Near-term Fetal Lambs Research Articles

Gestational long-term hypoxia induces metabolomic reprogramming and phenotypic transformations in fetal sheep pulmonary arteries.

Gestational long-term hypoxia increases the risk of myriad diseases in infants including persistent pulmonary hypertension. Similar to humans, fetal lamb lung development is susceptible to long-term intrauterine hypoxia, with structural and functional changes associated with the development of pulmonary hypertension including pulmonary arterial medial wall thickening and dysregulation of arterial reactivity, which culminates in decreased right ventricular output. To further explore the mechanisms associated with hypoxia-induced aberrations in the fetal sheep lung, we examined the premise that metabolomic changes and functional phenotypic transformations occur due to intrauterine, long-term hypoxia. To address this, we performed electron microscopy, Western immunoblotting, calcium imaging, and metabolomic analyses on pulmonary arteries isolated from near-term fetal lambs that had been exposed to low- or high-altitude (3,801 m) hypoxia for the latter 110+ days of gestation. Our results demonstrate that the sarcoplasmic reticulum was swollen with high luminal width and distances to the plasma membrane in the hypoxic group. Hypoxic animals were presented with higher endoplasmic reticulum stress and suppressed calcium storage. Metabolically, hypoxia was associated with lower levels of multiple omega-3 polyunsaturated fatty acids and derived lipid mediators (e.g., eicosapentaenoic acid, docosahexaenoic acid, α-linolenic acid, 5-hydroxyeicosapentaenoic acid (5-HEPE), 12-HEPE, 15-HEPE, prostaglandin E3, and 19(20)-epoxy docosapentaenoic acid) and higher levels of some omega-6 metabolites (P < 0.02) including 15-keto prostaglandin E2 and linoleoylglycerol. Collectively, the results reveal broad evidence for long-term hypoxia-induced metabolic reprogramming and phenotypic transformations in the pulmonary arteries of fetal sheep, conditions that likely contribute to the development of persistent pulmonary hypertension.

Continuous chest compressions with asynchronous ventilations increase carotid blood flow in the perinatal asphyxiated lamb model.

Background:The neonatal resuscitation program (NRP) recommends interrupted chest compressions (CC) with ventilation in the severely bradycardic neonate. The conventional 3:1 compression-to-ventilation (C:V) resuscitation provides 90 CC/min, significantly lower than the intrinsic newborn heart rate (120–160 beats/min). Continuous CC with asynchronous ventilation (CCCaV) may improve success of return of spontaneous circulation (ROSC).Methods:Twenty-two near-term fetal lambs were randomized to interrupted 3:1 C:V (90 CC + 30 breaths/min) or CCCaV (120 CC + 30 breaths/min). Asphyxiation was induced by cord occlusion. After five min of asystole, resuscitation began following NRP guidelines. The first dose of epinephrine was given at 6 minutes. Invasive arterial blood pressure and left carotid blood flow were continuously measured. Serial arterial blood gases were collected.Results:Baseline characteristics between groups were similar. Rate of and time to ROSC was similar between groups. CCCaV was associated with a higher PaO2 (22±5.3 vs. 15±3.5 mmHg, p<0.01), greater left carotid blood flow (7.5±3.1 vs. 4.3±2.6 ml/kg/min, p<0.01) and oxygen delivery (0.40±0.15 vs. 0.13±0.07 mL O2/kg/min, p <0.01) compared to 3:1 C:V.Conclusion:In a perinatal asphyxiated cardiac arrest lamb model, CCCaV showed greater carotid blood flow and cerebral oxygen delivery compared to 3:1 C:V resuscitation.

Evidence for placental-derived iron-nitrosyls in the circulation of the fetal lamb and against a role for nitrite in mediating the cardiovascular transition at birth.

