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.     

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.     

Temporal changes in serum prostaglandin F2alpha and oxytocin in dairy cows with short luteal phases after the first postpartum ovulation

Luteolysis in the cow depends upon an interaction between prostaglandin F(2alpha) (PGF(2alpha)) and oxytocin. The objectives of our study were 1) to determine oxytocin concentrations in postpartum dairy cows and 2) to identify the temporal relationship between oxytocin and PGF(2alpha) release patterns during luteolysis in normal and abbreviated estrous cycles in the postpartum period. Serum oxytocin and PGF(2alpha) metabolite (PGFM) concentrations from nine cows which had short estrous cycles (< 17 d) were compared with those of six cows which had normal estrous cycles. Serum basal oxytocin concentrations in short estrous cycle cows (23.7 to 31.1 pg/ml) were higher (P<0.05) than those of normal estrous cycle cows (14.6 to 19.8 pg/ml). Oxytocin concentrations increased to peak values in both short and normal cycle cows, during luteolysis. Basal PGFM concentrations (112.2 to 137.4 pg/ml) were higher in cows with short cycle (P<0.05) than in cows with normal cycles (62.9 to 87.5 pg/ml). The increase in PGFM concentrations during luteolysis was significant in both normal cycle and short cycle cows (P<0.05). Increases in serum PGFM concentrations were always associated with increases in serum oxytocin concentrations in normal cycle and short cycle cows and the levels decreased simultaneously before the subsequent estrus. Results support the idea of a positive relationship between PGF(2alpha) and oxytocin concentration during the estrous cycle as well as a possible synergistic action of these hormones in the induction of luteolysis in dairy cattle.     

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.     

Secretion patterns of oxytocin and PGF2alpha-metabolite in response to cervical dilatation in cyclic mares

We conducted the present study to establish a standardized method for cervical stimulation without affecting the endometrium, and to investigate the effect on estrous cycle pattern and concentrations of progesterone, oxytocin and PGF2alpha-metabolite of cervical dilatation in the mare. Six healthy Haflinger mares underwent three different treatments (control, insertion, dilatation) on Days 5 and 7 of the cycles in different orders according to a Latin square design. During dilatation, the balloon of the catheter was inflated stepwise every 30s with warm physiological saline to a maximum of 50 ml. At this stage the size of the balloon was 4.5 cm in diameter and 6 cm length. Estrous cycle length was significantly shortened by dilatation when compared to controls (control: 22.8+/-1.7, insertion: 21.8+/-2.5, dilatation: 20.0+/-1.3 days; P<0.05). Concentrations of progesterone at Days 10, 12 and 14 after ovulation were significantly lower in dilatation cycles. Calculation of the area under the curve (AUC) for progesterone secretion from Day 7 to Day 12 also revealed a significant decrease in progesterone secretion in the dilatation group (dilatation: 34.1+/-7.3, insertion: 35.6+/-7.8, control: 39.1+/-5.9 ng/ml; P<0.05). Cervical insertion and dilatation caused a rapid and pronounced increase in plasma concentrations of oxytocin from basal levels (1.0-6.1 pg/ml) to maximum peaks (insertion: 125.5 pg/ml and dilatation: 305.2 pg/ml). The AUC for oxytocin was significantly higher after insertion (Day 5: 858.4+/-469.9; Day 7: 411.9+/-213 pg/ml/h) and dilatation (Day 5: 1697+/-1725; Day 7: 1078.5+/-764 pg/ml/h) when compared to controls (Day 5: 186+/-98; Day 7: 156+/-23.5 pg/ml/h; P<0.05). Manipulations did not cause considerable changes in plasma PGF2alpha-metabolite concentrations. Because cervical dilatation up to a diameter of 4.5 cm did not cause any immediate PGF2alpha release, the luteolytic pathway is unlikely to be responsible for shortening the length of diestrus and the estrous cycle. The present data suggest an involvement of oxytocin in the shortening of the luteal phase in response to cervical manipulation.     

