Hemodynamic responses to asphyxia in spontaneously breathing newborn term and premature lamb

Underlying mechanism of subcortical brain protection during hypoxia and reoxygenation in a sheep model - Influence of α1-adrenergic signalling

While the cerebral autoregulation sufficiently protects subcortical brain regions during hypoxia or asphyxia, the cerebral cortex is not as adequately protected, which suggests that regulation of the cerebral blood flow (CBF) is area-specific. Hypoxia was induced by inhalation of 5% oxygen, for reoxygenation 100% oxygen was used. Cortical and subcortical CBF (by laser Doppler flowmetry), blood gases, mean arterial blood pressure (MABP), heart rate and renal blood flow were constantly monitored. Low dosed urapidil was used for α1A-adrenergic receptor blockade. Western blotting was used to determine adrenergic receptor signalling mediators in brain arterioles. During hypoxia cortical CBF decreased to 72 ± 11% (mean reduction 11 ± 3%, p < 0.001) of baseline, whereas subcortical CBF increased to 168±18% (mean increase 43 ± 5%, p < 0.001). Reoxygenation led to peak CBF of 194 ± 27% in the subcortex, and restored cortical CBF. α1A-Adrenergic blockade led to minor changes in cortical CBF, but massively reduced subcortical CBF during hypoxia and reoxygenation–almost aligning CBF in both brain regions. Correlation analyses revealed that α1A-adrenergic blockade renders all CBF-responses pressure-passive during hypoxia and reoxygenation, and confirmed the necessity of α1A-adrenergic signalling for coupling of CBF-responses to oxygen saturation. Expression levels and activation state of key signalling-mediators of α1-receptors (NOSs, CREB, ERK1/2) did not differ between cortex and subcortex. The dichotomy between subcortical and cortical CBF during hypoxia and reoxygenation critically depends on α1A-adrenergic receptors, but not on differential expression of signalling-mediators: signalling through the α1A-subtype is a prerequisite for cortical/subcortical redistribution of CBF.

Regional cerebral blood flow response to acute hypoxia changes with postnatal age in the rat

The quantitative autoradiographic [14C]iodoantipyrine technique was applied to measure the effects of an acute hypoxic exposure on rates of local cerebral blood flow (LCBF) in the 10 (P10)-, 14 (P14)- and 21 (P21)-day-old rat. The animals were exposed to hypoxic (7% O2/93% N2) or control gas mixtures (21% O2/79% N2) for 40 min before the initiation of the 1-min LCBF measurement. At P10, hypoxia induced a 142-415% increase in LCBF over control levels, which affected the 45 structures studied. The highest increases in LCBF were noticed in posterior midbrain and brainstem regions. These increases are in good accordance with hypoxia-induced increases in LCBF recorded during acute hypoxia exposure in both newborn and adult animals. At P14 and P21, rates of LCBF decreased with hypoxia. These decreases were significant in 23 and 21 brain regions, respectively, belonging to all systems studied. These changes in LCBF are in quite good correlation with our previous data on the effects of acute hypoxia exposure on cerebral glucose utilization but the decrease in LCBF is of higher amplitude than the one in cerebral glucose utilization translating into a relative hypoperfusion at a constant metabolic level at P14 and P21. However, arterial blood pressure was reduced by 16 mmHg and arterial pCO2 was significantly decreased at the two latter ages in hypoxic animals compared to controls. These two systemic factors, and mainly hypocapnia, are rather responsible for the cerebral hypoperfusion recorded at P14 and P21 in hypoxic rats whereas the circulatory response seems to be predominantly hypoxic at P10.

The renal response to acute asphyxia in spontaneously breathing newborn lambs

The effects of acute asphyxia on neonatal renal function were examined in spontaneously breathing newborn lambs. 25 min of asphyxia were induced by addition of a respiratory dead space to reduce PaO2 to 41 +/- 3 mm Hg, pH to 7.03 +/- 0.05 and increase PaCO2 to 68 +/- 5 mm Hg (mean +/- S.E.M.). Glomerular filtration rates did not change significantly during or after asphyxia. Immediately following asphyxia significant (P less than 0.05) increases over the baseline were found in urinary flow rates, fractional sodium excretion, absolute sodium excretion and osmolar clearances. These changes were in part secondary to significant (P less than 0.05) increases in plasma glucose concentrations associated with increases (P less than 0.05) in circulating arterial catecholamine concentrations. The percentage of tubular reabsorption of glucose decreased significantly (P less than 0.05) and urinary glucose excretion increased significantly. Renal blood flow was unchanged. Therefore, asphyxia induced significant hyperglycemia which contributed to the concomitant natriuresis and osmotic diuresis in these newborn lambs.