Increased fetal plasma prostaglandin E2 concentrations during fetal placental embolization in pregnant sheep

Postnatal effects of intrauterine treatment of the growth-restricted ovine fetus with intra-amniotic insulin-like growth factor-1

KEY POINTS

Fetal growth restriction increases the risk of fetal and neonatal mortality and morbidity, and contributes to increased risk of chronic disease later in life. Intra-amniotic insulin-like growth factor-1 (IGF1) treatment of the growth-restricted ovine fetus improves fetal growth, but postnatal effects are unknown. Here we report that intra-amniotic IGF1 treatment of the growth-restricted ovine fetus alters size at birth and mechanisms of early postnatal growth in a sex-specific manner. We also show that maternal plasma C-type natriuretic peptide (CNP) products are related to fetal oxygenation and size at birth, and hence may be useful for non-invasive monitoring of fetal growth restriction. Intrauterine IGF1 treatment in late gestation is a potentially clinically relevant intervention that may ameliorate the postnatal complications of fetal growth restriction.

ABSTRACT

Placental insufficiency-mediated fetal growth restriction (FGR) is associated with altered postnatal growth and metabolism, which are, in turn, associated with increased risk of adult disease. Intra-amniotic insulin-like growth factor-1 (IGF1) treatment of ovine FGR increases growth rate in late gestation, but the effects on postnatal growth and metabolism are unknown. We investigated the effects of intra-amniotic IGF1 administration to ovine fetuses with uteroplacental embolisation-induced FGR on phenotypical and physiological characteristics in the 2  weeks after birth. We measured early postnatal growth velocity, amino-terminal propeptide of C-type natriuretic peptide (NTproCNP), body composition, tissue-specific mRNA expression, and milk intake in singleton lambs treated weekly with 360 μg intra-amniotic IGF1 (FGRI; n = 13 females, 19 males) or saline (FGRS; n = 18 females, 12 males) during gestation, and in controls (CON; n = 15 females, 22 males). There was a strong positive correlation between maternal NTproCNP and fetal oxygenation, and size at birth in FGR lambs. FGR lambs were ∼20% lighter at birth and demonstrated accelerated postnatal growth velocity. IGF1 treatment did not alter perinatal mortality, partially abrogated the reduction in newborn size in females, but not males, and reduced accelerated growth in both sexes. IGF1-mediated upregulation of somatotrophic genes in males during the early postnatal period could suggest that treatment effects are associated with delayed axis maturation, whilst treatment outcomes in females may rely on the reprogramming of nutrient-dependent mechanisms of growth. These data suggest that the growth-restricted fetus is responsive to intra-amniotic intervention with IGF1, and that sex-specific somatotrophic effects persist in the early postnatal period.

Apoptosis in the Ovine Fetal Brain Following Placental Embolization and Intermittent Umbilical Cord Occlusion

The purpose of this study was to compare the regional distribution of apoptotic cells in the near term ovine fetal brain caused by prolonged moderate hypoxia, as seen in placental insufficiency, and intermittent severe hypoxia, as seen in umbilical cord compression, which may then contribute to adverse neurodevelopment in the postnatal life. We hypothesized that apoptosis in the fetal brain will be increased in response to both prolonged moderate hypoxia and intermittent severe hypoxia. Twenty-one near term (126-127 days) sheep were divided into 3 groups: control (CON; n = 7), placental embolization (EMB; n = 7), and umbilical cord occlusion (UCO; n = 8). The EMB group had microsphere injections into the umbilical arterial circulation until the oxygen content was at 50% of baseline value. The UCO group had complete cord occlusion for 2 minutes every hour, 6 times a day for 2 consecutive days. At 4 pm on day 2, the animals were euthanized; fetal brains were fixed and prepared for apoptosis staining using the terminal uridine deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay method. In the cerebellar white matter, there was a 3-fold increase in the number of TUNEL positive cells per 1000 cells in both EMB and UCO animals as compared to CON (P = .017). There was also a significant increase in the frontal cortical grey matter (layers 1-3) in EMB animals as compared to CON (P = .014). As such, apoptosis in the near term fetal sheep brain is altered with both sustained moderate hypoxia and intermittent severe hypoxia in the latter part of pregnancy, with potential for long-term neurological sequelae.

