Oxytocin levels during breast-feeding in established lactation

Serial blood samples were taken from eight lactating women while they were nursing their babies 1–4 months postpartum, and from four lactating controls while they were not nursing. The plasma was assayed for oxytocin by radioimmunoassay after extraction with activated Vycor glass powder. In the suckling mothers mean plasma oxytocin rose from 5.4 pg/ml before nursing to 13.0 pg/ml during nursing. Oxytocin levels changed rapidly from minute to minute, with individual peaks as high as 54 pg/ml. Oxytocin levels in the control mothers averaged 4.4 pg/ml.     

Oxytocin response to conditioned and nonconditioned stimuli in lactating ewes

We have measured oxytocin release during lactation in the ewe in response to normal tactile sucking stimuli as well as exteroceptive stimuli emanating from the lamb. Four puerperal ewes that had indwelling catheter inserted in the femoral artery while still pregnant were used. Each nursed a single lamb. Each was studied 2 or 3 times between Days 1 and 15 of lactation during a 2.5-h experimental period that was preceded by a 2-h separation from the lamb in the early morning. Samples were taken before and after the lamb was brought within sight and sound of the ewe but without contact, and then 0.5, 1, 5, 10, 15, 30, and 60 min after suckling began. When another suckling episode intervened, the same sampling schedule was immediately restarted. Suckling occurred in an intermittent fashion; 3 to 4 episodes of 2.9 +/- 2.0 (SD)-min duration took place with variable intervals during the 2.5-h experimental period. Exteroceptive stimuli emanating from the lamb caused plasma oxytocin to rise significantly from basal levels of 10.0 +/- 4.5 to 21.8 +/- 5.7 pg/ml (mean +/- SE, n 10, p less than 0.05). This rise was not seen on Day 1 and in only half of the ewes on Day 2, but thereafter the rise occurred in every instance. A further rise in plasma oxytocin was observed in almost all instances (86%) at suckling. Peak levels were usually observed within 1 min. They were quite variable, ranging from 15 pg/ml to 287 pg/ml, and not related to the milk yield, but were significantly greater than spontaneous pulses observed in nonlactating puerperal sheep.(ABSTRACT TRUNCATED AT 250 WORDS)     

Plasma oxytocin concentrations during late pregnancy and parturition in the dog

While oxytocin is widely used in the treatment of dystocia in dogs, there is little information about its secretion before and during normal unassisted whelping. We therefore measured plasma oxytocin concentrations during late pregnancy and the expulsive stage of parturition. Blood samples were collected from eight dogs at 3-min intervals during a 42-min period between the 2nd and 14th day before whelping and during parturition after the birth of 1-3 pups. The litters consisted of 5-15 pups and the progression of the expulsive stage was linear and nearly parallel in the eight bitches. The overall mean (+/-S.D.) plasma oxytocin concentration during late pregnancy was 3.6+/-2.1pg/ml. Mean values in individual dogs ranged from 1.2 to 7.4 pg/ml, but the intra-animal variation was rather small. During the expulsive stage the overall mean (+/-S.D.) plasma oxytocin concentration was 12.9+/-13.9 pg/ml, with mean values in individual dogs ranging from 3.5 to 46 pg/ml. The mean area under the oxytocin curve for parturient dogs was significantly higher (P<0.05) than for pregnant dogs. During the expulsive stage, the peak plasma oxytocin level in individual dogs ranged between 10 and 117 pg/ml. In six of the eight dogs a pup was born during blood collection and in five of these animals the plasma oxytocin concentration increased temporarily during periods of abdominal straining and expulsion. However, straining efforts and expulsion were not consistently associated with a rise in the circulating oxytocin level. It is concluded that in the dog plasma oxytocin levels are higher and more variable during the expulsive stage of parturition than during late pregnancy. Interrelationships between the secretion pattern of oxytocin, the level of uterine contractility, and the progress of fetal expulsion in dogs need further exploration.     

