78 to 1.14, two trials, 1339 women) or caesarean section (RR 0.96, 95% CI 0.81 to 1.14, eight trials, 2023 women). There was no difference in serious maternal morbidity or death (RR 1.24, 95% CI 0.55 to 2.82, one trial, 523 women), and no difference in serious neonatal morbidity or perinatal death (RR 0.84, 95% CI 0.23 to 3.12, one trial, 781 infants). Finally, no trials reported on the number of women who had uterine hyperstimulation with fetal heart rate changes.Results of secondary outcomes revealed no difference between time from induction to delivery (mean difference (MD) -0.90 hours, 95% CI -2.28 to +0.49 hours; five studies), uterine rupture (RR 3.10, 95% CI 0.50 to 19.33; three trials), epidural analgesia (RR 1.03, 95% CI 0.89 to 1.18; two trials), instrumental birth (RR 1.22, 95% CI 0.88 to 1.66; three trials), Apgar less than seven at five minutes (RR 1.25, 95% CI 0.77 to 2.01, five trials), perinatal death (RR 0.84, 95% CI 0.23 to 3.12; two trials), postpartum haemorrhage (RR 1.08, 95% CI 0.87 to 1.34; five trials), or endometritis (RR 1.35, 95% CI 0.53 to 3.43; three trials). Removal of high bias studies reveals a significant reduction of induction to delivery interval (MD -1.94 hours, 95% CI -0.99 to -2.89 hours, 489 women). A significant increase in hyperstimulation without specifying fetal heart rate changes was found in the high-dose group (RR 1.86, 95% CI 1.55 to 2.25).No other secondary outcomes were reported: unchanged/unfavourable cervix after 12 to 24 hours, meconium-stained liquor, neonatal intensive care unit admission, neonatal encephalopathy, disability in childhood, other maternal side-effects (nausea, vomiting, diarrhoea), maternal antibiotic use, maternal satisfaction, neonatal infection and neonatal antibiotic use. The findings of our review do not provide evidence that high-dose oxytocin increases either vaginal delivery within 24 hours or the caesarean section rate. There is no significant decrease in induction to delivery time at meta-analysis but these results may be confounded by poor quality trials. High-dose oxytocin was shown to increase the rate of uterine hyperstimulation but the effects of this are not clear. The conclusions here are specific to the definitions used in this review. Further trials evaluating the effects of high-dose regimens of oxytocin for induction of labour should consider all important maternal and infant outcomes.

Release of oxytocin contributes to the natriuretic action of aminophylline in rats.

Abstract Aminophylline, 0·25 to 2·0 mg intramuscularly, caused diuresis, natriuresis and increase in the Na/K of the urine in the 1 hr period after injection into normal hydrated or unhydrated rats. The urinary changes induced by 2 mg aminophylline and by 8 mU oxytocin equated. Salt-maintained adrenalectomized rats were fully sensitive to the diuretic, natriuretic action of aminophylline. The Na/K in the urine decreased at the 2 mg dose level. Hypophysectomy abolished and neuro-hypophysectomy markedly decreased the diuretic natriuretic action of aminophylline in unanaesthetized rats. Under ethanol-pentobarbitone anaesthesia the diuretic natriuretic action of 0·4 mg aminophylline intravenously lasted 30–40 min in normal rats, and for less than 10 min in neurohypophsectomized rats. The duration of the cardiovascular response to aminophylline was 7–8 min. Thioglycollate-labile oxytocic activity, not detectable in the arterial plasma of control animals, was demonstrable in the arterial plasma of normal rats 8–12 min after 0·5 mg aminophylline intravenously.