Fetal endocrine and metabolic adaptations to hypoxia: the role of the hypothalamic-pituitary-adrenal axis

In utero, hypoxia is a significant yet common stress that perturbs homeostasis and can occur due to preeclampsia, preterm labor, maternal smoking, heart or lung disease, obesity, and high altitude. The fetus has the extraordinary capacity to respond to stress during development. This is mediated in part by the hypothalamic-pituitary-adrenal (HPA) axis and more recently explored changes in perirenal adipose tissue (PAT) in response to hypoxia. Obvious ethical considerations limit studies of the human fetus, and fetal studies in the rodent model are limited due to size considerations and major differences in developmental landmarks. The sheep is a common model that has been used extensively to study the effects of both acute and chronic hypoxia on fetal development. In response to high-altitude-induced, moderate long-term hypoxia (LTH), both the HPA axis and PAT adapt to preserve normal fetal growth and development while allowing for responses to acute stress. Although these adaptations appear beneficial during fetal development, they may become deleterious postnatally and into adulthood. The goal of this review is to examine the role of the HPA axis in the convergence of endocrine and metabolic adaptive responses to hypoxia in the fetus.

Morphologic alterations in ovine placenta and fetal liver following induced severe placental insufficiency

OBJECTIVES

Umbilical-placental embolization with microspheres has been used as a model of placental insufficiency and intrauterine growth restriction (IUGR). However, the effects of embolization on placental structure and organ morphology of the resulting IUGR fetus are relatively unexplored. In this study using ovine fetuses, we determined the location and distribution of microspheres within the placenta and explored the extent of placental and fetal organ morphologic changes induced by placental embolization. We hypothesized that microspheres administered into the umbilical circulation over 4 days would cause placental damage without significant morphologic alterations in fetal kidney or liver.

METHODS

Eleven pregnant sheep at 118 +/- 1 (SE) days' gestation were studied. In six fetuses, embolization was induced by injections of 15-microm diameter microspheres on 4 successive days into the fetal descending aorta proximal to the umbilical arteries. Five fetuses served as time controls.

RESULTS

In embolized fetuses, microspheres were detected in the placenta embedded in the fetal cytotrophoblastic layer or maternal parenchyma adjacent to villous cytotrophoblasts. Fetal cytotrophoblasts appeared normal except for loss of distinct separation between fetal and maternal cell layers. Microspheres were also detected in the fetal membranes within capillaries. The body weights of embolized fetuses were lower than controls, as were the body weight-normalized liver but not kidney weights. In the liver of the embolized fetuses, the number of hematopoietic cell clusters was markedly reduced, whereas the fetal kidneys appeared normal.

CONCLUSIONS

We conclude that after 4 days of umbilical-placental embolization, microspheres were concentrated at the fetal villi proximal to the apical maternal-fetal interface and in the fetal membranes. There were noticeable morphologic changes in the embolized placentas, with no apparent gross damage to the placenta. The reduction in fetal liver weight and liver extramedullary hematopoietic cell abundance associated with embolization may predispose the fetus to alterations in liver function that could persist after birth.

Placental insufficiency and its consequences

Placental insufficiency is a process leading to progressive deterioration in placental function and a decrease in transplacental transfer of oxygen and nutrients to the fetus. The resulting fetal hypoxemia is the major stimulus involved in the reduction in fetal growth as an attempt to reduce metabolic demands by the growing fetus. Fetal growth restriction (FGR) is the second cause of perinatal death after prematurity and can complicate up to 6% of all pregnancies. It is becoming apparent that its occurrence has major impacts on the fetus and placenta with consequences on the cardiovascular, metabolic and neurological development up to adulthood. We are just starting to unveil some of the basic mechanisms involved in this complex adaptation that may lead to reprogramming of fetal organs development mostly the heart, pancreas, lungs and brain. It is becoming clear that future research is needed to develop strategies to improve antenatal detection of FGR, in addition to reduce the risk of abnormal neurodevelopment during childhood, and onset of common diseases in adulthood following pregnancies complicated with placental insufficiency.

