Up-regulation of mesotocin receptors in the tammar wallaby myometrium is pregnancy-specific and independent of estrogen

The oxytocin-like peptide of most Australian marsupials is mesotocin, which stimulates uterine contractions and is important for normal birth in the tammar wallaby. Female marsupials have two uteri and, in monovular species such as the tammar, one uterus is gravid with a single fetus, whereas the contralateral uterus is nongravid. A significant increase in myometrial mesotocin receptor concentrations occurs only in the gravid uterus on Day 23 of the 26-day gestation. This study examined whether or not mesotocin receptors are present in the myometrium and are up-regulated at the equivalent stage of the luteal phase in unmated tammars. In contrast to the marked increase in mesotocin receptor mRNA and protein concentrations in the myometrium of the gravid uterus during pregnancy, receptors did not increase in the unmated animals. There were also no significant differences between the two uteri, except on Day 27. Plasma profiles of peripheral estradiol-17beta and progesterone did not differ significantly between pregnant and nonpregnant cycles. However, progesterone concentrations were significantly lower on Day 1 postpartum compared with Day 27 of the nonpregnant cycle. In pregnant tammars, the molar ratio of circulating estradiol-17beta to progesterone increased significantly between Day 25 of gestation and 1 day postpartum, but was not correlated with an increase in mesotocin receptor concentrations in either uterus. The data confirm that a local fetal influence is more important than systemic factors, such as estrogen, in the regulation of uterine mesotocin receptors in the tammar wallaby.     

Adenylate cyclase of myometrium and its regulation by cAMP-dependent protein kinase

The purpose of this study was to examine activity of adenylate cyclase (AC) and its cAMP-dependent phosphorylation by cAMP-dependent protein kinase (PKA) in the membrane of rabbit myometrium. Isoproterenol (IP) significantly increased AC activity of nonpregnant rabbits myometrium plasma membranes. However, during pregnancy AC of myometrium plasma membranes did not respond to IP. Phosphorylation of the myometrium membrane by PKA was followed by significantly decreased response of AC to IP.     

Rabbit relaxin: the influence of pregnancy and ovariectomy during pregnancy on the plasma profile.

A porcine relaxin radioimmunoassay was developed to evaluate the profile of immunoreactive relaxin in rabbit plasma. Relaxin was nondetectable in pseudopregnant (Days 1, 4, 5-8, 12, and 16), nonpregnant, and male rabbits. However, in pregnant rabbits, relaxin was detected during the peri-implantation period (Days 4-9). Peak concentrations were reached on Day 15 and were maintained until parturition (Day 32). Relaxin concentrations abruptly decreased on Day 1 postpartum to low but detectable concentrations that were unchanged during the first week postpartum. In contrast, progesterone concentrations peaked earlier (Day 13), decreased after Day 25, and were not detectable on Day 1 postpartum. The effect of ovariectomy on the profile of plasma relaxin was evaluated. Four pregnant rabbits were ovariectomized (Day 13) and treated with medroxyprogesterone acetate to maintain pregnancy. As in normal pregnant rabbits, relaxin was observed initially during the peri-implantation period (Days 4-9) and increased to peak concentrations by Day 16. These concentrations were maintained until parturition and abruptly decreased on Day 1 postpartum to low yet detectable concentrations during the first week postpartum. The concentrations of relaxin in the plasma of ovariectomized medroxyprogesterone-treated rabbits were not different from those in three sham controls. These results indicate that the ovary is not a significant source of relaxin in pregnant rabbits. The unique observation of the presence of relaxin during the peri-implantation period suggests that this hormone has a role in preparing the rabbit uterus for implantation. The continued presence of relaxin during the first week postpartum may represent residual hormone, or it may suggest a physiological role during the early postpartum period.     

Regulation of cyclic nucleotide phosphodiesterase activity in myometrium from pregnant and spayed rhesus monkeys.

