Uteroplacental unit as a source of elevated circulating prorenin levels in normal pregnancy

Cord Serum Calcitriol Inversely Correlates with Maternal Blood Pressure in Urinary Tract Infection-Affected Pregnancies: Sex-Dependent Immune Implications

Urinary tract infections (UTI) during pregnancy are frequently associated with hypertensive disorders, increasing the risk of perinatal morbidity. Calcitriol, vitamin D3's most active metabolite, has been involved in blood pressure regulation and prevention of UTIs, partially through modulating vasoactive peptides and antimicrobial peptides, like cathelicidin. However, nothing is known regarding the interplay between placental calcitriol, cathelicidin, and maternal blood pressure in UTI-complicated pregnancies. Here, we analyzed the correlation between these parameters in pregnant women with UTI and with normal pregnancy (NP). Umbilical venous serum calcitriol and its precursor calcidiol were significantly elevated in UTI. Regardless of newborn's sex, we found strong negative correlations between calcitriol and maternal systolic and diastolic blood pressure in the UTI cohort (p < 0.002). In NP, this relationship was observed only in female-carrying mothers. UTI-female placentas showed higher expression of cathelicidin and CYP27B1, the calcitriol activating-enzyme, compared to male and NP samples. Accordingly, cord-serum calcitriol from UTI-female neonates negatively correlated with maternal bacteriuria. Cathelicidin gene expression positively correlated with gestational age in UTI and with newborn anthropometric parameters. Our results suggest that vitamin D deficiency might predispose to maternal cardiovascular risk and perinatal infections especially in male-carrying pregnancies, probably due to lower placental CYP27B1 and cathelicidin expression.

Prorenin periconceptionally and in pregnancy: Does it have a physiological role?

Pregnancy demands major cardiovascular, renal and endocrine changes to provide an adequate blood supply for the growing fetus. The renin-angiotensin-aldosterone system plays a key role in this adaptation process. One of its components, prorenin, is released in significant amounts from the ovary and uteroplacental unit. This review describes the sources of prorenin in the periconception period and in pregnancy, including its modulation by in-vitro fertilization protocols, and discusses its potential effects, among others focusing on preeclampsia. It ends with discussing the long-term consequences, even in later life, of inappropriate renin-angiotensin-aldosterone system activity in pregnancy and offers directions for future research. Ultimately, a full understanding of the role of prorenin periconceptionally and during pregnancy will help to develop tools to diagnose and/or prevent reproductive complications.

Prorenin periconceptionally and in pregnancy: Does it have a physiological role?

Pregnancy demands major cardiovascular, renal and endocrine changes to provide an adequate blood supply for the growing fetus. The renin-angiotensin-aldosterone system plays a key role in this adaptation process. One of its components, prorenin, is released in significant amounts from the ovary and uteroplacental unit. This review describes the sources of prorenin in the periconception period and in pregnancy, including its modulation by in-vitro fertilization protocols, and discusses its potential effects, among others focusing on preeclampsia. It ends with discussing the long-term consequences, even in later life, of inappropriate renin-angiotensin-aldosterone system activity in pregnancy and offers directions for future research. Ultimately, a full understanding of the role of prorenin periconceptionally and during pregnancy will help to develop tools to diagnose and/or prevent reproductive complications.

The 4-vessel Sampling Approach to Integrative Studies of Human Placental Physiology In Vivo

