Indicators of delayed maturation of rat heart treated prenatally with dexamethasone

Using human induced pluripotent stem cell-derived cardiomyocytes to understand the mechanisms driving cardiomyocyte maturation

Cardiovascular diseases are the leading cause of mortality and reduced quality of life globally. Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) provide a personalized platform to study inherited heart diseases, drug-induced cardiac toxicity, and cardiac regenerative therapy. However, the immaturity of CMs obtained by current strategies is a major hurdle in utilizing hiPSC-CMs at their fullest potential. Here, the major findings and limitations of current maturation methodologies to enhance the utility of hiPSC-CMs in the battle against a major source of morbidity and mortality are reviewed. The most recent knowledge of the potential signaling pathways involved in the transition of fetal to adult CMs are assimilated. In particular, we take a deeper look on role of nutrient sensing signaling pathways and the potential role of cap-independent translation mediated by the modulation of mTOR pathway in the regulation of cardiac gap junctions and other yet to be identified aspects of CM maturation. Moreover, a relatively unexplored perspective on how our knowledge on the effects of preterm birth on cardiovascular development can be actually utilized to enhance the current understanding of CM maturation is examined. Furthermore, the interaction between the evolving neonatal human heart and brown adipose tissue as the major source of neonatal thermogenesis and its endocrine function on CM development is another discussed topic which is worthy of future investigation. Finally, the current knowledge regarding transcriptional mediators of CM maturation is still limited. The recent studies have produced the groundwork to better understand CM maturation in terms of providing some of the key factors involved in maturation and development of metrics for assessment of maturation which proves essential for future studies on in vitro PSC-CMs maturation.

Glucocorticoids regulate mitochondrial fatty acid oxidation in fetal cardiomyocytes

The late gestational rise in glucocorticoids contributes to the structural and functional maturation of the perinatal heart. Here, we hypothesized that glucocorticoid action contributes to the metabolic switch in perinatal cardiomyocytes from carbohydrate to fatty acid oxidation. In primary mouse fetal cardiomyocytes, dexamethasone treatment induced expression of genes involved in fatty acid oxidation and increased mitochondrial oxidation of palmitate, dependent upon a glucocorticoid receptor (GR). Dexamethasone did not, however, induce mitophagy or alter the morphology of the mitochondrial network. In vivo, in neonatal mice, dexamethasone treatment induced cardiac expression of fatty acid oxidation genes. However, dexamethasone treatment of pregnant C57Bl/6 mice at embryonic day (E)13.5 or E16.5 failed to induce fatty acid oxidation genes in fetal hearts assessed 24 h later. Instead, at E17.5, fatty acid oxidation genes were downregulated by dexamethasone, as was GR itself. PGC-1α, required for glucocorticoid-induced maturation of primary mouse fetal cardiomyocytes in vitro, was also downregulated in fetal hearts at E17.5, 24 h after dexamethasone administration. Similarly, following a course of antenatal corticosteroids in a translational sheep model of preterm birth, both GR and PGC-1α were downregulated in heart. These data suggest that endogenous glucocorticoids support the perinatal switch to fatty acid oxidation in cardiomyocytes through changes in gene expression rather than gross changes in mitochondrial volume or mitochondrial turnover. Moreover, our data suggest that treatment with exogenous glucocorticoids may interfere with normal fetal heart maturation, possibly by downregulating GR. This has implications for clinical use of antenatal corticosteroids when preterm birth is considered a possibility. KEY POINTS: Glucocorticoids are steroid hormones that play a vital role in late pregnancy in maturing fetal organs, including the heart. In fetal cardiomyocytes in culture, glucocorticoids promote mitochondrial fatty acid oxidation, suggesting they facilitate the perinatal switch from carbohydrates to fatty acids as the predominant energy substrate. Administration of a synthetic glucocorticoid in late pregnancy in mice downregulates the glucocorticoid receptor and interferes with the normal increase in genes involved in fatty acid metabolism in the heart. In a sheep model of preterm birth, antenatal corticosteroids (synthetic glucocorticoid) downregulates the glucocorticoid receptor and the gene encoding PGC-1α, a master regulator of energy metabolism. These experiments suggest that administration of antenatal corticosteroids in anticipation of preterm delivery may interfere with fetal heart maturation by downregulating the ability to respond to glucocorticoids.

