Ultrastructural basis for maintenance and termination of pregnancy

Physiology and Pathology of Contractility of the Myometrium

Uterine atony is a serious obstetrical complication since it is the leading cause of postpartum hemorrhage. Postpartum hemorrhage (PPH) is one of the 5 major causes of postpartum mortality; therefore, it requires immediate medical intervention, independent of whether delivery occurs normally or with a cesarean section. While in the past years most cases of postpartum hemorrhage were caused due to uterine atony following vaginal delivery, in recent years most PPH cases indicate a significant association with cesarean delivery. There are several methods used in order to avoid such a life-threatening complication, ranging from risk assessment to prevention, and finally medical intervention and management, if such an event occurs. In this scientific paper emphasis is given on the so-called "uterotonic" agents that are currently used, including oxytocin among others. It is, therefore, important to be familiar with these agents as well as understand the physiological mechanism by which they work, since they are used in everyday practice, not only for managing but also for preventing PPH. There are several potential questions that arise from the use of such "uterotonic" agents, and most specifically of oxytocin. Maybe one of the most important issues is the determination of optimal dosing of oxytocin in order to avoid PPH after a cesarean section.

Gap junctions

Gap junctions are essential to the function of multicellular animals, which require a high degree of coordination between cells. In vertebrates, gap junctions comprise connexins and currently 21 connexins are known in humans. The functions of gap junctions are highly diverse and include exchange of metabolites and electrical signals between cells, as well as functions, which are apparently unrelated to intercellular communication. Given the diversity of gap junction physiology, regulation of gap junction activity is complex. The structure of the various connexins is known to some extent; and structural rearrangements and intramolecular interactions are important for regulation of channel function. Intercellular coupling is further regulated by the number and activity of channels present in gap junctional plaques. The number of connexins in cell-cell channels is regulated by controlling transcription, translation, trafficking, and degradation; and all of these processes are under strict control. Once in the membrane, channel activity is determined by the conductive properties of the connexin involved, which can be regulated by voltage and chemical gating, as well as a large number of posttranslational modifications. The aim of the present article is to review our current knowledge on the structure, regulation, function, and pharmacology of gap junctions. This will be supported by examples of how different connexins and their regulation act in concert to achieve appropriate physiological control, and how disturbances of connexin function can lead to disease.

Acute management of preterm labor

We are continually reminded that the preterm birth rate has failed to improve; in fact, it has increased over the last 20 years. Much of this increase is related to the tremendous strides made by neonatologists and the resulting increased willingness of obstetricians to deliver preterm babies from hostile intrauterine environments. However, there is still much to learn concerning the pathogenesis, accurate early detection, treatment, and prevention of spontaneous preterm labor. This article concentrates on the clinical diagnosis and acute management of this enigmatic clinical problem.

A randomised trial of progesterone prophylaxis after midtrimester amniocentesis

BACKGROUND

Midtrimester amniocentesis to investigate fetal karyotype carries a small risk of fetal loss.

AIM

To test the hypothesis that progesterone prophylaxis may reduce this.

STUDY DESIGN

A randomised controlled trial comparing a short prophylactic treatment with progesterone after amniocentesis with untreated controls.

RESULTS

There were no differences in frequency of miscarriage, preterm delivery or neonatal outcome.

CONCLUSION

Prophylactic progesterone treatment after amniocentesis does not improve obstetric outcome.