Circulating metabolites of nitric oxide, such as nitrite, iron nitrosyls (FeNO), and nitrosothiols, have vasodilatory bioactivity. In both human and sheep neonates, plasma concentrations of these NO metabolite (NOx) concentrations fall >50% within minutes after birth, raising the possibility that circulating NOx plays a role in maintaining low fetal vascular resistance and in the cardiovascular transition at birth. To test whether the fall in plasma NOx concentrations at birth is due to either ligation of the umbilical cord or oxygenation of the fetus to newborn levels, plasma NOx concentrations were measured during stepwise delivery of near-term fetal lambs. When fetal lambs were intubated and mechanically ventilated with 100% O2 to oxygenate the arterial blood while still in utero with the umbilical circulation still intact, there was no change in plasma NOx levels. In contrast, when the umbilical cord was ligated while fetal lambs were mechanically ventilated with O2 levels that maintained fetal arterial blood gases, plasma NOx levels decreased by nearly 50%. Characterization of the individual NOx species in plasma revealed that the overall fall in NOx at birth was attributable mainly to FeNO compounds. Finally, when the typical fall in NOx after birth was prevented by intravenous nitrite infusion, birth-related changes in blood pressure, heart rate, and carotid flow changes were little affected, suggesting the cardiovascular transition at birth is not dependent on a fall in plasma NOx. In conclusion, this study shows FeNO is released from the placenta and that its decline accounts for most of the measured fall in plasma NOx at birth.

Acute Hypoxia Alters Ryanodine Receptor Activity in Pulmonary Arterial Myocytes of High Altitude Acclimatized Fetal and Adult Sheep

Intrauterine stress, such as long term high altitude hypoxia (LTH), can alter pulmonary function and reprogram fetal pulmonary vascular development, potentially causing respiratory distress and pulmonary hypertension after birth. Our recent data demonstrates that newborn LTH lambs exhibit exaggerated hypoxia‐induced pulmonary vasoconstriction (HPV). Previous evidence shows Ryanodine Receptor (RyR) activation is central to the HPV response. Furthermore, LTH disrupts RyR activated Ca2+ sparks in pulmonary arterial (PA) myocytes of sheep. Given the relationship between RyR and HPV responses, we tested the hypothesis that acute hypoxia (exposure to 4% O2) and LTH increase RyR mediated Ca2+ spark activity. This was tested by measuring the Ca2+ spark activity in PA myocytes of near‐term fetal lambs and adult sheep that resided at either low (335m, LA) or high (3801m, HA) altitude. Intracellular Ca2+ was recorded in myocytes of isolated PA tissues loaded with Fluo‐4 using line‐scan techniques on a confocal microscope. Line‐scan images were analyzed using custom software (SparkLab) to determine the number of cells displaying sparks, spark frequency, and spatial‐ temporal characteristics. Adult myocytes had greater prevalence and frequency of Ca2+ sparks relative to those of fetuses, confirming previous studies. Acute hypoxia significantly decreased spark prevalence and frequency in myocytes from both HA fetal and LA adult groups; however, acute hypoxia increased spark prevalence and frequency in HA adult sheep. Spark amplitude was decreased by acute hypoxia in the LA fetus but increased in the HA adult. Acute hypoxia also reduced spark duration in the fetus independent of altitude but increased the duration in HA adult. These results provide new insight into the influence of high altitude reprogramming of RyR function in PA of the fetus and adult and the roles their activity have during acute hypoxic exposures.Support or Funding InformationThis work is supported by The National Institutes of Health, Eunice Kennedy Shriver National Institute of Child Health and Human Development grant number HD083132, by the National Science Foundation under Grant No. MRI 0923559, and the Loma Linda University School of Medicine. JT was a Loma Linda University Summer Undergraduate Research Fellow while TY was a Walter E Macpherson Society Medical Student Summer Research Fellow.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Major contribution of central pulmonary reservoir discharge to increased pulmonary arterial diastolic blood flow after birth in near-term lambs.