Effect of Preterm Birth on Cardiac and Cardiomyocyte Growth and the Consequences of Antenatal and Postnatal Glucocorticoid Treatment

Preterm birth coincides with a key developmental window of cardiac growth and maturation, and thus has the potential to influence long-term cardiac function. Individuals born preterm have structural cardiac remodelling and altered cardiac growth and function by early adulthood. The evidence linking preterm birth and cardiovascular disease in later life is mounting. Advances in the perinatal care of preterm infants, such as glucocorticoid therapy, have improved survival rates, but at what cost? This review highlights the short-term and long-term impact of preterm birth on the structure and function of the heart and focuses on the impact of antenatal and postnatal glucocorticoid treatment on the immature preterm heart.

Early life factors and their relevance to intima-media thickness of the common carotid artery in early adulthood

BACKGROUND

Early life factors may predispose an offspring to cardiovascular disease in later life; relevance of these associations may extend to ‟healthy" people in Western populations. We examined the prospective associations between early life factors and adult carotid intima-media thickness (IMT), a surrogate marker of atherosclerosis, in a healthy German population.

METHODS

We studied term participants (n = 265) of the DONALD Study, with bilateral sonographic measurements of IMT (4-8 measurements on both left and right carotid artery) at age 18-40 years and prospectively collected data on early life factors (maternal and paternal age at child birth, birth weight, gestational weight gain and full breastfeeding (>17weeks). Mean IMT values were averaged from mean values of both sides. Associations between early life factors and adult IMT were analyzed using multivariable linear regression models with adjustment for potential confounders.

RESULTS

Adult mean IMT was 0.56mm, SD 0.03, (range: 0.41 mm-0.78 mm). Maternal age at child birth was of relevance for adult IMT, which was sex specific: Advanced maternal age at child birth was associated with an increased adult IMT among female offspring only (β 0.03, SE 0.009 mm/decade, P = 0.003), this was not affected by adult waist circumference, BMI or blood pressure. Other early life factors were not relevant for IMT levels in males and females.

CONCLUSION

This study suggests that advanced maternal age at child birth is of prospective relevance for adult IMT levels in a healthy German population and this association may be of adverse relevance for females only.

Glucocorticoid Maturation of Fetal Cardiovascular Function

The last decade has seen rapid advances in the understanding of the central role of glucocorticoids in preparing the fetus for life after birth. However, relative to other organ systems, maturation by glucocorticoids of the fetal cardiovascular system has been ignored. Here, we review the effects of glucocorticoids on fetal basal cardiovascular function and on the fetal cardiovascular defense responses to acute stress. This is important because glucocorticoid-driven maturational changes in fetal cardiovascular function under basal and stressful conditions are central to the successful transition from intra- to extrauterine life. The cost-benefit balance for the cardiovascular health of the preterm baby of antenatal glucocorticoid therapy administered to pregnant women threatened with preterm birth is also discussed.

Glucocorticoids and programming of the microenvironment in heart

Glucocorticoids are primary stress hormones and can improve neonatal survival when given to pregnant women threatened by preterm birth or to preterm infants. It has become increasingly apparent that glucocorticoids, primarily by interacting with glucocorticoid receptors, play a critical role in late gestational cardiac maturation. Altered glucocorticoid actions contribute to the development and progression of heart disease. The knowledge gained from studies in the mature heart or cardiac damage is insufficient but a necessary starting point for understanding cardiac programming including programming of the cardiac microenvironment by glucocorticoids in the fetal heart. This review aims to highlight the potential roles of glucocorticoids in programming of the cardiac microenvironment, especially the supporting cells including endothelial cells, immune cells and fibroblasts. The molecular mechanisms by which glucocorticoids regulate the various cellular and extracellular components and the clinical relevance of glucocorticoid functions in the heart are also discussed.