Sensitivity of myometrium to CGRP varies during mouse estrous cycle and in response to progesterone

Calcitonin gene-related peptide (CGRP) inhibits contractions of the myometrium. Isometric force measurements on myometrial strips were carried out to monitor the inhibitory capacity of CGRP in the myometrium during the estrous cycle and in response to estrogen and progesterone in ovariectomized mice. CGRP inhibition of KCl-induced contractions was lowest at estrus and significantly increased during metestrus and diestrus. Progesterone treatment of ovariectomized mice resulted in a significant increase in the responsiveness of the myometrium to CGRP. Expression of CGRP-receptor component protein (CGRP-RCP), a marker of CGRP-receptor expression, was quantitated by Western and Northern blot analyses. The levels of inhibition exerted by CGRP during the various stages of the estrous cycle and in response to steroid hormone treatment correlated with the protein levels of CGRP-RCP. The mRNA levels did not change significantly during the estrous cycle or in response to hormone treatment, indicating that the regulation of CGRP-RCP protein does not occur at the transcriptional level. CGRP had an inhibitory effect both when applied before the stimulus for contraction and when applied during a sustained contracture induced by KCl. This suggests that CGRP-induced generation of second messengers can influence late events in electro-/chemomechanical coupling and/or the contractile machinery directly.

Ineffectiveness of oxytocin on intercellular communication between term pregnant human myometrial cells before labor

OBJECTIVE

The effects of the uterotonic agent oxytocin on intercellular communication between term pregnant human myometrial cells before labor were studied to investigate its contribution to the synchronicity of uterine contractions.

STUDY DESIGN

The membrane potential and the input resistance of myometrial cells were measured with intracellular micropipettes, and dye-coupling assays were concomitantly performed while the tissues were perfused with three concentrations of oxytocin (10(-8) mol/L, 10(-7) mol/L, and 10(-6) mol/L). The results were compared with those obtained from tissues perfused with gap junctional uncoupling agent octanol and those with Tyrode's solution (control).

RESULTS

Octanol increased input resistance and inhibited dye coupling but did not affect membrane potential compared with controls. Oxytocin depolarized membrane potential at all studied concentrations. The mean input resistance values and detectable dye propagation with oxytocin were similar to controls.

CONCLUSION

Oxytocin does not contribute to gap junctional intercellular communication in term pregnant human myometrium before labor but may affect contractions by depolarizing membrane potential.

Ethanol and parturition: a role for prostaglandins

A common pattern of birth defects was reported in children born to alcoholic women over 20 years ago. Shortly thereafter the constellation of defects became known as the Fetal Alcohol Syndrome, and reports from around the world served to acknowledge the pervasiveness of the disorder. Simultaneously with the clinical reports, animal models were developed to characterize the full spectrum of the teratogenic effects of ethanol. Not only did these animal models serve to define the actions of ethanol on fetal growth and development at the molecular pharmacological, neuroanatomical, and behavioral level, but unintentionally, they have resulted in renewed scientific interest in the effects of ethanol on pregnancy and parturition itself. The purpose of this review is twofold. First we will consolidate and summarize data from both clinical and basic research that pertains to ethanol and parturition. These data will demonstrate that ethanol consumption during pregnancy results in both delayed as well as premature delivery depending upon the pattern of consumption and timing of exposure. With these data as a background, the second objective will be to present a theoretical case for prostaglandins as possible mediators of ethanol-induced effects on the onset of parturition.

Increase of myometrial activity correlated with variations in 17 beta-estradiol and progesterone uterine concentrations in mid-term pregnant rat: estrogen agonist effect of 4-hydroxytamoxifen

The objective of this investigation was to examine the effects of 4-hydroxytamoxifen on the uterine activity. For this, we evaluated the electrical activity of the myometrium, chronically, in conscious unrestrained rats at mid-pregnancy. We also examined the tissular progesterone and 17 beta-estradiol concentrations in the myometrium and uterus 6 hours after administration of 4-hydroxytamoxifen. Comparison of myometrial electrical activities recorded during the control period with those obtained during the two periods (6 and 24 hours) after administration of 4-hydroxytamoxifen (80 micrograms.kg-1, s.c.) showed an increase in simultaneity of uterine contractions (P < 0.01). Tissular steroid hormone measurement by radioimmunoassay shows a fall of progesterone in the myometrium (P < 0.001) and of 17 beta-estradiol in the uterus (P < 0.01), 6 hours after administration of 4-hydroxytamoxifen. In the myometrium, for 50% of animals, 17 beta-estradiol concentration decreased (P < 0.01) and for 50% of animals it increased (P < 0.05). The decrease in progesterone is significant in the myometrium and in the whole uterus (respectively P < 0.001 and P < 0.01), 24 hours after administration of 4-hydroxytamoxifen. The 17 beta-estradiol concentration significantly decreased for all animals in the myometrium (P < 0.01) and in the uterus (P < 0.01), after this time. It appears that variation in progesterone induces the activation of uterine motility and exerts an effect on some factors involved in the regulation of the rat myometrium at mid-pregnancy.