Recent fetal lamb data have suggested that the pulmonary trunk (PT) region displays a reservoir function and that a pharmacologically induced fall in pulmonary vascular resistance (PVR) increases and redistributes diastolic discharge from this central pulmonary reservoir toward the lungs, thereby producing a positive diastolic offset in the pulmonary arterial (PA) blood flow profile. As a similar offset in PA flow characteristically occurs after birth, this study tested the hypotheses that 1) central pulmonary reservoir discharge is both redistributed toward the lungs and increased in magnitude during the birth transition and 2) discharge from this reservoir constitutes a major component of increased PA diastolic blood flow after birth. Six anesthetized near-term fetal lambs were instrumented with PT, ductal and left PA transit-time flow probes, and aortic, PT and left atrial catheters. Hemodynamic data were recorded in fetuses and at regular intervals during 2-h mechanical ventilation following cesarean section delivery. Diastolic PA blood flow rose from near zero in fetuses to 468 ± 188 ml/min by 15 min (P < 0.001). Central pulmonary reservoir discharge in fetuses (99 ± 44 ml/min) passed primarily right-to-left across the ductus. However, this reservoir discharge redistributed entirely to the lungs by 1 min after birth, and then doubled to a peak of 214 ± 167 ml/min at 15 min (P < 0.001). Reservoir discharge subsequently stabilized at 151 ± 60 ml/min at 30-120 min, which comprised ∼50% of diastolic and ∼20% of mean PA blood flow. These findings suggest that enhanced diastolic central pulmonary reservoir discharge plays a major role in supporting an increased pulmonary perfusion after birth.

Development of ovine chorionic somatomammotropin hormone-deficient pregnancies.

Intrauterine growth restriction (IUGR) is a leading cause of neonatal mortality and morbidity. Chorionic somatomammotropin hormone (CSH), a placenta-specific secretory product found at high concentrations in maternal and fetal circulation throughout gestation, is significantly reduced in human and sheep IUGR pregnancies. The objective of this study was to knock down ovine CSH (oCSH) expression in vivo using lentiviral-mediated short-hairpin RNA to test the hypothesis that oCSH deficiency would result in IUGR of near-term fetal lambs. Three different lentiviral oCSH-targeting constructs were used and compared with pregnancies (n = 8) generated with a scrambled control (SC) lentiviral construct. Pregnancies were harvested at 135 days of gestation. The most effective targeting sequence, "target 6" (tg6; n = 8), yielded pregnancies with significant reductions (P ≤ 0.05) in oCSH mRNA (50%) and protein (38%) concentrations, as well as significant reductions (P ≤ 0.05) in placental (52%) and fetal (32%) weights compared with the SC pregnancies. Fetal liver weights were reduced 41% (P ≤ 0.05), yet fetal liver insulin-like growth factor-I (oIGF1) and -II mRNA concentrations were reduced (P ≤ 0.05) 82 and 71%, respectively, and umbilical artery oIGF1 concentrations were reduced 62% (P ≤ 0.05) in tg6 pregnancies. Additionally, fetal liver oIGF-binding protein (oIGFBP) 2 and oIGFBP3 mRNA concentrations were reduced (P ≤ 0.05), whereas fetal liver oIGFBP1 mRNA concentration was not impacted nor was maternal liver oIGF and oIGFBP mRNA concentrations or uterine artery oIGF1 concentrations (P ≥ 0.10). Based on our results, it appears that oCSH deficiency does result in IUGR, by impacting placental development as well as fetal liver development and function.