Ghrelin is present in human colostrum, transitional and mature milk

Ghrelin and its mRNA have recently been found in numerous human tissues including breast. The aim of this study was to compare the ghrelin levels in colostrum, mature and transitional milk and plasma in lactating women with plasma samples from non-lactating women. Venous blood samples were obtained from 17 healthy lactating women aged 22-35 years and from 16 age-matched controls. Colostrum, transitional and mature milk samples were collected just before suckling. The level of bioactive ghrelin was determined by RIA. Comparison of ghrelin values for lactating women showed significantly lower concentrations in colostrum (70.3 +/- 18 pg/ml), transitional milk (83.8 +/- 18pg/ml) and mature milk (97.3 +/- 13 pg/ml) than in the corresponding plasma samples (first day 95 +/- 16 pg/ml, 10th day 111 +/- 13 pg/ml and 15th day 135 +/- 16 pg/ml). The plasma concentrations were lower in the lactating than in the non-lactating women. Thus, the ghrelin levels in colostrum, transitional and mature milk were elavated concomitantly with increasing plasma ghrelin after delivery. The origin of milk ghrelin is not known, but it probably comes from the plasma.     

Effect of the environment on the physiology of the sow during late pregnancy, farrowing and early lactation

We investigated the effect of two types of housing on the duration of farrowing and the physiology of sows before and after farrowing. We assigned 20 sows (PEN) to farrowing pens (210 cm x 335 cm) enriched with straw bedding and 18 sows (CRATE) were placed in farrowing crates (80 cm x 210 cm) with no bedding material. We sampled the animals during period A (from day -5 before farrowing to day +1 of lactation) and during period B (from days +2 to +5 of lactation). We took daily venous blood samples for progesterone measurements and four salivary samples per day (at 09:00, 11:00, 13:00 and 15:00) for cortisol determination. In addition, intensive blood sampling was performed in 18 catheterised sows (9 PEN and 9 CRATE) to determine the level of oxytocin during farrowing. The treatment had no effect on litter size, piglet mortality at birth and weaning, growth of the piglets between days 1 and 5 of life. We found no significant difference in the cortisol concentration between the two groups during period A (p=0.36). Significant difference was found in period B, where the CRATE group had a higher concentration of cortisol than the PEN group (p=0.03). Progesterone concentration did not differ between the two groups (p=0.80). The duration of farrowing was on average 93 min longer in the CRATE sows (n=15), with a mean of 311+/-35 min (mean+/-S.D.), than in the PEN sows (n=19), with a mean of 218+/-24 min (p=0.03). In addition, the mean interval between each piglet expulsion was longer in the CRATE group (n=11), with a mean of 25+/-4 min, than in the PEN group (n=15) with a mean of 16+/-2 min (p=0.05). During farrowing, the post-expulsion oxytocin pulses average tended to be higher in the PEN group (77.6+/-47.6 ng/ml) than in the CRATE group (38.1+/-24.6 pg/ml) (p=0.08). The concentration of oxytocin strongly affected the duration of farrowing (p<0.001). In conclusion, this study showed that the environment influences the physiology of the sow during farrowing and early lactation.     

A combined radioimmunoassay and immunocytochemical study of ovarian oxytocin production during the periovulatory period in the ewe

Corpora lutea and follicles were taken from the ovaries of 12 ewes at intervals from the start of luteolysis until 3 days after ovulation. RIA analysis of the tissue oxytocin content showed that luteal oxytocin concentrations declined during luteolysis to reach basal values at about the time of the next ovulation. Oxytocin was first measurable in the walls of 3 out of 6 preovulatory follicles during the LH surge, with a small increase in concentration to 26.1 +/- 6.6 pg/mg before ovulation, and a further increase in the young corpus luteum to concentrations exceeding 1 ng/mg 2-3 days later. After the LH surge, oxytocin was also found in the follicular fluid at a concentration of 3.4 +/- 0.3 ng/ml. Using immunocytochemical techniques, oxytocin and neurophysin were first detected in the follicle wall immediately before ovulation, and were localized in the granulosa cells. After ovulation the stained cells initially formed strands which appeared to break down to clusters and then to individual cells as the corpus luteum matured. The immunocytochemical picture also suggested that neurophysin immunoreactivity increased within a few hours of ovulation but that processing to oxytocin may be delayed. Measurements of circulating oxytocin concentrations revealed a pulsatile release pattern throughout the follicular phase with the height of the pulses decreasing from 25 +/- 5 pg/ml during luteolysis to a minimum of 11 +/- 2 pg/ml during the LH surge.     