Telemetric uterine contraction model in preterm cynomolgus monkeys

A telemetric-based model is presented for evaluation of uterine contractions and preterm labor (PTL) in pregnant cynomolgus monkeys. The model allows continuous monitoring of electromyography (EMG) and intrauterine pressure (IUP) as indicators of uterine activity. A pressure sensor was implanted into the amnion of pregnant monkeys on gestational day (GD) 120±3 and biopotential sensors were attached to the uterus. A telemetry transmitter was placed in a subcuticular pocket located in the flank. Venous catheters were tethered to the next room for dosing and blood sampling without disturbing the conscious animals. EMG and/or IUP were monitored continuously post-operatively. IUP is a reliable parameter for monitoring intrauterine activity, as demonstrated by a close relationship between bursts of activity in the EMG and increases in IUP. Animals close to term showed a basal level of uterine activity during the daytime, with irregular contractions of <10 mmHg. In the night, spontaneous contractions (10–40 mmHg; maximum between 18:00 and 01:00 h) appeared every 3–6 min. Artificial contractions of 15–40 mmHg that mimicked preterm labor were induced at any time of the day by infusion of 5–60 mU oxytocin (OT) per kilogram per hour. These contractions showed a dose-dependent response to OT, and were stable for up to 14 h of constant infusion of OT. Following withdrawal of oxytocin, contractions returned to baseline within 1 h. No desensitization of oxytocin-induced contractions was observed when oxytocin was administered daily for up to several weeks. This telemetric model characterizes uterine contractions in non-human primates and provides an excellent method to evaluate pharmacological characteristics of drug candidates intended to treat PTL.

Connections from the subfornical organ to the oxytocin and vasopressin systems in the lactating rat. A study using electrical stimulations, lesions and electrophysiological recordings

The medial septal area has been implicated in the control of the magnocellular neurosecretory cells of the hypothalamus, and in particular, in the regulation of neurons secreting oxytocin. The present study investigated the hypothesis that this medial septal pathway originates in the subfornical organ. Brief electrical stimulation of the subfornical organ or the medial septum both evoked a transient rise in intramammary pressure equivalent to that caused by an i.v. injection of 1 mU oxytocin. The optimal frequency was 5–20 Hz for 5–10 s. Prolonged stimulation also elicited at its onset a single transient response, similar to that evoked by brief stimulation. Extracellular recordings were made from neurosecretory cells of the supraoptic nucleus identified by antidromic stimulation of the neural stalk and further classified as vasopressinergic and oxytocinergic by their reaction at the time of reflex milk ejection induced by suckling. Single-pulse stimulation of the subfornical organ rarely produced excitation, but short trains of stimuli evoked a large excitation in most oxytocinergic and vasopressinergic neurons. To delineate further the pathway from the subfornical organ to the magnocellular neurons, stimulations were combined with various lesions of the medial forebrain. The effects of stimulation of the subfornical organ were abolished after a section immediately rostral to the organ, and in most cases after lesion of the medial septum. Stimulation of the medial septum no longer had an effect after the subfornical organ had been lesioned a week prior to experiments, a period sufficient to allow degeneration of subfornical efferents. This study shows that the excitatory afferent input to the oxytocin and vasopressin-secreting neurons of the hypothalamus from the medial septal area originates in the subfornical organ. This input is not involved in the main afferent control of the milk ejection reflex since lesions of the subfornical organ and of the medial septum had no effect on the reflex. It is suggested, therefore, that the subfornical input to both oxytocin and vasopressin cells intervenes to facilitate synergistic action of both hormones in non-reproductive functions.

Induction of uterine activity with oxytocin in late pregnant rats replicates the expression of c-fos in neuroendocrine and brain stem neurons as seen during parturition.

We investigated whether delivery could be induced with pulsatile oxytocin and whether such treatment activated neurons in the supraoptic nucleus (SON) and putative afferent neurons in the nucleus tractus solitarii and ventrolateral medulla, as seen during spontaneous parturition. Rats were implanted with a jugular venous cannula 1 day before the expected term and on the next morning were given a pulse of oxytocin or saline every 10 min for 4 h. Pulses of 10 and 20 mU oxytocin induced delivery in 77% (14 of 18) of rats, whereas none of the control animals (0 of 9) gave birth during the treatment. Lower doses of oxytocin were not effective at this time in inducing delivery. Animals were killed either before (prepartum groups) or during (parturient groups) delivery, and the brains were processed for immunocytochemistry. Oxytocin treatment induced Fos expression in SON and brain stem neurons in both parturient rats and rats in which parturition was not induced. Fos expression in all sites was significantly higher than that in control prepartum rats, but was similar in extent and distribution to that in spontaneous parturient rats. In the brain stem, a substantial proportion of Fos-immunoreactive cells contained tyrosine hydroxylase-like immunoreactivity, and the number of these cells was increased in response to oxytocin treatment. As only very few Fos-immunoreactive nuclei in either the SON or the nucleus tractus solitarii were observed in virgin rats injected with oxytocin, we suggest that intermittent oxytocin injections in late pregnant rats induce strong uterine activity, which can stimulate magnocellular and putative afferent neurons even before the expulsion of pups.