Differential Expression of Wnt‐pathway in Late Fetal Ovine Heart and Lung Development

Wnt signalling pathways critical to early cardiogenesis have not been extensively studied in sheep. The activity of Wnt signalling during the rapid heart growth occurring between late‐fetal and early newborn development is not known but may be important given that a right ventricular (RV) functional dominance occurs in utero, but a left ventricular (LV) dominance after birth. Thus our aims were to measure cardiopulmonary changes in 7 key proteins (Wnt3a, Wnt5a, Dvl‐1, phospho‐GSK3β, cdc42, ROCK2 and TCF7L2) and genes (Wnt4 and Wnt7b) in the Wnt signalling pathway in fetal and neonatal sheep. Lung, ventricular and atrial samples from pre‐term (age=128‐129 days; term=147 days), near‐term fetal (age=139‐141 days), and neonatal lambs (age= 5‐7 days post birth) were processed for western immunoblots and qRT‐PCR assays. Four main developmental patterns were evident. In the first pattern, the relative ventricular and atrial expression of Wnt3a, Dvl‐1, phospho‐GSK3β increased during fetal development (P&lt;0.05), but decreased after birth (P&lt;0.05). In the second pattern, fetal and newborn Wnt5a and ROCK2 expression was relatively constant, but with RV expression 2‐3 fold higher (P&lt;0.025 – P&lt;0.001). cdc42 expression decreased in both the ventricular and atrial samples. TCF7L2 expression decreased progressively in both ventricles from preterm to neonatal stages, but increased in the atria. Gene expression of Wnt4 and Wnt7b were greater in atrial samples than ventricular samples. Wnt3a, cdc42 and ROCK2 expression decreased in the lungs post birth but increased for Wnt5a and Dvl‐1. These data show that distinct patterns in Wnt signalling occur during ovine fetal growth, with evidence of differential expression between ventricles and atria.

Abnormal cardiac Wnt signalling in preterm and near-term fetal lambs with sustained in utero stress

Abnormal cardiac Wnt signalling in preterm and near-term fetal lambs with sustained in utero stress

Central Angiotensin I Increases Swallowing Activity and Oxytocin Release in the Near-Term Ovine Fetus

The brain renin-angiotensin system (RAS) plays an important role in hydromineral and neuroendocrine balance. Although previous studies showed that exogenous angiotensin (Ang) II increased dipsogenic and vasopressin responses in near-term fetuses, little is known about the functional development of fetal endogenous brain RAS in the regulation of body fluid homeostasis. To determine the functional development of the central angiotensin-converting enzyme (ACE) in utero, we investigated the electrocortical (ECoG) activity, swallowing activity, oxytocin (OT) release, and c-fos expression in response to intracerebroventricular Ang I administration in the near-term fetal lamb. Ang I did not change fetal low-voltage (LV) and high-voltage (HV) ECoG temporal distributions, but increased fetal swallowing activity during LV ECoG (1.0 ± 0.1 to 3.5 ± 0.4 swallows/min). Additionally, Ang I evoked an increase in c-fos-immunoreactivity in putative dipsogenic centers, including the supraoptic and paraventricular nuclei of the hypothalamus, accompanied by an increase in fetal plasma OT levels. The expression of c-fos was demonstrated in OT neurons in the hypothalamus. The Ang I-mediated increase in fetal swallowing and plasma OT was inhibited by captopril. These results demonstrate the functional development of the fetal brain ACE system in the last trimester of gestation, which plays an important role in the RAS-mediated dipsogenic response and OT release in the regulation of body fluid homeostasis.

Nitric oxide alterations following acute ductal constriction in the fetal lamb: a role for superoxide