Effects of estrogens on Ca channels in myometrial cells isolated from pregnant rats

Whole cell patch-clamp techniques were applied to cultured smooth muscle cells isolated from the longitudinal layer of the late pregnant rat myometrium. Effects of estrogens on Ca channels were examined. Inhibitory effects of beta-estradiol (1 microM) on Ca channel currents were recognized. The inhibitory effects of beta-estradiol depended on holding potentials. beta-Estradiol shifted the steady-state inactivation curve in the negative direction by 7 mV at mid potential (n = 9). Diethylstilbestrol, a synthetic estrogen, gave similar effects on Ca channel currents at lower concentration (2 microM) to those of beta-estradiol. Strong inhibitory effects on Ca channel currents were obtained by higher concentration (20 microM). Diethylstilbestrol shifted the steady-state inactivation curve in the negative direction by 7 mV at mid potential (n = 5). The results indicate that estrogens influence the voltage dependency and the whole cell conductance of Ca channels of pregnant rat myometrial cells. The acute effect of estrogens may cause both electrical and mechanical depression of myometrium.

Efficacy and safety of a 2-tier prostaglandin labor induction schedule

OBJECTIVES

To determine the efficacy and safety of a 2-tier, Bishop score-driven induction schedule using gel and tablet formulations of PGE2 to establish new monitoring guidelines.

DESIGN

Prospective descriptive study.

SETTING

eight Germany University obstetric departments.

SUBJECTS

467 high risk patients.

INTERVENTIONS

Bishop score < 5: intracervical 0.5 mg PGE2 gel; Bishop score 5-7: intravaginal 3 mg PGE2 tablet.

MAIN OUTCOME MEASURES

Uterine hyperstimulation: > 5 contractions/min, contraction duration > 2 min and emergency tocolytic therapy; perinatal outcome: secondary caesarean section rate, operative vaginal delivery and fetal acidosis.

RESULTS

PGE2 induction resulted in spontaneous delivery in 76.9% (6.4% fetal acidosis) and 14.1% secondary c. section. The gel was associated with more adverse uterine effects and a poorer fetal outcome than the tablet. Adverse uterine events increased significantly on repeat dosing with the gel (39.5 vs 29.3%), but not the tablet: they also increased with a dosing interval < 8 h with either formulation. Overall, adverse uterine events were fewer and perinatal outcome better in multiparae than in nulliparae.

CONCLUSIONS

The higher adverse uterine event rate with the gel should be viewed against the backdrop of the low Bishop score. The interval between repeat dosing should not be less than 8 h after PGE2 gel treatment. Primigravidae must be monitored with special care following labour induction with PGE2.

Molecular cloning of a rat uterine gap junction protein and analysis of gene expression during gestation

