Oxytocin Receptor Concentrations Research Articles

Receptors for and myometrial responses to oxytocin and vasopressin in preterm and term human pregnancy: Effects of the oxytocin antagonist atosiban

Our purpose was to study myometrial oxytocin and type V1 vasopressin receptors, the in vitro contractile effects of these hormones, and the influence of an oxytocin antagonist. Women delivered by cesarean section preterm (n = 51) and at term (n = 71), with and without labor contractions, gave myometrium for the estimation of oxytocin and V1 vasopressin receptors. The in vitro myometrial effects of the peptides and the influence on these of the competitive oxytocin receptor blocking agent 1-deamino-2-D-Tyr(OEt)-4-Thr-8-Orn-oxytocin were also tested. The median concentration of oxytocin receptors was 116 fmol/mg protein (range 15 to 372 fmol/mg protein) in patients delivered preterm not in labor, 134 fmol/mg protein (27 to 1421 fmol/mg protein) in the beginning of labor, and 46 fmol/mg protein (9 to 140 fmol/mg protein) in advanced labor. At term the corresponding concentrations were 172 (25 to 629), 223 (24 to 414), and 70 (21 to 92) fmol/mg protein. The concentration of V1 vasopressin receptors also decreased in advanced labor. In advanced labor after oxytocin infusion a reduction in the concentration of the receptor for this hormone was observed, which appeared to be related to the duration and dose of treatment. Oxytocin receptors did not vary between women with different indications for cesarean section. The oxytocin effects in vitro and the degree of inhibition by the antagonist of oxytocin responses correlated with the concentration of oxytocin receptors but not with that of V1 vasopressin receptors. No correlation was seen between the response to vasopressin and concentrations of oxytocin or V1 vasopressin receptors. The effect of oxytocin on the myometrium in pregnancy is mediated by an oxytocin receptor, whereas vasopressin acts on both oxytocin and vasopressin receptors. The initiation of labor both preterm and at term may be primarily related to increased release of oxytocin, which is locally produced in the uterus and not detectable in the plasma, but oxytocin and vasopressin receptors may play a role in the regulation of labor. The analog 1-deamino-2-D-Tyr(OEt)-4-Thr-8-Orn-oxytocin, which blocks both the oxytocin and the V1 vasopressin receptor, should inhibit labor both preterm and at term, the former confirming results of recent clinical studies in Sweden and the United States.

Use of repeated biopsies to monitor endometrial oxytocin receptors in the cow

A transcervical biopsy technique was used for the repeated collection of samples of bovine endometrium from individual cows to monitor the development of oxytocin receptors. The collection of biopsies from three cyclic cows at two-day intervals between days 13 and 17 after oestrus did not result in any shortening of the length of the cycle (mean +/- sem 22.0 +/- 0.5 days). Furthermore, the assay of serial blood samples from three cows around the time of biopsy on days 15 and 17 demonstrated that the technique did not induce any release of prostaglandin F2 alpha (PGF2 alpha). Oxytocin receptors were undetectable (< 20 fmol/mg protein) on days 13 and 15 of the cycle, increased to a mean of 121 +/- 16 fmol/mg protein on day 17 (before luteolysis), and were highest in samples collected at oestrus (498 +/- 84 fmol/mg protein). To determine whether a single biopsy of endometrium would give a representative measure of the concentration of uterine oxytocin receptors, the concentrations were determined in samples of caruncular and intercaruncular endometrium collected from the upper, middle and lower proportions of uteri from three oestrous animals at slaughter. The concentrations were similar in both types of tissue (caruncular 730 +/- 15 fmol/mg protein, intercaruncular 727 +/- 48 fmol/mg protein) and there were no differences between the three regions of the uterus.

Effects of progesterone pretreatment on the oxytocin receptor concentration and the response to oxytocin during the simulated early luteal phase in the ovariectomized ewe.