Acute partial compression of the fetal ductus arteriosus (DA) results in an initial abrupt increase in pulmonary blood flow (PBF), which is followed by a significant reduction in PBF to baseline values over the ensuing 2-4 h. We have previously demonstrated that this potent vasoconstricting response is due, in part, to an endothelin-1 (ET-1)-mediated decrease in nitric oxide synthase (NOS) activity. In addition, in vitro data demonstrate that ET-1 increases superoxide levels in pulmonary arterial smooth muscle cells and that oxidative stress alters NOS activity. Therefore, the objectives of this study were to determine the potential role of superoxide in the alterations of hemodynamics and NOS activity following acute ductal constriction in the late-gestation fetal lamb. Eighteen anesthetized near-term fetal lambs were instrumented, and a lung biopsy was performed. After a 48-h recovery, acute constriction of the DA was performed by inflating a vascular occluder. Polyethylene glycol-superoxide dismutase (PEG-SOD; 1,000-1,500 units/kg, n = 7) or PEG-alone (vehicle control group, n = 5) was injected into the pulmonary artery before ductal constriction. Six animals had a sham operation. In PEG-alone-treated lambs, acute ductal constriction rapidly decreased pulmonary vascular resistance (PVR) by 88%. However, by 4 h, PVR returned to preconstriction baseline. This vasoconstriction was associated with an increase in lung superoxide levels (82%), a decrease in total NOS activity (50%), and an increase in P-eNOS-Thr495 (52%) (P < 0.05). PEG-SOD prevented the increase of superoxide after ductal constriction, attenuated the vasoconstriction, preserved NOS activity, and increased P-eNOS Ser1177 (307%, P < 0.05). Sham procedure induced no changes. These data suggest that an acute decrease in NOS activity that is mediated, in part, by increased superoxide levels, and alterations in the phosphorylation status of the endothelial NOS isoform, underlie the pulmonary vascular response to acute ductal constriction.

ECG and Heart Rate Variability Changes in Preterm and Near-Term Fetal Lamb Following LPS Exposure

The aim of this study is to evaluate the myocardial response in the preterm and near-term fetal lamb with infection. Chronically instrumented fetal lambs were exposed to lipopolysaccharide (LPS), and the fetal electrocardiogram (FECG) ST waveform was examined using STAN. Fetal heart rate variability (FHRV) was automatically analyzed by adapting a polynomial function to the RR sequence in the FECG. Preterm fetuses exposed to >90 ng/kg LPS died within 8 hours of LPS administration, a response not seen in near-term fetuses. In both surviving and nonsurviving preterm fetuses, cardiovascular responses were characterized by decreased arterial pressure, negative T waves, and tachycardia accompanied by an increase in FHRV. Similar changes were not observed in the near-term fetuses after LPS. The study shows that preterm lambs are more sensitive to LPS in terms of myocardial/cardiovascular response than the more mature fetuses are. High FHRV and negative ST waveform seem to characterize the LPS-induced stress response in preterm fetuses.

Dexamethasone Alters Vascular Reactivity by Enhancing COX-Related Vasodilatation in Fetal Ovine Carotids

Based on preliminary studies that contractile efficacy was altered in vertebral and basilar arteries from neonatal donors treated with postnatal glucocorticoids, we examined the hypothesis that postnatal dexamethasone (DEX), a glucocorticoid used for respiratory disease in neonates, can alter vascular reactivity. Using near-term fetal lamb carotids, we measured 5-hydroxytryptamine (5-HT) dose-response relationship in DEX-treated and untreated arteries. We found that DEX incubation for 1 h had no effect on 5-HT sensitivity and agonist affinity but significantly reduced 5-HT contractile efficacy, a response that became even more pronounced after 4 h of DEX treatment. Coincubation of DEX-treated arteries with INDO for 4 h reversed this DEX-induced attenuation in 5-HT contractile efficacy, although DEX had no significant effects on cyclooxygenase (COX)-1 and COX-2 protein abundance. This data suggests that DEX alters vascular reactivity through a COX-related mechanism, with possible repercussions to neurological injury.