The activities of myometrial cyclic nucleotide phosphodiesterases (PDEs) and the sensitivity of these enzymes to the effector molecules, cGMP and cAMP, were determined in the 100,000 g supernatant of homogenates from pregnant and spayed rhesus monkeys. The specific activities (per mg nitrogen) of the myometrial cyclic nucleotide PDEs in the supernatant from spayed monkeys were higher than those from pregnant monkeys at all substrate levels studied. However, when calculated on the basis of the DNA content of the myometrium, which was 8 times higher in the spayed than in the pregnant animals, the specific activities were lower in the tissue from spayed animals. At substrate levels of 2 . 5 micron-cAMP, low levels of cGMP (0 . 1-1 . 0 micron) caused the same percentage increase in cGMP-PDE activity in both tissues. At high substrate levels of 100 micron-cAMP, 1 micron-cGMP inhibited only the cAMP-PDE from spayed monkeys, and the enzyme from spayed monkeys was more effectively inhibited by 10 and 40 micron-cGMP than was the enzyme from pregnant animals. The cGMP-PDE activity was inhibited by cAMP (1 . 0-50 . 0 micron), and the percentage inhibition with increasing levels of cAMP appeared to be similar in the two series. The levels of cGMP and cAMP that modify the rate of hydrolysis of the other nucleotide in rhesus myometrium seem to be within the physiological range for these compounds in situ. It therefore appears possible that cAMP and cGMP are each involved in regulating the degradation of the other nucleotide in rhesus myometrium.     

Localization and regulation of IL-1alpha in rat myometrium during late pregnancy and the postpartum period

Interleukin-1 (IL-1) has been implicated as a participant in preterm labor that is induced by bacterial infection. Previously, we showed that serotonin-induced production of IL-1alpha by myometrial smooth muscle cells in vitro is also essential for the synthesis of interstitial collagenase. It is therefore likely that IL-1alpha production in uterine tissues has implications for both the normal physiology of involution and for the pathophysiological mechanisms of preterm labor. The objective of this study was to characterize the serotonin-induced production of IL-1alpha by myometrial cultures in vitro and to assess the production of IL-1alpha and its relationship to collagenase production in vivo during pregnancy and the postpartum period. Immunohistochemistry demonstrated IL-1alpha protein in the nuclei and cytoplasm of serotonin-treated myometrial cells. IL-1alpha levels were decreased by treatment with progesterone or IL-1-receptor antagonist but were unaffected by lipopolysaccharide. Western analysis of myometrium from pregnant rats showed low levels of IL-1alpha during midpregnancy with increased concentrations at days 21 and 22 and postpartum. IL-1alpha mRNA levels also increased from days 15 to 22. Levels of mRNA for IL-1beta also increased, although to a lesser degree than IL-1alpha. Both mRNAs decreased postpartum. Conversely, mRNA for interstitial collagenase was barely detectable at term but increased postpartum. Together, these data show that serotonin stimulates IL-1alpha production in vitro and indicate that normal myometrium from pregnant rats is an identifiable source of IL-1 during late pregnancy. The findings are consistent with the possibility that myometrial IL-1alpha participates in normal labor as well as the postpartum production of interstitial collagenase.     

Activity of lysosomal hydrolases in human placenta, amnion, decidua and myometrium in the 2nd and 3rd trimester

Six lysosomal hydrolases were studied in tissues of human placenta, amnion, decidua, and myometrium obtained in the 2nd and 3rd trimester. No significant increase in the activity of any lysosomal hydrolase was found in the 3rd-trimester values compared with those obtained in the 2nd trimester. Thus, the increased activity of lysosomal hydrolases in maternal serum at term, previously described in several studies, seems not to be paralleled by a corresponding increase in the activity of these enzymes in the tissues of the pregnant uterus. We speculate that the increase in maternal serum towards term may reflect a macrophage activation initiated by elevated estrogen concentrations in the 3rd trimester.     

Up-regulation of Rnd1 during pregnancy serves as a negative-feedback control for Ca2+ sensitization of contractile elements in rat myometrium

We investigated the role of Rnd1, a member of the small GTP-binding Rho protein family, in the change in Ca(2+) sensitivity of contractile element in rat myometrium at estrus, gestation, and postpartum stages. In the permeabilized muscles, GTPgammaS or carbachol with GTP increased Ca(2+) sensitivity of contractile force in non-pregnant myometrium at the estrus stage, whereas these stimuli were ineffective in pregnant myometrium at day 21. After postpartum, the reduced Ca(2+) sensitization was recovered. Semi-quantitative RT-PCR analysis indicated that the expressions of RhoA, ROCKI, and ROCKII were not significantly different between non-pregnant and pregnant myometria. In contrast, the expression of Rnd1 was increased during the course of pregnancy, reaching a maximal at day 21, and rapidly declined after the delivery. On the other hand, Ca(2+) sensitization of contractile elements was decreased during the progress in gestation. These results suggest that Rnd1 may have an important role as a negative-feedback control of uterine contraction during gestation through the inhibition of RhoA-mediated increase in the Ca(2+) sensitivity of contractile elements.     