A steroid-treated ovariectomized ewe model was used to investigate the role of progesterone pretreatment in the control of functional oxytocin receptor concentrations during the early luteal phase. Ovariectomized ewes (n = 28) were injected with oestradiol for 2 days (final injection = day 0) with or without progesterone pretreatment (progestagen sponge for 10 days). Ewes were then given high or low concentrations of progesterone combined with high, low or zero concentrations of oestradiol in a pattern known to simulate the early luteal phase profile (n = 4 per group). Ewes were given 1 microgram oxytocin (i.v.) on day 4 and plasma was collected to assay 13,14-dihydro-15-keto PGF2 alpha. The concentration of progesterone and oestradiol administered had no effect on the concentration of 13,14-dihydro-15-keto PGF2 alpha following oxytocin administration (P > 0.05). However, the group that was not pretreated exhibited a small but significant 13,14-dihydro-15-keto PGF2 alpha response in comparison with the equivalent pretreated group (P < 0.05). In a subsequent study, ewes were divided into groups pretreated and not pretreated with progesterone; both groups were given oestrous concentrations of oestradiol and high concentrations of progesterone and oestradiol together. On day 0, 2, 3 or 4, ewes from each group (n = 3, 3, 4 and 4, respectively) were given 1 microgram of oxytocin i.v., and the endometrium was collected to measure the binding of oxytocin receptors. Oxytocin caused a significant (P < 0.05) increase in the concentration of 13,14-dihydro-15-keto PGF2 alpha in all ewes on day 0 but not on days 2, 3 or 4. Oxytocin receptor concentrations were maximal on day 0 and basal by day 4.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of ovariectomy and ovarian steroids on uterine oxytocin receptors in the anoestrous ewe

The effects of treatment with progesterone, oestradiol-17β or the presumptive oestrogen agonist Tamoxifen on the concentrations of uterine oxytocin receptors (OTr) were investigated in Merino ewes during the anoestrous season. Three groups of ewes were ovariectomised (Ovx) and subsequently given progesterone ( n = 5), oestradiol-17β ( n = 4) or no treatment (control-Ovx, n = 5) for 6 days. Two groups of ewes were sham-operated and given either Tamoxifen ( n = 5) or no treatment (control-intact, n = 5) for 6 days. All ewes were then given an intravenous (i.v.) injection of oxytocin (10 IU) to determine the prostaglandin (PG)F 2α metabolite, 13, 14-dihydro-15-keto-PGF 2α (PGFM) response. Immediately thereafter, the concentrations of uterine OTr were measured in uterine tissues after hysterectomy. The untreated ewes during the anoestrous season (control-intact) had high uterine OTr concentrations. There was no effect of ovariectomy on uterine OTr concentrations in anoestrous ewes (control-Ovx ewes). In contrast, the concentrations of uterine OTr were reduced ( P < 0.05) both in the progesterone-treated and oestradiol-17β-treated Ovx ewes compared with the control-Ovx ewes. The Tamoxifen-treated intact ewes had lower ( P < 0.05) uterine OTr concentrations than did the control-intact ewes. In all groups, the oxytocin-induced responses of PGFM were negligible. These results indicate that anoestrous ewes have high uterine OTr although the oxytocin-induced PGFM response was low. Ovariectomy of anoestrous ewes had no effect on these variables. In addition, treatment of the ovariectomised anoestrous ewe with progesterone or oestradiol-17β for 6 days reduced uterine OTr but did not affect the oxytocin-induced PGFM response.

Effect of ACTH on oxytocin-induced release of prostaglandin-F 2α and on uterine oxytocin receptors in the ewe

During the mid breeding season daily administration of 100 i.u. adrenocorticotrophic hormone (ACTH) from Days 13–16 of the oestrous cycle in multiparous, Welsh Mountain ewes (42–51 kg) significantly ( P<0.01) delayed oestrus and decreased ( P<0.01) oxytocin-induced release of 13,14-dihydro-15-keto-prostaglandin (PG)F 2α (PGFM) on Days 14 and 15. In ovariectomized Welsh Mountain ewes (40–50 kg) primed first with progesterone (10 mg day −1 × 6 days) and then oestradiol-17β (500 μg day −1 × 2 days), ACTH (50 i.u.) significantly ( P<0.001) reduced the ability of oxytocin to release PGFM into the peripheral circulation and also reduced oxytocin receptor concentrations in the myometrium ( P<0.05) and endometrium ( P<0.01). We conclude that ACTH can delay oestrus in the ewe by suppressing formation of uterine oxytocin receptors.