Pulsatile release of oxytocin into the circulation of the ewe during oestrus, mating and the early luteal phase

Two experiments were designed to investigate release patterns of oxytocin into plasma during oestrus and the early luteal phase. In Exp. 1, blood samples were collected from 5 ewes every 30 min for 10 h during 6 days around oestrus and the early luteal phase. During oestrus concentrations of oxytocin were generally low (1.27 +/- 0.54 pg/ml; mean +/- s.d.) but with occasional pulses up to 6 pg/ml. By Day 5 mean basal concentrations had risen to 4.5 +/- 2.1 pg/ml with a fluctuating release pattern. In Exp. 2, a method was developed for continuous blood sampling from conscious, unrestrained ewes. On the predicted day of oestrus following an untreated oestrous cycle, 8-ml blood samples were collected every minute for two 35-min periods (8 ewes: 16 sampling periods). For 6 ewes a ram was introduced to the pen for part of this time, and resulting behaviour was recorded. Additional blood samples were assayed for LH and progesterone to determine the stage of the cycle. Overall mean oxytocin concentrations ranged from 1.5 +/- 0.53 to 6.8 +/- 5.25 pg/ml in different animals. Ewes which were both in oestrus and exposed to the ram showed a pulsatile oxytocin release pattern consisting of low baseline concentrations with short-duration pulses superimposed (duration 1-4 min; amplitude 2.5-31.7 pg/ml; frequency 3.18/h). Coitus was not temporally associated with pulsatile release. However, the importance of the presence of the ram was indicated by total separation of 2 oestrous ewes from the ram until after experimentation. In these animals only 1 pulse of oxytocin was detected in 2.7 h of sampling. It is concluded that, although mean oxytocin concentrations at oestrus were low, short duration pulses were released into the plasma at this time. This effect may be dependent on the presence of a ram.     

Oxytocin infusion from day 10 after oestrus extends the luteal phase in non-pregnant cattle

Oestrus was synchronized in 8 cyclic heifers by progesterone treatment (PRID), after which the animals were monitored for one control cycle to measure the inter-oestrous interval. Osmotic minipumps containing saline (controls, N = 3) or oxytocin (N = 5) were implanted subcutaneously on Day 10 of the second cycle, and removed 12 days later. Jugular venous blood samples were collected daily for measurement of progesterone, and every 2 days for oxytocin. In addition, blood samples were taken every 10 min from 1 h before to 3 h after minipump insertion for measurement of plasma 15-keto-13,14-dihydroprostaglandin-F-2 alpha (PGFM) and every 30 min over the same period for measurement of progesterone and oxytocin. The lengths of the first untreated cycle in both groups of heifers were 20.2 +/- 0.56 (mean +/- s.e.m.) days compared with 25.4 +/- 0.81 days after oxytocin treatment (P less than 0.001). Oxytocin plasma concentrations in treated animals rose from less than 10 pg/ml to 70-500 pg/ml by 2 h after the start of oxytocin infusion and remained elevated until treatment was withdrawn. There was no increase in PGFM concentrations immediately after minipump insertion. Plasma progesterone concentrations were similar in treated and control animals but remained at mid-luteal levels for an average of 5 days longer in treated heifers. It is concluded that continuous administration of oxytocin can extend the luteal life-span in cattle.     