Hypoxia causes down-regulation of 11 beta-hydroxysteroid dehydrogenase type 2 by induction of Egr-1

Hypoxia causes several renal tubular dysfunctions, including abnormal handling of potassium and sodium and increased blood pressure. Therefore, we investigated the impact of hypoxia on 11beta-hydroxysteroid dehydrogenase (11beta-HSD2) enzyme, a crucial prereceptor gatekeeper for renal glucocorticosteroid-mediated mineralocorticoid action. The effect of hypoxia was assessed in vitro by incubating LLC-PK1 cells with antimycin A, an inhibitor of mitochondrial oxidative phosphorylation. Antimycin A induced a dose- and time-dependent reduction of 11beta-HSD2 activity. The early growth response gene, Egr-1, a gene known to be stimulated by hypoxia was investigated because of a potential Egr-1 binding site in the promoter region of 11beta-HSD2. Antimycin A induced Egr-1 protein and Egr-1-regulated luciferase gene expression. This induction was prevented with the MAPKK inhibitor PD 98059. Overexpression of Egr-1 reduced endogenous 11beta-HSD2 activity in LLC-PK1 cells, indicating that MAPK ERK is involved in the regulation of 11beta-HSD2 in vitro. In vivo experiments in rats revealed that Egr-1 protein increases, whereas 11beta-HSD2 mRNA decreases, in kidney tissue after unilateral renal ischemia and in humans the renal activity of 11beta-HSD2 as assessed by the urinary ratio of (tetrahydrocortisol+5alpha-tetrahydrocortisol)/tetrahydrocortisone declined when volunteers were exposed to hypoxemia at high altitude up to 7000 m. Thus, hypoxia decreases 11beta-HSD2 transcription and activity by inducing Egr-1 in vivo and in vitro. This mechanism might account for enhanced renal sodium retention and hypertension associated with hypoxic conditions.

Plasma adrenocorticotropin and cortisol concentrations during acute hypoxemia after a reversible period of adverse intrauterine conditions in the ovine fetus during late gestation

The present study determined the pituitary-adrenal responses to acute hypoxemia after a period of reversible adverse intrauterine conditions produced by partial compression of the umbilical cord for 3 days in the sheep fetus during late gestation. At 118 +/- 2 days gestation (term is approximately 145 days), 12 sheep fetuses were instrumented under halothane anesthesia with an occluder cuff around the umbilical cord, amniotic and vascular catheters, and a transit-time flow probe around an umbilical artery. In 6 of the fetuses at 125 days, umbilical blood flow was reduced by about 30% from baseline for 3 days (UCC), after which the occluder was deflated. The remaining 6 fetuses acted as sham-operated controls in which the occluder was not inflated. All fetuses were then subsequently subjected to 2 periods of acute hypoxemia, elicited by reducing the maternal inspired fraction of oxygen (FiO(2)) at 2 +/- 1 and 5 +/- 2 days after the end of cord compression or sham compression. In addition, 4 fetuses from each group were subjected to an ACTH challenge 1-2 days after the final episode of acute hypoxemia. Maternal and fetal arterial blood samples were taken at appropriate intervals during cord compression, acute hypoxemia, and ACTH challenge for analyses of blood gases, pH, and plasma ACTH and cortisol concentrations. Partial compression of the umbilical cord produced reversible mild fetal asphyxia, a transient increase in fetal plasma ACTH, and a progressive increase in fetal plasma cortisol. At 5 +/- 2 days after the end of compression, despite similar blood gas status between the groups, basal plasma cortisol, but not ACTH, concentrations were significantly greater in compressed fetuses relative to sham controls. However, this dissociation did not affect a similar increment in fetal plasma ACTH and cortisol concentrations during acute hypoxemia or in the fetal plasma cortisol response to the ACTH challenge in either group. An increase in adrenocortical mass occurred in fetuses preexposed to partial compression of the umbilical cord relative to sham controls. The data suggest that fetal exposure to a reversible period of adverse intrauterine conditions produced by partial compression of the umbilical cord does not affect the magnitude of the fetal hypothalamic-pituitary-adrenal axis response to subsequent acute hypoxemia, but it leads to resetting of basal hypothalamic-pituitary-adrenal axis function in the fetus. The mechanism for this resetting may include an increase in adrenocortical steroidogenic synthetic capacity, but it is not due to a change in adrenocortical sensitivity to ACTH. Inappropriate fetal glucocorticoid exposure after reversible periods of adverse intrauterine conditions has important implications for fetal and postnatal development.