To study the molecular mechanisms controlling the rapid increase in myometrial gap junctions observed in the parturient uterus, we have isolated a full-length cDNA clone corresponding to a rat uterine gap junction protein. Nucleotide sequence analysis of the cDNA clone reveals complete identity of the coding region with that of a previously reported heart gap junction protein (connexin43). Southern blot analysis suggests that the gene encoding this gap junction protein exists as a single copy in the rat haploid genome and contains no introns within the coding region. RNA blot analysis with this gap junction cDNA reveals a single 3.0-kb mRNA in uterine tissue without changes in transcript size throughout gestation. When normalized to the amount of 28S rRNA, the relative abundance of the connexin43 transcript in uterine tissue is quite constant between the nonpregnant state, during gestation, intrapartum, and postpartum. Similar size transcripts are shown by RNA blot analysis to be present in heart, lung, liver, brain, and skeletal muscle, and these transcripts are identified by the same 3'-nontranslated sequence probe. The results of these studies suggest that rat connexin43 is encoded by a single gene that is transcribed to identical transcripts in heart, uterus, and other tissues. They further suggest that changes in the abundance of connexin43 transcript are unlikely to be responsible for the abrupt increase in connexin43-containing myometrial gap junctions at term.

Variations in oestradiol and progesterone receptor levels in the porcine myometrium and cervix during pregnancy and parturition

In vitro binding and exchange methods were used to determine the levels of oestradiol (E) and progesterone (P) receptors in cytosolic and nuclear fractions of cells obtained from the porcine myometrium and cervix during pregnancy and parturition. The concentration of oestradiol cytosolic receptors (ERc) in non-placental and placental myometrium and the cervix was low in early and mid-pregnancy, increasing at late pregnancy and was highest at parturition. The oestradiol nuclear receptors (ERn) followed the same pattern in the 3 sampling areas. The levels of PRc in non-placental regions were low in early pregnancy, increased to late pregnancy but declined during parturition. In placental regions the PRc value was high in early and mid-pregnancy, but decreased at late pregnancy and parturition. The concentration of PRc in the cervix increased significantly at mid-pregnancy and declined afterwards. In early and mid-pregnancy the PRn content was high, and decreased at late pregnancy and parturition in the 3 sampling areas. The data obtained agree with the present theories on the endocrine mechanisms regulating receptor levels in the uterus. Moreover, these data support the concept that the uterine motility during pregnancy and parturition might be influenced by changes in myometrial steroid receptor concentrations.

Rapid formation of myometrial gap junctions during parturition in the unilaterally implanted rat uterus

In uterine smooth muscles, gap junction plaques rapidly form during the final stages of gestation. To investigate the related mechanisms, regional differences in myometrial gap junction development in rat uterus were examined quantitatively during delivery, using thin-section and freeze-fracture techniques in combination with light- and electron microscopy. Examination of implanted and nonimplanted horns in the unilaterally ligated rat bicornuate uteri, revealed no differences in the occurrence of gap junction plaques, but after 2 to 4 pups had been delivered, the contracted segments contained more gap junction plaques than did noncontracted segments examined immediately before delivery. In all segments, gap junctions were found more frequently in the circular muscle layers than in the longitudinal muscle layers. Gap junctions ranged in size from 0.002 micron 2 to 0.52 micron 2, but two-thirds were less than 0.1 micron 2. The frequency of small gap junction plaques (less than 0.1 micron 2) was higher in the noncontracted segment. These results suggest that gap junctions are dynamic structures, and that their formation is controlled not only by general hormonal factors, possibly involved in gap junction increases in the myometrium before delivery, but also by local factors, possibly related to the contraction, that may accelerate an increase in gap junction formation during delivery.

Hormonal control of gap junctions in the myometrium

Myometrial tissues from immature, ovariectomized mature, and pregnant rats were examined by thin-section or freeze-fracture electron microscopy for the presence of gap junctions following treatment with steroid hormones and inhibitors of prostaglandin synthesis. Gap junctions were present in the myometrium of immature rats after repeated injections of estradiol or diethylstilbestrol (500 micrograms/day). The frequency of gap junctions in immature rats after estradiol treatment was similar to that found during delivery. Concomitant administration of indomethacin or meclofenamate with estradiol (50 and 500 micrograms/day) for 3 or 4 days enhanced the effect of estradiol on gap junctions. Progesterone depressed the response of 500 micrograms/day estradiol and both 50 and 500 micrograms/day doses of estradiol when administered together with the inhibitors of prostaglandin synthesis. Gap junctions were also present in the myometria of ovariectomized mature rats after treatment with estradiol by itself or together with indomethacin, and these effects were suppressed by concomitant progesterone administration. These results indicate that the steroid hormones and prostaglandins may interact to modulate the presence of gap junctions in the myometrium.