Oestradiol concentration and the development of the uterine oxytocin receptor and oxytocin-induced PGF2 release in ewes

The control of temporal changes in oxytocin receptor concentrations and oxytocin-induced 13,14-dihydro-15-keto PGF2 alpha (PGFM) release was examined in ewes. One week after ovariectomy, 36 ewes were administered fluorogesterone acetate for 10 days followed by oestradiol (3 x 16 micrograms day-1) for 2 days (pretreatment cycle). Day 0 was designated as the time of the final 'oestrous' oestradiol injection. Ewes were then treated for up to 12 days with progesterone (24 mg day-1 maximum) with or without oestradiol (both hormones administered in 1 ml of corn oil i.m. at 8 h intervals) in a pattern known to simulate natural plasma profiles of the oestrous cycle. The three treatments were zero oestradiol, low oestradiol (12 micrograms day-1 maximum), and high oestradiol (36 micrograms day-1 maximum). Subgroups of four ewes from each treatment group were given 1 microgram of oxytocin (i.v.) on day 10, 11 or 12 of the simulated cycle, and endometrial oxytocin receptor concentrations were determined in samples collected within 3 h of oxytocin administration. On day 10 only one ewe in each group exhibited a PGFM response to oxytocin, and the mean response was unaffected by the concentration of oestradiol administered. On days 11 and 12 there was a significant effect of oestradiol concentration (P < 0.05) on the pattern of PGFM release in response to oxytocin, the high oestradiol concentration causing a rapid increase in the concentration of PGFM following oxytocin administration. On day 12 the oestradiol concentration was positively correlated with the PGFM mean response (P < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Up-regulation of oxytocin receptors in rabbit amnion by glucocorticoids: potentiation by cyclic adenosine 3',5'-monophosphate.

Oxytocin (OT) receptors (OTR) in rabbit amnion increase more than 200-fold at the end of gestation. In the present report, we studied the basis of this up-regulation. Incubation of amnion cells with cortisol (20 nM) for 24 h increased the amount of 125I-labeled OT antagonist bound by 16- to 18-fold. The effects of cortisol were dose and steroid dependent. Administration of glucocorticoid to pregnant does also increased the concentration of OTRs in amnion. The effects of cortisol in vitro were potentiated by the addition of forskolin (50 microM), so that OTR number increased by as much as 182 times. The effects of cortisol and forskolin, either separately or in combination, were inhibited by activation of protein kinase-C or coincubation with transforming growth factor-alpha (10 nM). Cyclosporin-A (5 microM) selectively inhibited cortisol-induced rises in the OTR concentration. The addition of cortisol to amnion cells increased OT-stimulated prostaglandin E2 (PGE2) release almost 100-fold; the combination of forskolin and cortisol increased the PGE2 response to OT about 5600 times. Judging from the greater effects on PGE2 release, these results suggest that forskolin and cortisol up-regulate the signal response mechanism to OT as well as the OTR concentration. The findings show that changes occurring in the amnion in vivo can be mimicked in vitro, and they elucidate the mechanism of up-regulation of OTR concentrations.

Prolonged progesterone treatment increases the concentration of uterine oxytocin receptors in ovariectomized ewes

The effect of prolonged progesterone treatment on the concentrations of endometrial oxytocin receptors (OTr) was studied in chronically ovariectomized ewes. The ewes were primed with progesterone and oestradiol-17β (E 2-17β) to induce oestrus (day of oestrus=Day 0) and then injected with graduated doses of progesterone, using vegetable oil as a vehicle, for 14, 16, 18 or 20 days. The control ewes were injected with the vehicle. The concentrations of OTr on the day of cessation of treatment were measured by a receptor binding assay. During the period of treatment, plasma progesterone concentrations rose above 1 ng ml −1 by Day 4 and remained above 2 ng ml −1 thereafter. Treatment with progesterone for 14 days significantly ( P < 0.01) reduced the concentrations of OTr in the endometrium compared with the control ewes. However, the concentrations of OTr were increased after 16 or more days of progesterone treatment. These results indicate that in ovariectomized ewes progesterone can exert a suppressive effect on the concentrations of endometrial OTr; however, prolongation of progesterone treatment would lead to OTr eventually becoming refractory to the suppressive effect of progesterone and an increase in the concentrations of OTr.

