Tissue specific responses that constrain glucose oxidation and increase lactate production with the severity of hypoxemia in fetal sheep

Fetal hypoxemia decreases insulin and increases cortisol and norepinephrine concentrations and may restrict growth by decreasing glucose utilization and altering substrate oxidation. Specifically, we hypothesized that hypoxemia would decrease fetal glucose oxidation and increase lactate and pyruvate production. We tested this by measuring whole-body glucose oxidation and lactate production, and molecular pathways in liver, muscle, adipose, and pancreas tissues of fetuses exposed to maternal hypoxemia for 9 days (HOX) compared with control fetal sheep (CON) in late gestation. Fetuses with more severe hypoxemia had lower whole-body glucose oxidation rates, and HOX fetuses had increased lactate production from glucose. In muscle and adipose tissue, expression of the glucose transporter GLUT4 was decreased. In muscle, pyruvate kinase (PKM) and lactate dehydrogenase B (LDHB) expression was decreased. In adipose tissue, LDHA and lactate transporter (MCT1) expression was increased. In liver, there was decreased gene expression of PKLR and MPC2 and phosphorylation of PDH, and increased LDHA gene and protein abundance. LDH activity, however, was decreased only in HOX skeletal muscle. There were no differences in basal insulin signaling across tissues, nor differences in pancreatic tissue insulin content, beta cell area, or genes regulating beta cell function. Collectively, these results demonstrate coordinated metabolic responses across tissues in the hypoxemic fetus that limit glucose oxidation and increase lactate and pyruvate production. These responses may be mediated by hypoxemia induced endocrine responses including increased norepinephrine and cortisol, which inhibit pancreatic insulin secretion resulting in lower insulin concentrations and decreased stimulation of glucose utilization.

Lower citrate synthase activity, mitochondrial complex expression, and fewer oxidative myofibers characterize skeletal muscle from growth restricted fetal sheep

Skeletal muscle from the late gestation sheep fetus with intrauterine growth restriction (IUGR) has evidence of reduced oxidative metabolism. Using a sheep model of placental insufficiency and IUGR, we tested the hypothesis that by late gestation, IUGR fetal skeletal muscle has reduced capacity for oxidative phosphorylation due to intrinsic deficits in mitochondrial respiration. We measured mitochondrial respiration in permeabilized muscle fibers from biceps femoris (BF) and soleus (SOL) from control and IUGR fetal sheep. Using muscles including BF, SOL, tibialis anterior (TA), and flexor digitorum superficialis (FDS), we measured citrate synthase (CS) activity, mitochondrial complex subunit abundance, fiber type distribution, and gene expression of regulators of mitochondrial biosynthesis. Ex vivo mitochondrial respiration was similar in control and IUGR muscle. However, CS activity was lower in IUGR BF and TA, indicating lower mitochondrial content, and protein expression of individual mitochondrial complex subunits was lower in IUGR TA and BF in a muscle specific pattern. IUGR TA, BF, and FDS also had lower expression of type I oxidative fibers. Fiber type shifts that support glycolytic instead of oxidative metabolism may be advantageous for the IUGR fetus in a hypoxic and nutrient deficient environment, whereas these adaptions may be maladaptive in postnatal life.

Central and Peripheral Cannulation for Cardiopulmonary Bypass in Fetal Sheep: A Comparative Study

Objective:In-utero correction is an option for treatment of critical congenital heart diseases (CHDs). Fetal cardiac surgery for CHDs is dependent on the reliable use of fetal cardiopulmonary bypass (CPB), but this technology remains experimental. In this study, we established fetal CPB models with central and peripheral cannulation to explore the differences between the two cannulation strategies.Methods: Ten fetal sheep with 90–110 gestational days were randomized into central cannulation (n = 5) and peripheral cannulation (n = 5) groups. All fetal CPB models were successfully established. At each time point (0, 30, and 60 min after initiation of CPB), echocardiography was performed. Blood samples were also collected for blood gas analysis and tests of myocardial enzymes and liver and kidney function.Results: In the central cannulation group, right ventricular Tei index significantly increased (p = 0.016) over time. Compared with the peripheral cannulation group, the left ventricular Tei index of the central cannulation group was significantly higher (1.96 ± 0.31 vs. 0.45 ± 0.19, respectively; p = 0.028) and the stroke volume was lower (0.46 ± 0.55 vs. 2.13 ± 0.05, respectively; p = 0.008) at 60 min after CPB. Levels of liver and kidney injury markers and of acid-base balance, including alanine aminotransferase (ALT), aspartate aminotransferase/ALT ratio, blood urea nitrogen (BUN), BUN/creatinine ratio, base excess and bicarbonates, were significantly higher for peripheral than for central cannulation. Other important physiologic parameters, including heart rate, blood pressure, myocardial enzymes, umbilical artery beat index and resistance index, left ventricular Tei index, and left and right ventricular stroke volume, were comparable between the two groups.Conclusions: Both central and peripheral cannulations can be used to establish fetal CPB models. Central cannulation causes more adverse impacts for cardiac function, whereas peripheral cannulation is more susceptible to complications related to inadequate organ perfusion.

