Serial relaxin concentrations in human pregnancy

Secretory products of the corpus luteum and preeclampsia

BACKGROUND

Despite significant advances in our understanding of the pathophysiology of preeclampsia (PE), there are still many unknowns and controversies in the field. Women undergoing frozen-thawed embryo transfer (FET) to a hormonally prepared endometrium have been found to have an unexpected increased risk of PE compared to women who receive embryos in a natural FET cycle. The differences in risk have been hypothesized to be related to the absence or presence of a functioning corpus luteum (CL).

OBJECTIVE AND RATIONALE

To evaluate the literature on secretory products of the CL that could be essential for a healthy pregnancy and could reduce the risk of PE in the setting of FET.

SEARCH METHODS

For this review, pertinent studies were searched in PubMed/Medline (updated June 2020) using common keywords applied in the field of assisted reproductive technologies, CL physiology and preeclampsia. We also screened the complete list of references in recent publications in English (both animal and human studies) on the topics investigated. Given the design of this work as a narrative review, no formal criteria for study selection or appraisal were utilized.

OUTCOMES

The CL is a major source of multiple factors regulating reproduction. Progesterone, estradiol, relaxin and vasoactive and angiogenic substances produced by the CL have important roles in regulating its functional lifespan and are also secreted into the circulation to act remotely during early stages of pregnancy. Beyond the known actions of progesterone and estradiol on the uterus in early pregnancy, their metabolites have angiogenic properties that may optimize implantation and placentation. Serum levels of relaxin are almost undetectable in pregnant women without a CL, which precludes some maternal cardiovascular and renal adaptations to early pregnancy. We suggest that an imbalance in steroid hormones and their metabolites and polypeptides influencing early physiologic processes such as decidualization, implantation, angiogenesis and maternal haemodynamics could contribute to the increased PE risk among women undergoing programmed FET cycles.

WIDER IMPLICATIONS

A better understanding of the critical roles of the secretory products of the CL during early pregnancy holds the promise of improving the efficacy and safety of ART based on programmed FET cycles.

Potential influence of the corpus luteum on circulating reproductive and volume regulatory hormones, angiogenic and immunoregulatory factors in pregnant women

Cardiovascular function is impaired and preeclampsia risk elevated in women conceiving by in vitro fertilization (IVF) in the absence of a corpus luteum (CL). Here, we report the serial evaluation of hormones and other circulating factors in women who conceived with (or without) IVF. After a prepregnancy baseline, the study participants (n = 19-24/cohort) were evaluated six times during pregnancy and once postpartum (~1.6 yr). IVF pregnancies were stratified by protocol and CL number, i.e., ovarian stimulation (>1 CL) or hypothalamic-pituitary suppression (0 CL) versus spontaneous conceptions (1 CL). Results include the following: 1) relaxin was undetectable throughout pregnancy (including late gestation) in the 0 CL cohort, but markedly elevated in ~50% of women in the >1 CL cohort; 2) progesterone, plasma renin activity, and aldosterone transiently surged at 5-6 gestational weeks in the >1 CL group; 3) soluble vascular endothelial growth factor-1 (sFLT-1) abruptly increased between 5-6 and 7-9 gestational weeks in all three participant cohorts, producing a marked elevation in sFLT-1/PLGF (placental growth factor) ratio exceeding any other time point during pregnancy; 4) sFLT-1 was higher throughout most of gestation in both IVF cohorts with or without abnormal obstetrical outcomes; 5) during pregnancy, C-reactive protein (CRP) increased in 0 and 1 CL, but not >1 CL cohorts; and 6) plasma protein, but not hemoglobin, was lower in the >1 CL group throughout gestation. The findings highlight that, compared with spontaneously conceived pregnancy, the maternal milieu of IVF pregnancy is not physiologic, and the specific perturbations vary according to IVF protocol and CL status.

