Fetal and maternal endocrine responses to prolonged reductions in uterine blood flow in pregnant sheep

Effect of maternal nutrient restriction on skeletal muscle mass and associated molecular pathways in SGA and Non-SGA sheep fetuses

Maternal nutrient restriction causes small for gestational age (SGA) offspring, which exhibit a higher risk for metabolic syndrome in adulthood. Fetal skeletal muscle is particularly sensitive to maternal nutrient restriction, which impairs muscle mass and metabolism. Using a 50% nutrient restriction treatment from gestational day (GD) 35 to GD 135 in sheep, we routinely observe a spectral phenotype of fetal weights within the nutrient-restricted (NR) group. Thus, our objective was to evaluate the effect of maternal NR on muscle mass, myofiber hypertrophy, myonuclear dotation, and molecular markers for protein synthesis and degradation, while accounting for the observed fetal weight variation. Within the NR group, we classified upper-quartile fetuses into NR(Non-SGA) (n = 11) and lower-quartile fetuses into NR(SGA) (n = 11). A control group (n = 12) received 100% of nutrient requirements throughout pregnancy. At GD 135, fetal plasma and organs were collected, and gastrocnemius and soleus muscles were sampled for investigation. Results showed decreased (P < 0.05) absolute tissue/organ weights, including soleus and gastrocnemius muscles, in NR(SGA) fetuses compared to NR(Non-SGA) and control. Myofiber cross-sectional area was smaller in NR(SGA) vs control for gastrocnemius (P = 0.0092) and soleus (P = 0.0097) muscles. Within the gastrocnemius muscle, the number of myonuclei per myofiber was reduced (P = 0.0442) in NR(SGA) compared to control. Cortisol may induce protein degradation. However, there were no differences in fetal cortisol among groups. Nevertheless, for gastrocnemius muscle, cortisol receptor (NR3C1; P = 0.0124), and FOXO1 (P = 0.0131) were upregulated in NR(SGA) compared to control while NR(Non-SGA) did not differ from the other 2 groups. KLF15 was upregulated (P = 0.0002) in both NR(SGA) and NR(Non-SGA); while FBXO32, TRIM63, BCAT2 or MSTN did not differ. For soleus muscle, KLF15 mRNA was upregulated (P = 0.0145) in NR(SGA) compared to control, and expression of MSTN was increased (P = 0.0259) in NR(SGA) and NR(Non-SGA) compared to control. At the protein level, none of the mentioned molecules nor total ubiquitin-labeled proteins differed among groups (P > 0.05). Indicators of protein synthesis (total and phosphorylated MTOR, EI4EBP1, and RPS6KB1) did not differ among groups in either muscle (P > 0.05). Collectively, results highlight that maternal NR unequally affects muscle mass in NR(SGA) and NR(Non-SGA) fetuses, and alterations in myofiber cross-sectional area and myonuclei number partially explain those differences.

Ovine uteroplacental and fetal metabolism during and after fetal cortisol overexposure in late gestation

Cortisol modifies fetal metabolism in preparation for delivery, but whether preterm cortisol exposure programs persisting changes in fetoplacental metabolism remains unknown. This study infused fetal sheep with saline ( n = 36) or cortisol ( n = 27) to raise fetal plasma cortisol to normal prepartum concentrations for 5 days from day 125 of gestation (term: ≈145 days). Fetal uptake and uteroplacental metabolism of glucose, oxygen, and lactate, together with fetal hepatic glucogenic capacity, were measured on the final day of infusion or 5 days later. Cortisol reduced adrenal weight and umbilical glucose uptake during infusion but increased fetal glucose concentrations, hepatic glycogen content, and hepatic glucogenic enzyme activity (fructose-1,6-bisphosphatase and glucose-6-phosphatase) and gene expression ( PC and G6PC) compared with saline infusion. Postcortisol infusion, umbilical glucose uptake, and hepatic glucose-6-phosphatase activity remained low and high, respectively, whereas fetal glucose levels normalized and hepatic glycogen was lower with higher adrenal weights than in controls. Cortisol infusion increased the proportion of total uterine glucose uptake consumed by the uteroplacental tissues, irrespective of age. Placental tracer glucose transport capacity was also increased after, but not during, cortisol infusion, without changes in placental glucose transporter gene expression. Blood lactate concentration and Pco2 were higher, whereas pH and O2 content were lower in cortisol-infused than saline-infused fetuses, although uteroplacental metabolism and fetal uptake of oxygen and lactate were unaltered. The results suggest that preterm cortisol overexposure alters fetoplacental metabolism and adrenal function subsequently with persisting increases in uteroplacental glucose consumption at the expense of the fetal supply.

The highs and lows of programmed cardiovascular disease by developmental hypoxia: studies in the chicken embryo

It is now established that adverse conditions during pregnancy can trigger a fetal origin of cardiovascular dysfunction and/or increase the risk of heart disease in later life. Suboptimal environmental conditions during early life that may promote the development of cardiovascular dysfunction in the offspring include alterations in fetal oxygenation and nutrition as well as fetal exposure to stress hormones, such as glucocorticoids. There has been growing interest in identifying the partial contributions of each of these stressors to programming of cardiovascular dysfunction. However, in humans and in many animal models this is difficult, as the challenges cannot be disentangled. By using the chicken embryo as an animal model, science has been able to circumvent a number of problems. In contrast to mammals, in the chicken embryo the effects on the developing cardiovascular system of changes in oxygenation, nutrition or stress hormones can be isolated and determined directly, independent of changes in the maternal or placental physiology. In this review, we summarise studies that have exploited the chicken embryo model to determine the effects on prenatal growth, cardiovascular development and pituitary-adrenal function of isolated chronic developmental hypoxia.