Oxytocin receptor binding activity in cultured ovine endometrium

Oxytocin receptor binding activity in explants of caruncular and intercaruncular endometrium collected from luteal phase ewes increased during culture. An initial rise in binding activity occurred during the first 24 h of culture in both tissues; binding activity in intercaruncular endometrium continued to increase until day 6, remained unchanged on day 8 and had decreased by day 10 of culture. The maximum concentration of receptors in caruncular endometrium was significantly lower than that in intercaruncular endometrium (P < 0.001) and did not change significantly between days 4 and 10 of culture. Apparent dissociation constants and maximal binding of oxytocin receptor in cultured caruncular and intercaruncular endometrium were 3.09 and 2.72 nmol l-1 and 249 and 459 fmol [3H]oxytocin bound mg-1 protein, respectively. Concentrations of oxytocin receptor remained constant in myometrium during 96 h of culture. The rise in endometrial oxytocin receptor concentration did not result from exposure to fetal calf serum, phenol red or insulin in the culture medium. Substituting fetal calf serum with sheep serum or BSA did not block the rise in receptor binding activity. Actinomycin D and cycloheximide inhibited the rise in receptor concentration in both tissues. Co-culture of lung or kidney with endometrium had no effect on binding activity, whereas co-culture with luteal tissue effectively reduced the rise in oxytocin receptor concentration. To establish whether synthesis of functional oxytocin receptors occurred during culture, the effect of oxytocin on prostaglandin F2 alpha (PGF2 alpha) production was assessed in fresh tissue and after 48 h in culture.(ABSTRACT TRUNCATED AT 250 WORDS)

Oxytocin receptors and prostaglandin release in rabbit amnion.

Besides stimulating uterine myometrial and mammary myoepithelial cell contraction, oxytocin (OT) causes the release of prostaglandins (PGs) from uterine endometrium/decidua and amnion cells. Lacking information about OT receptors eliciting PG release, we don't know how they are related to OT receptors involved in smooth muscle contraction. The amnion offers great potential for characterizing OT receptors associated with PG release, as the amount of iodinated OT antagonist ([125I]OTA) bound to rabbit amnion membranes during labor is among the greatest of any tissue yet studied, reaching about 10 pmol/mg membrane protein. The relative affinities of several OT analogues for binding sites on amnion membranes are the same as those on decidual membranes. There are differences in the ligand profile between amnion and myometrium, but they could be due to the additional presence of vasopressin receptors on myometrial membranes. An increase in the sensitivity of PGE2 release from amnion cells in culture to OT and analogues accompanies the rise in OT receptor concentration at the end of gestation. Increases in [125I]OTA binding in vivo can be mimicked with cultured amnion cells by addition of agents that elevate intracellular cAMP levels. Based on the time course and inhibition of the increase with cycloheximide, cAMP might induce OT receptor gene expression. The increase also is reflected by a marked elevation in the covalent labeling of a 50-kDa electrophoretic band with a photoactivated derivative of [125I]OTA. Because of the homogeneity of cell types in the amnion, the ease of culturing amnion cells, and the high concentration of OT receptors that can be induced, this tissue should be very useful in characterizing OT receptors associated with PG synthesis.

Effects of pregnancy and systemic or intrauterine oxytocin infusion on the distribution of endometrial oxytocin receptors in the ewe: an autoradiographic study

Previous autoradiographic studies have suggested that the regulation of oxytocin receptors differs between endometrial cell types during the ovine oestrous cycle, and that those present on luminal epithelial cells are of particular importance to the regulation of prostaglandin F2 alpha release during luteal regression. The present autoradiographic study compares the distribution of the endometrial oxytocin receptor in day-15 non-pregnant and pregnant ewes. The distribution of the endometrial oxytocin receptor in day-15 non-pregnant ewes infused with systemic or intrauterine oxytocin has also been investigated. Continuous, s.c. infusion of oxytocin (150 mmol/24 h) into ewes (n = 6) between days 10 and 15 of the oestrous cycle significantly increased plasma oxytocin concentrations (to approximately 100 pmol/l). There was no similar increase in systemic oxytocin concentrations in ewes receiving intrauterine (i.u.) oxytocin infusions (10 nmol/24 h) between days 10 and 15 of the oestrous cycle (n = 6). Luteolysis was inhibited in all six animals infused with oxytocin (s.c.) and endometrial oxytocin receptor concentrations were significantly lower on day 15 in these animals (12.8 +/- 6.5 (S.E.M.) fmol/mg protein; P < 0.001) and in pregnant ewes (18.4 +/- 15.4 fmol/mg protein; P < 0.001; n = 8) than in ewes infused with saline (248.6 +/- 67.1 fmol/mg protein; n = 6).(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in progesterone and oestrogen receptor mRNA and protein and oxytocin receptors in endometrium of ewes after intrauterine injection of ovine trophoblast interferon.