Interactions between oxytocin, glucagon and glucose in normal and streptozotocin-induced diabetic rats

Oxytocin has been suggested to have glucoregulatory functions in rats, man and other mammals. The hyperglycemic actions of oxytocin are believed to be mediated indirectly through changes in pancreatic function. The present study examined the interaction between glucose and oxytocin in normal and streptozotocin (STZ)-induced diabetic rats, under basal conditions and after injections of oxytocin. Plasma glucose and endogenous oxytocin levels were significantly correlated in cannulated lactating rats (r = 0.44, P less than 0.01). To test the hypothesis that oxytocin was acting to elevate plasma glucose, adult male rats were injected with 10 micrograms/kg oxytocin and killed 60 min later. Oxytocin increased plasma glucose from 6.1 +/- 0.1 to 6.8 +/- 0.2 mM (P less than 0.05), and glucagon from 179 +/- 12 to 259 +/- 32 pg/ml (P less than 0.01, n = 18). There was no significant effect of oxytocin on plasma insulin, although the levels were increased by 30%. A lower dose (1 microgram/kg) of oxytocin had no significant effect on plasma glucose or glucagon. To eliminate putative local inhibitory effects of insulin on glucagon secretion, male rats were made diabetic by i.p. injection of 100 mg/kg STZ, which increased glucose to greater than 18 mM and glucagon to 249 +/- 25 pg/ml (P less than 0.05). In these rats, 10 micrograms/kg oxytocin failed to further increase plasma glucose, but caused a much greater increase in glucagon (to 828 +/- 248 pg/ml) and also increased plasma ACTH. A specific oxytocin analog, Thr4,Gly7-oxytocin, mimicked the effect of oxytocin on glucagon secretion in diabetic rats. The lower dose of oxytocin also increased glucagon levels (to 1300 +/- 250 pg/ml), but the effect was not significant. A 3 h i.v. infusion of 1 nmol/kg per h oxytocin in conscious male rats significantly increased glucagon levels by 30 min in normal and STZ-rats; levels returned to baseline by 30 min after stopping the infusion. Plasma glucose increased in the normal, but not STZ-rats. The relative magnitude of the increase in glucagon was identical for normal and diabetic rats, but the absolute levels of glucagon during the infusion were twice as high in the diabetics. To test whether hypoglycemia could elevate plasma levels of oxytocin, male rats were injected i.p. with insulin and killed from 15-180 min later. Plasma glucose levels dropped to less than 2.5 mM by 15 min. Oxytocin levels increased by 150-200% at 30 min; however, the effect was not statistically significant.(ABSTRACT TRUNCATED AT 400 WORDS)     

Extension of oestrous cycles and prolonged secretion of progesterone in non-pregnant cattle infused continuously with oxytocin

The experimental objective was to evaluate how continuous infusion of oxytocin during the anticipated period of luteolysis in cattle would influence secretion of progesterone, oestradiol and 13,14-dihydro-15-keto-prostaglandin F-2 alpha (PGFM). In Exp. I, 6 non-lactating Holstein cows were infused with saline or oxytocin (20 IU/h, i.v.) from Day 13 to Day 20 of an oestrous cycle in a cross-over experimental design (Day 0 = oestrus). During saline cycles, concentrations of progesterone decreased from 11.0 +/- 2.0 ng/ml on Day 14 to 2.0 +/- 1.3 ng/ml on Day 23; however, during oxytocin cycles, luteolysis was delayed and progesterone secretion remained near 11 ng/ml until after Day 22 (P less than 0.05). Interoestrous interval was 1.6 days longer in oxytocin than in saline cycles (P = 0.07). Baseline PGFM and amplitude and frequency of PGFM peaks in blood samples collected hourly on Day 18 did not differ between saline and oxytocin cycles. In Exp. II, 7 non-lactating Holstein cows were infused with saline or oxytocin from Day 13 to Day 25 after oestrus in a cross-over experimental design. Secretion of progesterone decreased from 6.8 +/- 0.7 ng/ml on Day 16 to less than 2 ng/ml on Day 22 of saline cycles; however, during oxytocin cycles, luteolysis did not occur until after Day 25 (P less than 0.05). Interoestrous interval was 5.9 days longer for oxytocin than for saline cycles (P less than 0.05). In blood samples taken every 2 h from Day 17 to Day 23, PGFM peak amplitude was higher (P less than 0.05) in saline (142.1 +/- 25.1 pg/ml) than in oxytocin cycles (109.8 +/- 15.2 pg/ml). Nevertheless, pulsatile secretion of PGFM was detected during 6 of 7 oxytocin cycles. In both experiments, the anticipated rise in serum oestradiol concentrations before oestrus, around Days 18-20, was observed during saline cycles, but during oxytocin cycles, concentrations of oestradiol remained at basal levels until after oxytocin infusion was discontinued. We concluded that continuous infusion of oxytocin caused extended oestrous cycles, prolonged the secretion of progesterone, and reduced the amplitude of PGFM pulses. Moreover, when oxytocin was infused, pulsatile secretion of PGFM was not abolished, but oestrogen secretion did not increase until oxytocin infusion stopped.     