Noninvasive high-intensity focused ultrasound treatment of twin-twin transfusion syndrome: A preliminary in vivo study

We investigated the efficacy, maternofetal responses, and safety of using high-intensity focused ultrasound (HIFU) for noninvasive occlusion of placental vasculature compared to sham treatment in anesthetized pregnant sheep. This technique for noninvasive occlusion of placental vasculature may be translatable to the treatment of conditions arising from abnormal placental vasculature, such as twin-twin transfusion syndrome (TTTS). Eleven pregnant sheep were instrumented with maternal and fetal arterial catheters and time-transit flow probes to monitor cardiovascular, acid-base, and metabolic status, and then exposed to HIFU (n = 5) or sham (n = 6) ablation of placental vasculature through the exposed uterine surface. Placental vascular flow was occluded in 28 of 30 targets, and histological examination confirmed occlusion in 24 of 30 targets. In both HIFU and sham exposures, uterine contact reduced maternal uterine artery flow, but delivery of oxygen and glucose to the fetal brain remained normal. HIFU can consistently occlude in vivo placental vessels and ablate blood flow in a pregnant sheep model. Cardiovascular and metabolic fetal responses suggest that the technique is safe in the short term and potentially translatable to human pregnancy.

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.

Leptin alters adrenal responsiveness by decreasing expression of ACTH-R, StAR, and P450c21 in hypoxemic fetal sheep

The late gestation increase in adrenal responsiveness to adrenocorticotropin (ACTH) is dependent upon the upregulation of the ACTH receptor (ACTH-R), steroidogenic acute regulatory protein (StAR), and steroidogenic enzymes in the fetal adrenal. Long-term hypoxia decreases the expression of these and adrenal responsiveness to ACTH in vivo. Leptin, an adipocyte-derived hormone which attenuates the peripartum increase in fetal plasma cortisol is elevated in hypoxic fetuses. Therefore, we hypothesized that increases in plasma leptin will inhibit the expression of the ACTH-R, StAR, and steroidogenic enzymes and attenuate adrenal responsiveness in hypoxic fetuses. Spontaneously hypoxemic fetal sheep (132 days of gestation, PO(2) ≈ 15 mm Hg) were infused with recombinant human leptin (n = 8) or saline (n = 7) for 96 hours. An ACTH challenge was performed at 72 hours of infusion to assess adrenal responsiveness. Plasma cortisol and ACTH were measured daily and adrenals were collected after 96 hours infusion for messenger RNA (mRNA) and protein expression measurement. Plasma cortisol concentrations were lower in leptin- compared with saline-infused fetuses (14.8 ± 3.2 vs 42.3 ± 9.6 ng/mL, P < .05), as was the cortisol:ACTH ratio (0.9 ± 0.074 vs 46 ± 1.49, P < .05). Increases in cortisol concentrations were blunted in the leptin-treated group after ACTH(1-24) challenge (F = 12.2, P < .0001). Adrenal ACTH-R, StAR, and P450c21 expression levels were reduced in leptin-treated fetuses (P < .05), whereas the expression of Ob-Ra and Ob-Rb leptin receptor isoforms remained unchanged. Our results indicate that leptin blunts adrenal responsiveness in the late gestation hypoxemic fetus, and this effect appears mediated by decreased adrenal ACTH-R, StAR, and P450c21 expression.

Fetal cardiovascular reflex responses to hypoxaemia

The fetus mounts a coordinated cardiovascular response to an insult of acute hypoxaemia which involves neural and endocrine components. During acute hypoxaemia in late pregnancy there is a transient bradycardia, a gradual increase in arterial blood pressure and an increase in heart rate variability. In addition, there is a redistribution of the combined ventricular output favouring the cerebral, myocardial and adrenal circulations by shunting blood away from the peripheral circulations. A component of the increase in peripheral vascular resistance and the increase in arterial blood pressure during acute hypoxaemia is mediated via increases in plasma concentrations of vasoconstrictor hormones such as vasopressin, angiotensin II and neuropeptide Y. Whilst an increase in plasma ACTH and cortisol is also seen during acute hypoxaemia, their contribution to cardiovascular control in fetal sheep is less clear.

Evidence has been presented to suggest that a number of these cardiovascular and endocrine responses to acute hypoxaemia are chemorefiex in nature, mediated principally by carotid chemoreceptor afferents. In addition, this reflex may be modifiable in terms of changes in magnitude and gain: first, by an influence of the intrauterine environment during chronic hypoxaemia and second, through genetic influences, in animals adapted to life at high altitude.

Perinatal asphyxia pathophysiology in pig and human: A review

In utero fetuses are evidently exposed to several factors that cause an interruption of the oxygen flow through the umbilical cord causing asphyxia leading to hypoxia and metabolic acidosis. These conditions are important causes of intra-partum and neonatal mortality. The main objective of this review is to provide current information regarding the pathophysiology of asphyxia in piglets around parturition; the physiological mechanisms invoked by affected piglets to compensate perinatal hypoxemia are discussed. This review also addresses some similarities and differences of asphyxia between piglets and other mammals, including human neonates. Metabolic acidosis and hypoxia are sequela to asphyxia and can cause profound health effects in postnatal performance because of an abnormal suckling, a reduced absorption of colostrum and inadequate passive transfer of neonatal immunity. Acidosis also cause hypothermia, increased mortality and reduced survival in neonates. One of the first deleterious effects of intrauterine hypoxia is the expulsion of meconium into the amniotic sac leading to meconium staining of the skin, and in severe cases, meconium aspiration into the lungs. Even though there have been technological changes and improvements in husbandry, piglet mortality due to asphyxia remains a major problem. One potential alternative to reduce neonatal mortality in pigs is the monitoring of fetal stress during birth and the implemention of strategies such as the Apgar score, that is often used in human pediatrics. It is also important to consider the physiological, behavioral and biochemical changes that take place during parturition which subsequently impact the vitality, maturity and development of neonatal pigs. Understanding the pathophysiology of fetal hypoxia should help practitioners and farmers implement more effective delivery techniques aimed at reducing neonatal mortality and improving postnatal performance.