EMG activity of the muscular and stromal layer of the cervix in relation to EMG activity of the myometrium and cervical dilatation in PGF2alpha induced parturition in the cow

The goal of this study was to quantify and characterize the electromyographic (EMG) activities in the cervical outer muscular layer and in the cervical stromal layer, and to characterize their relationship with myometrial EMG activity and cervical dilatation during PGF2alpha-induced parturition in term pregnant cows. We continuously measured the EMG activity of the uterine myometrium and cervical outer muscular layer as well as the cervical stromal layer in five cows using bipolar electrodes while at the same time measuring changes in the cervical diameter with ultrasound cervimetry. This we did from the moment a prostaglandin analogue was injected until the expulsion of the calf. In contrast to the cervical stromal layer, the cervical outer muscular layer showed distinct EMG activity, which began to increase at about the same time as the EMG activity of the myometrium, i.e. some 12 h before the start of cervical dilatation. However, the rate of this increase was lower than in the myometrium and it was not characterized, like in the myometrium, by an increase in maximum EMG amplitude. Although the cervical outer muscular layer showed contracture and contraction like EMG activity in unison with in the myometrium, it was also characterized by a more irregular EMG activity, which occurred independently from the myometrium. These data suggest that while the outer muscular layer of the cervix may be considered to be a caudal continuation of the myometrium, it also displays activity independently from the myometrium. The physiological relevance of this activity remains to be explored.     

Corticotropin-releasing hormone effects on human pregnant vs. nonpregnant myometrium explants estimated from a mathematical model of uterine contraction.

In this paper, we applied a new theoretical model of uterine contraction to a large panel of human pregnant and nonpregnant myometrial strips, treated or not by corticotrophin-releasing hormone (CRH). This model is based on a fine analysis of the contraction curves. This analysis yielded four mathematical parameters (beta, theta, tau 1, and tau 2) related to excitability, duration of plateau phase, and time constants for relaxation describing, respectively, the different portions of the contraction cycle. This leads to specific differences in spontaneous contractile activity between pregnant and nonpregnant states. The relaxing effect of CRH in the pregnant state is presumably correlated with the origin of the strips (the lower uterine segment). Besides our observation of a specific receptor-dependent relaxing effect of CRH in both pregnant and nonpregnant myometrium, we could identify highly significant effects at given CRH concentration for beta in nonpregnant myometrium and for theta, tau 1, and tau 2 in pregnant myometrium. In addition, highly significant differences were found between pregnant and nonpregnant myometrium. Also, we discovered a strong correlation between theta and tau 1, specifically in the pregnant state. Although the biochemical signification of these results remains to be elucidated, they contribute to emphasize the complex network of CRH action at the myometrial level. Furthermore, our approach could pave the way toward a better analysis of the efficacy of the uterine contractile behavior.     

Spontaneous electrical activity and effects of noradrenaline on pregnant human myometrium recorded by the single sucrose-gap method

Spontaneous intracellular electrical activity and contraction of pregnant human myometrium were recorded by the single sucrose-gap method, and the effects of noradrenaline on the muscle were studied. Human myometrium obtained at various stages of gestation showed three types of action potentials, which were spike-, plateau- and intermediate-type in configuration. In human isthmic myometrium at full-term pregnancy dissected from the endometrial side of the uterine wall, both plateau and spike types of action potential were observed. All contractions were well synchronized with each action potential. These results indicate the possibility of the intertwinning of two different types of muscle bundles. The magnitude of contraction depended on the frequency of the spikes in spike-type or the plateau duration in plateau-type action potentials. The alpha-excitatory action of noradrenaline (2 X 10(-7) g/ml) was found to affect the configuration of the action potential; even the spike-type configuration became plateau.     