This study determined whether twice-daily intrauterine injections of ovine conceptus secretory proteins (oCSP) containing type-I trophoblast interferon (25 micrograms/uterine horn) from day 11 to day 15 post-oestrus (oestrus = day 0) could alter the binding capacities of endometrial receptors for oxytocin, progesterone and oestrogen in cyclic ewes when compared with control ewes receiving serum protein (SP) injections. Injections of oCSP on days 11-15 post-oestrus decreased concentrations of oestrogen receptors (P < 0.06), oestrogen receptor mRNA (P < 0.05) and progesterone receptors (P < 0.08) in endometrium on day 16 when compared with SP-infused control ewes, which were undergoing corpus luteum regression on days 14-16. Injection of oCSP also decreased the number (P < 0.10) and affinity (P < 0.06) of oxytocin receptors. Inositol phosphate formation induced in the endometrium on day 16 by 100 nM oxytocin in vitro was highly correlated with the concentration (r > or = 0.93, P < 0.001) and Kd (r = -0.91, P < 0.01) of oxytocin receptors in SP-infused ewes, but was not as highly correlated with concentration (r < or = 0.83, P < 0.06) and Kd (r < or = 0.40, P > 0.40) of oxytocin receptors in oCSP-infused ewes. This indicates that oCSP disrupted the relationship between oxytocin receptor binding and oxytocin-induced activation of its second messenger system. These results indicate that antiluteolytic type-I trophoblast interferon may prevent oxytocin-induced luteolytic pulsatile secretion of prostaglandin F2 alpha during maternal recognition of pregnancy in sheep, by reducing the synthesis and affinity of endometrial oxytocin receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Relationships among endometrial oxytocin receptors, oxytocin-stimulated phosphoinositide hydrolysis and prostaglandin F2 alpha secretion in vitro, and plasma concentrations of ovarian steroids before and during corpus luteum regression in cyclic heifers.

Oxytocin stimulates secretion of endometrial prostaglandin (PG) F2 alpha and induces endometrial phosphoinositide hydrolysis around the time of regression of the corpus luteum (CL) in cows. This study investigated the relationship between endometrial oxytocin receptors, oxytocin-stimulated phosphoinositide hydrolysis and PGF 2 alpha secretion in vitro, and plasma concentrations of ovarian steroids before and during CL regression in cyclic heifers (i.e., Days 13-19 post-estrus). Reproductive tracts were collected from 16 cyclic heifers on Days 13 (n = 5), 16 (n = 5), and 19 (n = 6). Decreases in mean CL weight (4.7, 4.6, and 3.0 +/- 0.6 g for Days 13, 16, and 19, respectively; p < 0.05) and plasma progesterone (12.2, 10.5, and 4.3 +/- 1.5 ng/ml for Days 13, 16, and 19, respectively; p < 0.01) were detected between Days 16 and 19, indicating that CL regression had been initiated in the group of Day 19 heifers. Mean concentration of plasma estradiol (3.4, 3.8, and 5.6 +/- 1.7 pg/ml for Days 13, 16, and 19, respectively) did not differ significantly (p > 0.5) among days of the estrous cycle. Concentration of endometrial oxytocin receptors increased (p < 0.06) during the estrous cycle (36, 49, and 789 +/- 234 fmol/mg protein on Days 13, 16, and 19, respectively), but the greatest increase (p < 0.05) occurred between Days 16 and 19. Similarly, incorporation of [3H]inositol into endometrial inositol phosphates induced in vitro with 100 nM oxytocin also increased (p < 0.01) during the estrous cycle (5121 vs. 5819, 4529 vs. 7239, and 5888 vs 68,096 +/- 18,564 dpm/g tissue for control vs. oxytocin-treated endometrium on Days 13, 16, and 19, respectively) with the greatest increase (p < 0.01) in responsiveness to oxytocin occurring between Days 16 and 19.(ABSTRACT TRUNCATED AT 250 WORDS)

Concentrations of progesterone and oxytocin receptors in endometrium of postpartum cows expected to have a short or normal oestrous cycle.