Chronic umbilical cord compression results in accelerated maturation of lung and brown adipose tissue in the sheep fetus during late gestation

Umbilical cord compression (UCC) sufficient to reduce umbilical blood flow by 30% for 3 days, results in increased fetal plasma cortisol and catecholamines that are likely to promote maturation of the fetal lung and brown adipose tissue (BAT). We determined the effect of UCC on the abundance of uncoupling protein (UCP)1 (BAT only) and -2, glucocorticoid receptor (GR), and 11beta-hydroxysteroid dehydrogenase (11beta-HSD)1 and -2 mRNA, and mitochondrial protein voltage-dependent anion channel (VDAC) and cytochrome c in these tissues. At 118 +/- 2 days of gestation (dGA; term approximately 145 days), 14 fetuses were chronically instrumented. Eight fetuses were then subjected to 3 days of UCC from 125 dGA, and the remaining fetuses were sham operated. All fetuses were then exposed to two 1-h episodes of hypoxemia at 130 +/- 1 and 134 +/- 1 dGA before tissue sampling at 137 +/- 2 dGA. In both tissues, UCC upregulated UCP2 and GR mRNA, plus VDAC and cytochrome c mitochondrial proteins. In lung, UCC increased 11beta-HSD1 mRNA but decreased 11beta-HSD2 mRNA abundance, a pattern reversed for BAT. UCC increased UCP1 mRNA and its translated protein in BAT. UCP2, GR, 11beta-HSD1 and -2 mRNA, plus VDAC and cytochrome c protein abundance were all significantly correlated with fetal plasma cortisol and catecholamine levels, but not thyroid hormone concentrations, in the lung and BAT of UCC fetuses. In conclusion, chronic UCC results in precocious maturation of the fetal lung and BAT mitochondria, an adaptation largely mediated by the surge in fetal plasma cortisol and catecholamines that accompanies UCC.

Prenatal stress and risk for psychopathology: specific effects or induction of general susceptibility?

This review focuses on prenatal stress as a risk factor for psychopathology. Evidence from animal studies is summarized, and the relevance of prenatal stress models in animals for human studies is discussed. In the offspring of prenatally stressed animals, overactivity and impaired negative feedback regulation of the hypothalamic-pituitary-adrenal axis are consistent findings and may reflect a pathophysiological mechanism involved in the development of psychopathology. Reduced activity of the opioid GABA/benzodiazepine, serotonin, and dopamine systems and increased activity of the sympathico-adrenal system have been found as well. These alterations have been linked to a diverse spectrum of psychopathology. Therefore, the evidence supports the view that exposure to prenatal stress may result in a general susceptibility to psychopathology, rather than exerting a direct effect on a specific form of psychopathology.

The fetal circulation

Accumulating data on the human fetal circulation shows the similarity to the experimental animal physiology, but with important differences. The human fetus seems to circulate less blood through the placenta, shunt less through the ductus venosus and foramen ovale, but direct more blood through the lungs than the fetal sheep. However, there are substantial individual variations and the pattern changes with gestational age. The normalised umbilical blood flow decreases with gestational age, and, at 28 to 32 weeks, a new level of development seems to be reached. At this stage, the shunting through the ductus venosus and the foramen ovale reaches a minimum, and the flow through the lungs a maximum. The ductus venosus and foramen ovale are functionally closely related and represent an important distributional unit for the venous return. The left portal branch represents a venous watershed, and, similarly, the isthmus aorta an arterial watershed. Thus, the fetal central circulation is a very flexible and adaptive circulatory system. The responses to increased afterload, hypoxaemia and acidaemia in the human fetus are equivalent to those found in animal studies: increased ductus venosus and foramen ovale shunting, increased impedance in the lungs, reduced impedance in the brain, increasingly reversed flow in the aortic isthmus and a more prominent coronary blood flow.

Nitric oxide plays a role in the regulation of adrenal blood flow and adrenocorticomedullary functions in the llama fetus

The hypothesis that nitric oxide plays a key role in the regulation of adrenal blood flow and plasma concentrations of cortisol and catecholamines under basal and hypoxaemic conditions in the llama fetus was tested. At 0.6-0.8 of gestation, 11 llama fetuses were surgically prepared for long-term recording under anaesthesia with vascular and amniotic catheters. Following recovery all fetuses underwent an experimental protocol based on 1 h of normoxaemia, 1 h of hypoxaemia and 1 h of recovery. In nine fetuses, the protocol occurred during fetal I.V. infusion with saline and in five fetuses during fetal I.V. treatment with the nitric oxide synthase inhibitor L-NAME. Adrenal blood flow was determined by the radiolabelled microsphere method during each of the experimental periods during saline infusion and treatment with L-NAME. Treatment with L-NAME during normoxaemia led to a marked fall in adrenal blood flow and a pronounced increase in plasma catecholamine concentrations, but it did not affect plasma ACTH or cortisol levels. In saline-infused fetuses, acute hypoxaemia elicited an increase in adrenal blood flow and in plasma ACTH, cortisol, adrenaline and noradrenaline concentrations. Treatment with L-NAME did not affect the increase in fetal plasma ACTH, but prevented the increments in adrenal blood flow and in plasma cortisol and adrenaline concentrations during hypoxaemia in the llama fetus. In contrast, L-NAME further enhanced the increase in fetal plasma noradrenaline. These data support the hypothesis that nitric oxide has important roles in the regulation of adrenal blood flow and adrenal corticomedullary functions during normoxaemia and hypoxaemia functions in the late gestation llama fetus.

