Placental metabolism: substrate requirements and the response to stress

The placenta is a dynamic, metabolically active organ with significant nutrient and energy requirements for growth, nutrient transfer and protein synthesis. It uses a range of substrates to meet its energy needs and has a higher rate of oxygen (O2) consumption than many other foetal and adult tissues. Placental metabolism varies with species and alters in response to a range of nutritional and endocrine signals of adverse environmental conditions. The placenta integrates these signals and adapts its metabolic phenotype to help maintain pregnancy and to optimize offspring fitness by diversifying the sources of carbon and nitrogen available for energy production, hormone synthesis and foeto-placental growth. The metabolic response of the placenta to adversity depends on the nature, severity and duration of the stressful challenge and on whether the insult is maternal, placental or foetal in origin. This review examines placental metabolism and its response to stresses common in pregnancy with particular emphasis on farm species like the sheep. It also considers the consequences of changes in placental metabolism for the supply of O2 and nutrients to the foetus.The authors are grateful to the CTR and the Biotechnology and Biological Sciences Research Council for research funding

Glucocorticoids and equine pancreatic β cell function

REASONS FOR PERFORMING STUDY: Synthetic glucocorticoids are used to treat inflammatory conditions in horses. In other pregnant animals, glucocorticoids are given to stimulate fetal maturation with long-term metabolic consequences for the offspring if given preterm. However, their metabolic effects during equine pregnancy remain unknown. OBJECTIVE: Thus, this study investigated the metabolic effects of dexamethasone administration on pregnant pony mares and their foals after birth. STUDY DESIGN: Experimental study. METHODS: A total of 3 doses of dexamethasone (200 μg/kg bwt i.m.) were given to 6 pony mares at 48 h intervals beginning at ≈270 days of pregnancy. Control saline injections were given to 5 mares using the same protocol. After fasting overnight, pancreatic β cell responses to exogenous glucose were measured in the mares before, during and after treatment. After birth, pancreatic β cell responses to exogenous glucose and arginine were measured in the foals at 2 and 12 weeks. RESULTS: In mares during treatment, dexamethasone but not saline increased basal insulin concentrations and prolonged the insulin response to exogenous glucose. Basal insulin and glucose concentrations still differed significantly between the 2 groups 72 h post treatment. Dexamethasone treatment significantly reduced placental area but had little effect on foal biometry at birth or subsequently. Foal β cell function at 2 weeks was unaffected by maternal treatment. However, by 12 weeks, pancreatic β cell sensitivity to arginine, but not glucose, was less in foals delivered by dexamethasone- than saline-treated mares. CONCLUSIONS: Dexamethasone administration induced changes in maternal insulin-glucose dynamics, indicative of insulin resistance and had subtle longer term effects on post natal β cell function of the foals. The programming effects of dexamethasone in horses may be mediated partially by altered maternal metabolism and placental growth.These studies were funded by the Horserace Betting Levy Board.This is the author accepted manuscript. The final version is available from Wiley via http://dx.doi.org/10.1111/evj.12560

Glucocorticoid programming of intrauterine development.

Glucocorticoids (GCs) are important environmental and maturational signals during intrauterine development. Toward term, the maturational rise in fetal glucocorticoid receptor concentrations decreases fetal growth and induces differentiation of key tissues essential for neonatal survival. When cortisol levels rise earlier in gestation as a result of suboptimal conditions for fetal growth, the switch from tissue accretion to differentiation is initiated prematurely, which alters the phenotype that develops from the genotype inherited at conception. Although this improves the chances of survival should delivery occur, it also has functional consequences for the offspring long after birth. Glucocorticoids are, therefore, also programming signals that permanently alter tissue structure and function during intrauterine development to optimize offspring fitness. However, if the postnatal environmental conditions differ from those signaled in utero, the phenotypical outcome of early-life glucocorticoid receptor overexposure may become maladaptive and lead to physiological dysfunction in the adult. This review focuses on the role of GCs in developmental programming, primarily in farm species. It examines the factors influencing GC bioavailability in utero and the effects that GCs have on the development of fetal tissues and organ systems, both at term and earlier in gestation. It also discusses the windows of susceptibility to GC overexposure in early life together with the molecular mechanisms and long-term consequences of GC programming with particular emphasis on the cardiovascular, metabolic, and endocrine phenotype of the offspring.We would also like to thank the BBSRC, the Horserace Betting Levy Board and the Centre for Trophoblast for their financial support