Luteal lifespan is short after first postpartum ovulation in early-weaned beef cows unless cows are pretreated with a progestogen. Regression of the short-lived corpus luteum in the postpartum beef cow is due to a premature release of prostaglandin F2 alpha (PGF2 alpha) from the uterus. The premature release of PGF2 alpha may be mediated through lower concentrations of receptors for progesterone, higher concentrations of oxytocin receptors, or both, in the endometrium. Thirty-one beef cows were randomly assigned to four groups at parturition. Calves from cows assigned to the short cycle group (n = 6; control) and the short cycle/endometrium group (n = 10) were weaned at 30-32 days post partum. Cows in the normal cycle group (n = 5; control) and the normal cycle/endometrium group (n = 10) received norgestomet implants for 9 days beginning 21-23 days post partum and calves were weaned at implant insertion. Duration of oestrous cycle (x +/- SEM; P < 0.01) following first postpartum ovulation for the short cycle group was 11.5 +/- 1.9 days compared with 18.8 +/- 0.6 days for the normal cycle group. On day 5 following first postpartum ovulation, cows in the short cycle/endometrium and the normal cycle/endometrium groups were hysterectomized and endometrial tissue collected for measurement of progesterone and oxytocin receptors. Mean number of total progesterone receptors per cell was lower (P < 0.05) in the short cycle/endometrium group than in the normal cycle/endometrium group. Mean concentration of oxytocin receptors (fmol mg-1 protein) in the short cycle/endometrium group was higher (P < 0.05) than that in the normal cycle/endometrium group.(ABSTRACT TRUNCATED AT 250 WORDS)

Oxytocin receptor concentrations, inositol phosphate turnover and prostaglandin release by endometrium from ewes induced to ovulate during the early post-partum period

Induction of ovulation early post partum in sheep is associated with a high incidence (30-40%) of premature luteolysis. The present study was designed to characterize oxytocin receptor levels, oxytocin-stimulated inositol phosphate (IP) turnover (second messenger) and oxytocin-stimulated prostaglandin F2 alpha (PGF2 alpha) release in the endometrium of post-partum ewes induced to ovulate 21 days after parturition and expected to exhibit a range of corpus luteal functions subsequently. Ovulation was induced on day 21 post partum using a controlled internal drug release device and pregnant mare serum gonadotrophin, and uterine tissues were collected on days 5, 10 or 15 of the cycle (n = 4/day). A further 12 ewes whose interval from previous parturition exceeded 150 days were similarly treated and acted as controls. Measurement of daily peripheral progesterone concentrations revealed that while all control ewes exhibited normal luteal function, abnormal luteal function was evident in two, two and one post-partum ewes studied on days 5, 10 and 15 of the cycle respectively. Oxytocin receptor binding was detected (by receptor-binding assay and in-vitro autoradiography) in the endometrium and myometrium of post-partum ewes at all three stages of the oestrous cycle but only at day 15 in control ewes. To determine IP turnover, 100 mg caruncular endometrium was incubated in duplicate for 2.5 h with 10 microCi [3H]inositol and treated with 0 or 2 mumol oxytocin/l for 30 min, then [3H]inositol mono-, bis- and trisphosphates were quantified.(ABSTRACT TRUNCATED AT 250 WORDS)

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)

Concentration of oxytocin receptors in the placenta and fetal membranes of cows during pregnancy and labour.