The human placenta is highly inaccessible for research while still in utero. The current understanding of human placental physiology in vivo is therefore largely based on animal studies, despite the high diversity among species in placental anatomy, hemodynamics and duration of the pregnancy. The vast majority of human placenta studies are ex vivo perfusion studies or in vitro trophoblast studies. Although in vitro studies and animal models are essential, extrapolation of the results from such studies to the human placenta in vivo is uncertain. We aimed to study human placenta physiology in vivo at term, and present a detailed protocol of the method. Exploiting the intraabdominal access to the uterine vein just before the uterine incision during planned cesarean section, we collect blood samples from the incoming and outgoing vessels on the maternal and fetal sides of the placenta. When combining concentration measurements from blood samples with volume blood flow measurements, we are able to quantify placental and fetal uptake and release of any compound. Furthermore, placental tissue samples from the same mother-fetus pairs can provide measurements of transporter density and activity and other aspects of placental functions in vivo. Through this integrative use of the 4-vessel sampling method we are able to test some of the current concepts of placental nutrient transfer and metabolism in vivo, both in normal and pathological pregnancies. Furthermore, this method enables the identification of substances secreted by the placenta to the maternal circulation, which could be an important contribution to the search for biomarkers of placenta dysfunction.

Renin-angiotensin system in pre-eclampsia: everything old is new again

This review presents an update of the role of the renin-angiotensin system in normal pregnancy and pre-eclampsia. We have known for years that the circulatory renin-angiotensin system in pre-eclampsia is suppressed. We now know that the circulating renin-angiotensin system does not only have a vasoconstrictor arm, but also a vasodilator arm, which is upregulated in normal pregnancy; this balance is probably disturbed in pre-eclampsia. Recent studies show the importance of the local renin-angiotensin system in the uteroplacental unit for early placentation and regulation of placental blood flow. We discuss the possible role of autoantibodies against the AT1-receptor in pre-eclampsia and the suggestion that activation of the AT1-receptor in the placenta may lead to placental dysfunction and the clinical syndrome of pre-eclampsia.

Fetal sex affects expression of renin-angiotensin system components in term human decidua

The maternal decidua expresses the genes of the renin-angiotensin system (RAS). Human decidua was collected at term either before labor (i.e. cesarean delivery) or after spontaneous labor. The mRNA for prorenin (REN), prorenin receptor (ATP6AP2), angiotensinogen (AGT), angiotensin-converting enzymes 1 and 2 (ACE1 and ACE2), angiotensin II type 1 receptor (AGTR1), and angiotensin 1-7 receptor (MAS1) were measured by quantitative real-time RT-PCR. Decidual explants were cultured in duplicate for 24 and 48 h, and all RAS mRNA, and the secretion of prorenin, angiotensin II, and angiotensin 1-7 was measured using quantitative real-time RT-PCR, ELISA, and radioimmunoassay, respectively. In the decidua collected before labor, REN mRNA levels were higher if the fetus was female. In addition, REN, ATP6AP2, AGT, and MAS1 mRNA abundance was greater in decidual explants collected from women carrying a female fetus, as was prorenin protein. After 24 h, ACE1 mRNA was higher in the decidual explants from women with a male fetus, whereas after 48 h, both ACE1 and ACE2 mRNA was higher in decidual explants from women with a female fetus. Angiotensin II was present in all explants, but angiotensin 1-7 levels often registered below the lower limits of sensitivity for the assay. After labor, decidua, when compared with nonlaboring decidua, demonstrated lower REN expression when the fetus was female. Therefore, the maternal decidual RAS is regulated in a sex-specific manner, suggesting that it may function differently when the fetus is male than when it is female.