The Role of Placental Hormones in Mediating Maternal Adaptations to Support Pregnancy and Lactation

During pregnancy, the mother must adapt her body systems to support nutrient and oxygen supply for growth of the baby in utero and during the subsequent lactation. These include changes in the cardiovascular, pulmonary, immune and metabolic systems of the mother. Failure to appropriately adjust maternal physiology to the pregnant state may result in pregnancy complications, including gestational diabetes and abnormal birth weight, which can further lead to a range of medically significant complications for the mother and baby. The placenta, which forms the functional interface separating the maternal and fetal circulations, is important for mediating adaptations in maternal physiology. It secretes a plethora of hormones into the maternal circulation which modulate her physiology and transfers the oxygen and nutrients available to the fetus for growth. Among these placental hormones, the prolactin-growth hormone family, steroids and neuropeptides play critical roles in driving maternal physiological adaptations during pregnancy. This review examines the changes that occur in maternal physiology in response to pregnancy and the significance of placental hormone production in mediating such changes.

Effect of Mode of Conception on Maternal Serum Relaxin, Creatinine, and Sodium Concentrations in an Infertile Population

OBJECTIVE

To investigate how the mode of conception affects maternal relaxin, creatinine, and electrolyte concentrations.

BACKGROUND

Pregnancies achieved by fertility treatment often begin in a nonphysiologic endocrine milieu with no corpus luteum (CL) or with many corpora lutea. The CL produces not only estradiol and progesterone but is also the sole source of relaxin in early pregnancy, a hormone that may contribute to maternal systemic and renal vasodilation. There is limited data about maternal physiology in early pregnancy during fertility treatment, and studies have rarely considered the potential effect of the absence of the CL. To begin to address this gap in knowledge, we sought to investigate how the mode of conception affects maternal relaxin, creatinine, and electrolyte concentrations.

METHODS

One hundred eighty-four women who received care at an academic infertility practice provided serum samples. Levels of relaxin 2, creatinine, and electrolytes were compared between 4 groups defined on the basis of mode of conception which corresponded to categories of CL number: (1) absence of the CL, (2) single CL, (3) multiple CL from ovarian stimulation not including in vitro fertilization (IVF), and (4) multiple CL from IVF with fresh embryo transfer.

RESULTS

Relaxin-2 levels were undetectable in patients lacking a CL. Creatinine, sodium, and total CO₂ levels were significantly higher in the 0 CL group (relaxin absent) compared to all other groups (relaxin present). Compared to clomiphene, use of letrozole was associated with a lower relaxin level.

CONCLUSION

Early creatinine and sodium concentrations are increased in the absence of relaxin. Given the increasing utilization of frozen embryo transfer, further studies comparing programmed with natural cycles are warranted.

Neuroendocrine mechanisms in pregnancy and parturition

The complex mechanisms controlling human parturition involves mother, fetus, and placenta, and stress is a key element activating a series of physiological adaptive responses. Preterm birth is a clinical syndrome that shares several characteristics with term birth. A major role for the neuroendocrine mechanisms has been proposed, and placenta/membranes are sources for neurohormones and peptides. Oxytocin (OT) is the neurohormone whose major target is uterine contractility and placenta represents a novel source that contributes to the mechanisms of parturition. The CRH/urocortin (Ucn) family is another important neuroendocrine pathway involved in term and preterm birth. The CRH/Ucn family consists of four ligands: CRH, Ucn, Ucn2, and Ucn3. These peptides have a pleyotropic function and are expressed by human placenta and fetal membranes. Uterine contractility, blood vessel tone, and immune function are influenced by CRH/Ucns during pregnancy and undergo major changes at parturition. Among the others, neurohormones, relaxin, parathyroid hormone-related protein, opioids, neurosteroids, and monoamines are expressed and secreted from placental tissues at parturition. Preterm birth is the consequence of a premature and sustained activation of endocrine and immune responses. A preterm birth evidence for a premature activation of OT secretion as well as increased maternal plasma CRH levels suggests a pathogenic role of these neurohormones. A decrease of maternal serum CRH-binding protein is a concurrent event. At midgestation, placental hypersecretion of CRH or Ucn has been proposed as a predictive marker of subsequent preterm delivery. While placenta represents the major source for CRH, fetus abundantly secretes Ucn and adrenal dehydroepiandrosterone in women with preterm birth. The relevant role of neuroendocrine mechanisms in preterm birth is sustained by basic and clinic implications.