The significance of prostacyclin produced by pregnant rat myometrium: the relationship between myometrial prostacyclin producing activity and passive stretch of myometrium by growing conceptus

The present experiment was performed to elucidate the significance of prostacyclin (PGI2) produced by pregnant rat myometrium. PGI2-like substance producing activity of various portions of the uterus was measured at selected gestational stages by platelet bioassay; surface area per 1 gm of uterine wall enveloping one conceptus was calculated; and spontaneous contractility of myometrium of both conceptus and non-conceptus regions and the effects of authentic PGI2 on it were examined. PGI2-like substance producing activity increased with advancing pregnancy, but the activity varied according to area of the myometrium, being highest in the area where it was most greatly stretched by the growing conceptus and lowest where no conceptus was contained. Spontaneous contractility was reduced in regions with high PGI2 producing activity. Though authentic PGI2 generally exhibited a stimulatory effect, it had an inhibitory effect on Day 10 pregnant myometrium. From these results, it may be concluded that the producing activity of PGI2, which remarkably increases in the conceptus region with the advance of pregnancy, keeps the uterine wall relaxed, making the uterus adapt to the growth of the fetus. Passive myometrial stretch by the growing conceptus is thought to be one of factors which enhance myometrial PGI2 producing activity.     

Bovine Steroid Hormone and SHBG Concentrations Postpartum and during the Oestrous Cycle

Changes in consecutive estimates of milk progesterone concentrations and serum steroid hormone and sex hormone-binding globulin (SHBG) concentrations in the postpartum period were examined in Finnish Ayrshire and Friesian dairy cows which were divided according to feeding into a hay group and a silage group. Milk progesterone concentrations rose above 10 nmol/1, indicating the start of ovarian luteal activity, slightly earlier in the silage group (28.4 ± 8.7 (S.D.) days, n = 19) than in the hay group (33.4 ± 10.3, n = 28) after calving. Likewise, the first normal oestrous cycles began slightly earlier in cows fed with silage. On the other hand, no differences in the beginning of ovarian luteal activity were observed between the breeds. Serum oestradiol-17β, oestrone, testosterone, 5α-dihydrotestosterone (5α-DHT), pregnenolone and progesterone concentrations were fairly unchanged during postpartum anoestrus after uterine involution and before ovarian cyclic activity. After first ovulation, considerable increases in milk and serum progesterone concentrations were observed. The increase was accompanied by elevations in serum pregnenolone and 5α-DHT concentrations. In the late luteal phase, progesterone, 5α-DHT and pregnenolone concentrations rapidly declined, leading to low hormone levels in pro-oestrus. Thereafter, serum pregenolone and 5α-DHT concentrations slightly increased during the follicular phase. On the other hand, oestradiol-17β concentrations were elevated in pro-oestrus and decreased after that, being lowest at met-oestrous. Serum testosterone concentrations appeared to be unchanged during postpartum anoestrus and over the oestrous cycle. Serum SHBG concentrations were unchanged during postpartum anoestrus and over the oestrous cycle, as well as in pregnant animals. The serum SHBG concentrations were about double those found in women with normal menstrual cycles, whereas oestradiol concentrations were much lower. At present, it cannot be explained how the biological effects of oestradiol become evident under such conditions.     

Bovine Uterine,Cervical and Ovarian Cytosol Estrogen and Progesterone Receptor Concentrations in Cystic Ovarian Disease

Bovine cytosol estrogen (ERC) and progesterone receptor (PRC) concentrations were measured simultaneously in various regions of the uterus and in ovarian stromal tissue in cows with cystic ovarian disease (follicular cysts), arid the concentrations compared with those in animals with normal cycles. In cystic ovarian disease, ERC concentrations in endometrium (550 fmol/mg cytosol protein (c.p.)) and in myometrium (405) were significantly higher than in control animals. Very high PRC contents were measured in the endometrium (3115) and myometrium (2761) of cows with cystic ovarian disease. In control animals, PRC concentrations in the endometrium and myometrium were significantly lower than in diseased animals. No statistical differences were observed in ERC or PRC contents between the endometrium and the myometrium in cows with cystic ovarian disease. ERC and PRC concentrations in the uterine cervix and ovaries were low compared to those detected in the uterus. Bovine serum estradiol-17ß concentrations were higher (p<0.001) in cows with cystic ovarian disease than in control animals in postpartum anestrus or during the normal estrous cycle. Serum sex hormone-binding globulin (SHBG) concentrations were of the same magnitude as in control cows during their estrous cycles. These findings show that long standing low endogenous progesterone and elevated estradiol concentrations in serum are associated with elevated ERC and PRC concentrations in bovine uterus.     

