Effect of 2-chloroadenosine on cerebrovascular reactivity to hypercapnia in newborn pig

Molecular hydrogen alleviates asphyxia-induced neuronal cyclooxygenase-2 expression in newborn pigs

Cyclooxygenase-2 (COX-2) has an established role in the pathogenesis of hypoxic-ischemic encephalopathy (HIE). In this study we sought to determine whether COX-2 was induced by asphyxia in newborn pigs, and whether neuronal COX-2 levels were affected by H₂ treatment. Piglets were subjected to either 8 min of asphyxia or a more severe 20 min of asphyxia followed by H₂ treatment (inhaling room air containing 2.1% H₂ for 4 h). COX-2 immunohistochemistry was performed on brain samples from surviving piglets 24 h after asphyxia. The percentages of COX-2-immunopositive neurons were determined in cortical and subcortical areas. Only in piglets with more severe HIE, we observed significant, region-specific increases in neuronal COX-2 expression within the parietal and occipital cortices and in the CA3 hippocampal subfield. H₂ treatment essentially prevented the increases in COX-2-immunopositive neurons. In the parietal cortex, the attenuation of COX-2 induction was associated with reduced 8'-hydroxy-2'-deoxyguanozine immunoreactivity and retained microglial ramifcation index, which are markers of oxidative stress and neuroinfiammation, respectively. This study demonstrates for the first time that asphyxia elevates neuronal COX-2 expression in a piglet HIE model. Neuronal COX-2 induction may play region-specific roles in brain lesion progression during HIE development, and inhibition of this response may contribute to the antioxidant/anti-infiammatory neuroprotective effects of H₂ treatment.

Effect of propentofylline on free radical generation during cerebral hypoxia in the newborn piglet

The present study tests the hypothesis that propentofylline, an adenosine re-uptake inhibitor, will reduce free radical generation during cerebral hypoxia. Ten newborn piglets were pretreated with propentofylline (10 mg/kg), five of which were subjected to hypoxia, while the other five were maintained at normoxia. Five untreated control piglets underwent the same conditions. Hypoxia was induced through a decrease in FiO2 to 0.11 and documented biochemically by a decrease in ATP and phosphocreatine levels. Free radical formation in the cortex was detected directly using electron spin resonance spectroscopy with a spin trap technique. Results demonstrate that free radicals, corresponding to the alkoxyl radical, increased significantly following hypoxia, and that this increase was inhibited by pretreatment with propentofylline. Conjugated dienes, a lipid peroxidation product, also increased following hypoxia and were subsequently inhibited by propentofylline. The administration of propentofylline also significantly limited the hypoxia-induced decrease in tissue levels of ATP and phosphocreatine. These data demonstrate that pretreatment with propentofylline decreased free radical generation and lipid peroxidation as well as preserved high energy phosphates during cerebral hypoxia.

Effect of hypoglycemia on postischemic cortical blood flow, hypercapnic reactivity, and interstitial adenosine concentration

Hypoglycemia increases the vulnerability of the perinatal brain to asphyxia, but it is not known if hypoglycemia-induced changes in cerebral hemodynamics and vascular reactivity underlie this vulnerability. This study tested the hypothesis that hypoglycemia exacerbates postischemic hypoperfusion, and impairs postischemic CO2 reactivity. The authors also examined the hypothesis that postischemic hypoperfusion is associated with a reduction in the interstitial concentration of the vasodilator metabolite adenosine. Global cerebral ischemia of 10 minutes duration was induced in newborn pigs anesthetized with isoflurane by occlusion of subclavian and brachiocephalic arteries; cortical cerebral blood flow (CBF) and interstitial adenosine concentration were evaluated simultaneously using the combined hydrogen clearance/microdialysis technique. Hypoglycemia (blood glucose < 25 mg/dl) was induced by regular insulin (25 IU/kg) administered intravenously 2 hours prior to induction of ischemia. In the eight normoglycemic animals, baseline CBF was 38 +/- 4 ml/min/100 gm and baseline adenosine concentration was 1.2 +/- 0.1 microM; in the eight hypoglycemic animals, these values were 39% (p < 0.05) and 62% (p < 0.05) greater, respectively, under baseline conditions. At 1 hour of postischemic reperfusion in normoglycemic animals, CBF was reduced 39% relative to the preischemic baseline (p < 0.01), concomitant with a 27% reduction (p < 0.05) in adenosine concentration, suggesting that this lowered concentration may underlie delayed hypoperfusion. These postischemic reductions in CBF and interstitial adenosine concentration were significantly greater in hypoglycemic animals, with CBF and adenosine concentration reduced 70% (p < 0.001) and 71% (p < 0.01), respectively, relative to baseline. In nine animals preischemic reactivity to hypercapnia was unaffected by hypoglycemia. Postischemic hypercapnic reactivity was retained in the eight normoglycemic animals, but was attenuated 73% (p < 0.05) in hypoglycemic animals. Thus, in the newborn pig, hypoglycemia exacerbates postischemic cortical hypoperfusion and impairs postischemic cerebrovascular reactivity to hypercapnia.

The effect of carbon dioxide on cerebral arteries

The constancy of cerebral blood flow and volume relies heavily upon the cerebral arteries' intrinsic ability to respond to changes in the partial pressure of arterial CO2. The physiologic mechanisms underlying these responses have not been determined, although changes in extracellular and intracellular pH, mediation by prostanoids and neural activity have been suggested. CO2 reactivity can be influenced by oxygen status and blood pressure and can vary according to age and brain region. In certain pathological conditions or diseases, it can be severely altered. Modern techniques, which measure CBF in cases of cerebral hemodynamic insufficiency, head injury or tumor, rely on the inherent ability of the cerebral circulation to respond to changing levels of CO2.