Relaxin has a role in establishing a renal response in pregnancy

Women with normal ovarian function (n = 9) and women who conceived with ovum donation (no circulating relaxin; n = 9) had serial measurements of renal function made during the first trimester of pregnancy by using 24-hour creatinine clearance (CrCl) and plasma osmolality (P(osm)). All pregnancies were associated with increasing CrCl and reduced P(osm), but the change from baseline was significantly greater in the women with normal ovarian function, indicating that in contrast to the rodent model, other factors in addition to circulating relaxin contribute to gestational renal adaptation to human pregnancy.

Maternal relaxin concentrations in diabetic pregnancy

Maternal serum concentrations of relaxin, an insulin homologue produced both by the corpus luteum of pregnancy and by the fetoplacental unit, are highest in the first trimester and fall to their lowest level in the third trimester. Relaxin is thought to influence carbohydrate metabolism in the uterus, and it has been suggested that serum concentrations of relaxin in diabetic women are higher than those of non-diabetic women. We show that maternal serum relaxin concentrations are significantly higher at each stage of pregnancy in insulin-dependent diabetic mothers than in non-diabetic mothers. This elevation in relaxin concentrations is not related to other indices of diabetic control. The physiological importance of the higher concentrations of relaxin in the serum of diabetic women--in particular, whether they contribute to the higher incidence of major anomalies in the fetuses of diabetic mothers--is yet to be determined.

Morphology of the pubis and preauricular area in relation to parity and age at death in Macaca mulatta

Some adult human females show bone resorption (pitting) at the dorsal aspect of the pubis and preauricular area of the ilium. The etiology of pelvic bone resorption is attributed alternatively to reproduction and to pelvic anatomy. While most researchers infer that pelvic pitting is related to reproduction, only a few studies are based on women of known parity. Degree of pubic resorption is directly associated with both parity (Suchey et al.: Am. J. Phys. Anthropol. 51:517-539, 1979; Bergfelder and Hermann: J. Hum. Evol. 9:611-613, 1980) and age (Suchey et al.: Am. J. Phys. Anthropol. 51:517-539, 1979). The relationship between parity and degree of resorption of the preauricular area is equivocal, found to be significant by Dunlap (A Study of the Preauricular Sulcus in a Cadaver Population, Ph.D. dissertation, East Lansing, Michigan State University, 1981) but not by Spring et al. (Am. J. Phys. Anthropol. 79:247-252, 1989); both studies report that age is not associated with resorption of the preauricular area. Other mammals evidence public resorption, but the morphology of the preauricular area is less well known. This study addresses the issue on the etiology of pelvic bone resorption using a sample of Macaca mulatta (the free-ranging population from Cayo Santiago) for which parity and age at death are known for all specimens. The following results are reported. Resorption of the pubis is common among females but infrequent among males. Contrary to Rawlins (Am. J. Phys. Anthropol. 42:477-488, 1975), the degree of pubic resorption in female macaques is significantly related to both parity and age at death.(ABSTRACT TRUNCATED AT 250 WORDS)

Relaxin as an aetiological factor in diabetic embryopathy

Relaxin, an insulin homologue, has effects on collagen resembling those of certain teratogenic agents. It is suggested that diabetic embryopathy could be due to disturbances of relaxin secretion during fetal organogenesis.

Serum relaxin in pregnancy

Serum relaxin immunoactivity was measured by means of a porcine radioimmunoassay in a cross-sectional study of 302 normal singleton pregnancies. Concentrations in the third trimester were lower than in early and mid pregnancy. At term, relaxin levels in patients who went into spontaneous labour within a week of sampling were significantly lower than in those who did not. However, relaxin levels were highest during labour and fell almost to non-pregnant levels by the third postnatal day. Levels in twin pregnancies in the third trimester were higher than those in singleton pregnancies. Serial samples from 4 patients with a history of premature labour showed declining, very low levels in the only patient who subsequently had a preterm delivery. These results are compatible with the proposed roles of relaxin during pregnancy: namely, to maintain myometrial quiescence, facilitate uterine stromal remodelling during uterine growth, and promote cervical ripening at the onset of parturition.