Expression of prostanoid receptors in human lower segment pregnant myometrium

Prostanoids, especially prostaglandin (PG) E(2), are important mediators of uterine relaxation and contractions during gestation and parturition. Inhibitors of PG formation as well as PG analogues are used to modulate uterine tonus. So far, only limited data are available regarding the expression of prostanoid receptors in human pregnant myometrium. In the present study, the expression of the receptors for PGE(2) (EP1, EP2, EP3, EP4), PGF(2alpha) (FP), prostacyclin (IP), and thromboxane A(2) (TP) in human pregnant myometrium was studied by RT-PCR, in situ hybridization and immunohistochemistry. Myometrial tissue was obtained from five women at term and not in labour and from two women who delivered preterm. Tissue specimens were excised from the upper edge of the transverse lower uterine segment incision. In all tissues analysed, EP1, EP2, EP3, EP4, FP, TP and IP receptor mRNA and protein was detected. mRNA expression for PGD(2) (DP) receptor was not detected in the majority of tissue specimens. EP1, EP2, EP4, IP, TP and FP receptor protein was detected on myometrial smooth muscle cells, whereas EP3 receptor protein was only expressed by stromal and endothelial cells. In situ hybridization experiments yielded similar results. The expression of the EP2 receptor mRNA was inversely related to gestational age. We suggest that the contractile effect of PGE(2) at term is probably mediated directly by the EP1 receptor expressed in myometrial smooth muscle cells and indirectly by the EP3 receptor expressed in stromal cells and a decrease in EP2 receptor expression.     

Nitric oxide synthase activity in human trophoblast, term placenta and pregnant myometrium

To investigate the possible role of nitric oxide (NO) produced locally or intramurally in the quiescence of the pregnant myometrium, nitric oxide synthase (NOS) activity was measured in samples from first trimester (villous, and non villous-trophoblast), term placenta and pregnant myometrium. Trophoblast tissue was obtained from psychosocial termination of pregnancy (9 – 12 weeks' gestation) whereas placenta and myometrium, from the same patient, at deliveries by Caesarean section. NOS activity was measured in both cytosolic and particulate fractions by the formation of ¹⁴C-citrulline from ¹⁴C-arginine. Western immunoblotting was used to identify the endothelial NOS (eNOS) and neuronal (nNOS) isoforms. The activity of NOS in particulate fractions from all preparations was considerably higher than the cytosolic fractions. Activity in all fractions except the myometrium was highly Ca-dependent. More than 50% of particulate NOS from the myometrium was Ca-independent. NOS activity was highest in the villous trophoblast and there was a significant difference between the villous and non-villous trophoblast. In placenta and myometrium, NOS was 2–4 fold and 20–28-fold lower than the villous trophoblast, respectively. Western blot analysis showed clearly eNOS in the particulate fraction and a weak eNOS band in the cytosolic fractions, whereas nNOS was not detectable in any of the fractions. In view of the marginal activity of NOS in the myometrium, NO produced by the trophoblast and placenta could play a significant role in maintaining uterine quiescence by paracrine effect.     

Influence of oxidative stress and metabolic adaptation on PON1 activity and MDA level in transition dairy cows

Serum PON1 is a HDL-associated enzyme that protects lipoproteins, both LDL and HDL, against oxidation and it is considered as an antioxidative/anti-inflammatory component of HDL. Dairy cows are highly susceptible to oxidative stress which commonly occurs in late pregnancy and early lactation. During the transition period, increased production of reactive oxygen species is associated to processes of metabolic adaptation to a low-energy balance. We investigated serum paraoxonase-1 (PON1) activity and malondialdehyde (MDA) concentration to assess the antioxidative/prooxidative status during pregnancy and the postpartum period. In order to evaluate metabolic homeostasis, common metabolic parameters (glucose, triglyceride, total cholesterol, HDL-C and albumin concentrations) were determined as well. A significantly lower PON1 activity was found in late pregnancy and early postpartum (P<0.05) compared to the first and the second trimester of pregnancy and the mid-lactation. MDA level was significantly higher (P<0.05) in the dry period compared to pregnant lactating and postpartum cows. Serum glucose concentration (P<0.001) was lower in the early and late puerperium indicating low-energy balance in the early lactation. Serum triglyceride and albumin concentrations were lower in late puerperium (P<0.001), while total cholesterol and HDL-C were lower during the dry period (P<0.05) as well as in early postpartum (P<0.001). Significant correlations of PON1 activity with glucose (P<0.05), albumin (P<0.05), total cholesterol (P<0.001) and HDL-C (P<0.001) were also found. The observed lower serum PON1 activity and higher MDA level in late pregnancy and early postpartum could indicate a prooxidants/antioxidants imbalance influenced by reproductive stress and metabolic adaptation in the transition period of dairy cows.     