Endocrine and paracrine regulation of birth at term and preterm

We have examined factors concerned with the maintenance of uterine quiescence during pregnancy and the onset of uterine activity at term in an animal model, the sheep, and in primate species. We suggest that in both species the fetus exerts a critical role in the processes leading to birth, and that activation of the fetal hypothalamic-pituitary-adrenal axis is a central mechanism by which the fetal influence on gestation length is exerted. Increased cortisol output from the fetal adrenal gland is a common characteristic across animal species. In primates, there is, in addition, increased output of estrogen precursor from the adrenal in late gestation. The end result, however, in primates and in sheep is similar: an increase in estrogen production from the placenta and intrauterine tissues. We have revised the pathway by which endocrine events associated with parturition in the sheep come about and suggest that fetal cortisol directly affects placental PGHS expression. In human pregnancy we suggest that cortisol increases PGHS expression, activity, and PG output in human fetal membranes in a similar manner. Simultaneously, cortisol contributes to decreases in PG metabolism and to a feed-forward loop involving elevation of CRH production from intrauterine tissues. In human pregnancy, there is no systemic withdrawal of progesterone in late gestation. We have argued that high circulating progesterone concentrations are required to effect regionalization of uterine activity, with predominantly relaxation in the lower uterine segment, allowing contractions in the fundal region to precipitate delivery. This new information, arising from basic and clinical studies, should further the development of new methods of diagnosing the patient at risk of preterm labor, and the use of scientifically based strategies specifically for the management of this condition, which will improve the health of the newborn.

Differential effects of placental restriction on IGF-II, ACTH receptor and steroidogenic enzyme mRNA levels in the foetal sheep adrenal

We have investigated the effects of restriction of placental growth on foetal adrenal growth and adrenal expression of mRNAs for Insulin-like Growth Factor II (IGF-II), the IGF binding protein IGFBP-2, Steroidogenic Factor 1 (SF-1) and adrenocorticotrophic hormone (ACTH) receptor (ACTH-R) and the steroidogenic cytochrome P-450 enzymes: cholesterol side chain cleavage (CYP11A1), 17alpha-hydroxylase (CYP17) and 21-hydroxylase (CYP21A1); and 3beta-hydroxysteroid dehydrogenase/Delta5Delta4 isomerase (3betaHSD). Endometrial caruncles were removed from non-pregnant ewes before mating (placental restriction group; PR). The total adrenal: foetal weight ratio was higher in PR (n=6 foetuses) than in control foetuses (n=6 foetuses). There was no difference in plasma ACTH concentrations between the PR and control foetuses between 130 and 140 days gestation. Adrenal IGF-II mRNA levels were lower (P<0.05) in the PR group, however, adrenal IGFBP-2 mRNA levels were not different between the PR and control groups. Adrenal ACTH-R mRNA levels were also lower whilst CYP11A1 mRNA levels were increased (P<0.005) in the PR group. We conclude that foetal adrenal growth and steroidogenesis are stimulated as a consequence of foetal growth restriction and that factors other than ACTH are important in foetal adrenal activation during chronic, sustained hypoxaemia.