Actions of oxytocin and vasopressin on oestrogen-induced electromyographic activity recorded from the uterus and oviduct of anoestrous ewes

Oxytocin and the related peptide [Arg8]vasopressin (AVP) have previously been shown to bind with equally high affinity to oxytocin binding-sites (presumed oxytocin receptors) present within the uterus and oviduct of oestrous ewes. There is a possibility, therefore, that AVP mediates oxytocic actions through these binding sites. For the present study, ewes in seasonal anoestrus were treated with oestradiol-17 beta (50 micrograms subcutaneously, daily for 2-4 days). It was shown initially that this treatment stimulated the development of high-affinity oxytocin binding-sites (Kd 4.4 +/- 0.8 nmol L-1) which had similar affinity for AVP (Kd 4.2 +/- 0.9 nmol L-1) in the myometrium. The efficacy of oxytocin and AVP in vivo were compared by recording electromyographic (EMG) activity from the ampullary-isthmic junction of the left oviduct and the left uterine horn of four conscious ewes. Before oestradiol treatment there was no EMG response to oxytocin even at supraphysiological (1000 mU) doses. During oestradiol treatment, EMG activity was consistently increased in response to injections of 25 mU and 100 mU oxytocin via the jugular vein, but not to saline or 100 mU AVP. Higher doses of AVP were not investigated because of the possibility of cardiovascular side effects. A subsequent blood sampling experiment showed that maximal concentrations of oxytocin and AVP (achieved in peripheral plasma during the first 2 min following injection into the jugular vein) were of a similar order of magnitude after injection of equivalent doses of the two peptides. It is concluded that AVP probably does not mediate biological activity through the oxytocin receptor in non-pregnant ewes.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of inactivation of oxytocin receptor and inhibition of prostaglandin synthesis on uterine oxytocin receptor and gap junction formation and labor in the rat.

Normal term labor is associated with a surge in myometrial oxytocin receptor formation and gap junction development. We have previously shown that inhibition of prostaglandin synthesis by naproxen sodium, 2.0 mg/day, suppressed oxytocin receptor formation but not gap junction formation and prolonged gestation. In this study, we investigated the effects of a specific oxytocin antagonist on oxytocin receptor formation, gap junction formation, and labor in the rat. [Pen1,Phe(Me)2,Thr4,Orn8]oxytocin, a specific oxytocin antagonist, was infused subcutaneously during the last 3 days of pregnancy at 300 micrograms/day. Measurements of myometrial oxytocin receptor concentrations and gap junction formation on days 21 and 22 and days 22-23 (in labor) pregnant uteri showed no significant differences in the Bmax and Kd values between the control and the treated group. Gestation period was not prolonged by the oxytocin antagonist. However, in a separate group of day 23 pregnant rats, the uterine contractile response to 60 mU of oxytocin i.v. was found completely blocked by 10 micrograms of the oxytocin antagonist. These findings suggest that although functional oxytocin receptors did not appear to be essential for the initiation of labor, oxytocin antagonists may still be effective in the prevention of premature contractions. We also examined the effects of a higher dose of naproxen sodium, 5.0 mg/day, on gap junction formation. At this dose, naproxen sodium suppressed both oxytocin receptor and gap junction formation, prolonged gestation, and delayed parturition by 24 h or longer. Prostaglandin appears to be an important regulator or mediator of oxytocin receptor and gap junction formation and plays a critical role in the initiation of labor.