Effects of prevailing hypoxaemia, acidaemia or hypoglycaemia upon the cardiovascular, endocrine and metabolic responses to acute hypoxaemia in the ovine fetus

Although it is established that the fetus can successfully withstand a single, acute hypoxaemic challenge during gestation, little is known about what effects prevailing adverse intrauterine conditions might have on the fetal response to acute hypoxaemia. The aims of this study were therefore: (1) to characterise the effects of prevailing and sustained hypoxaemia, acidaemia or hypoglycaemia on the fetal cardiovascular responses to an episode of acute hypoxaemia; and (2) to determine the effects of these adverse intrauterine conditions on mechanisms mediating these cardiovascular responses. Thirty-three Welsh Mountain sheep fetuses were chronically instrumented (1-2 % halothane) between 117 and 125 days of gestation (term is ca 145 days) with amniotic and vascular catheters and with a transit-time flow probe around a femoral artery. The animals were divided retrospectively into four groups based upon post-surgical, sustained, basal blood oxygen (chronically hypoxaemic; P(a,O2), 17.3 +/- 0.5 mmHg; n = 8), glucose (chronically hypoglycaemic; blood glucose, 0.49 +/- 0.03 mmol l(-1); n = 6) and acid-base (chronically acidaemic; pH(a), 7.25 +/- 0.01; n = 5) status. Values for compromised fetuses were -2 S.D. from a group of control (n = 14) fetuses. At 130 +/- 4 days, a 1 h episode of acute, isocapnic hypoxaemia (9 % O(2) in N(2), to reduce carotid P(a,O2) to 12 +/- 1 mmHg) was induced in all fetuses by reducing the maternal inspired O(2) fraction (F(I,O2)). Fetal cardiovascular variables were recorded at 1 s intervals throughout the experimental protocol and arterial blood samples taken at appropriate intervals for biophysical (blood gases, glucose, lactate) and endocrine (catecholamines, vasopressin, cortisol, ACTH) measures. During acute hypoxaemia all fetuses elicited hypertension, bradycardia and femoral vasoconstriction. However, prevailing fetal compromise altered the cardiovascular and endocrine responses to a further episode of acute hypoxaemia, including: (1) enhanced pressor and femoral vasoconstriction; (2) greater increments in plasma noradrenaline and vasopressin during hypoxaemia; and (3) basal upward resetting of hypothalamic-pituitary-adrenal axis function. Only chronically hypoxaemic fetuses had significantly elevated basal concentrations of noradrenaline and enhanced chemoreflex function during acute hypoxaemia. These data show that prevailing adverse intrauterine conditions alter the capacity of the fetus to respond to a subsequent episode of acute hypoxaemia; however, the partial contributions of hypoxaemia, acidaemia or hypoglycaemia to mediating these responses can vary.

A novel method for controlled and reversible long term compression of the umbilical cord in fetal sheep

1. In fetal sheep during late gestation the aims of the present study were to (1) develop a technique for inducing prolonged but reversible periods of controlled compression of the umbilical cord and (2) characterise the cardiovascular, endocrine and metabolic responses to this challenge. 2. Under 1-2 % halothane anaesthesia, 16 Welsh Mountain sheep fetuses were chronically instrumented at 118 +/- 2 days of gestation (term is ca 145 days) with an inflatable occluder cuff around the umbilical cord, amniotic and femoral vascular catheters and with transit-time flow probes around the contra-lateral femoral artery and an umbilical artery. At 125 days, umbilical blood flow was reduced by 30 % from a pre-determined 24 h baseline for 3 days by automated servo-controlled inflation of the occluder cuff (n = 8). The occluder was then deflated allowing return of umbilical blood flow to baseline. The remaining eight fetuses were used as sham-operated controls in which the occluder was not inflated throughout the protocol. Fetal cardiovascular variables were recorded at 8 s intervals and arterial blood samples taken for measurement of blood gases, glucose and lactate and plasma adrenaline, noradrenaline and vasopressin concentration throughout the study. 3. Automated servo-controlled inflation of the occluder cuff, programmed to reduce umbilical blood flow by 30 % from baseline, reduced umbilical blood flow by 30.2 +/- 1.7 %, with a coefficient of variation during compression of 6.5 +/- 1.1 %. Sustained partial compression of the umbilical cord produced falls in fetal arterial pH, P(a,O2), percentage O(2) saturation of haemoglobin, and hindlimb oxygen delivery, and increases in P(a,CO2), haemoglobin concentration, arterial blood oxygen carrying capacity and in blood glucose and lactate concentrations. While the reductions in P(a,O2), percentage saturation of haemoglobin and hindlimb oxygen delivery and the increase in P(a,CO2) were sustained throughout compression, the reduction in arterial pH and the increase in arterial oxygen carrying capacity had returned towards baseline values by 48 h compression. Fetal blood lactate concentrations reached a peak at 8 h of compression and, thereafter, were maintained at an elevated level relative to baseline. 4. Partial compression of the umbilical cord produced fetal hypertension, a reduction in femoral blood flow and, consequently, an increase in calculated fetal femoral vascular resistance for the duration of the challenge. In addition, the fall in heart rate measured in sham control fetuses by the end of the study, did not occur in cord-compressed fetuses. Cosinor analysis on 24 h rhythms of cardiovascular data indicated a significant increase in the amplitude of the 24 h rhythm in heart rate in cord-compressed fetuses relative to sham controls during the period of compression or sham-compression. Furthermore, cord compression led to an increase in fetal plasma noradrenaline, but not adrenaline and vasopressin concentrations relative to sham control fetuses. 5. In conclusion, a novel reversible method for controlled, long-term compression of the umbilical cord in sheep has been developed. The data show that sustained, partial compression of the umbilical cord produced moderate but sustained asphyxia, which resolved after the end of the compression period, and induced changes in fetal cardiovascular, endocrine and metabolic functions.

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.