Relaxin, oxytocin, and prostaglandin effects on progesterone secretion from bovine luteal cells during different stages of gestation

To determine the effects of relaxin, oxytocin, and prostaglandin F2 alpha on progesterone secretion, bovine luteal cells from different stages of gestation were dispersed in Medium 199 with 200 units/ml penicillin, 1.0% kanamycin, 0.5% bovine serum albumin, and 400 units/ml collagenase. Cells (10(5) were cultured in 400 microliters of Dulbecco's modified Eagle's medium and Ham's F-12 medium containing fetal bovine serum and antibiotics, in Falcon multiwell plates, in a humidified environment of 95% O2 and 5% CO2 at 37 degrees C. Cells were cultured for 24 hr without treatment and thereafter with medium-hormone replacement every 24 hr. Progesterone was quantified from unextracted media by radioimmunoassay. Basal progesterone secretion after 24 hr was 1.81 +/- 0.14, 1.76 +/- 0.17, 0.54 +/- 0.49, and 0.57 +/- 0.21 pg/ml per viable luteal cell from 145-, 165-, 185-, and 240-day-old corpora lutea, respectively. Basal progesterone secretion increased (P less than 0.05) with time in culture. Relaxin induced a dose-dependent (greater than 100 ng/ml) increase in progesterone release, compared with the controls. Oxytocin and prostaglandin F2 alpha induced greater release (P less than 0.05) of progesterone than relaxin at all stages of gestation, but progesterone release was dependent on the stage of gestation and the duration in culture. Luteinizing hormone (100 ng/ml) stimulated whereas 17 beta-estradiol (50 ng/ml) inhibited progesterone secretion by luteal cells at all stages of gestation examined. Relaxin obliterated the prostaglandin- and oxytocin-induced progesterone secretion by bovine luteal cells from 145 to 214 days of gestation. Thus, relaxin, cloprostenol, and oxytocin regulate progesterone production by cultured bovine luteal cells, but hormone secretion was dependent on the stage of gestation.     

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)     

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.     

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.     

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)     

Simultaneous measurement of multiple Th1 and Th2 serum cytokines in psoriasis and correlation with disease severity

BACKGROUND: Psoriatic plaques have been shown to contain increased levels of proinflammatory cytokines. Serum levels of interleukin (IL)-6, IL-7, IL-8, and interferon (IFN)-gamma have been reported elevated in psoriatic patients. AIM: To evaluate serum cytokine profiles in psoriasis patients by improved enzyme-linked immunosorbent assay (ELISA) technique and to correlate these levels with disease severity. METHODS: We analyzed single serum samples from 10 patients with active untreated psoriasis, two patients with active treated psoriasis, and five healthy volunteers for major T helper type 1 and T helper type 2 cytokines using the LINCOplex ELISA multi-analyte detection system that permits simultaneous detection of multiple cytokines from a single sample. The disease severity, including erythema, induration, scale, and surface area, was assessed. RESULTS: IFN-gamma was markedly elevated in all sera from psoriasis patients, 33.8 +/- 1.3 pg/ml (mean +/- standard error) versus 8 +/- 1.5 pg/ml for normal controls (p < 0.01), and positively correlated with all indices of disease severity (Spearman r > 0.6). IL-8 was also increased in psoriasis patients (24.4 +/- 1.8 pg/ml) versus normal controls (3.6 +/- 1.2 pg/ml) (p < 0.05) and positively correlated with the degree of erythema (Spearman r > 0.6). Mean IL-12 levels were decreased in sera from psoriasis patients (8.5 +/- 1.2 pg/ml) compared with normal controls (42.2 +/- 5.3 pg/ml) (p < 0.01). Also, serum IL-10 levels were below detection levels in psoriatics compared with controls (6.4 +/- 1.3 pg/ml). CONCLUSIONS: This new ELISA system allowed rapid and reliable detection of numerous cytokines in single serum samples from patients with psoriasis. We observed that IFN-gamma and IL-8 cytokines were elevated in psoriatics and correlated with parameters of disease severity while IL-10 and IL-12 were decreased.     

