A. López Bernal

11β-hydroxysteroid dehydrogenase activity (E.C. 1.1.1.146) in human placenta and decidua

Abstract 11β-Hydroxysteroid dehydrogenase activity (E.C. 1.1.1.146) was studied in homogenates of placenta, fetal membranes and decidua obtained at term, using cortisol and cortisone as substrates. For activity in the direction cortisol to cortisone, the distribution of enzyme was choriodecidua > placenta. While choriodecidua catalysed the conversion in both directions, cortisone reduction was negligible in placenta. Amnion contained no measurable enzyme, using either substrate. The activity in the choriodecidua was thought to be of decidual rather than chorionic origin. In both decidua and placenta, the enzyme was localised by subcellular fractionation in the microsomal fraction, and microsomal fractions were used to study the dependence of activity on pH, substrate and enzyme concentration and the inhibitory effect of other steroids. Data on the K M values for cortisol and the inhibition of the enzyme by some steroids, notably androgens, suggest that the enzymes in decidua and placenta may be different. The findings are discussed in relation to the possible role of the enzyme in the control of fetal corticosteroid concentrations.

Overview of Current Research in Parturition

The mechanism of human parturition is not understood and further research into this important physiological process is needed. Preterm labour remains a major cause of perinatal mortality and morbidity and there is controversy about the effectiveness of current tocolytic agents. In some species, notably the sheep, parturition is preceded by an activation of the fetal hypothalamic‐pituitary‐adrenal axis. However, in primates this axis has a supportive, rather than essential, role. A fall in maternal progesterone levels is a prerequisite for parturition in most mammals and this takes place either through increased conversion of progesterone to oestrogens in the placenta, or through the demise of the corpus luteum of pregnancy, depending on the species. In primates and guinea‐pigs parturition occurs without an apparent fall in maternal progesterone levels. Gene targeting experiments in mice have demonstrated the critical role of prostaglandin FP receptors, necessary to mediate the luteolytic effect of PGF2α before parturition. Prostaglandin synthesis is required for the onset and progress of labour as demonstrated by experiments with cPLA2‐ and PGHS‐1‐deficient mice. The importance of local tissue conversion of progesterone to reduced androgens in the regulation of cervical ripening has been demonstrated in 5α‐reductase‐deficient mice. The chronic and ubiquitous gene inactivation obtained with conventional methods has disadvantages, in that it may allow the activation of compensating pathways, making the interpretation of results difficult. This problem may be overcome by using pulsed and tissue‐selective gene knockout strategies. The study of human parturition is complicated by the lack of access to direct experimentation, whereas the endocrine differences between species make it difficult to extrapolate animal data to humans. However, the development of genomic/proteomic technologies that allow the simultaneous screening of thousands of genes and gene products in small samples of tissue, and new methods to study the biochemistry of receptors and proteins involved in smooth muscle physiology promise new insights into the control of human labour. Nevertheless, the integration of rapidly expanding knowledge into a complete understanding of the roles of the mother and the fetus in the initiation of parturition, and the development of selective medication for the effective management of preterm labour remain an arduous challenge for the next decade.

Effect of oxytocin antagonists on the activation of human myometrium in vitro: Atosiban prevents oxytocin-induced desensitization

OBJECTIVEnOur purpose was to investigate whether the sensitivity of myometrial cells to oxytocin is affected by prolonged exposure to oxytocin antagonists.nnnSTUDY DESIGNnTissue slices or cultured myometrial cells were exposed to peptides in vitro. Myometrial activation was studied by measuring the formation of inositol phosphates and the changes in intracellular calcium. Oxytocin binding was measured by saturation analysis.nnnRESULTSnAtosiban and related peptides inhibited oxytocin-induced myometrial activation as pure antagonists (inhibition constant 10 nmol/L) but had no effect on prostaglandin E2-induced activation. Long-term (> or = 24 hours) exposure to atosiban had no residual effect on oxytocin sensitivity. However, long-term exposure to oxytocin resulted in homologous desensitization and loss of oxytocin receptors. Oxytocin-induced desensitization was prevented by coincubation with atosiban.nnnCONCLUSIONSnAtosiban is a pure oxytocin antagonist and has a specific, reversible effect on myometrial cells in vitro. Its potential use for the management or even prevention of idiopathic preterm labor or to reverse uterine hypertony during oxytocin-induced labor should be tested in controlled clinical trials.

Prostaglandins, chorioamnionitis and preterm labour

Summary. The production of prostaglandin E (PGE) and leukotriene B4 (LTB4) by amnion was measured in vitro in 12 women delivered after spontaneous preterm labour and in 15 women delivered after spontaneous labour at term. The placenta and fetal membranes were examined histologically in all cases. PGE output (fmol/mg dry weight/2 h) in the amnions from uncomplicated preterm deliveries was low (median 486, range 232–3203, n= 7) compared with the values obtained after spontaneous labour at term (5529, 1722–14226, n= 15). In amnions from preterm deliveries complicated by acute chorioamnionitis or round cell infiltration there was massive release of PGE (15 262,10 905–27 640, n= 5) which was accompanied by an increase in LTB4 production. Inflammatory infiltration of the fetal membranes results in a huge increase in PG production which could cause preterm labour.