Exposure of immature rat heart to antenatal glucocorticoid results in cardiac proliferation

BACKGROUND

ATP synthesis and cardiac contraction-related protein production are accelerated in the immature fetal heart by antenatal glucocorticoids (GC). This study investigated the structural maturity of the myocardium and underlying signal pathway associated with cardiac growth in fetal rats that received antenatal GC.

METHODS AND RESULTS

Dexamethasone (DEX) was given to pregnant rats for 2 days from day 17 or day 19 of gestation, and the hearts of 19 and 21 day fetuses and 1-day-old neonates were analyzed. Although irregular myofibril orientation was observed morphologically in 19 day fetal hearts, the myofibril components were organized in fetuses after DEX. The cross-sectional area of the myocardium and Ki-67-positive cells were significantly increased in fetal DEX groups, suggesting that cardiac enlargement resulted from myocyte proliferation. Glycogen synthase kinase-3β (GSK-3β) protein was significantly decreased in fetal DEX groups. β-Catenin and vascular endothelial growth factor protein were also significantly increased. Furthermore, increased cardiomyocyte proliferation appeared to be mediated by GC receptors after culture with DEX in vitro.

CONCLUSIONS

Antenatal DEX induces structural maturity accompanying cardiomyocyte proliferation in the premature fetal rat heart, and GSK-3β and β-catenin are thought to contribute to cardiac growth.

Gestational Hypoxia and Developmental Plasticity

Hypoxia is one of the most common and severe challenges to the maintenance of homeostasis. Oxygen sensing is a property of all tissues, and the response to hypoxia is multidimensional involving complicated intracellular networks concerned with the transduction of hypoxia-induced responses. Of all the stresses to which the fetus and newborn infant are subjected, perhaps the most important and clinically relevant is that of hypoxia. Hypoxia during gestation impacts both the mother and fetal development through interactions with an individual's genetic traits acquired over multiple generations by natural selection and changes in gene expression patterns by altering the epigenetic code. Changes in the epigenome determine "genomic plasticity," i.e., the ability of genes to be differentially expressed according to environmental cues. The genomic plasticity defined by epigenomic mechanisms including DNA methylation, histone modifications, and noncoding RNAs during development is the mechanistic substrate for phenotypic programming that determines physiological response and risk for healthy or deleterious outcomes. This review explores the impact of gestational hypoxia on maternal health and fetal development, and epigenetic mechanisms of developmental plasticity with emphasis on the uteroplacental circulation, heart development, cerebral circulation, pulmonary development, and the hypothalamic-pituitary-adrenal axis and adipose tissue. The complex molecular and epigenetic interactions that may impact an individual's physiology and developmental programming of health and disease later in life are discussed.

Dexamethasone Treatment of Newborn Rats Decreases Cardiomyocyte Endowment in the Developing Heart through Epigenetic Modifications