Effect of oxytocin infusion on secretion of progesterone and luteinizing hormone and the concentration of uterine oxytocin receptors during the periovulatory period in cloprostenol-treated ewes.

Oxytocin infusions were initiated on day 10 of the oestrous cycle in ewes, and luteal regression was induced by injection of 100 micrograms cloprostenol on day 12. Blood samples were collected at frequent intervals via an indwelling jugular vein cannula to measure concentrations of progesterone and luteinizing hormone (LH) during the luteal and follicular phases in saline (n = 6) and oxytocin (n = 5) infused animals. The oxytocin infusion maintained peripheral plasma concentrations of 53 +/- 3.2 pg oxytocin ml-1 (mean +/- SEM) compared with values of about 1 pg ml-1 during oestrus in control ewes. Oxytocin infusion had no effect on luteal phase progesterone concentrations, the timing of luteolysis, basal luteinizing hormone (LH) secretion, LH pulse frequency, or the timing or height of the LH surge. Treated ewes came into oestrus significantly earlier than controls (P < 0.05) but ovulated normally. Uterine samples collected 96 h after cloprostenol injection (approximately day 2 of the cycle) showed that oxytocin receptor concentrations were significantly higher in the endometrium in ewes that had been given a 5 day oxytocin infusion than in control animals (556 and 262 fmol mg-1 protein, respectively: geometric means from ANOVA, P < 0.001), whereas myometrial receptor concentrations were not affected (113 and 162 fmol mg-1 protein, respectively). We conclude that the previously reported delay in luteal development caused by oxytocin infusion (Wathes et al., 1991) is not due to the inhibition or delay of ovulation, but must instead occur via a direct influence on the developing corpus luteum.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of oestradiol — 17β and oxytocin treatment on prostaglandin F alpha release in the anoestrous ewe

The concentration of prostaglandin Fα in the vena caval blood at a point anterior to the entry of uterine venous blood was determined by radioimmunoassay in nine anoestrous ewes. At all times the concentrations were much greater than those in jugular blood. The ewes were given oestradiol-17β or oxytocin or oestradiol followed 24 hours later by oxytocin.Oxytocin alone had negligible effect on prostaglandin release. Oestradiol alone increased the concentration of prostaglandin in the posterior vena cava about threefold to 160 pg/ml. When oxytocin was given to oestrogen primed ewes the concentration in the posterior vena cava rose dramatically to 1000 pg/ml within 5 minutes after which it declined to pre-treatment values: repetition of oxytocin treatment in one hour was much less effective.     

Oestrogenic effects of ICI 182,780, a putative anti-oestrogen,on the secretion of oxytocin and prostaglandin F2α during oestrous cycle in the intact ewe