Expression of the calcium-activated potassium channel in upper and lower segment human myometrium during pregnancy and parturition

Background  

Large conductance calcium-activated potassium channel (BKCa) plays an important role in the control of uterine contractility during pregnancy. The change from uterine quiescence to enhanced contractile activity may be associated with the spatial and temporal expression of BKCa within myometrium. The objectives of this study were to examine the expression of BKCa alpha- and beta-subunit in upper segment (US) and lower segment (LS) regions of uterus, and to investigate for the possibly differential expression of these proteins in US and LS myometrium obtained from three functional states: (1) non-pregnant (NP); (2) term pregnant not in labour (TNL) and (3) term pregnant in labour (TL).     

Specific glucocorticoid binding in human uterine tissues, placenta and fetal membranes

The binding of [3H]dexamethasone to cytosol fractions of human myometrium, endometrium, decidua, chorion, amnion and placenta has been studied. All tissues examined contained high affinity, low capacity binding sites with high specificity for glucocorticoids. Maximum specific binding of [3H]dexamethasone was reached after about 10 h at 0-4 degrees C and remained stable for at least the next 12 h. Sucrose density gradient analysis showed that the binding macromolecules sedimented at 7.9 S in hypotonic solutions and at 4.35 in solutions containing 0.4 M KCl. In the presence of sodium molybdate, the sedimentation coefficients shifted both in the absence and presence of 0.4 M KCl to 8.9 and 5.7 S, respectively. The apparent equilibrium dissociation constants (Kd) of the glucocorticoid binding sites were similar in most tissues, ranging between 1 and 6 nM, with the exception of the placenta in which the binding sites showed a higher Kd (13-22 nM). In all tissues studied, the binding affinities were similar in nonpregnant and pregnant patients and in patients at different stages of pregnancy or in labor. The concentration of the binding sites in the different tissues ranged from 11 to 268 fmol/mg protein, higher concentrations being found in myometrium, placenta and amnion and lower concentrations found in endometrium, chorion and decidua. The number of binding sites was higher in the myometrium of nonpregnant than pregnant women, but was similar in the myometrium of women at term pregnancy before or during labor. In the placenta, the number of binding sites increased significantly from early pregnancy to midpregnancy, while in chorion, amnion and decidua the number of binding sites did not change during pregnancy. It is concluded that human uterine tissues, placenta and fetal membranes contain specific binding sites with properties characteristic of glucocorticoid receptors suggesting that these tissues may respond directly to glucocorticoids.     

Effects of 17 beta-estradiol on myometrial gap junctions and pregnancy in the rat

The effects of estradiol treatment on the development of myometrial gap junctions and premature labour were investigated using timed pregnant rats. In control animals myometrial gap junctions were infrequent between days 17 and 20 of pregnancy, but began to develop on day 21 and were at maximum frequency, size, and membrane area on day 22 during delivery. Gap junctions were completely absent from the myometrium 48 h after delivery. Animals treated with 500 micrograms 17 beta-estradiol/day starting on day 16 of pregnancy developed numerous myometrial gap junctions and delivered their pups prematurely on day 19. Similarly, treatment with 50 micrograms estradiol/day resulted in the development of myometrial gap junctions on day 20 of pregnancy and premature labour. However, treatment with various doses of estradiol up to and including 500 micrograms/day for 3 days beginning 1 day before delivery was not able to maintain the presence of myometrial gap junctions during the postpartum period. These results support the hypothesis that estradiol stimulates the development of myometrial gap junctions and that the presence of gap junctions in the myometrium is a requirement for the occurrence of term, as well as preterm labour. Furthermore, it is evident from this study that the postpartum regression of myometrial gap junctions is not dependent on the decrease in estradiol.