Distribution of vasopressin and oxytocin in rat brain

Arginine-vasopressin and oxytocin in various portions of rat brain were determined by radioimmunoassays. The hormones were extracted from tissue samples into 0.1 N HCl and then purified partially with acetone-petroleum ether extraction. The non-equilibration method was used for the assays. In this method recovery rates of arginine-vasopressin and oxytocin were 73.0 +/- 4.4% and 75.0 +/- 3.8%, respectively. Sensitivities of the assays were 1 pg of arginine-vasopressin and 0.75 pg (0.3 microU) of oxytocin per assay tube. The higher concentrations of arginine-vasopressin and oxytocin were confirmed in the hypothalamo-neurohypophyseal system, where these hormones are synthesized, transported and stored. Relatively high concentrations of these hormones, especially oxytocin, were detected in spinal cord. Amygdala, hippocampus, limbic forebrain and pineal body contained a certain amount of arginine-vasopressin (2-20 pg/mg protein). Oxytocin (1-7 pg/mg protein) was also detected in amygdala, pons and medulla oblongata, pineal body and midbrain. The low concentrations of these hormones were also found in cerebral cortex and cerebellum.     

Relaxin regulates oxytocin secretion in late-pregnant beef heifers

The effects of porcine relaxin (3000 units/mg) on oxytocin (OT) and progesterone secretion were studied in beef heifers on Day 274 (10 days before expected parturition). Heifers (n = 11) were randomly assigned to three treatments: relaxin iv infusions combined with im injection (RLX-INF, 9000 units), relaxin im injection (RLX-im, 6000 units), and phosphate-buffered saline-treated controls (PBS). RLX-INF heifers received infusions of PBS and 1000 units of relaxin for 165 min, followed by 2000 units of relaxin im and finally 2000 units of relaxin infusion followed by 4000 units of relaxin im. Endogenous relaxin (immunoreactive) in the PBS-treated group was 0.2-0.9 ng/ml peripheral plasma. For the RLX-im group, peak relaxin was 81 +/- 12 ng/ml (+/- SE) at 45 min after treatment. There were two peaks of relaxin, 18 +/- 5.3 ng/ml and 74 +/- 7.5 ng/ml, 3.5-4.5 hr apart in the RLX-INF group. Significant peak releases of OT were evident in the relaxin-treated heifers. For the RLX-im group, an OT peak (42 +/- 16 pg/ml) occurred within 30 min after relaxin treatment. For the RLX-INF heifers, 2000 and 4000 units of relaxin were associated with major peaks of 14 +/- 0.5 and 43 +/- 1.7 pg/ml OT, respectively. Basal OT plasma levels in the PBS group were 2.5-3.1 pg/ml. Mean plasma progesterone for all heifers was 6.2 +/- 2.11 ng/ml before treatment. There was a significant decrease in progesterone (-2.5 ng/ml) in the RLX-im group within 60 min after relaxin treatment and 45 min after peak OT secretion. The maximum decrease in progesterone (-3.2 +/- 0.68 ng/ml) occurred 135 min after treatment in the RLX-im group. In the RLX-INF group, 2000 units of relaxin infusion combined with 4000 units of relaxin im significantly decreased progesterone (-3.2 +/- 1.59 ng/ml) in peripheral plasma. These results clearly indicate that relaxin causes an acute peak release of oxytocin within 30 min, followed by a marked decrease in plasma progesterone concentration in late-pregnancy cattle.     

Activation of subfornical organ efferents stimulates oxytocin secretion in the rat

The effects of activation of subfornical organ (SFO) efferents on plasma oxytocin concentrations were examined in conscious freely moving male Sprague-Dawley rats. Blood samples were obtained through chronically implanted atrial catheters and SFO efferents were activated electrically using chronically implanted bipolar stimulating electrodes. Hormone concentrations were measured by radioimmunoassay, and experimental animals were assigned to one of 3 experimental groups according to histologically verified anatomical locations of stimulating electrodes in either the SFO, the hippocampal commissure (HC), or the medial septum (MS). Electrical stimulation in the SFO resulted in increased plasma concentrations of oxytocin from control values of 2.54 +/- 0.9 pg/ml, to a post-stimulation level of 65.6 +/- 27.0 pg/ml. In contrast, stimulation in immediately adjacent structures including HC and MS was found to be without effect on plasma concentrations of oxytocin. These studies provide the first definitive evidence that SFO efferents may play a significant role in controlling the secretion of oxytocin from the posterior pituitary.     