The potential adverse effect of synthetic glucocorticoid, dexamethasone therapy on the developing heart remains unknown. The present study investigated the effects of dexamethasone on cardiomyocyte proliferation and binucleation in the developing heart of newborn rats and evaluated DNA methylation as a potential mechanism. Dexamethasone was administered intraperitoneally in a three day tapered dose on postnatal day 1 (P1), 2 and 3 to rat pups in the absence or presence of a glucocorticoid receptor antagonist Ru486, given 30 minutes prior to dexamethasone. Cardiomyocytes from P4, P7 or P14 animals were analyzed for proliferation, binucleation and cell number. Dexamethasone treatment significantly increased the percentage of binucleated cardiomyocytes in the hearts of P4 pups, decreased myocyte proliferation in P4 and P7 pups, reduced cardiomyocyte number and increased the heart to body weight ratio in P14 pups. Ru486 abrogated the effects of dexamethasone. In addition, 5-aza-2'-deoxycytidine (5-AZA) blocked the effects of dexamethasone on binucleation in P4 animals and proliferation at P7, leading to recovered cardiomyocyte number in P14 hearts. 5-AZA alone promoted cardiomyocyte proliferation at P7 and resulted in a higher number of cardiomyocytes in P14 hearts. Dexamethasone significantly decreased cyclin D2, but not p27 expression in P4 hearts. 5-AZA inhibited global DNA methylation and blocked dexamethasone-mediated down-regulation of cyclin D2 in the heart of P4 pups. The findings suggest that dexamethasone acting on glucocorticoid receptors inhibits proliferation and stimulates premature terminal differentiation of cardiomyocytes in the developing heart via increased DNA methylation in a gene specific manner.

Glucocorticoids and foetal heart maturation; implications for prematurity and foetal programming

Glucocorticoids are steroid hormones, essential in mammals to prepare for life after birth. Blood levels of glucocorticoids (cortisol in most mammals including humans; corticosterone in rats and mice) rise dramatically shortly before birth. This is mimicked clinically in the routine administration of synthetic glucocorticoids to pregnant women threatened by a preterm birth or to preterm infants to improve neonatal survival. Whilst effects on lung are well documented and essential for postnatal survival, those on heart are less well known. In this study, we review recent evidence for a crucial role of glucocorticoids in late gestational heart maturation. Either insufficient or excessive glucocorticoid exposure before birth may alter the normal glucocorticoid-regulated trajectory of heart maturation with potential life-long consequences.

Binucleation of cardiomyocytes: the transition from a proliferative to a terminally differentiated state

Cardiomyocytes possess a unique ability to transition from mononucleate to the mature binucleate phenotype in late fetal development and around birth. Mononucleate cells are proliferative, whereas binucleate cells exit the cell cycle and no longer proliferate. This crucial period of terminal differentiation dictates cardiomyocyte endowment for life. Adverse early life events can influence development of the heart, affecting cardiomyocyte number and contributing to heart disease late in life. Although much is still unknown about the mechanisms underlying the binucleation process, many studies are focused on molecules involved in cell cycle regulation and cytokinesis as well as epigenetic modifications that can occur during this transition. Better understanding of these mechanisms could provide a basis for recovering the proliferative capacity of cardiomyocytes.

Glucocorticoid receptor is required for foetal heart maturation

Glucocorticoids are vital for the structural and functional maturation of foetal organs, yet excessive foetal exposure is detrimental to adult cardiovascular health. To elucidate the role of glucocorticoid signalling in late-gestation cardiovascular maturation, we have generated mice with conditional disruption of glucocorticoid receptor (GR) in cardiomyocytes and vascular smooth muscle cells using smooth muscle protein 22-driven Cre recombinase (SMGRKO mice) and compared them with mice with global deficiency in GR (GR(-/-)). Echocardiography shows impaired heart function in both SMGRKO and GR(-/-) mice at embryonic day (E)17.5, associated with generalized oedema. Cardiac ultrastructure is markedly disrupted in both SMGRKO and GR(-/-) mice at E17.5, with short, disorganized myofibrils and cardiomyocytes that fail to align in the compact myocardium. Failure to induce critical genes involved in contractile function, calcium handling and energy metabolism underpins this common phenotype. However, although hearts of GR(-/-) mice are smaller, with 22% reduced ventricular volume at E17.5, SMGRKO hearts are normally sized. Moreover, while levels of mRNA encoding atrial natriuretic peptide are reduced in E17.5 GR(-/-) hearts, they are normal in foetal SMGRKO hearts. These data demonstrate that structural, functional and biochemical maturation of the foetal heart is dependent on glucocorticoid signalling within cardiomyocytes and vascular smooth muscle, though some aspects of heart maturation (size, ANP expression) are independent of GR at these key sites.