The effect of ICI 182,780, oestrogen antagonist, on the concentrations of oxytocin and uterine PGF_2α was investigated in intact Border Leicester Merino cross ewes during the late oestrous cycle. Twelve cyclic ewes (n=6 per group) were randomly assigned to receive, at 6 h intervals, intra-muscular injection of either peanut oil or ICI 182,780 (1.5 mg kg⁻¹ day⁻¹) in oil for 2 days, starting at 1900 h on day 13 until 1300 h on day 15 post-oestrus. Hourly blood samples were collected via a jugular catheter from 0800 h on day 14 for 37 h and then daily over days 16, 17 and 18 post-oestrus. Peripheral plasma concentrations of oxytocin, the metabolite of prostaglandin F_2α, 15-keto-13,14-dihydro-prostaglandin F_2α, (PGFM) and progesterone were measured by radioimmunoassay. All ewes treated with ICI 182,780 exhibited functional luteal regression as indicated by a marked reduction in plasma progesterone concentrations to less than 1000 pg/ml over the period of 18–36 h during sampling period on days 14 and 15 of the oestrous cycle. In five of six vehicle-treated ewes, progesterone concentrations declined between day 16 and day 18 post-oestrus. In the remaining control ewe, progesterone concentrations reach less than 1000 pg/ml within 36 h of the commencement of the sampling period. During the frequent sampling period, the number of oxytocin pulses in the ICI 182,780 treated ewes was significantly higher compared to control ewes (2.7±0.3 vs. 0.8±0.3). The mean amplitude of oxytocin pulses observed was also greater (70.4±19.5 pg/ml) in ewes treated with ICI 182,780, but was not significantly different from control ewes (33.5±12.9 pg/ml). Oxytocin pulses may however have occurred following the initial two ICI 182,780 injections but before commencing blood sampling. The oxytocin pulses were detected at a mean of 3.2±0.2 h following each injection with ICI 182,780 during blood sampling. In the ICI 182,780-treated ewes, the pulsatile pattern of plasma PGFM in jugular blood samples over the 37 h sampling period on days 14 and 15 post-oestrus had a higher amplitude (512.9±158.9 vs. 121.7±78.7 pg/ml) and pulse area (618.1±183.3 vs. 151.5±102.9 (pg/ml)τ) compared to the vehicle-treated ewes (P<0.05) respectively. The average number of PGFM pulses observed per ewe was 3.0±0.7 in the ICI 182,780-treated group and was significantly (P<0.02) higher than the number of pulses (0.5±0.3) observed in ewes treated with vehicle alone. The PGFM pulses were detected at 4.2±0.6 h following each injection with ICI 182,780 during blood sampling. The percentage of PGFM pulses that occurred coincidently with a significant elevation of oxytocin concentrations was 44.4% in ICI 182,780-treated compared to 66.6% in control ewes. We conclude that administration of oestrogen antagonist ICI 182,780 accelerated development of the luteolytic mechanism by enhancing pulsatile secretion of oxytocin and PGFM which suggests that ICI 182,780 acts as an agonist for oxytocin and prostaglandin F_2α release in intact ewes when administered at 1.5 mg/kg/day over Day 13 to 15 post-oestrus.     

Oxytocin and bovine parturition: a steep rise in endometrial oxytocin receptors precedes onset of labor.

Oxytocin receptors were measured in myometrium and intercaruncular endometrium of cows during pregnancy and parturition. Concentrations of estradiol-17 beta, estrone, and progesterone in peripheral blood were also measured. Receptor concentrations in the endometrium rose almost 200-fold from Day 20 to term (p < 0.0001, ANOVA), from 40 +/- 11 to 7300 +/- 1430 fmol/mg protein. Myometrial receptor concentrations increased 10-fold from 180 +/- 36 fmol/mg on Day 20 to 1850 +/- 360 fmol/mg protein at term (p < 0.0001, ANOVA). During labor, endometrial receptors (6600 +/- 1300 fmol/mg) remained at prelabor values, whereas myometrial receptor concentrations had decreased to 1190 +/- 316 fmol/mg (not significant) and declined further postpartum. Plasma concentrations of progesterone declined from 4-5 ng/ml to about 2 ng/ml between Days 250 and 282 and dropped to < 0.2 ng/ml shortly before delivery. Plasma concentrations of estrone and estradiol-17 beta were below 10-20 pg/ml until Day 230. Estrone concentrations were significantly (p < 0.05) increased by Day 250 and estradiol-17 beta by Day 270, and then both rose rapidly. During labor, plasma estrone was 1135 +/- 245 pg/ml and plasma estradiol-17 beta was 226 +/- 131 pg/ml. The molar ratio of estrone and estradiol-17 beta to progesterone rose from less than 0.01 to 4.4 during labor, and was correlated with oxytocin receptor concentrations in endometrium (r = 0.5160, p < 0.001), but not those in myometrium (r = 0.0122). The regulation of oxytocin receptors by ovarian hormones in the two tissues may therefore differ.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of intra-ovarian infusion of oxytocin on plasma progesterone concentrations in pregnant ewes.