Effect of Oxytocin and Estradiol on Uterine Prostaglandin Release in Nonpregnant and Early-Pregnant Ewes1

The effects of exogenous oxytocin (OT) and estradiol-17 beta (E2) on plasma concentrations of prostaglandin (PG) E2 and 13, 14-dihydro-15-keto-PGF2 alpha (PGFM) were investigated on Day 14-15 (NP) of the estrous cycle and Days 14-16 (PI) and 21-25 (EP) of pregnancy in the ewe. Basal concentrations of PGFM were significantly elevated in utero-ovarian venous (UOV) plasma on Day 14 of pregnancy (4.05 +/- 0.81 nM, mean +/- SEM) compared to that observed on Day 14 of the cycle or Days 21-25 of pregnancy (2.29 +/- 1.3 nM and 1.06 +/- 0.56 nM, respectively). PGFM release increased significantly following intera-arterial bolus injections of 50, 500, and 5000 mU OT at 2-h intervals in all experimental groups. There was no significant difference in area and peak height of the PGFM response between the 3 groups studied. The time to peak PGFM response was, however, significantly longer in the PI group. No significant changes in concentration of PGFM were observed in any experimental group following 1-h infusions of E2 at 5, 50, and 500 pmol/min. Long-term (15-18 h) infusion of E2 at 83 pmol/min increased the peak height of the OT-induced PGFM response at both stages of gestation studied. PGE2 concentrations in UOV plasma were less than 0.05 nM in all samples studied. These results demonstrate that PG release can be induced in response to OT during the period in which ovine trophoblastic protein-1 (oTP-1) is released by the conceptus. During pregnancy, oTP-1 does not appear to inhibit the E2 induction of uterine OT receptors.

Effect of Centrally Administered Oxytocin on the Association Between Cortical Electroencephalogram and Milk Ejection in the Rat

Abstract The milk-ejection reflex of the rat is closely associated with synchronized activity of the cortical electroencephalogram (EEG), and the frequency of milk ejections has been shown to be greatly facilitated by central oxytocin. The following experiments were undertaken to examine the changes in the EEG during facilitation of the reflex by central oxytocin. Intracerebroventricular injection of 1 mU (2.2 ng) oxytocin during suckling caused a rapid increase in the frequency of milk ejections but no change in the predominantly synchronized pattern of the EEG. However, after a delay (11.3 +/- 0.8 min, mean +/- SE) there appeared to be an increasing proportion of desynchronization, which correlated with cessation of the facilitated milk-ejection responses. Hence, the observed EEG desynchronization may signal activation of mechanisms inhibitory to the milk-ejection reflex. In the absence of the suckling stimulus oxytocin also caused a change to desynchronization. However, this effect was more pronounced and commenced after a much shorter latency (1.7 +/- 0.4 min, mean +/- SE; P < 1.001), suggesting that the desynchronizing effect of oxytocin on the EEG can be attenuated by the suckling stimulus. These results demonstrate two phases in the action of central oxytocin in the suckled rat. During the initial phase, the milk-ejection reflex is facilitated and although there may be a concomitant desynchronizing influence on the EEG this is prevented by the influence of the suckling stimulus. In the later phase, this desynchronizing influence predominates and is accompanied by cessation of milk-ejection responses. Although milk ejections were generally restricted to periods of a synchronized EEG as previously reported, during the oxytocin-induced change to desynchronization a number of milk ejections were observed to occur in the absence of a synchronized EEG. These results provide further evidence that the association between milk ejection and the EEG state is not an absolute causal relationship.

Oxytocin release evoked by electrical stimulation of the medial forebrain in the rat: Analysis of stimulus parameters and supraoptic neuronal activity

The role of the medial forebrain area (vertical limb of the diagonal band, medial septum and medial nucleus accumbens) in the control of oxytocin secretion in lactating rats was investigated. Electrical stimulation of the medial forebrain evoked a reproducible rise in intramammary pressure, equivalent to that caused by i.v. injection of 1 mU oxytocin. No presser effect accompanied this response. Radio-immunoassay of plasma samples showed that stimulation caused a significant rise in the concentration of circulating oxytocin. The effects of changing the parameters of stimulation to the medial forebrain were compared with those evoked by stimulation of the neural stalk. The optimal frequency for stimulation of the forebrain was found to be four-fold lower (10–20 Hz) than that for stimulation of the neural stalk (50 Hz). During continuous prolonged stimulation of the forebrain (20 Hz; 2 min) only a single transient response was obtained, whereas a protracted response was obtained as a result of prolonged stimulation of the stalk. Recordings were made from antidromically identified neurosecretory cells in the supraoptic nucleus. Electrophysiological responses to electrical stimulation of the medial forebrain were characterized by two main features. (1) Single-pulse stimulation produced only a small excitation (one or two action potentials), while high-frequency trains produced a profound facilitation of this response, with each pulse evoking short-duration ‘bursting’ behaviour in the supraoptic neurons. (2) During long trains of stimulation this frequency-dependent facilitation declined and could only be renewed after a period of rest. The study indicates that the medial forebrain provides a major excitatory afferent input to the oxytocin-secreting neurons of the supraoptic nucleus, and that the pulse-like secretion of oxytocin following medial forebrain stimulation results from the complex facilitatory and adaptive characteristics of this input.