A physiological increase in maternal cortisol alters uteroplacental metabolism in the pregnant ewe

Fetal nutrition is determined by maternal availability, placental transport and uteroplacental metabolism of carbohydrates. Cortisol affects maternal and fetal metabolism, but whether maternal cortisol concentrations within the physiological range regulate uteroplacental carbohydrate metabolism remains unknown. This study determined the effect of maternal cortisol infusion (1.2 mg kg−1 day−1 I.V. for 5 days, n = 20) on fetal glucose, lactate and oxygen supplies in pregnant ewes on day 130 of pregnancy (term = 145 days). Compared to saline infusion (n = 21), cortisol infusion increased maternal, but not fetal, plasma cortisol (P < 0.05). Cortisol infusion also raised maternal insulin, glucose and lactate concentrations, and blood pH, PCO2 and HCO3 − concentration. Although total uterine glucose uptake determined by Fick’s principle was unaffected, a greater proportion was consumed by the uteroplacental tissues, so net fetal glucose uptake was 29% lower in cortisol-infused than control ewes (P < 0.05). Concomitantly, uteroplacental lactate production was > 2-fold greater in cortisol- than saline-treated ewes (P < 0.05), although uteroplacental O2 consumption was unaffected by maternal treatment. Materno-fetal clearance of non-metabolizable [3H]methyl-D-glucose and placental SLC2A8 (glucose transporter 8) gene expression were also greater with cortisol treatment. Fetal plasma glucose, lactate or α-amino nitrogen concentrations were unaffected by treatment although fetal plasma fructose and hepatic lactate dehydrogenase activity were greater in cortisol- than saline-treated ewes (P < 0.05). Fetal plasma insulin levels and body weight were also unaffected by maternal treatment. During stress, cortisol-dependent regulation of uteroplacental glycolysis may allow increased maternal control over fetal nutrition and metabolism. However, when maternal cortisol concentrations are raised chronically, prolonged elevation of uteroplacental lactate production may compromise fetal wellbeing

Dexamethasone treatment of pregnant F0 mice leads to parent of origin-specific changes in placental function of the F2 generation.

Dexamethasone treatment of F0 pregnant rodents alters F1 placental function and adult cardiometabolic phenotype. The adult phenotype is transmitted to the F2 generation without further intervention, but whether F2 placental function is altered by F0 dexamethasone treatment remains unknown. In the present study, F0 mice were untreated or received dexamethasone (0.2µgg(-1)day(-1), s.c.) over Days 11-15 or 14-18 of pregnancy (term Day 21). Depending on the period of F0 dexamethasone treatment, F1 offspring were lighter at birth or grew more slowly until weaning (P<0.05). Glucose tolerance (1gkg(-1), i.p.) of adult F1 males was abnormal. Mating F1 males exposed prenatally to dexamethasone with untreated females had no effect on F2 placental function on Day 19 of pregnancy. In contrast, when F1 females were mated with untreated males, F2 placental clearance of the amino acid analogue (14)C-methylaminoisobutyric acid was increased by 75% on Day 19 specifically in dams prenatally exposed to dexamethasone on Days 14-18 (P<0.05). Maternal plasma corticosterone was also increased, but F2 placental Slc38a4 expression was decreased in these dams (P<0.05). F0 dexamethasone treatment had no effect on F2 fetal or placental weights, regardless of lineage. Therefore, the effects of F0 dexamethasone exposure are transmitted intergenerationally to the F2 placenta via the maternal, but not paternal, line.This is the accepted manuscript. The final version is available at http://dx.doi.org/10.1071/RD14285

A physiological increase in maternal cortisol alters uteroplacental metabolism in the pregnant ewe.