The function of oxytocin receptors in the corpus luteum of pregnant ewes was investigated by infusing saline or oxytocin (100 ng/min) into the utero-ovarian artery of pregnant ewes (62 +/- 5 days, n = 12). During a 4-h infusion, plasma oxytocin (OT) concentration increased to 268 +/- 80 pg OT/ml in the OT-infused group and remained unchanged at 2.5 +/- 1.5 pg OT/ml in the saline-infused group. Progesterone concentration in jugular venous plasma (17 +/- 9 ng/ml) rapidly decreased during oxytocin infusion to 59 +/- 10% and 26 +/- 9% of control at 1.5 and 2 h, respectively; the utero-ovarian venous concentration of 64 +/- 38 ng/ml decreased by a similar magnitude during oxytocin infusion. Electron microscopy of corpora lutea, removed at the end of the experiments, showed no indication of luteolytic changes following oxytocin infusion. It was concluded that oxytocin markedly and rapidly reduces progesterone secretion in pregnant ewes.     

In vivo oxytocin release from microdialyzed bovine corpora lutea during spontaneous and prostaglandin-induced regression

The release of luteal oxytocin during spontaneous and prostaglandin-induced luteolysis was investigated in cows. A continuous-flow microdialysis system was used in 11 cows to collect dialysates of the luteal extracellular space between Days 12 and 24 postestrus. Seven cows were untreated and were expected to exhibit spontaneous luteolysis during sampling, whereas 4 cows received prostaglandin F(2alpha) (PGF(2alpha)) systemically between Days 13 and 15 to induce luteolysis during sampling. Oxytocin was detectable in the dialysate of all cows before Day 16 postestrus and occurred as 2 or 3 discrete pulses per 12-h sampling period. For non-PGF(2alpha)-treated cows, dialysate oxytocin content began to decline spontaneously on Day 15 postestrus and was undetectable by Day 17 postestrus. Oxytocin decay curves preceded onset of serum progesterone decline by at least 72 h and were not related temporally with onset of progesterone decline within cow. Exogenous PGF(2alpha) (25 mg, i.m.) produced a 10-fold increase in dialysate oxytocin within 1 h (1.9 +/- 0.3 pg/ml to 20.8 +/- 3.0 pg/ml; P < 0. 01). Dialysate oxytocin then declined to pretreatment concentrations within 2 h and was undetectable within 8 h posttreatment. A second PGF(2alpha) injection given 20 h after the first did not result in a measurable increase in dialysate oxytocin, probably because luteolysis was underway. Although robust luteal oxytocin release was observed after treatment with a pharmacological dose of PGF(2alpha), the lack of detectable oxytocin secretion during spontaneous luteolysis suggests that the contribution of luteal oxytocin in the cow may be less than that proposed for the ewe.     

Prevention of the normal expansion of maternal plasma volume: a model for chronic fetal hypoxaemia

The effects of inadequate expansion of maternal blood volume on uterine blood flow, fetal oxygen levels and vasoactive mediators during the third trimester were studied in 8 pregnant sheep. Results were compared to those obtained during 15 normal pregnancies. Prevention of the normal (20 ml/day) increase in maternal plasma volume was achieved by repeated haemorrhage and injections of furosemide. These treatments also reduced the rise in blood flow to the pregnant uterine horn that normally occurs during this period of gestation: at term flow was only 508 +/- 61 (SEM) compared to 838 +/- 83 ml/min in the control group (P greater than 0.01). This reduction in uterine blood flow caused a gradual fall in fetal PaO2, and rise in fetal levels of plasma renin activity, vasopressin, catecholamines and angiotensin II without change in pHa or base excess. Four to 5 days prior to delivery, the difference from control in PaO2 was -3.9 +/- 0.5 mmHg, plasma renin activity +2.9 +/- 1.7 ng/ml.h, vasopressin +4.2 +/- 1.1 pg/ml, catecholamines +957 +/- 145.3 pg/ml and angiotensin II +243 +/- 108.2 pg/ml. Furthermore, the fall in PaO2 and rise in vasoactive mediators that normally occur 3-5 days prior to the onset of labour was either absent (PaO2 and plasma renin activity) or blunted. Thus when expansion of blood volume during pregnancy is inadequate, blood flow to the uterus is adversely affected. This leads to various degrees of chronic fetal hypoxaemia and stimulation of vasoactive mediator systems. However, the normal stimulation of vasoactive mediator systems that occurs 3-5 days before delivery appears to be blunted. Experimental prevention of blood volume expansion during pregnancy produces an excellent model for the study of chronic mild fetal hypoxaemia.     