Corticosteroid metabolism in vitro by humanplacenta, fetal membranes and decidua in early and late gestation

Corticosteroid conversion by early (7 to 10 weeks) and late (36 to 42 weeks) human placenta, fetal membranes and decidua was studied. Tritiated cortisol, cortisone, corticosterone, 11-dehydrocorticosterone (DHC) and 11-deoxycorticosterone (DOC) were incubated in vitro with homogenates of these tissues and extracts chromatographed on Sephadex LH20 columns. 11 beta-hydroxysteroid dehydrogenase activity (11 beta-HSD; EC 1.1.1.146) was present in all these tissues from as early as the seventh week of pregnancy. In the fetal membranes the enzyme activity was present in the chorion, the amnion showing no activity. Only the decidua could convert cortisone and DHC to the biologically more active cortisol and corticosterone, respectively. DOC was not transformed by any of the tissues. No 11 beta-HSD activity was found in amniotic fluid, retroplacental serum, umbilical cord plasma or cord tissue.

Preterm labour: a pharmacological challenge

Preterm labour is a major cause of perinatal mortality and morbidity, but its prevention is difficult because most of the available drugs lack uterine selectivity and have potentially serious side-effects for the mother or the foetus. In this article, Andrés López Bernal and colleagues discuss new evidence that shows pregnancy is associated with changes in G protein signalling and second messenger formation in human myometrium. During gestation uterine relaxation is favoured by a pronounced increase in G alpha s levels, thereby facilitating the effect of agonists that increase cAMP formation. The change in G alpha s is reversed in spontaneous labour enabling the uterus to become responsive to contractile agents. Although it is not established that these changes in G protein function are causally related to the spontaneous onset of labour, nevertheless they provide a novel viewpoint towards increased understanding of the cellular mechanisms of uterine contractility, which may result in better drugs for the management of preterm labour.

Topographical distribution of prostaglandin E receptors in human myometrium

Summary. The binding of radiolabelled prostaglandin (PG) F2α and PGE2 by human myometrium was measured in vitro and the distribution and characteristics of the binding sites in non‐pregnant and pregnant uteri were studied. PGF2α binding sites were of low affinity (Kd 30 nM) and could be occupied by PG of the E series with higher affinity than PGF2α itself. PGE binding sites were of high affinity (Kd 1·5 nM) and highly specific for PG of the E series, suggesting that they represent true PGE receptors. The concentration of PGE receptors was higher in nonpregnant than in pregnant uteri at term. In non‐pregnant uteri the concentration of PGE receptors was highest in the fundus and decreased towards the cervix; in term pregnant uteri the concentration was constant in all areas. In both non‐pregnant and pregnant uteri there was a significantly lower PGE binding affinity in cervix than in myometrium from the fundus‐corpus area. The concentrations and affinity of PGE receptors were similar during the proliferative and secretory phases of the menstrual cycle and were not influenced by age of the patient. PGE receptors were not influenced by the presence or absence of primary dysmcnorrhoca but appeared to be increased in unexplained menorrhagia.

Corticotrophin Releasing Hormone: Its Potential for a Role in Human Myometrium

Aside from its role as a hypothalamic stress hormone, corticotrophin releasing hormone (CRH) is also a placental hormone, at least in primates. Although the function of placentally derived CRH remains to be fully elucidated, elevated CRH levels have been associated with premature labour, suggesting that the hormone may be involved in regulating the duration of pregnancy. Indeed, pregnant human myometrium expresses functional CRH receptors (CRH R1 and CRH R2 subtypes) thought to signal predominantly via the second messenger cAMP. Thus, like other cAMP‐producing hormones in the myometrium such as β2 agonists, CRH may play a part in maintaining uterine quiescence. However, several of the CRH receptor isoforms identified to date have a reduced ability to activate adenylate cyclase, raising the question as to whether they are linked to other signal transduction pathways. Here, we discuss critically the evidence for the peptides role in regulating contractility, both directly at the myometrium and indirectly via the fetal membranes and decidua. The possibility of a role in myometrial growth modulation is also described.

Surfactant stimulates prostaglandin E production in human amnion

Summary. Discs of human amnion prepared from nine women delivered by elective caesarean section at term were incubated with and without purified human amniotic fluid surfactant (9 μg lipid P/ml), and the output of prostaglandin E (PGE) was estimated by radioimmunoassay. Surfactant stimulated the release of PGE from 3·8 (SD 2·9) to 5·4 (SD 2·5) pmol/mg dry weight/3 h (P<0·01). Arachidonic acid accounted for 2% of the fatty acids of surfactant lecithin. These results show that surfactant has a stimulatory effect on amniotic PGE production and that it is an important source of arachidonic acid in amniotic fluid.

Prostaglandin E production by amniotic cells in relation to term and preterm labour

Summary. The rate of prostaglandin E (PGE) production was measured in collagenase‐dispersed amniotic cells obtained from 14 women after spontaneous labour at term—seven after spontaneous preterm labour, nine after delivery by elective caesarean section at term and six after induction of labour at term. Cells were incubated with and without arachidonic acid and PGE was estimated by specific radioimmunoassay. Basal PGE output (pmol/106 cells per 3 h) was highest in the spontaneous labour group, 27·5 (SEM 5·5) and lowest in the preterm labour group, 4 (SEM 1·2) (P<0·001). Values in the elective section and induction groups were 13·6 (SEM 2·7) and 10 (SEM 3·1), respectively; these values were significantly higher than in the preterm labour group and the values after induction were significantly lower than after spontaneous labour. Addition of arachidonic acid resulted in a significant increase in PGE output in all groups, but the values after preterm labour remained significantly lower than those of any group at term. These data indicate that towards term there is a maturation in the PG synthetase activity of the amnion and that PGE output in this tissue is increased in spontaneous labour.