KEY POINTS: Fetal nutrient supply is dependent, in part, upon the transport capacity and metabolism of the placenta. The stress hormone, cortisol, alters metabolism in the adult and fetus but it is not known whether cortisol in the pregnant mother affects metabolism of the placenta. In this study, when cortisol concentrations were raised in pregnant sheep by infusion, proportionately more of the glucose taken up by the uterus was consumed by the uteroplacental tissues while less was transferred to the fetus, despite an increased placental glucose transport capacity. Concomitantly, the uteroplacental tissues produced lactate at a greater rate. The results show that maternal cortisol concentrations regulate uteroplacental glycolytic metabolism, producing lactate for use in utero. Prolonged increases in placental lactate production induced by cortisol overexposure may contribute to the adverse effects of maternal stress on fetal wellbeing. ABSTRACT: Fetal nutrition is determined by maternal availability, placental transport and uteroplacental metabolism of carbohydrates. Cortisol affects maternal and fetal metabolism, but whether maternal cortisol concentrations within the physiological range regulate uteroplacental carbohydrate metabolism remains unknown. This study determined the effect of maternal cortisol infusion (1.2 mg kg(-1)  day(-1) i.v. for 5 days, n = 20) on fetal glucose, lactate and oxygen supplies in pregnant ewes on day ∼130 of pregnancy (term = 145 days). Compared to saline infusion (n = 21), cortisol infusion increased maternal, but not fetal, plasma cortisol (P  2-fold greater in cortisol- than saline-treated ewes (P < 0.05), although uteroplacental O2 consumption was unaffected by maternal treatment. Materno-fetal clearance of non-metabolizable [(3) H]methyl-d-glucose and placental SLC2A8 (glucose transporter 8) gene expression were also greater with cortisol treatment. Fetal plasma glucose, lactate or α-amino nitrogen concentrations were unaffected by treatment although fetal plasma fructose and hepatic lactate dehydrogenase activity were greater in cortisol- than saline-treated ewes (P < 0.05). Fetal plasma insulin levels and body weight were also unaffected by maternal treatment. During stress, cortisol-dependent regulation of uteroplacental glycolysis may allow increased maternal control over fetal nutrition and metabolism. However, when maternal cortisol concentrations are raised chronically, prolonged elevation of uteroplacental lactate production may compromise fetal wellbeing

Hypoxia, fetal and neonatal physiology: 100 years on from Sir Joseph Barcroft.

This is the author accepted manuscript. The final version is available from Wiley via http://dx.doi.org/10.1113/JP27200

Effects of stress during pregnancy on hepatic glucogenic capacity in rat dams and their fetuses

Stress during pregnancy is associated with metabolic dysfunction in the adult offspring in human and other animals. However, little is known about the metabolic effects of pregnancy stress on the mothers and fetuses during pregnancy itself. This study aimed to determine the consequences of the common experimental procedures of injection and single housing in pregnant rats on fetal and maternal hepatic glucogenic capacities. On day (D) 20 of pregnancy, feto-placental weights and the glycogen content and activities of phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase) of fetal and maternal liver were measured in rats pair or single housed from D1 with or without saline injection from D15 to D19. Housing and saline injection both affected hepatic glucogenic capacity. In maternal liver, saline injection but not housing reduced glycogen content and raised G6Pase activity, whereas housing but not treatment increased PEPCK activity. In fetuses, housing and injection interacted in regulating PEPCK activity and reducing hepatic glycogen content and placental weight. Body weight was decreased and hepatic G6Pase increased by injection but not housing in the fetuses. Single-housed dams ate less than those pair-housed near term while saline injection elevated maternal plasma corticosterone concentrations. Thus, single housing and saline injection are both stresses during rat pregnancy that alter feto-placental weight and hepatic glucogenic capacity of the fetuses and dams near term. Routine experimental procedures per se may, therefore, have consequences for offspring hepatic phenotype as well as modifying the outcomes of dietary and other environmental challenges during pregnancy.The work was part funded by the Gates Cambridge Trust (Scholarship for KLF)