Maternal oxygen administration and fetal cerebral oxygenation: Studies on near-term fetal lambs at both low and high altitude

We tested the hypothesis that O2 administration to the mother would increase arterial O2 tension (PaO2) and cortical tissue O2 tension (tPO2) in both the normoxic control fetus and that acclimatized to high altitude, and that this effect might be greater in the high altitude fetus than that at sea level. Pregnant ewes were divided into either low-altitude group or high-altitude group (3820 m; maintained from day 30 of gestation to near-term, term = 145 days; n = 6 each). In near-term fetuses we instrumented the cerebral cortex with tissue PO2-laser Doppler flow probes, and placed arterial catheters. With the ewe breathing either air or 50% O2, we measured fetal arterial and brain tissue PO2 and cerebral blood flow (CBF). In response to maternal O2 administration, in both low- and high-altitude fetuses, PaO2, O2 content, and brain tissue PO2 increased significantly. Nonetheless, the fetuses at high altitude showed significantly greater increase in both PaO2 and cerebral tPO2, than that seen in fetuses at low altitude. In neither group was there a change in CBF or cerebral metabolic rate for O2. Maternal O2 administration significantly increased PaO2 and cerebral tPO2 of the fetus both at low altitude and at high altitude, with this increase being greater in the high altitude animals. We suggest that maternal O2 administration can have an important effect in ameliorating nonreassuring fetal status, and perhaps be of value in instances of mothers who smoke heavily or with cardiopulmonary disease.

Selective type 5 phosphodiesterase inhibition alters pulmonary hemodynamics and lung liquid production in near-term fetal lambs.

Nitric oxide causes dilation of the pulmonary circulation and reduction in net lung liquid production in the fetal lamb, two critical perinatal events. Phosphodiesterase inhibition alone causes similar changes and also enhances the effects of nitric oxide. To better define the cyclic guanosine 5'-monophosphate (GMP) pathway in these events, we studied the effects of a specific phosphodiesterase inhibitor, E4021, on pulmonary arteries and veins isolated from near-term fetal lambs, as well as in intact, chronically instrumented late-gestation fetal lambs. In the in vitro experiments, both pulmonary arteries and veins relaxed to E4021 in a dose-dependent manner, although pulmonary veins were significantly more sensitive to E4021. Pretreatment with N(G)-nitro-l-arginine (L-NNA) abolished this response in arteries but not in veins. In both arteries and veins, pretreatment with beta-phenyl-1,N2-etheno-8-bromoguanosine-3',5'-cyclic monophosphorothionate blunted relaxations to E4021. In the in vivo experiments, E4021 infusion into either the pulmonary artery or central venous circulation increased pulmonary blood flow and decreased pulmonary vascular resistance, and these responses were blunted by pretreatment with L-NNA. Net lung liquid production, measured by a dye-dilution technique using blue dextran, decreased when E4021 was infused directly into the pulmonary artery and this effect was not altered by L-NNA. There was no effect on lung liquid production when E4021 was infused into the central venous circulation. Taken together, these results suggest that the pulmonary hemodynamic effects of E4021 involve the cyclic GMP pathway and are primarily nitric oxide synthase dependent. In contrast, the effects on E4021 on net lung liquid production appear to be independent of nitric oxide synthase, suggesting that these two critical perinatal events might be modulated independently.

Cerebral blood flow and morphological changes after hypoxic‐ischaemic injury in preterm lambs

To evaluate the effect of cerebral hypoxia-ischaemia induced by partial occlusion of the umbilical cord on the relationship of the regional cerebral blood flow and the cerebral cell death in near-term fetal lambs. Fifteen near-term lambs were assigned to two hypoxic-ischaemic groups with or without life support (3 h), and a healthy one. Hypoxia-ischaemia was induced by partial occlusion of the umbilical cord (60 min). Routine light and electron microscopy, and the TUNEL method for apoptosis were performed. Regional cerebral blood flow was measured by coloured microspheres. Cardiovascular, gas exchange and pH parameters were also evaluated. Both hypoxic-ischaemic groups produced a transient acidosis and a decrease of base excess in comparison to the healthy group. Cortical and cerebellar zones, where the regional cerebral blood flow values were similar to baseline, showed an increased number of oligodendrocyte-like apoptotic cells. In contrast, in the inner zones, where regional cerebral blood flow was increased, the number of apoptotic cells did not increase. Necrotic neurons were observed in the basal nuclei, mesencephalon, pons and deep cerebellar nuclei. Our results suggest that regional cerebral blood flow and the presence of apoptotic cells, 3 h after hypoxic-ischemic injury, are correlated.