Effect of reduced inspired oxygen on fetal growth and maternal glucose metabolism in rat pregnancy

The effect of prolonged exposure to a reduced fraction of inspired oxygen ([FiO2] 0.17 for 3 days) on maternal glucose kinetics, placental glucose transporters GLUT1 and GLUT3, and fetal growth was examined in rat pregnancy. Arterial and venous catheters were placed 3 days before the study. [3-(3)H]glucose tracer and deuterium labeling of water were used to measure the rates of glucose turnover and gluconeogenesis (GNG), respectively. Glucose uptake by maternal tissues was measured using [14C]2-deoxyglucose. Exposure to a reduced FiO2 resulted in a significant decrease (mean +/- SE) in fetal weight (room air, 4.02 +/- 0.04 g; 0.17 FiO2, 3.27 +/- 0.6 g, P < .02). There was a significant increase in the maternal-fetal glucose gradient (maternal-fetal glucose ratio: room air, 1.48 +/- 0.11; 0.17 FiO2, 2.26 +/- 0.24, P < .05), but there was no change in the maternal or fetal blood lactate concentration. No significant change in maternal blood pH was observed; however, a significant decrease in the blood partial pressure of O2 (PO2) occurred (room air, 97 +/- 0.5 torr; 0.17 FiO2, 81 +/- 1.8) on day 3. There was no change in the rate of turnover of glucose or GNG in the maternal compartment, nor was there any effect on glucose uptake by the maternal tissues. Placental GLUT1 and GLUT3 mRNA were not different in the control or experimental animals. We conclude that a mild reduction in the FiO2 for 3 days in rat pregnancy results in a significant fetal growth restriction that is not related to any observed alteration in maternal glucose metabolism. The lower glucose concentration in the fetal blood may be the consequence of an increase in fetal glucose metabolism, thereby resulting in an increased maternal-fetal gradient of glucose.

Altered fetal cardiovascular responses to prolonged hypoxia after sinoaortic denervation

This study examines the role of the peripheral chemoreceptors in mediating fetal cardiovascular responses to prolonged hypoxia secondary to reduced uterine blood flow (RUBF). Fetal sheep were chronically instrumented for continuous heart rate (FHR), blood pressure (FBP), and carotid blood flow (CBF) measurements after bilateral sectioning of the carotid sinus and vagus nerves (denervated, n = 7) or sham denervation (intact, n = 7). Four days postoperatively, uterine blood flow was mechanically restricted, reducing fetal arterial oxygen saturation by 47.3% (P < 0.01). An initial bradycardia was observed in intact (184.0 +/- 10.7 to 160.5 +/- 10.7 beats/min, not significant) but not denervated fetuses, followed by a tachycardia (180.0 +/- 2.2 to 193.7 +/- 2.7 beats/min, P < 0.05). FHR increased in denervated fetuses (175.5 +/- 8.7 to 203. 0 +/- 17.9 beats/min, P < 0.05). FBP increased transiently in intact fetuses from 45.1 +/- 1.0 to 55.4 +/- 3.0 mmHg at 2 h (P < 0.01), whereas denervated fetuses demonstrated a decrease in FBP from 47.1 +/- 4.2 to 37.2 +/- 3.7 mmHg (not significant). CBF increased (P < 0. 05) in both intact and denervated fetuses from 39.3 +/- 2.8 and 29.7 +/- 3.8 ml. min-1. kg-1 to 47.7 +/- 0.4 and 39.1 +/- 0.3 ml. min-1. kg-1, respectively, whereas carotid vascular resistance decreased only in denervated fetuses (1.7 +/- 0.1 to 1.1 +/- 0.02 mmHg. ml-1. min. kg-1, P < 0.05). We conclude that the peripheral chemoreceptors play an important role in mediating fetal cardiovascular responses to prolonged RUBF.

Chemoreflex contribution to adrenocortical function during acute hypoxemia in the llama fetus at 0.6 to 0.7 of gestation

This study tested the hypothesis that the fetal llama, a species adapted to the chronic hypoxia of life at high altitude, demonstrates a potent carotid chemoreflex influence on adrenocortical responses during acute hypoxemia. Plasma ACTH and cortisol concentrations, and mesencephalic and adrenal blood flows were measured during a 1-h period of acute hypoxemia in six intact and four carotid sinus-denervated llama fetuses at 0.6-0.7 of gestation. Fetal PaO2 was reduced from approximately 23 to about 14 mm Hg in both intact and carotid-denervated groups during acute hypoxemia. During hypoxemia, fetal plasma ACTH, adrenal blood flow, and, therefore, delivery of ACTH to the adrenals increased to similar extents in both intact and carotid-denervated fetal llamas. Despite this, the increase in plasma cortisol in hypoxemia in intact fetuses was absent in carotid-denervated fetuses. In addition, the increase in delivery of cortisol to the mesencephalon calculated in intact fetuses during hypoxemia did not occur in the carotid-denervated group. These data suggest that the integrity of the carotid chemoreceptors is indispensable to cortisol release during acute hypoxemia in the llama fetus, even at 0.6-0.7 of gestation.

Divergent changes in plasma ACTH and pituitary POMC mRNA after cortisol administration to late-gestation ovine fetus

Plasma concentrations of cortisol and adrenocorticotropic hormone (ACTH) rise in the late-gestation sheep fetus at approximately the same time as there is an increase in the plasma levels of corticosteroid-binding globulin (CBG). We hypothesized that intrafetal cortisol infusion during late pregnancy would stimulate an increase in fetal plasma CBG, which in turn would bind cortisol and diminish glucocorticoid negative-feedback regulation of the fetal pituitary, leading to an increase in plasma ACTH concentrations. Cortisol was infused into chronically catheterized fetal sheep beginning at 126.1 +/- 0.5 days of gestation and continued for 96 h. Control fetuses were infused with saline. In cortisol-infused fetuses, the plasma cortisol concentrations rose significantly from control levels (4.4 +/- 0.6 ng/ml) to 19.3 +/- 3.1 ng/ml within 24 h and remained significantly elevated throughout the infusion period. Plasma immunoreactive (i.r.) ACTH concentrations were significantly elevated in cortisol-infused fetuses within 24-48 h and remained significantly higher than in controls throughout the 96-h experimental period. Plasma free cortisol concentrations increased 10-fold and remained significantly elevated in cortisol-infused animals, despite a rise in plasma corticosteroid-binding capacity. Levels of pituitary proopiomelanocortin (POMC) mRNA in the fetal pars distalis and pars intermedia were 96 and 38% lower, respectively, after 96 h of cortisol infusion. Therefore physiological elevations of plasma cortisol, in the late-gestation ovine fetus, lead to increases in mean plasma irACTH concentrations, but this is not associated with increases in fetal pituitary POMC mRNA levels.