Radioimmunoassay of relaxin in pregnancy with an analogue of human relaxin

A radioimmunoassay for relaxin was developed in which a synthetic analogue of human relaxin was used as standard, tracer, and immunogen. Relaxin could not be measured in sera from men or non-pregnant women, but was measurable in pregnant women from the tenth week of gestation until term. Concentrations ranged from 0.19-1.18 ng/ml, with highest levels measured in the first trimester.

Paracrine regulation of follicular maturation in primates

Taken together, the studies reviewed here suggest that although gonadotropins are necessary for follicular growth, they are insufficient by themselves to explain the dynamics of folliculogenesis. Indeed, the role of gonadotropins in follicular maturation must necessarily be permissive: that is LH and FSH initiate a synchronized cascade of follicular events directly mediated by paracrine and autocrine factors.

The occurrence of relaxin in hyperstimulated human preovulatory follicles collected in an in vitro fertilization program

The avidin-biotin immunoperoxidase method and antisera against highly purified porcine relaxin were utilized to localize relaxin-like immunoreactivity in biopsied specimens from six preovulatory follicles from four women undergoing laparoscopy for oocyte retrieval in an in vitro fertilization program. By histological criteria, three of the follicles were luteinized and three were not. Relaxin was found in the granulosa cells of those cells which showed histological luteinization, whereas no relaxin was found in the nonluteinized preovulatory follicles. Our results show, for the first time, the occurrence of relaxin in the human ovary before ovulation and they suggest that the appearance of relaxin is related to the luteinization process.

Immunohistochemical demonstration of relaxin in gynecologic tumors

The immunoperoxidase technique was utilized to study the occurrence and localization of relaxin in malignant and nonmalignant trophoblastic disease and nontrophoblastic gynecologic cancer. Tissue specimens were studied from normal placenta at various weeks of gestation (N = 10), hydatidiform mole (N = 10), invasive mole (N = 10), choriocarcinoma (N = 10), normal endometrium (N = 28), decidua (N = 5), adenocarcinoma of the endocervix (N = 10), of the endometrium (N = 10), corpus luteum (N = 2), ovarian serous papillary cystadenocarcinoma (N = 10), and mucinous cystadenocarcinoma (N = 10). Relaxin was identified in the syncytiotrophoblast of normal placenta at all stages of gestation and also, for the first time, in hydatidiform mole, invasive mole, and choriocarcinoma. No relaxin was found in any nontrophoblastic cancer, including secretory-type adenocarcinoma. The absence of relaxin in the secretory-type endometrial adenocarcinoma is intriguing in light of the finding of relaxin in the normal secretory endometrium. The difference may be due to absence of the relaxin-producing corpus luteum in cancer patients or to loss of relaxin synthesis in the malignant endometrium. The results of this study suggest that relaxin may be synthesized by the normal syncytiotrophoblast and trophoblastic tumors, although differential absorption of relaxin produced in another site cannot be excluded.

Relaxin--a review

Relaxin is a polypeptide hormone, similar in structure to insulin and has been found in the female of all species studied. The corpus luteum of pregnancy is the main source of relaxin in many species but in others the decidua is apparently of greater importance. It has also been found in other tissues; e.g. prostatic fluid, testis and ovary. First discovered and extracted from the corpora lutea of pregnant sows in an impure form in 1926, it was found to relax the pubic ligament of the oestrogen primed guinea-pig. It was named after this action, but has since been found to have many other possible roles, including preparation of the endometrium for implantation, inhibition of uterine activity in early pregnancy, remodelling of the uterine stroma during pregnancy, cervical ripening and the initiation of parturition. Relaxin's main cellular action in pregnancy may be to drive collagen biosynthesis in its target organs, thus facilitating the remodelling of the connective tissue. Due to the impurity of relaxin preparations used in clinical trials until the mid-1970's, the role of relaxin in the human has been in doubt. Porcine and rat relaxins have now been highly purified and their detailed structure is known. Human relaxin awaits adequate isolation, purification and characterization, and is not yet available for laboratory and clinical trials. However, the recent preparation of purified porcine relaxin for clinical trials and the availability of specific radioimmunoassays for this relaxin together with the identification of relaxin receptor sites, are rapidly helping to establish the concept that relaxin is indeed an important hormone both in human reproduction and in other physiological processes.