Embryonic stem cell-derived cardiomyocytes as a model system to study cardioprotective effects of dexamethasone in doxorubicin cardiotoxicity

Embryonic stem cell (ESC)-derived beating cardiomyocytes may be considered as a suitable model for in vitro assessment of pharmacological and toxicological studies. In this model, laboratory animals are not required. In addition, physiological functions, such as heart beat, are assessed rather than single parameters such as cell viability. Here we report that doxorubicin (DOX) cardiotoxicity on mouse ESC-derived beating cardiomyocytes can be ameliorated by treatment with dexamethasone (DEX) when DEX is administrated only before DOX and not in combination with DOX. DEX effect appears to be mediated via glucocorticoid receptor and increases cardiomyocyte-specific gene expression. Cardiotoxicity of DOX can be augmented by calcium channel blocker, verapamil (VER) which also decreases the expression of cardiac gene markers. This model provides us with a clinical suggestion which proposes that the beneficial effect of DEX is obtained when DEX was added before DOX administration.

Cardiac corticosteroid receptors mediate the enlargement of the ovine fetal heart induced by chronic increases in maternal cortisol

Previous studies have demonstrated that modest, physiologically relevant increases in maternal cortisol in late gestation result in enlargement of the fetal heart. In this study, we investigated the role of mineralocorticoid receptor (MR) or glucocorticoid receptor (GR) in this enlargement. Ewes with single fetuses were randomly assigned at approximately 120 days of gestation to one of four groups: maternal cortisol infusion (1 mg/kg per day, cortisol); maternal cortisol infusion with fetal intrapericardial infusion of the MR antagonist (MRa) potassium canrenoate (600 microg/day; cortisol+MRa); maternal cortisol infusion with fetal intrapericardial infusion of the GR antagonist (GRa) mifepristone (50 microg/day, cortisol+GRa); and maternal saline infusion (control). At approximately 130 days of gestation, fetal heart to body weight ratio and right ventricular (RV) and left ventricular (LV) free wall thicknesses were increased in the cortisol group when compared with control group. Fetal hearts from the cortisol+MRa group weighed significantly less, with thinner LV, RV, and interventricular septum walls, when compared with the cortisol group. Fetal hearts from the cortisol+GRa group had significantly thinner RV walls than the cortisol group. Fetal arterial pressure and heart rate were not different among groups at 130 days. Picrosirius red staining of fetal hearts indicated that the increased size was not accompanied by cardiac fibrosis. These results suggest that physiologic increases in maternal cortisol in late gestation induce fetal cardiac enlargement via MR and, to a lesser extent, by GR, and indicate that the enlargement is not secondary to an increase in fetal blood pressure or an increase in fibrosis within the fetal heart.

Prenatal dexamethasone 'programmes' hypotension, but stress-induced hypertension in adult offspring

Low birth weight in humans is predictive of hypertension in adult life. Although the mechanisms underlying this link remain unknown, fetal overexposure to glucocorticoids has been implicated. We previously showed that prenatal dexamethasone (DEX) exposure in the rat lowers birth weight and programmes adult hypertension. The current study aimed to further investigate the nature of this hypertension and to elucidate its origins. Unlike previous studies, we assessed offspring blood pressure (BP) with radiotelemetry, which is unaffected by stress artefacts of measurement. We show that prenatal DEX during the last week of pregnancy results in offspring of low birth weight (14% reduction) that have lower basal BP in adulthood ( approximately 4-8 mmHg lower); with the commonly expected hypertensive phenotype only being noted when these offspring are subjected to even mild disturbance or a more severe stressor (up to 30 mmHg higher than controls). Moreover, DEX-treated offspring sustain their stress-induced hypertension for longer. Promotion of systemic catecholamine release (amphetamine) induced a significantly greater rise of BP in the DEX animals (77% increase) over that observed in the vehicle controls. Additionally, we demonstrate that the isolated mesenteric vasculature of DEX-treated offspring display greater sensitivity to noradrenaline and other vasoconstrictors. We therefore conclude that altered sympathetic responses mediate the stress-induced hypertension associated with prenatal DEX programming.