Term ovine placental vasculature: comparison of sea level and high altitude conditions by corrosion cast and histomorphometry

The placental vascular architecture differs significantly at high altitude from that at sea level in the human and guinea-pig. Four sheep between 137 and 140 days of gestation, kept near sea level throughout gestation, were used as a normoxic control group for comparison of the placental vasculature with 10 other ewes, kept at high altitude (3820 m above sea level; Barcroft Laboratory, White Mountain Research Station, CA, USA). Placentomes from both groups were prepared for histology and scanning electron microscopy of vascular corrosion casts. Singular perfusion of fetal placentae, as well as combined maternal/fetal injection was performed. The influence of long-term hypoxaemia was determined by qualitative and semi-quantitative evaluation of corrosion casts and histological sections. The fetal vessel casts show a distinct difference in the arrangement of vessels of all sizes in response to long-term hypoxaemia. In the control group, stem arteries and veins are straight and parallel. In contrast, this is much less evident in the hypoxaemic group because arterioles and venules branch off the stem vessels more frequently and in an irregular manner. This leads to a capillary bed that is much more dense due to increased branching and capillary coiling. These observations are confirmed by histomorphometry. In the fetal vessels of high altitude sheep placentomes, we observed a decreased number of vascular cross sections (21.6 +/- 4.7 SEM versus 27.7 +/- 4.0 SEM; P = 0.02). However, the average luminal size per cross section (77.9 +/- 10.5 microns2 SEM versus 59.4 +/- 7.4 microns2 SEM; P = 0.004) was increased at high altitude and the percentage of lumina of the total area (5.7 +/- 0.5 SEM versus 5.3 +/- 0.3 SEM; P = 0.09) indicated a trend towards an increase. In maternal vessels of high altitude placentomes, the number of vessel cross sections (6.5 +/- 0.7 SEM versus 6.0 +/- 0.5 SEM; P = 0.2) remained unchanged, whereas the average luminal size (1108 +/- 122 microns2 SEM versus 844 +/- 77 microns2 SEM; P < 0.001) and the percentage of lumina out of the total area (20.9 +/- 1.8 SEM versus 17.5 +/- 1.7 SEM; P < 0.001) were increased. The interhaemal distance appeared to be slightly but not significantly increased at high altitude. These findings indicate that at high altitude the sheep placenta develops an increased materno-fetal absorptive surface to help guarantee substance exchange.

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.

Corticotropin-releasing hormone-like axons in the adrenal glands of fetal and postnatal sheep

The presence of corticotropin-releasing hormone outside of the brain suggests that it may have functional roles besides the stimulation of pituitary adrenocorticotropin. The purpose of the present study was to determine immunohistochemically, the ontogeny of corticotropin-releasing hormone-like immunoreactivity in sheep adrenals from fetal day 100 to adult. Adrenal corticotropin-releasing hormone-like immunoreactivity: 1) was found in all animals examined, 2) existed as clusters of darkly beaded axon-like fibers over medullary cells at the medullary-cortical interface or over islands of medullary cells surrounded by cortex or rays of medullary cells extending into the cortex, 3) showed a definite increase with age, 4) was depleted in newborns < 12 h old, and 5) was found in pre- and postnatal splanchnic nerves. In conclusion, corticotropin-releasing hormone-like immunopositive fibers are present in the ovine adrenal medulla at various stages of development, supporting the concept that corticotropin-releasing hormone in pre- and postnatal sheep may play a direct local part in the regulation of adrenal function.

Fetal circulatory responses during maternal bleeding

The purpose of this study was to investigate the fetal circulatory responses during maternal hemorrhage. Five pregnant goats with fetuses with a mean gestational age of 132 +/- 2 days were used. The maternal blood was withdrawn at 350 ml/h for 2 hours (e.g. 700 ml, 11.9 ml/kg maternal weight) and reinfused at the same speed. During maternal bleeding, maternal arterial pressure (MAP) gradually decreased. As a result of this maternal hypotension, fetal arterial pO2 and pH decreased, and PCO2 increased. After the reinfusion, fetal pO2 recovered but pH and PCO2 did not recover. Fetal arterial pressure (FAP) increased and heart rate (FHR) decreased during maternal bleeding and returned to the control level, after the reinfusion. Fetal arginine vasopressin (AVP) concentration increased to 401.2 +/- 318.5 pg/ml at the maximum bleeding. There were significant positive correlation between AVP concentration and FAP, and negative correlation between AVP and FHR during maternal bleeding. Therefore, we concluded that 700 ml maternal bleeding for 2 hours resulted the decrease in fetal pH, pO2, and FHR, and increase in PCO2, FAP, and AVP concentration. Fetal pH, PCO2, and AVP did not return to the control level in spite of reinfusion.

Effect of arginine vasopressin on regional adrenal blood flow and plasma cortisol concentration in fetal sheep

OBJECTIVE

To determine whether arginine vasopressin (AVP) at plasma concentrations measured during moderate hypoxemia affects adrenal blood flow.

STUDY DESIGN

Regional blood flows were measured in 5 unanesthetized normoxemic fetuses (124-128 days' gestation) during a 24-h intravenous infusion of AVP in isotonic saline solution. Another 5 fetuses received an infusion of vehicle. Blood flows were determined before the infusion and at 2 h and 24 h from its onset using radionuclide-labeled microspheres.