Characteristics of milk ejection, associated intramammary pressure changes and oxytocin release in the mare.

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 less than 5 min to greater than 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 (greater than 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.

Baboon corpus luteum oxytocin: An intragonadal peptide modulator of luteal function

Oxytocin concentrations were determined in baboon (Papio anubis) corpora lutea, and the effect of oxytocin on dispersed luteal cell progesterone production was evaluated. Oxytocin concentrations increased significantly from an early luteal phase value of 2.1 ± 1.1 ng/gm to a peak concentration of 18.1 ± 4.3 ng/gm wet weight in midluteal phase corpora lutea. Corpora albicantia and ovarian stroma had comparatively low oxytocin concentrations. Reverse-phase high-pressure liquid chromatography of corpora lutea extracts gave a peptide peak (retention time, 17.25 min) similar to a standard oxytocin peak. Plasma oxytocin levels, which were significantly higher in the ovarian vein draining a corpus luteum than in the contralateral side or the femoral vein, declined significantly after luteectomy. Oxytocin was localized by immunocytochemical methods in luteal cells. In the early luteal phase oxytocin (4 to 800 mU; 1 mU is equivalent to 2 ng) inhibited basal and human chorionic gonadotropin-stimulated progesterone production by dispersed luteal cells, but in the late luteal phase 200 to 800 mU oxytocin inhibited only human chorionic gonadotropin-stimulated progesterone output. Oxytocin did not affect luteal cell progesterone production in the midluteal phase. Thus oxytocin is present in corpora lutea, can be localized in the luteal cells, is probably produced locally, and may modulate luteal cell progesterone production.

Hormonal, estrual, ovulatory and milk traits in postpartum dairy cows following multiple daily injections of oxytocin.

Release of oxytocin at suckling or milking may delay onset of estrous cycles in postpartum cows. Twenty lactating Holsteins of mixed parity were given 100 mU oxytocin iv (n = 10) or 2 ml saline (control; n = 10) via jugular catheters at 0530, 0930, 1730 and 2130 daily from calving (d o) until 28 d postpartum. All cows were milked twice daily at 0130 and 1330. Blood was collected thrice weekly (Monday, Wednesday, Friday at 0530) for 12 wk and analyzed by radioimmunoassay for progesterone and 13,14-dihydro-15-keto-prostaglandin F2 alpha (PGFM) in serum. On d 12, blood was collected every 15 min for 6 h via jugular catheters and concentrations of luteinizing hormone (LH), cortisol and PGFM were determined. Rate of involution of the reproductive tract was estimated twice weekly by palpation per rectum. Overall mean, baseline concentrations, number of pulses/6 h, and pulse duration of LH on d 12 were similar among treatment groups. However, oxytocin seemed to reduce (P less than .10) pulse amplitude of LH in multiparous cows (.4 +/- .2 vs .8 +/- .1 ng/ml), but not in primiparous cows. Concentrations of cortisol and PGFM in serum on d 12 were unaffected by treatment. The average intervals from calving to first ovulation, based on changes of progesterone in serum and the intervals to first estrus, were similar between treatment groups. Rates of involution of the cervix and uterus also were similar between treatments. Milk yield, percent protein in milk and somatic cell counts did not differ between treatment groups. However, percent fat in milk tended to be higher (P less than .10) in cows given oxytocin than in controls (3.99 +/- .22 vs 3.68 +/- .21). These data indicate that multiple daily injections of oxytocin did not affect: 1) length of anestrus and anovulation in postpartum dairy cows, 2) LH release and 3) rates of cervical and uterine involution.

Mammary gland growth and milk ejection in the agile wallaby, Macropus agilis, displaying concurrent asynchronous lactation.