Effect of estradiol-17 beta and prostaglandins on rat myometrial gap junctions

The effects of estradiol-17 beta and indomethacin on myometrial gap junction development, plasma estradiol levels and uterine PGF2 alpha content were evaluated in immature and/or ovariectomized, mature rats. High doses of estradiol stimulated the development of gap junctions in the myometrium of animals from both groups. Concomitant injections of estradiol and indomethacin to ovariectomized rats potentiated the estradiol stimulation of gap junctions. Plasma estradiol levels were lower in ovariectomized rats treated with both estradiol and indomethacin than in animals treated with estradiol alone. Indomethacin also enhanced the uptake and retention of 3H-estradiol into uterine tissues. Uterine PGF2 alpha content of ovarectomized rats was stimulated with the initial injection of estradiol but thereafter, the PGF2 alpha content declined with repeated injections to values lower than that observed in controls. Prostaglandin F2 alpha content in tissues from rats treated with estradiol plus indomethacin were also higher than that observed in rats treated with indomethacin alone, however, the values obtained in both groups were significantly lower compared to those from control animals. These results are consistent with the hypothesis that steroid hormones and prostaglandins regulate myometrial gap junction formation. Regulation of myometrial gap junctions by prostaglandins is discussed with respect to a down regulation of the steroid-receptor mechanism and effects on cyclo-oxygenase or lipoxygenase products.

Biological implications of gap junction structure, distribution and composition: a review

Traditionally, all gap junctions have been considered to be identical in structure and function throughout the animal kingdom. Functions ascribed to these membrane specializations have been fundamental and have not been thought to differ significantly with respect to their mechanism of action. More recent studies support the view, however, that structural and compositional diversity may reflect significant functional differences between gap junctions in different classes of tissue but no clear and definitive patterns have yet emerged. This review does not attempt to comprehensively analyze the totality of the vast gap junction and coupling literature but focuses instead upon those recent observations which raise new questions related to the biological activities of gap junctions in different tissues.

Improved electrical coupling in uterine smooth muscle is associated with increased numbers of gap junctions at parturition

We have studied some passive electrical properties of uterine smooth muscle to determine whether a change in electrical parameters accompanies gap junction formation at delivery. The length constant of the longitudinal myometrium increased from 2.6 +/- 0.8 mm (X +/- SD) before term to 3.7 +/- 1 mm in tissues from delivering animals. The basis of the change was a 33% decrease in internal resistance and a 46% increase in membrane resistance. Axial current flow in an electrical syncytium such as myometrium is impeded by the cytoplasm of individual cells plus the junctions between cells. Measurement of the longitudinal impedance indicated that the specific resistance of the myoplasmic component was constant at 319 +/- 113 omega . cm before term and 340 +/- 93 omega . cm at delivery. However, a decrease in junctional resistance was apparent from 323 +/- 161 omega . cm to 134 +/- 64 omega . cm at delivery. 1.5-2 d after delivery, the junctional resistance was increased, as was the myoplasmic resistance. Thin-section electron microscopy of some of the same muscle samples showed that gap junctions were present in significantly greater numbers in the delivering tissues. Therefore, our results support the hypothesis that gap junction formation at delivery is associated with improved electrical coupling of uterine smooth muscle.