The concentrations of oxytocin receptors were measured in intercaruncular and caruncular endometrium, fetal cotyledons, chorioallantois and amnion during pregnancy and parturition in cows. Tissues were obtained on days 20 (endometrium only), 50, 100, 150, 200, 225, 250, 275, at term (days 280-284), during labour and within 24 h after calving. Receptor concentrations in intercaruncular endometrium were low on day 20 of pregnancy, 39 +/- 11 fmol mg-1 protein. By day 50, receptor concentrations had increased more than tenfold to 572 +/- 52 fmol and rose steadily until day 250 and then levelled off at about 4500 fmol mg-1. Shortly before parturition, on day 282 +/- 1, a further rise to 7300 +/- 1418 fmol mg-1 was observed, these concentrations were maintained throughout labour. By contrast, caruncular endometrial receptor concentrations remained low until term, mean 145 +/- 15 fmol mg-1, and then rose to 720 +/- 163 fmol mg-1 during labour (cervix 17 cm--fully dilated). Fetal cotyledons and membranes had very low oxytocin receptor concentrations during most of pregnancy, on average only 20 fmol mg-1 protein. At term and during labour, receptor concentrations were significantly increased in both tissues. Mean concentrations during labour were 163 +/- 36 fmol mg-1 for cotyledons, 270 +/- 61 fmol mg-1 for chorioallantois and 311 +/- 121 fmol mg-1 for amnion.(ABSTRACT TRUNCATED AT 250 WORDS)

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)

Increase in concentration of uterine oxytocin receptors and decrease in response to 13,14-dihydro-15-keto prostaglandin F2 alpha in ewes after withdrawal of exogenous progesterone.

Ovariectomized ewes were treated with progesterone and oestradiol to induce oestrus (day of expected oestrus = day 0) and with progesterone on days 1 to 12. The concentrations of endometrial oxytocin receptors and the 13,14-dihydro-15-keto prostaglandin F2 alpha (PGFM) response induced by oxytocin were measured on days 12, 14, 16 and 18 after the cessation of progesterone treatment on day 12, by a receptor binding assay and direct radioimmunoassay, respectively. During the period of treatment, the concentrations of plasma progesterone were high and remained above 2 ng ml-1 until day 13 when they dropped rapidly to less than 0.5 ng ml-1 by day 14. The concentrations of oxytocin receptors in endometrium of control ewes were high (820.7 +/- 91.7 (SEM) fmol mg-1 protein). Treatment with progesterone significantly (P < 0.01) reduced the concentrations of the receptors on days 12 and 14 (144.1 +/- 65.0 and 200.4 +/- 45.4 fmol mg-1 protein, respectively). The receptor concentrations then increased to relatively high values on day 16 (1021.4 +/- 216.6 fmol mg-1 protein) and remained high until day 18 (677.7 +/- 103.4 fmol mg-1 protein).(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of oxytocin receptors in rabbit amnion involved in the production of prostaglandin E2.

We characterized oxytocin (OT) receptors in purified plasma membranes from amnion, decidua, and myometrium of late pregnant rabbits using an iodinated OT antagonist (OTA). Saturation studies showed similar Kd values for specific binding sites in the 100- to 250-pM range in all three tissues. OT receptor concentrations in decidua and myometrium did not change until the day of labor (day 31), when they rose about 2.5- and 18-fold, respectively. Increases in amnion receptors were first apparent on day 28 and continued to maximal levels on day 31. There was an increase of about 230-fold from day 26 to labor, reaching 9.5 pmol/mg protein. Competition studies using analogs showed that ligand specificities of amnion and decidual membranes were indistinguishable. Those of myometrial membranes were somewhat different, possibly owing to the presence of both AVP receptors and OT receptors in the myometrium. Binding of OTA corresponded to the OT-induced release of prostaglandin E2 (PGE2) by amnion cells in culture. The effects of OT were dose dependent, agonist specific, and selectively inhibited by OTA. Amnion cells from days 22 and 28 did not respond significantly to either OT or phorbol 12-myristate 13-acetate (PMA), but cells from day 30 pregnant rabbits responded strongly to both. In contrast, calcium ionophore stimulated comparable amounts of PGE2 release from cells cultured on day 22, 28, or 30. These studies show that specific, high affinity OT receptors are associated with the release of PGE2 from rabbit amnion cells. Increases in amnion OT receptor and protein kinase-C activity precede by several days the increases in receptor concentrations in decidua and myometrium, suggesting important roles for the amnion and OT in the initiation of labor in rabbits.