Veterinary pharmacovigilance. Part 6. Predictability of adverse reactions in animals from laboratory toxicology studies

Toxicological studies are conducted on constituents of veterinary medicinal products for a number of reasons. Aside from being a requirement of legislation, they are carried out for predictive purposes in the assessment of user safety or for the determination of consumer safety, for example, in the elaboration of maximum residue limits or tolerances. Alternatively, the results of toxicology studies may be available as they have been generated for registration of the drug for human medicinal purposes. This paper examines if the results of such studies have any predictive value for adverse reactions, which might occur during clinical use in animals. A number of adverse reactions, notably the Type A (toxicology or pharmacology dependent) should be predictable from these laboratory studies. However, as with human pharmaceutical products, they have less utility in predicting Type-B reactions (idiosyncratic in nature).

Prenatal exposure to dexamethasone alters Leydig cell steroidogenic capacity in immature and adult rats

This study examines the effects of prenatal exposure to dexamethasone (DEX) on postnatal testosterone production in male rats. Pregnant female rats were treated on gestation days 14-19 with DEX (100 microg/kg body weight per day; n = 9) or vehicle (n = 9). Results show that 35-day-old male offspring from DEX-treated pregnant females (n = 42) had decreased levels of serum testosterone (45.6% lower, P < .05) compared with control offspring (n = 43), although serum luteinizing hormone (LH) levels were not significantly altered. These findings suggest that a direct programming of developing gonadal cells occurs in response to high levels of maternal glucocorticoid. Indeed, testosterone production was significantly reduced in Leydig cells isolated from immature offspring of DEX-treated pregnant females compared with controls (48.3%, P < .001), and LH stimulation of these cells did not compensate for the lowered steroidogenic capacity. The hypothalamic-pituitary-adrenal axis was also affected, because significant reductions in both serum adrenocorticotropic hormone (ACTH; 26.2%, P < .001) and corticosterone (CORT; 32.3%, P < .001) were measured in DEX-exposed immature male offspring. In contrast, adult male offspring from DEX-treated dams had significantly higher levels of serum ACTH (39.2%, P <. 001) and CORT (37.8%, P < .001). These same animals had higher serum testosterone (31.6%, P < or = .05) and a significant reduction in serum LH (30.8%, P < .001). Moreover, Leydig cells isolated from these adult offspring exhibited an increased capacity for testosterone biosynthesis under basal (38.6%, P < .001) and LH-stimulated conditions (33.5%, P < .001). In summary, sustained changes in steroidogenic capacity were observed in male rats exposed to high levels of glucocorticoid during prenatal development. More specifically, DEX exposure in utero perturbed Leydig cell testosterone production in both pubertal and adult rats.

Prenatal dexamethasone rescues heart hypoplasia in fetal rats with congenital diaphragmatic hernia

BACKGROUND/PURPOSE

Patients and rats with congenital diaphragmatic hernia (CDH) have lung and heart hypoplasia. Prenatal steroids improve lung hypoplasia in CDH rats. The current study tests the hypothesis that prenatal dexamethasone could rescue heart hypoplasia in rats with CDH.

METHODS

Timed pregnant rats received intragastrically either 100 mg nitrofen or oil on day 9.5, and other animals had the same treatment with, in addition, either 0.25 mg/kg dexamethasone intraperitoneally or no treatment on days 19 and 20. Fetuses were recovered on day 21, and heart weight to body weight ratios, heart DNA, protein, and glycogen were measured in fresh specimens. Left-to-right ventricular diameter and aortic-to-pulmonary diameter ratios were measured after formalin fixation.