Electrical activity of the pregnant uterus in the cynomolgus monkey

Electromyographic activity in six pregnant cynomolgus monkeys was recorded in vivo by means of bipolar electrodes individually attached to the surface of the uterine corpus. These data were compared with recordings of intra-amniotic pressure obtained simultaneously by means of an open-ended, fluid-filled catheter placed in the amniotic cavity. During the last third of pregnancy, the electromyograms showed recurrent electrical complexes (trains of potentials, TOP) which lasted for about 2 minutes and whose rate of occurrence varied during the nyctohemeral period. The initiation of electrical activity in various uterine areas was always synchronous with and was related to mechanical contraction. The complex, polyphasic contractions that were observed can be explained, largely, by the variations of potentials within a train of potentials. However, a simple quantitative relationship between the mean frequency of the potentials within the TOP and the mean amplitude of the change in pressure could not be demonstrated. In contrast to data available on the woman, the increase in contractile strength at the onset of labor in the macaque appears to be due to a higher frequency of excitation of individual cells rather than the result of an improved functional coordination between the active muscular elements.

Biochemistry and pharmacology of the myometrium and labor: regulation at the cellular and molecular levels

This review presents the essentials of cell biology, biochemistry, and pharmacology of the myometrium and labor, especially as they relate to premature labor. The aim is to explore the neuroendocrine and pharmacologic regulation of myometrial muscle cells in the context of cell membrane function and contractile processes. In the first section, smooth muscle protein structure and interactions are described, the central roles of myosin light-chain kinase, calcium, and cyclic adenosine monophosphate (cAMP) are reviewed. Subsequently, events relating to hormone/drug receptor function and implications for pharmacologic applications are discussed. Finally, the processes of information transfer via hormones and drugs are incorporated into a model which explains how the hormones and drugs used in clinical practice may mediate myometrial contractility. The reader is introduced to current concepts of the cellular, molecular, and pharmacologic aspects of labor in order to facilitate the understanding of medical augmentation of labor and tocolytic therapy.

Appearance of gap junctions in the myometrium of women during labor

Myometrial tissue from 29 women who underwent cesarean section in various stages of labor were examined quantitatively in the electron microscope. Gap injections were presented between smooth muscle cells with low frequency in women with a closed cervix and infrequent contractions of the uterus. There was a statistically significant correlation between increased cervical dilation or increased frequency of uterine contractions and increased area of gap junctions. Tissues from women in premature labor had more gap junctions than tissues from other women who underwent cesarean section either prematurely or at term. We propose that the formation of gap junctions may be stimulated by some physiologic change. The appearance of gap junctions may then terminate pregnancy by resulting in coordinated, synchronized muscle activity and dilatation of the cervix followed by expulsion of the fetus.

Presence of gap junctions in the myometrium of women during various stages of menstruation

Myometrial tissues from 36 women who underwent hysterectomy at various stages of the menstrual cycle were examined in the electron microscope for the presence of gap junctions. Gap junctions were present between smooth muscle cells in some of the tissues from women with dysmenorrhea, during menstruation and at other times. The suggestion is made that gap junctions may form between myometrial cells in nonpregnant women in response to physiologic or pathologic stimuli, such as the production of prostaglandins, and the presence of the contacts may result in contractions of the uterus.

Gap junction formation and regulation in myometrium

Myometrial tissues from pregnant rats were examined by electron microscopy for the presence of gap junctions after incubation in vitro with a variety of agents. Gap junctions were present in low frequency or absent prior to incubation in vitro. The junctions were present in control tissues in high frequency after 48 h incubation. The addition of cycloheximide or actinomycin D inhibited the incorporation of [3H]leucine into TCA-precipitable proteins and prevented gap junction formation. A prostacyclin analog (carbacyclin), a thromboxane synthesis inhibitor, and indomethacin also prevented gap junction formation. Oxytocin had no effect on gap junction formation but isoxsuprine decreased their number and increased their size. Isoxsuprine and isoproterenol also produced electron opaque crystals associated with the gap junctions. Dibutyryl cAMP treatment but not monobutyryl cGMP also increased the size of gap junctions. Based upon this and previous studies, we propose at least four sites for regulation of gap junctions and possible control of labor.