RESULTS

At 2 h and 24 h of AVP infusion, fetal plasma concentrations of IR-AVP had risen from 4.7 +/- 0.9 pg/ml to 9.8 +/- 1.1 pg/l and 9.4 +/- 0.7 pg/ml, respectively. Thus we achieved plasma concentrations of IR-AVP comparable to those previously reported during moderate hypoxemia. There was no significant effect of treatment on fetal plasma concentrations of immunoreactive adrenocorticotropic hormone (ACTH) or cortisol. AVP infusion significantly decreased fetal heart rate and raised cotyledonary blood flow from 198 +/- 18 ml/min per 100 g to 235 +/- 17 ml/min and 218 +/- 10 ml/min per 100 g at 2 h and 24 h, respectively, from the start of the AVP infusion. Basal values for adrenal medullary and cortical blood flows were similar in the AVP and saline groups, and did not change significantly during the infusions.

CONCLUSION

These findings suggest that the rise in adrenal blood flow seen after hypoxemia is not due to a direct action of systemic AVP, but is attributable to other influences, likely including changes in circulating ACTH.

Inhibition of ACTH secretion blocks hypoxia-induced increase of adrenal cortical blood flow in fetal sheep

To examine the role of endogenous adrenocorticotropic hormone (ACTH) in adrenal blood flow responses to hypoxia, we studied unanesthetized ovine fetuses during an intravenous infusion of cortisol or vehicle. Fetal hypoxia was induced after 5 h of cortisol or vehicle infusion. Control fetuses were not made hypoxic. Blood flows were determined before and at three time points during the infusions. At 2 and 6 h of hypoxia, in vehicle-infused fetuses, fetal plasma concentrations of immunoreactive ACTH (irACTH) had risen from 9 +/- 3 (SE) pg/ml to 68 +/- 25 and 127 +/- 37 pg/ml, respectively. No significant change in fetal plasma irACTH occurred in the other groups. Adrenal cortical blood flow rose three- to fourfold during hypoxia in vehicle-infused fetuses but did not change from prehypoxia levels in cortisol-infused fetuses (P < 0.005). Medullary flow rose with hypoxemia, and this was not affected by concurrent cortisol infusion. Adrenal blood flows did not change in the control groups. Thus prior infusion of cortisol suppressed the rise in fetal plasma ACTH during hypoxia and selectively blocked the increase in adrenal cortical blood flow.

Distribution of calbindin-D28K in the brain of the fetal sheep in late gestation

Immunocytochemical distribution of calbindin-D28K was examined in the brains of fetal sheep at 100-105 and 120-125 days of gestation (dGA) and at term in labor (term approximately 150 dGA) and compared to adult sheep. Brains were perfused, cut frozen and immunostained as freely floating sections with a polyclonal anti-calbindin-D28K antibody. Light microscopy revealed that calbindin-D28K immunogenicity in fetal cerebellum, rhombencephalon, mesencephalon, diencephalon and telencephalon was overall very similar to that seen in the brains of adult sheep. In addition even though fetal sheep peripheral plasma cortisol concentrations increase 10-20 times over the last 3-4 weeks of gestation to peak during labor and delivery, no glucocorticoid dependent increases in calbindin-D28K in the hippocampal formation could be seen in the sheep fetus as have been demonstrated for the adult rat given exogenous glucocorticoids. It is concluded that: (1) the basic pattern of calbindin-D28K distribution seen in adult sheep brains is established by at least 100 dGA in the sheep fetus; and (2) the pattern of calbindin-D28K expression in fetal sheep hippocampal formation is not related to peripheral plasma glucocorticoid concentrations.

Effects of intravenous infusions of noradrenaline into the pregnant ewe on uterine blood flow, fetal renal function, and lung liquid flow

To determine the effects on the fetus of high maternal levels of noradrenaline, experiments were carried out in 17 pregnant ewes (123-137 days gestation). Intravenous infusion of 40 mg/min of norepinephrine to the ewe for 1.5 h increased maternal arterial pressure and significantly decreased maternal placental blood flow (p < 0.05). Fetal arterial pressure did not change, but fetal arterial PO2 fell (p < 0.01) and PCO2 rose (p < 0.01). Fetal urine flow fell and osmolality rose (p < 0.01), fetal lung liquid flow and osmolar excretion fell (p < 0.01, p < 0.05, respectively), and the lung sodium:potassium ratio changed. These effects of high levels of maternal noradrenaline were transient, i.e., 2.5 h after the infusion of noradrenaline had finished, fetal urine flow and lung liquid flow had both returned to control values and fetal PCO2 was significantly depressed relative to control values (p < 0.01). It is concluded that high levels of maternal catecholamines reduce placental blood flow and cause small changes in fetal oxygenation. These changes are sufficient to transiently affect fetal water excretion and to reduce lung liquid flow.

Endocrinologic and biophysical responses to prolonged (24-hour) hypoxemia in fetal goats

OBJECTIVE

The purpose of this study was to observe some fetal endocrinologic and biophysical responses to prolonged (24-hour) nonacidemic hypoxemia in fetal goats.

STUDY DESIGN

Endocrinologic and biophysical changes during continuous infusion of nitrogen into the maternal trachea were examined in six chronically instrumented goat fetuses at 123 to 131 days' gestation. We measured arginine vasopressin, adrenocorticotropic hormone, cortisol, epinephrine, and norepinephrine as endocrinologic parameters and fetal heart rate, fetal blood pressure, and fetal breathing movements as biophysical parameters.

RESULTS

Fetal arterial PO2 declined from 25.1 +/- 2.1 mm Hg at baseline to 16.8 +/- 0.9 mm Hg at 1 hour of hypoxemia and remained at this level (p < 0.05). Arginine vasopressin, adrenocorticotropic hormone, and epinephrine initially rose in response to hypoxic stress, gradually declined, and by 24 hours were not significantly different from control values. Cortisol and norepinephrine remained significantly elevated through the experiments. Fetal breathing movements as a percentage of total time were 34.9% +/- 1.2% at baseline, 12.9% +/- 3.7% at 1 hour, and 70% to 80% of control values from 4 hours. Fetal heart rate was significantly reduced for the first 10 minutes, then progressively became tachycardiac thereafter. Systolic fetal blood pressure was significantly elevated for only 30 minutes.