RESULTS

Wet heart weight to body weight, left-to-right ventricular diameter, and aortic-to-pulmonary root diameter ratios, which were lower in fetuses exposed only to nitrofen than in their oil controls, were similar in those exposed to nitrofen plus dexamethasone than in their corresponding oil plus dexamethasone controls. Total heart DNA, which was decreased in fetuses exposed to nitrofen with CDH in comparison with their controls, was increased in those receiving nitrofen and dexamethasone in comparison with theirs. Protein to DNA ratio was decreased in all rats with CDH irrespective of their exposure or not to dexamethasone. Glycogen to DNA ratio was higher in all dexamethasone-treated fetuses than in those without this treatment. No gross histologic differences were seen among groups.

CONCLUSIONS

Heart hypoplasia in rats with CDH is in part rescued by prenatal dexamethasone treatment as expressed by increased number of smaller myocytes with higher glycogen content. Prenatal steroids could modify heart involvement in human fetuses with CDH as well.

Dexamethasone induced cardiac hypertrophy in newborn rats is accompanied by changes in myosin heavy chain phenotype and gene transcription

Cardiac hypertrophy has been observed in newborn infants treated with dexamethasone (DEX). This study was undertaken to examine whether DEX-induced hypertrophy in newborn rats is associated with redistribution of cardiac myosin heavy chain (MHC) isoforms and, if so, the effects involve transcriptional regulation. Newborn rats were injected with either DEX (1 mg/kg/day; s.c.) or equivalent volume normal saline for 1, 3, 5, 7 or 9 days. Hypertrophy was quantified by heart dry/wet wt ratios, heart/body wt ratios, and total protein content of the myocardium. Changes in the expression of cardiac MHC mRNA were characterized by northern blot and slot blot analyses, using isoform specific probes for alpha- and beta-MHC genes. DEX effect on alpha-MHC gene transcription was analyzed by transiently transfecting various alpha-MHC promoter/CAT reporter constructs into primary cultures of cardiac myocytes derived from one day old rat pups. DEX administration into newborn rats produced significant cardiac hypertrophy ranging from 23% at day 1 to 59% at 9 days. The hypertrophy was accompanied by immediate increase (83%) in steady state level of the alpha-MHC mRNA within one day and a maximum increase (148%) at 7 days of treatment. The steady state level of beta-MHC mRNA declined by 25% at day 1 and a maximum decrease of 54% at day 7 of DEX treatment. The changes in MHC mRNA were also reflected in their protein levels as determined by V1 and V3 isozyme analysis. DEX treatment of primary cultures of cardiomyocytes following transfection with alpha-MHC promoter/CAT reporter constructs resulted in increased CAT expression in a dose dependent manner. The minimum alpha-MHC gene sequences responding to DEX treatment were located between the -200 to -74-bp region of the gene, resulting in 2-fold and 6-fold activation of CAT reporter after 0.05 and 0.1 mM doses of DEX, respectively. Our data indicate that DEX induced cardiac hypertrophy in newborn rats is accompanied by increased expression of alpha-MHC and decreased expression of beta-MHC. The alpha-MHC effects are mediated in part through transcriptional mechanisms.

Perinatal myocardial DNA and protein changes in the lamb: effect of cortisol in the fetus

Myocardial growth during fetal life is accomplished by proliferation of the number of myocytes (hyperplasia). Shortly after birth, normal growth of the heart is predominantly due to increase in cell size (hypertrophy), and myocytes largely lose the capability to replicate. This change is characterized by a decrease in myocardial DNA concentration and an increase in protein/DNA concentration ratio. Among many of the events associated with birth is an increase in plasma cortisol concentrations in the few days before delivery of the fetus. To determine the possible role of cortisol in the postnatal change in myocardial growth, we measured DNA and protein concentrations in the free walls of the left (LV) and right (RV) ventricles in normal fetal lambs, normal newborn lambs, and in fetal lambs in which cortisone was infused for 72-80 h into the left coronary artery, which we showed does not perfuse the RV free wall. Normally, fetal RV DNA is higher than LV DNA concentration, and DNA/protein ratio is lower in RV than in LV. It is suggested that this could be related to the greater load on the RV. Postnatally, protein concentrations increase progressively, but DNA remains the same in both ventricles, and protein/DNA ratios increase. Cortisol, infused to achieve normal prenatal levels in LV myocardium, markedly decreases LV DNA without affecting RV DNA concentrations. The present study indicates that cortisol inhibits myocyte replication and that cortisol simulates the change in myocardial growth pattern normally occurring after birth. It raises concerns regarding prenatal administration of glucocorticoids to mothers to mature the fetal lungs before preterm delivery.

