Developmental Changes in Newborn Lamb Brain Mitochondrial Activity and Postasphyxial Lipid Peroxidation

Abstract

Abstract

Previously, our laboratory has demonstrated inhibition of mitochondrial state 3 (ADP-dependent) respiration 5 min after resuscitation from an asphyxial insult in lambs less than 3 days of age. Older lambs were resistant to this transient mitochondrial dysfunction. This study was designed to examine if age-related differences in baseline state 3 mitochondrial respiration, electron transport chain activity, or susceptibility to oxygen free radical-mediated lipid peroxidation were related to the previously observed differences in postasphyxial mitochondrial respiration. Mitochondrial respiration was measured in 24 nonasphyxiated control lambs aged 1-10 days using four different substrates. Electron transport chain activity was assessed in 15 of these lambs, and lipid peroxidation measured as conjugated diene production was measured in 11 of these lambs. These lambs were all ventilated to maintain normal blood gases for a time period equal to the length of the hypoxic insult in asphyxiated lambs (see below), after which samples of brain were removed for isolation of mitochondria. A second group of 11 lambs (seven ≤3 days of age and four >3 days of age) were asphyxiated. The insult was a 75-to-90-min episode of hypoxia and hypercarbia that resulted in bradycardia and systemic hypotension over the final 15 min of the insult. At the end of asphyxia, the lambs were resuscitated and returned to control ventilator settings. Samples of brain were removed 5 min after resuscitation. Postas-phyxia electron transport chain activity and lipid peroxidation were measured. All measurements described above were done in both nonsynaptic (primarily glial in origin) and synaptic mitochondria. State 3 mitochondrial respiration varied significantly with age, decreasing by an average of 41.2% ± 11.1% (mean ± SEM) from Day 2 to Day 5-6 and then increasing back to levels similar to Day 2 by Day 8-10 in nonsynaptic mitochondria. State 3 respiration in synaptic mitochondria decreased 60.6% ± 5.2% from Day 2 to Day 5-6 before returning to levels similar to Day 2 by Day 8-10. Resting (nonADP-dependent) state 4 respiration demonstrated similar developmental patterns. Electron transport chain activities did not vary with age in the nonasphyxiated control animals. In addition, an asphyxial insult did not diminish electron transport chain activities in either lambs ≤3 days old or those >3 days of age. In the ≤3-day-old animals, lipid peroxidation (measured as conjugated diene production) was 0.755 ± 0.19 (Abs 233/mg lipid; mean ± SEM) in controls and 1.31 ± 0.28 post-asphyxia (P <0.03) in nonsynaptic mitochondria. Synaptic mitochondria conjugated diene productions were 0.561 ± 0.05 and 1.369 ± 0.27 (P <0.004), respectively. No differences in conjugated diene production between control and postasphyxia lambs were present in animals >3 days old. These data are consistent with the conclusion that lambs ≥3 days old were more susceptible to oxygen free radical-mediated lipid peroxidation than those >3 days of age.