The effect of days postpartum, indomethacin and oxytocin on prostaglandin metabolite concentrations in postpartum suckled beef cows

In Experiment 1, blood samples were collected on days 1, 4, 7, 10, 13, 16, 19, 22, and 25 postpartum from the jugular veins of 10 suckled beef cows to determine 13, 14-dihydro-15-keto prostaglandin F(2)alpha (PGFM) concentrations during the early postpartum period. PGFM concentrations on days 1 and 4 were 207.8 +/- 33.9 and 283.6 +/- 45.6 pg/ml and then declined linearly (r = -0.71; P < 0.05) to 44.1 +/- 5.7 and 44.0 +/- 5.3 pg/ml on days 22 and 25 postpartum. Two groups of postpartum (25.3 +/- 0.5 and 37.7 +/- 1.1 days) suckled beef cows (10 cows/group) were used in the second experiment. Five cows of each group received intrauterine infusions of indomethacin for 5.5 days while the other five cows of each group served as controls. All cows had calves removed at the time of the last indomethacin infusion and were subcutaneously administered oxytocin six hours later. During the infusion period, PGFM concentrations decreased (P < 0.01) across time for both groups of indomethacin-treated cows. Concentrations of PGFM increased (P < 0.05) after oxytocin treatment for both groups of control and indomethacin-treated cows, but concentrations were higher for the control cows than for the indomethacin-treated cows.     

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.     

Preovulatory biosynthesis and granulosa cell secretion of immunoreactive oxytocin by goat ovaries

Oestrous cycles of goats were synchronized hormonally. Immunoreactive oxytocin was undetectable (less than 0.1 ng/mg protein) in media from granulosa cells isolated before the LH surge for small (1-2 mm), medium (3-5 mm) and large (greater than 5 mm diameter) follicles when cultured for 24 h without or with added hormones. Granulosa cells from large and medium, but not small, follicles isolated 6-12 h after spontaneous preovulatory LH surges secreted high concentrations of oxytocin (4-12 ng/mg protein). Addition of PGE-2 (1 microgram/ml) caused a further significant (P less than 0.05) increase in oxytocin secretion by cultured granulosa cells, whereas PGF-2 alpha, FSH and LH were ineffective when added to culture media. Ovarian venous blood and granulosa cells were collected at 0, 6, 12 or 18 h after GnRH injection in hormonally synchronized goats. Peripheral serum LH values were increased significantly in all but 2 of 22 goats within 2 h of GnRH injection. At the earliest sampling time after GnRH (6 h), ovarian venous levels of oxytocin were increased significantly from basal levels of 0.4 pg/ml to 2.4 pg/ml. Oxytocin concentrations in follicular fluid increased from a basal value of 67 pg/ml to 155 pg/ml by 6 h and to 372 pg/ml by 18 h after GnRH injection. Oxytocin secretion by cultured granulosa cells was not increased significantly by 6 h (0.1 ng/mg protein) but rose to 1.4 and 3.5 ng/mg protein at 12 and 18 h, respectively. Approximately parallel increases occurred in progesterone in ovarian venous blood and granulosa cell culture media over the same time period. (ABSTRACT TRUNCATED AT 250 WORDS)     