Fetal sheep endocrine responses to sustained hypoxemic stress after chronic fetal placental embolization

The purpose of this study was to determine the endocrine and circulatory responses of the ovine fetus, near term, to sustained hypoxemic stress superimposed on chronic hypoxemia. Fetal sheep were chronically embolized (n = 7) for 10 days between 0.84 and 0.91 of gestation via the descending aorta until arterial oxygen content was decreased by approximately 30%. Control animals (n = 8) received saline only. On experimental day 10, both groups were embolized over a 6-h period until fetal arterial pH decreased to approximately 7.00. Regional distribution of lower body blood flows was measured on day 10, before and at the end of acute embolization. On day 10, the chronically embolized group had lower arterial oxygen content (P < 0.05), Po2 (P < 0.01), and placental blood flow (P < 0.05) than controls and higher prostaglandin E2 (PGE2) and norepinephrine plasma concentrations (both P < 0.05). In response to a superimposed sustained hypoxemic stress, there was a twofold greater increase in PGE2 in the chronically embolized group than in the control group (P < 0.05). However, the increase in fetal plasma cortisol in response to superimposed hypoxemic stress was similar in both groups, despite significantly lower adrenocorticotropic hormone and adrenal cortex blood flow responses in the chronically hypoxemic group (both P < 0.05). We conclude that PGE2 response to a sustained superimposed reduction in placental blood flow, leading to metabolic acidosis, is enhanced under conditions of chronic hypoxemia and may play an important role for the maintenance of the fetal cortisol response to an episode of superimposed acute stress.

Effects of long-term hypoxemia on pituitary-adrenal function in fetal sheep

To test the hypothesis that long-term hypoxemia causes premature activation of the fetal pituitary-adrenal function, we embolized the fetal side of the placenta in pregnant sheep and examined the changes in concentrations of immunoreactive adrenocorticotropic hormone (irACTH), cortisol, and prostaglandin E2 (PGE2) in fetal plasma, and levels and localization of proopiomelanocortin (POMC) mRNA in the pars distalis and the pars intermedia of the fetal pituitary. Twelve fetal sheep were studied (6 embolized and 6 control) for 21 days between 0.74 and 0.88 of gestation. Daily injections of nonradiolabeled microspheres were given into the fetal abdominal aorta to decrease fetal arterial oxygen content by 40-50% of the preembolization values. In the embolized group, concentrations of irACTH, PGE2, and cortisol in fetal plasma increased gradually and were significantly (P < 0.05) elevated above those of controls after day 10, day 16, and day 20, respectively. POMC mRNA levels in the pars distalis of the fetal pituitary were not different from those of controls but were significantly reduced in the pars intermedia (P < 0.05). We conclude that levels of POMC mRNA in the pars distalis are unchanged during long-term hypoxemia possibly because of negative feedback effects of elevated cortisol on the pituitary gland. During long-term fetal hypoxemia, there is a differential regulation of POMC mRNA expression in the pars distalis and pars intermedia.

Regulation of the hypothalamo-pituitary-adrenocortical axis in fetal sheep

Development of the fetal hypothalamo-pituitary-adrenal (HPA) axis is required for normal fetal life and subsequent neonatal health. Activation of the fetal pituitary gland results in the synthesis and release of glucocorticoids from the adrenal cortex. Glucocorticoids promote maturation of several organ systems, are important in responses of the fetus to stress, and are involved in the initiation of parturition in several species. The expression of hypothalamic and pituitary genes associated with HPA function is apparent early in gestation in fetal sheep, although the endocrine changes associated with maturation and parturition do not occur until the last fifth of gestation. In this connection, the fetal HPA axis can be activated by treatment with hypophysiotrophic factors or moderate stress throughout gestation. This review focuses on the development of neuroendocrine mechanisms controlling HPA function during fetal life.