Placental phenotype and the insulin-like growth factors: resource allocation to fetal growth

The placenta is the main determinant of fetal growth and development in utero. It supplies all the nutrients and oxygen required for fetal growth and secretes hormones that facilitate maternal allocation of nutrients to the fetus. Furthermore, the placenta responds to nutritional and metabolic signals in the mother by altering its structural and functional phenotype which can lead to changes in maternal resource allocation to the fetus. The molecular mechanisms by which the placenta senses and responds to environmental cues are poorly understood. This review discusses the role of the insulin-like growth factors (IGFs) in controlling placental resource allocation to fetal growth, particularly in response to adverse gestational environments. In particular, it assesses the impact of the IGFs and their signalling machinery on placental morphogenesis, substrate transport and hormone secretion, primarily in the laboratory species, although it draws on data from human and other species where relevant. It also considers the role of the IGFs as environmental signals in linking resource availability, to fetal growth through changes in the morphological and functional phenotype of the placenta. As altered fetal growth is associated with increased perinatal morbidity and mortality and a greater risk of developing adult-onset diseases in later life, understanding the role of IGFs during pregnancy in regulating placental resource allocation to fetal growth is important for identifying the mechanisms underlying the developmental programming of offspring phenotype by suboptimal intrauterine growth.ANS-P is funded by a Royal Society Dorothy Hodgkin Research Fellowship

AMPK and uterine artery vasodilation

Genes near adenosine monophosphate-activated protein kinase-α1 (PRKAA1) have been implicated in the greater uterine artery (UtA) blood flow and relative protection from fetal growth restriction seen in altitude-adapted Andean populations. Adenosine monophosphate-activated protein kinase (AMPK) activation vasodilates multiple vessels but whether AMPK is present in UtA or placental tissue and influences UtA vasoreactivity during normal or hypoxic pregnancy remains unknown. We studied isolated UtA and placenta from near-term C57BL/6J mice housed in normoxia (n = 8) or hypoxia (10% oxygen, n = 7-9) from day 14 to day 19, and placentas from non-labouring sea level (n = 3) or 3100 m (n = 3) women. Hypoxia increased AMPK immunostaining in near-term murine UtA and placental tissue. RT-PCR products for AMPK-α1 and -α2 isoforms and liver kinase B1 (LKB1; the upstream kinase activating AMPK) were present in murine and human placenta, and hypoxia increased LKB1 and AMPK-α1 and -α2 expression in the high- compared with low-altitude human placentas. Pharmacological AMPK activation by A769662 caused phenylephrine pre-constricted UtA from normoxic or hypoxic pregnant mice to dilate and this dilatation was partially reversed by the NOS inhibitor l-NAME. Hypoxic pregnancy sufficient to restrict fetal growth markedly augmented the UtA vasodilator effect of AMPK activation in opposition to PE constriction as the result of both NO-dependent and NO-independent mechanisms. We conclude that AMPK is activated during hypoxic pregnancy and that AMPK activation vasodilates the UtA, especially in hypoxic pregnancy. AMPK activation may be playing an adaptive role by limiting cellular energy depletion and helping to maintain utero-placental blood flow in hypoxic pregnancy.Funding for these studies was provided by the Wellcome Trust (084804/2/08/Z) to G.J.B., the British Heart Foundation and the Wellcome Trust to D.A.G., the Biotechnology and Biological Sciences Research Council (BBSRC) to A.L.F., a UK Wellcome Trust Programme Grant (WT081195MA) to A.M.E. and A.D.M., a BBSRC studentship and in vivo skills award to J.S.H., a National Health Medical Research Council and Centre for Trophoblast Research fellowship to A.N.S.-P., and a NIH RO1 grant (HLBI-079647) to L.G.M. along with sabbatical support from Wake Forest University.This is the author accepted manuscript. The final version is available from Wiley via http://dx.doi.org/10.1113/JP27099