Plasma oxytocin concentrations in cyclic mares and sexually aroused stallions

An experiment was conducted to measure plasma oxytocin concentrations at 4 different stages of the estrous cycle in 11 pony mares. Plasma oxytocin concentrations (muU/ml +/- SE) were found to be higher (P<.01) on day 2 of estrous (39.8 +/- 12.5) and day 5 post-ovulation (33.1 +/- 12.0) than on day 10 (2.3 +/- 1.6) and day 15 post-ovulation (6.8 +/- 4.1). A second experiment was conducted to measure jugular plasma oxytocin concentrations before and after sexual arousal in six pony stallions. Oxytocin concentrations (muU/ml +/- SE) were higher (P<0.06) after sexual arousal (50.5 +/- 8.9) than before sexual arousal (23.8 +/- 2.9). These data suggest plasma oxytocin concentrations may be associated with ovarian function in mares and with sexual behavior in stallions.     

Application of sensitive enzymeimmunoassays for oxytocin and prolactin determination in blood plasma of yaks (Poephagus grunniens L.) during milk let down and cyclicity

Highly sensitive and specific enzymeimmunoassays for oxytocin and prolactin determination in yak plasma using the biotin-streptavidin amplification system and the second antibody coating technique were validated and applied for determining their profiles during milk let down and cyclicity in yaks. Oxytocin EIA was conducted taking duplicate 200 microl of unknown plasma samples and standards per well. The lowest detection limit was 0.2 pg/well, which corresponded to 1pg/ml plasma. Prolactin EIA was carried out directly in 50 microl of yak plasma. The sensitivity of EIA procedure was 5 pg/well prolactin, which corresponded to 0.1 ng/ml plasma. Mean plasma prolactin concentrations although high at estrus were not statistically different (P > 0.05) from the hormone concentrations on other days. Mean plasma prolactin concentrations during non-breeding season were significantly higher (P < 0.001) than that recorded in breeding season. Oxytocin and prolactin profiles were also obtained in two yaks before, during and after milking. A sharp release of oxytocin and prolactin shortly after udder stimulation was observed. High levels of oxytocin and prolactin were maintained during milking, falling sharply thereafter.     

Concentrations and origin of oxytocin in breast milk

Samples of human milk obtained from lactating women in the early postpartum period were assayed for oxytocin concentrations by specific RIA, following extraction procedures with Florisil. Mean oxytocin concentrations in human milk at postpartum day 1 to 5 were 4.5 +/- 1.1, 4.7 +/- 1.1, 4.0 +/- 1.3, 3.2 +/- 0.4, 3.3 +/- 0.6 microunits/ml (+/- SE), respectively. Oxytocin levels in milk were significantly increased by nursing (3.1 +/- 0.6, 5.3 +/- 1.0 microunits/ml, respectively). 3H-oxytocin in human milk was stable even after incubation at 37 degrees C for 2 hours. The dilution curve for milk was parallel to the curve for the standard oxytocin. The chromatographic fraction of immunoreactive oxytocin was identical to that of 3H-oxytocin. 3H-oxytocin was administered to lactating rats. Radioactivity in the neonatal gastric contents and plasma were 12.8% and 4.4% of the counts in the maternal plasma. It was made clear that oxytocin is stable in milk and that oxytocin in maternal blood can be transferred to mik and then to neonates.