Summary. Lactation was initiated and confined to the mammary gland to which the neonate attached at birth, and continued for about 360 days. The lactating gland continued to grow for over 200 days, increasing 7- to 10-fold in size between Days 50 and 250. The young at these times weighed approximately 35 and 2500 g respectively. The young left the pouch permanently at 200–220 days, although they continued to suck intermittently for a further 160 days. Twenty-six days after evacuation of the pouch a second young was born and this attached to one of the three unused nipples. Thereby a second lactation developed in parallel to the previous one, but over 200 days in arrears. The nipples of the non-lactating mammary glands were about 6 mm long and 2 mm in diameter: these increased to >50 mm long and 4 mm in diameter at Day 100 of lactation. Each nipple contained 18–33 separate galactophores. Recordings were made of intramammary pressure in cannulated galactophores of wallabies under barbiturate anaesthesia. Bolus injections of 2–30 mU oxytocin caused increases in intramammary pressure after a latency of 23–60 sec, and pressures peaked at 20–60 mmHg after 50–100 sec. Infusions of oxytocin produced multiple increases in intramammary pressure with peaks at intervals of about 4 min. The threshold dose of oxytocin for the induction of a mammary contraction increased from approximately 2 to 15 mU oxytocin between Days 50 and 250 of lactation. Likewise, there was an increase in the latency to the onset of contraction and a decline in the peak pressure generated between early and late lactation. Injections of small doses of oxytocin induced contraction only of mammary glands in early lactation, whereas larger doses caused the contraction of both 'young' and 'old' glands. The decline in oxytocin sensitivity during lactation permits milk ejection in response to small releases of oxytocin to be confined to the mammary gland to which the neonate is continuously attached. Conversely, the release of a large quantity of oxytocin, in response to the sucking of the juvenile at foot, would cause milk ejection in both lactating mammary glands.

Modification by magnesium and manganese ions of the effects of oxytocin on the electrical and mechanical activity of the longitudinal muscle of estrogen-treated rat uterus.

Ovariectomized rats were given one-shot injections of 10 microgram estradiol-17 beta benzoate, and the longitudinal muscle of the uterus was used for experiment 4 days later. The membrane potential was 58.8 +/- 2.4 mV, and the membrane activity consisted of repetitive spike potentials carried on a plateau potential. Oxytocin (1 mU/ml) caused a depolarization of 7.6 +/- 3.7 mV in a Mg-free Locke solution, a prolongation of burst discharge and increase in the frequency of spontaneous activity. The effects were potentiated by 1-3 mM Mg. Burst discharge was abolished by the application of 0.5 mM Mn, and resumed when 0.1 mU/ml oxytocin was applied. Depolarization up to about - 30 mV and a contracture were caused by 1 mU oxytocin when applied with 1-2 mM Mn. The excitatory effect of oxytocin was depressed by 5 mM Mn. Marked depolarization and contracture were produced by 1 mU oxytocin when the application of Mn was discontinued (rebound phenomenon). The above observations indicate the superficial site of Mg and Mn at low concentration in cooperative action with oxytocin, and an additional site of Mn for the rebound potentiation of contraction and depolarization. It is proposed that oxytocin accelerated Mn influx, and that intracellular Mn participates in electrical and contractile responses due to the application of oxytocin. In this respect, acetylcholine caused a similar effect.

Evidence that thyrotropin-releasing hormone and a hypothalamic prolactin-releasing factor may function in the release of prolactin in the lactating rat.

We have compared the effectiveness of TRH and a rat hypothalamic PRL-releasing factor (PRF; previously incubated with rat serum to destroy TRH) in stimulating the release of PRL into the plasma of conscious lactating rats when injected before and after pituitary PRL had been depleted and transformed into releasable PRL by 10 min of suckling. TRH (1.25 microgramsss) and PRF [equivalent to 2.5 stalk median eminence (SME) fragments] each caused a small increase (38 and 30 ng, respectively) in the plasma PRL concentration within 10 min when injected into nondepleted mothers. The levels then fell quickly. Suckling, by comparison, caused a sustained 175 ng/ml increase above basal levels. Though PRL depletion occurred, as expected, as a result of suckling, there was no measurable depletion within the pituitaries of TRH- or PRF-injected rats. By contrast, the iv administration of TRH (doses ranging from 2-250 ng) and hypothalamic PRF (doses ranging from 0.2-1.0 SME equivalent) after depletion-transformation had been effected by 10 min of suckling resulted in a rapid and, in most instances, a sustained elevation in the plasma PRL concentration comparable to that seen after suckling. Dose-response relationships, though, were not clearly evident with either PRF or TRH. Neither saline, 1.25 microgram TRH previously incubated in serum, 50 mU oxytocin, 1 microgram dopamine, 25 microgram LHRH, nor an extract of cerebral cortex prepared in the same manner as hypothalamic TRH caused plasma PRL to rise after PRL depletion. We conclude that TRH and possibly a separate hypothalamic PRF have a stimulatory action upon the releasable, but not upon the depletion-transformation, phase of PRL secretion in the lactating rat.

