Effect of Moderate Hypocapnic Ventilation on Nuclear DNA Fragmentation and Energy Metabolism in the Cerebral Cortex of Newborn Piglets

Abstract

Previous studies have shown that severe hypocapnic ventilation [arterial carbon dioxide partial pressure (PaCO 2 ) 7-10 mm Hg] in newborn animals results in decreased cerebral blood flow and decreased tissue oxidative metabolism. The present study tests the hypothesis that moderate hypocapnic ventilation (PaCO 2 20 mm Hg) will result in decreased cerebral oxidative metabolism and nuclear DNA fragmentation in the cerebral cortex of normoxemic newborn piglets. Studies were performed in 10 anesthetized newborn piglets. The animals were ventilated for 1 h to achieve a PaCO 2 of 20 mm Hg in the hypocapnic (H) group (n ϭ 5) and a PaCO 2 of 40 mm Hg in the normocapnic, control (C) group (n ϭ 5). Tissue oxidative metabolism, reflecting tissue oxygenation, was documented biochemically by measuring tissue ATP and phosphocreatine (PCr) levels. Cerebral cortical nuclei were purified, nuclear DNA was isolated, and DNA content was determined. DNA samples were separated, stained, and compared with a standard DNA ladder. Tissue PCr levels were significantly lower in the H group than the C group (2.32 Ϯ 0.66 versus 3.73 Ϯ 0.32 mol/g brain, p Ͻ 0.05), but ATP levels were preserved. Unlike C samples, H samples displayed a smear pattern of small molecular weight fragments between 100 and 12,000 bp. The density of DNA fragments was eight times higher in the H group than the C group, and DNA fragmentation varied inversely with levels of PCr (r ϭ 0.93). These data demonstrate that moderate hypocapnia of 1 h duration results in decreased oxidative metabolism that is associated with DNA fragmentation in the cerebral cortex of newborn piglets. We speculate that hypocapnia-induced hypoxia results in increased intranuclear Ca 2ϩ flux, which causes protease and endonuclease activation, DNA fragmentation, and periventricular leukomalacia in newborn infants. Preterm and ill term infants are at risk for brain injury, subsequent neurodevelopmental delay, and CP, partially because of alterations in CBF (1). In neonates, white matter ischemia often results in ultrasonographic evidence of PVL, which has been associated with the development of CP in human infants (2).Brain ischemia has been associated with hypocapnia owing to the effect of pH and CO 2 on cerebral vascular tone (3). In multiple studies both the degree and duration of hypocapnia have been associated with an increased incidence of PVL and CP in preterm infants (4 -6). In one study of 251 infants Ͻ34 wk gestation, 50% of ventilated preterm infants with a PaCO 2 Ͻ17 mm Hg at least once during the first 3 d of life, and 30% of those with PaCO 2 of 7-20 mm Hg, developed CP or PVL or both (6). In a study of 26 ventilated preterm infants Ͻ1500 g, 23% of infants with PaCO 2 values Ͻ20 mm Hg during the first 24 h of life developed cystic PVL compared with 6% of infants who had normal PaCO 2 values during the same period (7). In another study of 103 preterm infants, the duration of hypocapnia (defined as a PaCO 2 Յ30 mm Hg) during the first 72 h of life was an independent predictor of PVL and ...

Relationship between PCO2 and unfavorable outcome in infants with moderate-to-severe hypoxic ischemic encephalopathy

“…This possibility is supported by observations in animal models of cerebral hypoxia-ischemia. Hypocapnia from overventilation decreased oxidative metabolism and increased DNA degradation in the cerebral cortex of normoxic newborn piglets (2). Normocapnia protected the immature rat brain compared to hypocapnia after hypoxia-ischemia; mild hypercapnia was even more protective than normocapnia (19).…”

“…Hypercapnia leads to cerebral vasodilation and hyperperfusion and, reciprocally, hypocapnia is a potent mediator of cerebral vasoconstriction leading to decreased cerebral blood flow (1). Hypocapnia has been associated with brain injury in animals and human infants (2)(3)(4)(5)(6)(7)(8)(9)(10)(11). In a large retrospective cohort study of infants with hypoxic ischemic encephalopathy (HIE), severe hypocapnia was associated with increased risk of subsequent death or severe neurodevelopmental impairment, with an odds ratio of 2.3 (12).…”

Brain Changes to Hypocapnia Using Rapidly Interleaved Phosphorus-Proton Magnetic Resonance Spectroscopy at 4 T

“…As sufentanil, which has dose related vasoactive properties (Hanel et al, 1997), was used in that study, it is very likely that CBF decreases had occurred before HV-initiation, leading to the observed highly variable CBF effects. As the other described animal studies (Fritz et al, 2001(Fritz et al, , 2004) also used a 1 h anesthesia (halothane 0.8%) baseline before HV, the possibility of such a confounding factor exaggerating CBF seems likely. Although control groups (anesthesia baseline plus normocapnia) were included for comparison (Fritz et al, 2001(Fritz et al, , 2004, a third control group, brain measures from animals that have had no anesthesia, would be necessary to interrogate this factor.…”

“…While 31 P MRS provides a sensitive measure of the intracellular environment, techniques such as microdialysis measure the extracellular environment. Other approaches, such as histologic assays (Fritz et al, 2001(Fritz et al, , 2004, measure both compartments combined, plus additional pH sources (e.g., blood). As these compartments have different baselines and normalize at different rates during sustained HV (Arieff et al, 1976), it may not be possible to directly infer the intracellular compartment from an extracellular measure.…”

“…In contrast, animal models using invasive measurement techniques and anesthesia find much greater increases in tissue pH (both intra/extracellular contributions) (0.27 U (Fritz et al, 2001(Fritz et al, , 2004) after 1 h of HV (to 20 mm Hg). As the induced pH increase during sustained HV is greater at 20 mins than at 30 mins or over hours (Albrecht et al, 1987), these reported values are likely less than the maxima.…”