Effect of oxygen on cyclic GMP-dependent protein kinase-mediated relaxation in ovine fetal pulmonary arteries and veins.

Cyclic GMP-dependent protein kinase (PKG) plays an important role in regulating pulmonary vasomotor tone in the perinatal period. In this study, we tested the hypothesis that a change in oxygen tension affects PKG-mediated pulmonary vasodilation. Isolated intrapulmonary arteries and veins of near-term fetal lambs were first incubated for 4 h under hypoxic and normoxic conditions (Po2 of 30 and 140 mmHg, respectively) and then contracted with endothelin-1. 8-Bromoguanosine 3',5'-cyclic monophosphate (8-BrcGMP), a cell membrane-permeable analog of cGMP, induced a greater relaxation in vessels incubated in normoxia than in hypoxia. beta-Phenyl-1,N2-etheno-8-bromoguanosine-3',5'-cyclic monophosphorothioate, Rp isomer (Rp-8-Br-PET-cGMPS), a selective inhibitor of PKG, attenuated relaxation induced by 8-BrcGMP (10-4 and 3 x 10-4 M). In the presence of Rp-8-Br-PET-cGMPS, the differential responses to 8-BrcGMP between hypoxia and normoxia treatment were abolished in veins but not in arteries. cGMP-stimulated PKG activity was present in arteries but not in veins after 4 h of hypoxia. Both vessel types showed significant increase in cGMP-stimulated PKG activity after 4 h of normoxia. PKG protein (Western blot analysis) and PKG mRNA levels (quantitative RT-PCR) were greater in veins but not in arteries after 4-h exposure to normoxia vs. hypoxia. These results demonstrate that oxygen augments cGMP-mediated vasodilation of fetal pulmonary arteries and veins. Furthermore, the effect of oxygen on response of the veins to cGMP is due to an increase in the activity, protein level, and mRNA of PKG.

The response of the lamb ductus arteriosus to endothelin: developmental changes and influence of light

Endothelin-1 (ET-1) is a putative messenger of oxygen in the ductus arteriosus. Since the ability of the vessel to contract to oxygen increases with gestation, we wished to ascertain whether ET-1 action is also developmentally regulated. A corollary objective was to assess whether any gestational variation in the ET-1 contraction is due to a change in the ET A-mediated action or to a shift in the balance between opposing, contractile (ET A - mediated) and relaxant (ET B-mediated), actions. Experiments were performed with isolated ductal strips from preterm (0.7 gestation) and near-term fetal lambs. ET-1 contracted the ductus dose-dependently (10 −10–10 −7 M) at both ages; however, the peak contraction was about double in magnitude at term. Regardless of age, ET-1 contraction was greater with preparations kept in the dark compared to those exposed to light. This effect of light was not seen after removing the endothelium or when treating the intact tissue with the ET B antagonist BQ788 (1 μM). In the dark, however, BQ788 did not modify significantly the ET-1 response at either age. We conclude that ET-1 becomes a stronger ductus constrictor with fetal age, conceivably by acting on ET A receptors. Hence, the concept of ET-1 mediating the oxygen contraction is further validated. Peculiarly, the ET-1 contraction is curtailed by light through a hitherto undefined ET B receptor-linked process.

Reduced postnatal cerebral glucose metabolism measured by PET after asphyxia in near term fetal lambs.