Physiology of Local Renin-Angiotensin Systems

Since the first identification of renin by Tigerstedt and Bergmann in 1898, the renin-angiotensin system (RAS) has been extensively studied. The current view of the system is characterized by an increased complexity, as evidenced by the discovery of new functional components and pathways of the RAS. In recent years, the pathophysiological implications of the system have been the main focus of attention, and inhibitors of the RAS such as angiotensin-converting enzyme (ACE) inhibitors and angiotensin (ANG) II receptor blockers have become important clinical tools in the treatment of cardiovascular and renal diseases such as hypertension, heart failure, and diabetic nephropathy. Nevertheless, the tissue RAS also plays an important role in mediating diverse physiological functions. These focus not only on the classical actions of ANG on the cardiovascular system, namely, the maintenance of cardiovascular homeostasis, but also on other functions. Recently, the research efforts studying these noncardiovascular effects of the RAS have intensified, and a large body of data are now available to support the existence of numerous organ-based RAS exerting diverse physiological effects. ANG II has direct effects at the cellular level and can influence, for example, cell growth and differentiation, but also may play a role as a mediator of apoptosis. These universal paracrine and autocrine actions may be important in many organ systems and can mediate important physiological stimuli. Transgenic overexpression and knock-out strategies of RAS genes in animals have also shown a central functional role of the RAS in prenatal development. Taken together, these findings may become increasingly important in the study of organ physiology but also for a fresh look at the implications of these findings for organ pathophysiology.

Mechanisms regulating angiotensin II responsiveness by the uteroplacental circulation

Pregnancy is associated with increases in cardiac output and uterine blood flow (UBF) and a fall in systemic vascular resistance. In ovine pregnancy, UBF rises from approximately 3% of cardiac output to approximately 25% at term gestation, reflecting a >30-fold rise in UBF by term. This increase in UBF supports exponential fetal growth during the last trimester and maintains fetal well-being by providing excess oxygen and nutrient delivery. These hemodynamic changes are associated with numerous hormonal changes, including increases in placental steroid hormones and enhanced activation of the renin-angiotensin and sympathetic nervous systems, all of which are believed to modulate systemic and uterine vascular adaptation and vascular reactivity. Systemic pressor responses to infused ANG II are attenuated in normotensive pregnancies and the uteroplacental vasculature is even less sensitive, suggesting development of mechanisms to maintain basal UBF and permit the rise in UBF necessary for fetal growth and well-being. The effects of ANG II on the uteroplacental vasculature are reviewed, and the mechanisms that may account for attenuated vascular sensitivity are examined, including ANG II metabolism, vascular production of antagonists, ANG II-receptor subtype expression, and the role of indirect mechanisms.

Current topic: the uteroplacental renin-angiotensin system

The components of the renin-angiotensin system (RAS) are expressed in the uteroplacental unit. The expression varies between species, probably due to the marked species differences in placental architecture. The conditions for angiotensin (Ang) II formation exist and Ang II receptors are present throughout the human uteroplacental unit, indicating the presence of a functional local RAS. The uteroplacental RAS interacts with other regulatory systems and in this way modulates various aspects of tissue function. It is suggested that the uteroplacental RAS is important for the regeneration of the endometrium after shedding, and for decidualization, implantation and placentation. The RAS participates in the regulation of the uteroplacental blood flow, prostaglandin synthesis and oestradiol secretion. Disturbances of the uteroplacental RAS may lead to dysfunctional bleeding and to reduced uteroplacental blood flow in pregnancies complicated by pre-eclampsia and intrauterine growth retardation.

Autoradiographic localization and characterization of angiotensin II receptors in the bovine placenta and fetal membranes

Autoradiography and angiotensin (Ang) II receptor binding studies showed that all parts of the bovine placenta and fetal membranes contained high densities of Ang II receptors throughout gestation. The receptors were predominantly subtype 2 (AT2) receptors in the fetal and subtype 1 (AT1) receptors in the maternal compartment. In the allantoamnionic membrane, Ang II receptors were evenly distributed in the mesenchymal tissue, with the highest expression around the few arteries. In the intercotyledonary and cotyledonary allantochorionic membrane, AT2 receptors as well as the less-expressed AT1 receptors were located on mesenchymal cells, especially adjacent to the allantoic endoderm, trophoblast cell layer, and arteries. In the mesenchymal tissue of the placentome, Ang II receptors were mostly expressed at the main branches of the fetal villi of the cotyledons. In the maternal part of the placentome, mainly AT1 receptors but also low densities of AT2 receptors and non-AT1/non-AT2 Ang II binding sites were found close to the stalk and at the main branches of the maternal crypts. Autoradiography revealed no changes in the pattern of distribution of the Ang II receptors throughout gestation. It is suggested that Ang II has an effect on regulatory as well as growth processes in these tissues.