Glucocorticoids downregulate cyclooxygenase-1 gene expression and prostacyclin synthesis in fetal pulmonary artery endothelium

Prostacyclin (prostaglandin I2 [PGI2]) is a key mediator of pulmonary vascular function during early postnatal life, and its production in the pulmonary vasculature rises markedly during that period because of increasing expression of cyclooxygenase type 1 (COX-1). The postnatal rise in COX-1 may be due to the release of inhibition by glucocorticoids, since plasma glucocorticoid levels fall after birth and glucocorticoids decrease PGI2 synthesis in certain nonpulmonary cell types. We therefore studied the direct effects of dexamethasone (DEX) on COX-1 expression in early-passage ovine fetal pulmonary-artery endothelial cells (PAECs). DEX (10(-10) to 10(-6) mol/L) caused a dose-related decrease in COX-1 mRNA expression that was evident by 24 hours, was maximal at 10(-6) mol/L (50% inhibition), and was not due to changes in mRNA stability. There was a parallel decline in COX-1 protein expression. COX-1 protein rose following DEX withdrawal, and DEX blunted the stimulatory effect of 17beta-estradiol on COX-1 expression. DEX alone (10(-8) mol/L for 48 hours) caused a 93% fall in basal PGI2 production, and bradykinin- and A23187-stimulated PGI2 were diminished 96% and 94%, respectively. Similarly, PGI2 synthesis from arachidonic acid fell 86% with DEX; all of the above effects are consistent with COX-1 downregulation. The glucocorticoid receptor (GR) antagonist mifepristone (RU-486; 10(-6) mol/L) blocked the inhibitory effect of DEX, and GR expression was evident by immunoblot analysis. These findings indicate that glucocorticoids downregulate COX-1 expression and PGI2 synthesis in fetal PAECs through the activation of PAEC GR and effects on COX-1 gene transcription. This mechanism may modulate pulmonary PGI2 production in the perinatal period, and it may also play a role in the effects of glucocorticoids on the systemic circulation at a variety of ages.

Neonatal catecholamine content of adrenal and extra-adrenal chromaffin tissue after prenatal exposure to dexamethasone

We investigated the effects of prenatal exposure to dexamethasone on paraganglia and adrenal catecholamine stores in rabbit neonates. We compared pregnant rabbits injected with 0.01 mg x kg(-1) of dexamethasone (Dex) from day 24 to day 27 of gestation to an untreated group of unmanipulated rabbits. A group injected with 0.9% saline solution was added to evaluate the effect of injection and handling. Catecholamines were assessed by HPLC in offspring paraganglia and adrenal glands on days 0, 1, and 7 after birth. Data were analyzed by a two-factor ANOVA and Bonferroni-Dunn and t tests. Statistical significance was accepted at p < 0.05. Paraganglia catecholamine levels were significantly higher in the Dex animals than in the untreated ones at every maturational stage studied. For saline animals, the levels were lower than in the Dex group and higher than in the untreated one. In adrenal glands, the same pattern was observed for noradrenaline only. These findings suggest that such a treatment has a positive long-term effect on catecholamine levels of both structures with a more marked effect on paraganglia, an extra-adrenal structure exerting a main function during the perinatal period in providing the child with catecholamine stores.