Function of myometrial graft at time of parturition in the rabbit.

Thirteen rabbits equipped with ear chambers containing myometrial grafts were studied during late pregnancy, spontaneous and induced parturition and early puerperium. Spontaneous electrical activity of the graft was recorded by means of F.-M. telemetry transmission in the unrestrained animals. Furthermore the reaction to intravenously injected oxytocin (threshold determination) was recorded daily. During the last 5 days before spontaneous delivery increasing spontaneous activity of the tissue was observed. During the same period the response to oxytocin increased. Concomitantly with the delivery of the litter a period of rhythmic activity was observed. This period had a sudden onset and lasted 12-55 min, while the delivery itself only lasted 6-16 min. Similar activity patterns could be evoked in the early post-partum period by 100 mU oxytocin given iv. In 3 rabbits, doses from 80-300 mU oxytocin induced complete delivery. During the 48 hours' period after delivery spontaneous activity (single spikes and series of bursts) was observed. The response to oxytocin was further increased. It is concluded that as the graft is not influenced by nervous regulation, mechanical influence or local humoral factors the spontaneous parturition and the change in excitability must therefore be due to one (or more) systemic, humoral factor(s).

The effect of luteectomy-induced progesterone-withdrawal on the oxytocin and prostaglandin response of the first trimester pregnant human uterus

In 9 normal first trimester pregnant patients strong stimulation with a single i.v. dose of 250 mU oxytocin, or 100 μg PG F2α, only provoked barely detectable and transient contractures, rather than advanced cyclic IUP. Evidently, the normal early pregnant human uterus is not an “inert” muscle organ, but is “refractory” to stimulants. Luteectomy-induced rapid and continued P-withdrawals in 5 of the 9 patients “converted” these “refractory” uteri to “responsive” organs. This “conversion” was revealed by advanced cyclic IUP during the OR and PGR within 24–48 hours after luteectomy. Abortion occurred in 60.6±7.6 hours. “Conversion” was delayed in those 2 patients whose P-withdrawal was slow and who only aborted after 140.5±13.5 hours. The remaining 2 patients, whose luteectomy induced P-withdrawal was moderate and transient showed no “conversion” and failed to abort. The rate and degree of P-withdrawal in these 3 groups of patients was a function of gestational age. Apparently the normal pregnant human uterus is a suppressed organ, refractory to stimulants. Continued P-withdrawal suspends suppression, lowers threshold and thus “converts” the refractory uterus to a reactive organ. Once “converted” the pregnant human uterus becomes spontaneously active under endogenous stimulation and expels its contents, a process which can be hastened by exogenous stimulants.

Interaction between paraventricular nucleus extracts and median eminence extracts on the formation of melanocyte stimulating hormone-release-inhibiting factor

Median eminence extracts (ME) incubated with oxytocin produced the formation of a melanocyte-stimulating hormone-release-inhibiting factor (MSH-R-IF). When extracts of paraventricular nucleus (PVN) were added to that system and incubated together no formation of MSH-R-IF was detected. The effect of PVN extracts is due not to the inactivation of MSH-R-IF, and is proportional to the amount of extract used. PVN extracts were effective in preventing the formation of MSH-R-IF when 150 mU oxytocin were used as substrate but not when 1000 mU were used, suggesting that the mechanism by which the blocking effect is exerted is similar to that of two enzymes competing for the same substrate. It is postulated that MSH-R-IF formation depends of two hypothalamic agents of antagonistic influence acting on oxytocin.