Is ANP responsible for the hemoconcentration in preeclampsia?

A hypothesis is presented that high plasma concentration of alpha-human Atrial Natriuretic Peptide (ANP) might be responsible for hemoconcentration of preeclampsia (PE), mediated by increased diuresis and shift of fluid to the interstitium. ANP is known as a peptide regulating blood volume by diuresis and natriuresis, and perhaps through increased permeability in vessels. Moreover ANP is an antagonist to the renin-angiotensin-aldosterone (RAA) system, especially when this system is activated quantitatively as in pregnancy or qualitatively as in PE.

Sites of renin production in fetal, neonatal, and postnatal Syrian hamster kidneys

Renin is first observed in the 14-day fetal kidney. There is a sharp increase in the number of renin positive cells in the 15-day fetal kidney. Renin is located in the smooth muscle cells of arterioles, interlobular arteries, and branches of the renal artery. In the neonatal kidney, the amount of renin appears to be equal to that observed in the 15-day fetal kidney and is still located in the same blood vessels. In the 24-hour postnatal kidney, there is a sharp decrease in the total amount of renin. Renin positive cells are now observed at the vascular pole. In the 48-hour postnatal kidney, there is a sharp increase in the total amount of renin. Most of the renin positive cells are located at the vascular poles; however, a few renin positive cells are seen in the interlobular arteries.

Renin and prorenin in reproductive tissues during gestation in pigs and cattle

1. High concentrations of prorenin and active renin were previously found in ovarian follicular fluid from cattle but not from pigs. In the present study female reproductive tissues and fluids from cattle and pigs during gestation were investigated to clarify a possible species difference in active renin and prorenin concentrations. 2. Very high concentrations of active renin but no prorenin were found in corpus luteum from both species. 3. Relatively low concentrations of active renin, in the same order as in maternal blood plasma, were found in myometrium, endometrium, placenta and fetal membranes from both species. Prorenin was undetectable in these tissues except for bovine myometrium and porcine endometrium in some animals. 4. The concentrations of active renin and prorenin in amnionic fluid from both species were below the maternal plasma values. In allantoic fluid the concentrations were higher than in amnionic fluid. 5. The plasma concentrations of active renin and prorenin did not change during gestation in pigs. This finding is in contrast to the observations in humans and does not support a systemic effect of prorenin during gestation. 6. The presence of renin in the reproductive tissues, especially the very high concentrations in the corpus luteum, indicates a local function of the renin-angiotensin system during gestation.

Effect of gonadotropins and pregnancy on prorenin and renin in the rat

1. Stimulation of adult female rats with pregnant mare serum gonadotropin (PMSG) and human chorion gonadotropin (hCG) increased active plasma renin about two-fold, but caused only a slight increase of plasma prorenin. The concentrations of active renin and prorenin in the ovaries, and active renin in the uterus all increased about two-fold 2 days after stimulation with PMSG. The prorenin in the uterus was below detection in unstimulated rats and did not change consistently after PMSG. 2. Active renin and prorenin in plasma were unchanged in relation to pregnancy, except for a slight decrease of prorenin in the third trimester. In the first and third trimester the concentration in the ovaries of active renin and prorenin was decreased to about one-third of that in normal female rats. In contrast active renin in the uterus was increased about two-fold in the first trimester, whereas prorenin did not change consistently. 3. Our results confirm that gonadotropins and pregnancy affect the renin-angiotensin system in rats. However, the changes in the plasma seem to be much smaller than those previously reported in humans. Accordingly, our results do not support a systemic role of prorenin for reproduction in the rat.