A Two-Week Insulin Infusion in Intrauterine Growth Restricted Fetal Sheep at 75% Gestation Increases Skeletal Myoblast Replication but Did Not Restore Muscle Mass or Increase Fiber Number

Intrauterine growth restricted (IUGR) fetuses are born with lower skeletal muscle mass, fewer proliferating myoblasts, and fewer myofibers compared to normally growing fetuses. Plasma concentrations of insulin, a myogenic growth factor, are lower in IUGR fetuses. We hypothesized that a two-week insulin infusion at 75% gestation would increase myoblast proliferation and fiber number in IUGR fetal sheep. Catheterized control fetuses received saline (CON-S, n=6), and the IUGR fetuses received either saline (IUGR-S, n=7) or insulin (IUGR-I, 0.014 ± 0.001 units/kg/hr, n=11) for 14 days. Fetal arterial blood gases and plasma amino acid levels were measured. Fetal skeletal muscles (biceps femoris, BF; and flexor digitorum superficialis, FDS) and pancreases were collected at necropsy (126 ± 2 dGA) for immunochemistry analysis, real-time qPCR, or flow cytometry. Insulin concentrations in IUGR-I and IUGR-S were lower vs. CON-S (P ≤ 0.05, group). Fetal arterial PaO2, O2 content, and glucose concentrations were lower in IUGR-I vs. CON-S (P ≤ 0.01) throughout the infusion period. IGF-1 concentrations tended to be higher in IUGR-I vs. IUGR-S (P=0.06), but both were lower vs. CON-S (P ≤ 0.0001, group). More myoblasts were in S/G2 cell cycle stage in IUGR-I vs. both IUGR-S and CON-S (145% and 113%, respectively, P ≤ 0.01). IUGR-I FDS muscle weighed 40% less and had 40% lower fiber number vs. CON-S (P ≤ 0.05) but were not different from IUGR-S. Myonuclear number per fiber and the mRNA expression levels of muscle regulatory factors were not different between groups. While the pancreatic β-cell mass was lower in both IUGR-I and IUGR-S compared to CON-S, the IUGR groups were not different from each other indicating that feedback inhibition by endogenous insulin did not reduce β-cell mass. A two-week insulin infusion at 75% gestation promoted myoblast proliferation in the IUGR fetus but did not increase fiber or myonuclear number. Myoblasts in the IUGR fetus retain the capacity to proliferate in response to mitogenic stimuli, but intrinsic defects in the fetal myoblast by 75% gestation may limit the capacity to restore fiber number.

Single versus continuous sustained inflations during chest compressions and physiological-based cord clamping in asystolic lambs

BackgroundThe feasibility and benefits of continuous sustained inflations (SIs) during chest compressions (CCs) during delayed cord clamping (physiological-based cord clamping; PBCC) are not known. We aimed to determine whether continuous SIs during CCs would reduce the time to return of spontaneous circulation (ROSC) and improve post-asphyxial blood pressures and flows in asystolic newborn lambs.MethodsFetal sheep were surgically instrumented immediately prior to delivery at ~139 days’ gestation and asphyxia induced until lambs reached asystole. Lambs were randomised to either immediate cord clamping (ICC) or PBCC. Lambs then received a single SI (SIsing; 30 s at 30 cmH2O) followed by intermittent positive pressure ventilation, or continuous SIs (SIcont: 30 s duration with 1 s break). We thus examined 4 groups: ICC +SIsing, ICC +SIcont, PBCC +SIsing, and PBCC +SIcont. Chest compressions and epinephrine administration followed international guidelines. PBCC lambs underwent cord clamping 10 min after ROSC. Physiological and oxygenation variables were measured throughout.ResultsThe time taken to achieve ROSC was not different between groups (mean (SD) 4.3±2.9 min). Mean and diastolic blood pressure was higher during chest compressions in PBCC lambs compared with ICC lambs, but no effect of SIs was observed. SIcont significantly reduced pulmonary blood flow, diastolic blood pressure and oxygenation after ROSC compared with SIsing.ConclusionWe found no significant benefit of SIcont over SIsing during CPR on the time to ROSC or on post-ROSC haemodynamics, but did demonstrate the feasibility of continuous SIs during advanced CPR on an intact umbilical cord. Longer-term studies are recommended before this technique is used routinely in clinical practice.

Oxygen and Steroids Affect the Regulatory Role of Natriuretic Peptide Receptor-C on Surfactant Secretion by Type II Cells

Atrial natriuretic peptide (ANP) and its receptors Natriuretic peptide receptor (NPR)-A and NRP-C are all highly expressed in alveolar epithelial type II cells (AEC2s) in the late gestation ovine fetal lung and are dramatically decreased postnatally. However, of all the components, NPR-C stimulation inhibits ANP-mediated surfactant secretion. Since alveolar oxygen increases dramatically after birth, and steroids are administered to mothers antenatally to enhance surfactant lung maturity, we investigated the effects of O2 concentration and steroids on NPR-C-mediated surfactant secretion in AEC2s. NPR-C expression was highest at 5% O2, while being suppressed by 21% O2, in cultured mouse lung epithelial cells (MLE-15s) and/or human primary AEC2s. Surfactant protein-B (SP-B) was significantly elevated in media from both in vitro and ex-vivo culture at 13% O2 versus 21% O2 in the presence of ANP or terbutaline (TER). Both ANP and C-ANP (an NPR-C agonist) attenuated TER-induced SP-B secretion; this effect was reversed by dexamethasone (DEX) pretreatment in AEC2s and by transfection with NPR-C siRNA in MLE-15 cells. DEX markedly reduced AEC2 NPR-C expression, and pregnant ewes treated with betamethasone showed reduced ANP in fetal sheep lung fluid. These data suggest that elevated O2 downregulates AEC2 NPR-C, and that steroid-mediated NPR-C downregulation in neonatal lungs may provide a novel mechanism for their effect on perinatal surfactant production.