The effects of fetal asphyxia on cerebral function and development, involve the transition from fetal to neonatal life. Changes in cerebral glucose metabolism may be an early postnatal indicator of fetal asphyxia. The objective is to develop an experimental lamb model involving the transition from fetal to neonatal life and to examine the effect of fetal asphyxia with cerebral hypoxic ischemia on early postnatal cerebral glucose metabolism. Fetal asphyxia was induced by total umbilical cord occlusion in eight near-term fetal lambs (134-138 days) with the ewe under isoflurane-opiate anesthesia. The mean occlusion time until cardiac arrest was 14.5 (4.2) min (SD). Lambs were immediately delivered and standardized resuscitation was instituted after 2 min asystole. At 4 hr postnatal age, [18-F]Fluoro-2-deoxy-glucose (18-FDG) was injected intravenously in eight asphyxiated lambs and in eight controls. Cerebral glucose metabolism was examined by positron emission tomography (PET). As a result the mean arterial blood pressure, acid-base values, blood glucose and serum lactate at 4 hr postnatal age did not differ significantly between lambs subjected to umbilical cord occlusion and controls. EEG was abnormal in all lambs subjected to cord occlusion and normal in the controls at 4 hr postnatal age. Global cerebral metabolic rate (CMRgl) as determined by PET was significantly lower in lambs subjected to cord occlusion mean/median (SD) 22.2/19.6 (8.4) micromol/min/100 g) than in controls mean/median (SD) 37.8/35.9 (6.1); P < 0.01). Global CMRgl is significantly reduced in newborn lambs 4 hr after fetal asphyxia induced by umbilical cord occlusion. A reduction in CMRgl is an early indicator of global hypoxic cerebral ischemia.

Role of ketone bodies in perinatal myocardial energy metabolism

Metabolic changes at around the time of birth are crucial for life. Here we review the energy utilization in the myocardium, emphasizing ketone body metabolism. Before birth, glucose and lactate are the major energy substrates for the myocardium. Long-chain fatty acids (LCFA) are normally not available as an energy substrate for the fetal heart; however, when LCFA are supplied artificially in near-term fetal lambs, they are readily oxidized. Hence the myocardium has no limitation to its ability to use LCFA before birth. After birth, lactate remains an important energy source for the myocardium, whereas the contribution of glucose to myocardial energy production decreases despite an increase in the supply of glucose. The oxidation of ketone bodies increases after birth in relation to an increase in supply. However, ketone bodies account for only 7% of left ventricular oxygen consumption. The supply and contribution of LCFA to the myocardium increases after birth; the oxidation of LCFA accounts for most of the left ventricular oxygen consumption. Hence the role of ketone bodies in myocardial metabolism is limited. However, there are interesting observations on interference between the uptake of different substrates and the release of ketone bodies, which might have consequences for our interpretation of ketone body utilization.

Hypoxia in fetal lambs: a study with (1)H-MNR spectroscopy of cerebrospinal fluid.

In fetal lambs, severe hypoxia (SH) will lead to brain damage. Mild hypoxia (MH) is thought to be relatively safe for the fetal brain because compensating mechanisms are activated. We questioned whether MH, leading to mild acidosis, induces changes in cerebral metabolism. Metabolites in cerebrospinal fluid (CSF) samples, as analyzed by proton magnetic resonance spectroscopy, were studied in two groups of seven anesthetized near-term fetal lambs. In group I, SH leading to acidosis with an arterial pH <7.1 was achieved. In group II, MH with an intended pH of 7.23--7.27 was reached [start of MH (SMH)], and maintained during 2 h [end of MH (EMH)]. During SH, choline levels in CSF, a possible indicator of cell membrane damage, were increased. Both during SH and at EMH, CSF levels of lactic acid, alanine, phenylalanine, tyrosine, lysine, branched chain amino acids, and hypoxanthine were increased compared with control values and with SMH, respectively. At EMH, the hypoxanthine CSF-to-blood ratio was increased as compared with SMH. These results indicate that prolonged MH leads to energy degradation in the fetal lamb brain and may not be as safe as assumed.