Prorenin secretion from human placenta perfused in vitro

We examined whether renin (prorenin plus renin) is secreted from the human placenta into the maternal or fetal circulation and compared the secretion rate to that of human chorionic gonadotropin (hCG), progesterone, and estradiol. While estradiol and progesterone passed into both circulations, renin (mostly prorenin) and hCG were secreted predominantly into the maternal circulation. To examine if prorenin passed from the maternal to the fetal circulation and vice versa, we perfused both circuits separately with exogenous recombinant human prorenin. No prorenin passed from maternal to fetal circulations, but a small amount (less than 10%) slowly passed from fetal to maternal, beginning 15 min after the addition of prorenin. Exogenous prorenin was not converted to renin in either circulation. Perfusate total renin had close to 10% active renin, whereas that of tissue extracts was closer to 50%. In conclusion, the results are consistent with some tissue activation of prorenin, either in vitro or in vivo, but no activation in the maternal or fetal circulations. The human placenta may secrete prorenin into the maternal but not into the fetal circulation. The possibility that the placenta may secrete a small amount of active renin into the maternal circulation was not ruled out.

Effect of progesterone on renin secretion in endometrial stromal, chorionic trophoblast, and mesenchymal monolayer cultures

Renin messenger ribonucleic acid expression has been shown in decidual tissue and endometrium, but weakly in chorion laeve, suggesting decidua as a primary source of uteroplacental renin. We proposed to confirm active renin secretion by endometrial stromal cells and examined the effect of progesterone-induced decidualization on secretion. Separate monolayer cultures of endometrial stroma, trophoblast, and mesenchymal cells were established and maintained for 10 to 15 days. Active renin concentration was measured with radioimmunoassay for angiotensin I generation and expressed as picograms per milliliter of angiotensin I generation per 10(5) cells. Active renin concentration was high in control secretory endometrial stromal cells (513 pg/ml) compared with trophoblast (none) and mesenchymal cell cultures (74 pg/ml). Marked decrease in active renin secretion occurred in control endometrial stromal cells (days 13 to 15, 86 pg/ml), whereas progesterone-induced decidualization sustained and increased secretion (days 13 to 15, 1017 pg/ml). This renin activity was quantitatively inhibited in culture fluid assays by specific human renal renin antibody in serial dilutions. Renin activity measurements before and after trypsin activation showed the majority of renin (91.15%) in the inactive form and a smaller fraction (8.85%) in the active form. Immunohistochemistry with the use of specific human renal renin antibody confirmed the presence of renin and its stimulation by progesterone in endometrial stromal cells. Progesterone had minimal effect on mesenchymal cells. These data confirmed endometrial stromal cells as a significant source of active renin and showed that progesterone induced a marked increase in this production.

Human prorenin

Human prorenin is the enzymatically inactive biosynthetic precursor of renin. Recent interest has focused on the posttranslational sorting and processing of prorenin to renin since markedly increased levels of circulating prorenin have been associated with both physiological and pathological changes. These observations raise the question of whether prorenin processing may be a regulatory event in renin production in the kidney. In the juxtaglomerular cells of the kidney, prorenin can be sorted to either of two pathways: 1) the regulated pathway, which is mediated by secretory granules, where a thiol protease resembling cathepsin B processes prorenin to renin by cleavage of the amino terminal 43-amino acid prosegment, which allows exposure of the active site of renin, or 2) the constitutive pathway, which is not regulated and does not involve conversion of prorenin to renin. Studies in which segments of prorenin are modified by site-directed mutagenesis suggest that the prosegment and glycosylation are not required for sorting, although they may influence or participate in sorting, or both. Certain areas in the prosegment are important determinants of enzyme activity and ability to cleave the prosegment. Further structural analysis of prorenin will be useful to assess details of its sorting and processing. In addition, a number of extrarenal tissues such as uterine lining, ovarian theca, corpus luteum, pituitary, and adrenal, express the renin gene. These tissues have different capabilities to sort and process prorenin compared with kidney, and some tissues secrete only prorenin. Whether prorenin-to-renin conversion is necessary to activate these local renin-angiotensin systems is a key issue.(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of the renin-angiotensin system during ritodrine treatment in preterm labor