CONCLUSION

Arginine vasopressin, adrenocorticotropic hormone, and epinephrine of the endocrinologic parameters and fetal heart rate, fetal blood pressure, and fetal breathing movements are normalized during prolonged nonacidemic hypoxemia in fetal goats.

Fetal endocrine responses to chronic placental embolization in the late-gestation ovine fetus

OBJECTIVE

The purpose of this study was to examine the effect of chronic fetal placental embolization on the fetal corticotropin, cortisol, and catecholamines concentrations and on myometrial contractility pattern.

STUDY DESIGN

Fourteen fetal sheep were studied (seven embolized, seven controls) for 10 days between 0.84 and 0.91 of gestation. Daily injections of nonradioactive microspheres were performed to decrease fetal arterial oxygen content by 30% to 35% of the preembolization value. Umbilical artery Doppler flow velocity waveforms were measured daily.

RESULTS

Chronic fetal placental embolization produced progressive fetal hypoxemia (p < 0.001) with changes in umbilical artery Doppler flow velocity waveforms indicative of a 25% increase in placental vascular resistance (p < 0.01). In response to chronic fetal hypoxemia there was a progressive increase in baseline fetal plasma norepinephrine concentration (p < 0.001). There was a transient fourfold to fivefold increase in baseline fetal plasma cortisol levels concomitant with a significant decrease in baseline immunoreactive corticotropin between days 7 and 9 of embolization (both p < 0.05), with a return to control values by day 10. There was a 57% increase in myometrial contracture frequency in the embolized group when compared with controls (p = 0.001).

CONCLUSIONS

During repetitive chronic placental damage that led to fetal hypoxemia, the fetal endocrine environment changed with time in a direction that would prevent the onset of premature activation of the hypothalamic-pituitary-adrenal axis and premature delivery.

Regional adrenal blood flow responses to adrenocorticotropic hormone in fetal sheep

To determine whether adrenocorticotropic hormone (ACTH) at plasma concentrations measured during mild hypoxemia and at term affects adrenal blood flow, we measured regional blood flows in five unanesthetized normoxemic fetuses (125-130 days gestation) during a 24-h intravenous infusion of ACTH-(1-24) in isotonic saline solution. Another five fetuses received an infusion of vehicle. Blood flows were determined before the infusion, at 2 and 24 h from its onset, and 24 h afterward using radionuclide-labeled microspheres. Blood flow to the adrenal medulla was fivefold greater than that to the adrenal cortex. Adrenal blood flow rose 99% at 24 h of the ACTH infusion. There was a large increase in adrenal cortical blood flow of 272% at this time but medullary blood flow did not change significantly during ACTH infusion. The rise in cortical blood flow was attributable to decreased vascular resistance. No significant alterations occurred in fetal arterial blood pressure and heart rate, or in blood flow to other lower body organs of the fetus or to the placental cotyledons. These findings are consistent with the hypothesis that the increase in adrenal blood flow observed during fetal hypoxia is associated with changes in plasma ACTH concentration. They are also indicative of selective regulation of cortical and medullary blood flows in the sheep fetus at this stage of gestation.

Acute intrauterine hypoxia increases amniotic fluid prostaglandin F metabolites in the pregnant sheep

OBJECTIVE

Amniotic fluid infection promotes cytokine release, prostaglandin production, and premature labor. In several tissues local hypoxia also activates the secretion of cytokines. Many patients initially seen in premature labor carry small-for-gestational-age fetuses, a condition associated with intrauterine hypoxia. The purpose of our study was to determine whether a reduction in placental blood flow and subsequent acute hypoxia affects prostaglandin secretion by the placenta.

STUDY DESIGN

We chronically catheterized six pregnant sheep at 120 days of gestation. We placed catheters in the maternal and fetal femoral arteries and in the amniotic fluid cavity. A flow probe and snare were placed around the common uterine artery.

RESULTS

A 30-minute uterine circulation occlusion of 30% of its control value produced an increase in prostaglandin F metabolite from 790 +/- 157 to 944 +/- 184 pg/ml within 10 minutes (p < 0.01). Additional uterine blood flow reduction to 60% of control increased the amniotic fluid prostaglandin F metabolites concentration to 894 +/- 202 (p < 0.05, analysis of variance). No increase in mean intrauterine pressure was detected (p > 0.1).

CONCLUSIONS

We speculate that the prostaglandin increase in amniotic fluid in response to intrauterine hypoxia could eventually lead to premature labor. Whether the increase in prostaglandins is mediated by changes in cytokines is unknown at the present time.

Adaptive changes in the developing brain during intrauterine stress

Maturation of neurological performance in moderately to severely growth-retarded newborn infants (SGA) can be accelerated by 3 to 4 weeks or more when compared to the development of appropriately grown infants (AGA) of the same gestation. This is particularly the case in multiple pregnancies or pregnancies characterized by maternal hypertension. This clinical finding has been confirmed by neurophysiological studies on the maturation of brainstem auditory evoked responses (BAERs). The possible mechanisms which underly this phenomenon are not yet elucidated. Glucocorticoids, other steroid hormones and catecholamines are elevated in pregnancies with placental dysfunction, and it is known that these substances have multiple actions on neuronal maturation, particularly on mechanisms of release of neurotransmitters. These observations suggest that the acceleration of brain maturation, and lung maturation, in SGA infants reflects an adaptation of the fetus to early extrauterine life. However, if the placental dysfunction progresses, these mechanisms of adaptation will be overwhelmed by severe malnutrition and anoxia which result in cerebral lesions and risk of death. The clinical goal at the present time for obstetric management of these risk pregnancies is to distinguish between these two periods.