Transient uterine refractoriness after oxytocin administration in ewes

Steroid-primed, ovariectomized ewes were treated intravenously with 2 doses of 1 microgram oxytocin at intervals of 1, 2, 4 or 6 h. The initial dose resulted in increases in 13,14-dihydro-15-keto-PGF-2 alpha in the peripheral circulation from 173 to 667 pg/ml within 5 min; subsequent doses caused responses of 23 +/- 1, 23 +/- 6, 54 +/- 12 and 62 +/- 10% respectively of the initial dose. Concentrations of oxytocin receptor in myometrium, caruncular endometrium and intercaruncular endometrium were, respectively, 185 +/- 33, 128 +/- 7 and 105 +/- 14 fmol/mg protein at 2 h after saline injection and 147 +/- 27, 195 +/- 52 and 170 +/- 50 fmol/mg protein at 2 h after administration of 1 microgram oxytocin. The dose of oxytocin administered was shown to raise circulating concentrations to levels characteristic of those observed during spontaneous episodes of release of oxytocin at luteolysis. Oxytocin administration therefore results in transitory uterine refractoriness which may be due to failure of a post-receptor response and this may contribute to the episodic nature of uterine prostaglandin secretion.     

Angiotensin acts at the subfornical organ to increase plasma oxytocin concentrations in the rat

We have examined the effects of systemic angiotensin II (AII) on plasma oxytocin (OXY) concentrations in freely moving male Sprague-Dawley rats. We have also examined the role of the subfornical organ (SFO) as a CNS site at which circulating AII acts to influence secretion of this neurohypophysial peptide. OXY concentrations were measured by radioimmunoassay in plasma samples obtained by drawing blood samples through indwelling atrial catheters. In SFO intact animals (n = 8) AII infusion (1.0 microgram/kg/min) resulted in increases in plasma OXY concentrations from baseline values of 6.8 +/- 2.5 pg/ml to postinfusion concentrations of 44.9 +/- 11.9 pg/ml. In a second series of experiments electrolytic lesions were placed in the region of the SFO prior to testing the effects of AII infusion on OXY concentrations. Two further experimental groups were thus established according to the histologically verified location of lesions in either the rostral or caudal SFO. In the caudal SFO lesioned group AII infusion resulted in increases in plasma OXY concentrations from control values of 6.9 +/- 1.4 pg/ml to postinfusion levels of 45.1 +/- 9.8 pg/ml. These changes were not significantly different from the SFO intact group. In contrast rostral SFO lesions resulted in significantly elevated basal concentrations of OXY (17.4 +/- 3.4 pg/ml, n = 6) while postinfusion concentrations were found to be 22.8 +/- 4.9 pg/ml indicating that AII infusion was without effect following such lesions. These data are in accordance with the hypothesis that circulating AII acts at the SFO to influence SFO efferents which in turn activate OXY secreting neurons in the hypothalamic supraoptic (SON) and paraventricular (PVN) nuclei. These neuroendocrine cells then release this peptide into the systemic circulation from the posterior pituitary.     

Secretion of oxytocin and progesterone by ovine corpora lutea in vitro

The mechanisms involved in the control of oxytocin and progesterone secretion by the ovine corpus luteum have been investigated in vitro using luteal slice incubations. Oxytocin and progesterone were secreted at constant rates from luteal slices for 2 h of incubation (366 +/- 60 pg X mg X h and 18.9 +/- 0.18 ng X mg X h, respectively). Secretion of progesterone, but not of oxytocin, was significantly (p less than 0.02) stimulated in the presence of ovine luteinizing hormone. Incubation of luteal slices in medium containing 100 mM potassium, however, resulted in increased secretion of oxytocin and, to a lesser extent, of progesterone (294 +/- 59% and 142 +/- 15%, respectively, p less than 0.05). Basal oxytocin secretion was reduced during incubation in calcium-free medium, compared to secretion in the presence of calcium (70 +/- 15 and 175 +/- 25 pg X mg X 20 min, respectively, p less than 0.01), whereas progesterone secretion was not altered in the absence of calcium. Secretion of both hormones by luteal slices was stimulated by the addition of the calcium ionophore A23187 (p less than 0.05). Addition of prostaglandin F2 alpha (2.8 microM) had no effect on secretion of either oxytocin or progesterone. We have demonstrated that oxytocin and progesterone can be stimulated, independently, from corpus luteum slices incubated in vitro. The pattern of release is consistent with the proposal that oxytocin, but not progesterone, is associated with and actively released from luteal secretory granules. Our results also indicated that prostaglandin F2 alpha does not directly stimulate release of oxytocin or progesterone from luteal cells in vitro.