Plasma renin activity, active renin concentrations, and total renin concentrations were measured serially in 15 women in preterm labor treated with ritodrine. All three parameters showed rapid increases during treatment that depended on ritodrine doses and paralleled changes in maternal pulse rate. The slope of the correlation between plasma renin activity and active renin concentrations suggested an increase in both active renin and renin substrate during ritodrine treatment. In contrast, levels of inactive renin remained unchanged for several hours during ritodrine infusion, showing only a late and much smaller response than that of either total or active renin concentrations. Thus ritodrine treatment results in further activation of an already activated renin-angiotensin system in pregnancy. This activation relates to an increase in both renin enzyme and its substrate. It may be responsible for the disturbance in water balance and the risk of pulmonary edema associated with beta-sympathomimetic drug treatment in preterm labor.

Human decidua is a major source of renin

Plasma prorenin levels are elevated in normal pregnant women. Current evidence suggests renin production by tissues of the uteroplacental unit contribute to this elevation. The purpose of this investigation was to define the source of renin biosynthesis within the human uteroplacental unit and to characterize the renin produced. RNA extraction and Northern blot analysis consistently demonstrated renin mRNA expression in uterine lining both in the pregnant (decidua) and nonpregnant states (endometrium) and in fetal chorion laeve, which is inseparable from the decidua. In contrast, renin mRNA expression was not detected in basal plate and intertwin chorion (which is separate from decidua), amnion, myometrium, or placental villi. The total renin content in decidual homogenates was two- to threefold greater than in endometrial homogenates, and cultured human decidual cells produced significantly more total renin than cultured human endometrial cells, suggesting that pregnancy enhanced renin production by the cells lining the uterus. Immunoblot analysis and [3H]leucine incorporation identified 47,000-mol wt prorenin as the major form of renin produced by cultured human decidual cells. These studies indicate that maternal decidua is the major source of prorenin in the uteroplacental unit.

Renin in the female reproductive system

The female reproductive system is characterized by the presence of local renin-angiotensin systems within the ovary and within the uterine lining. In the human ovary, renin arises from theca cells. In the uterus, the endometrium (uterine lining in the nonpregnant state) and the decidua (uterine lining in the pregnant state) are the major sources of renin. The primary form of renin produced by these extrarenal sources is prorenin. Angiotensin II, the active component of the renin cascade has a number of potential roles in the ovary and uterus. A key observation is that the human ovum and fetus is bathed in fluids rich in prorenin and angiotensin II.

Increased fetoplacental active renin production in pregnancy-induced hypertension

Since pregnancy-induced hypertension is associated with impaired uteroplacental blood flow, we studied fetoplacental and maternal renin production in controls and subjects with pregnancy-induced hypertension. We measured total, active, and inactive (pro-) renin in maternal serum, fetal arterial and venous blood, and chorion homogenate in eight normotensive term patients and 18 patients with pregnancy-induced hypertension. No differences in active or prorenin were found in maternal blood from normal women or patients with pregnancy-induced hypertension. In contrast, fetal artery and vein, as well as chorionic tissue, contained significantly higher active renin in pregnancy-induced hypertension compared with normal subjects. No difference in fetal or chorionic prorenin was seen in the two groups. Thus active to total renin ratio was higher in the fetus and chorion of subjects with pregnancy-induced hypertension, which suggests enhanced active renin production. These results suggest that pregnancy-induced hypertension is associated with increased activity of the renin-angiotensin system in the fetoplacental unit, which is not reflected in the maternal circulation. This may be an attempt by the fetus and chorionic membranes to maintain vascular homeostasis in the face of altered uteroplacental blood flow.