Onset of pulmonary ventilation in fetal sheep produces pial arteriolar constriction dependent on cytochrome p450 omega-hydroxylase activity

Abstract

With the onset of ventilation at birth, cerebral blood flow decreases as oxygenation increases, but the mechanism of cerebral vasoconstriction is unknown. Cytochrome P-450 omega-hydroxylase activity metabolizes arachidonic acid to 20-HETE, a potent vasoconstrictor, in a physiologically relevant O(2)-dependent manner. We tested the hypothesis that the omega-hydroxylase inhibitor, 17-octadecynoic acid (17-ODYA), reduces cerebral vasoconstriction during in utero ventilation with O(2) in fetal sheep. In anesthetized pregnant sheep near term, the fetal head was exposed with the rest of the body remaining in utero. Pial arteriolar diameter was measured by intravital microscopy through a closed cranial window superfused with vehicle or 17-ODYA. Mechanical ventilation of the fetal lungs with a high O(2) mixture to increase arterial Po(2) from approximately 20 to approximately 90 Torr markedly decreased pial arteriolar diameter by 24 + or - 3% (+ or - SE) without a change in arterial pressure. In contrast, superfusion of 17-ODYA completely blocked the decrease in diameter (2 + or - 3%) with increased oxygenation. Vasoconstriction to hypocapnia was intact after returning to the baseline intrauterine oxygenation state, thereby indicating that the effect of 17-ODYA was selective for increased oxygenation. In cerebral arteries isolated from fetal sheep, increasing oxygenation increased 20-HETE production. We conclude that cytochrome P-450 omega-hydroxylase activity makes an important contribution to cerebral vasoconstriction associated with the onset of ventilation at birth.

Fig. 1.

Schematic representation of the timing of measurements of arteriolar diameter in relation to the interventions of cranial window superfusion with vehicle or 17-octadecynoic acid (17-ODYA) in two groups of fetuses, onset of fetal lung ventilation, fetal lung ventilation with a high O₂ mixture, and fetal hyperventilation to produce hypocapnia.

Fig. 2.

Percent change in large and small pial arteriolar diameter (±SE) during the onset of mechanical ventilation of fetal lungs with a low O₂ mixture to maintain arterial blood gases at baseline intrauterine levels in fetal sheep with cranial windows superfused with vehicle (n = 6) or 17-ODYA (n = 6). Diameters were not significantly changed and were not different between groups for large vessels (P = 0.60; 1-β = 0.51 for group difference of 20% and α of 0.05) and for small vessels (P = 0.59; 1-β = 0.57 for group difference of 20% and α of 0.05).

Fig. 3.

Diameter response of large and small pial arterioles during mechanical ventilation of fetal lungs with a high O₂ mixture to increase arterial oxygenation to postnatal levels in fetal sheep with cranial windows superfused with vehicle (n = 6) or 17-ODYA (n = 6). Values are percent change (±SE) from diameter during low O₂ ventilation. *P < 0.001 between groups.

Fig. 4.

A: Pa_CO2 (±SE) during ventilation of the fetal lungs with a low O₂ mixture and during hyperventilation in fetal sheep with cranial windows superfused with vehicle (n = 4) or 17-ODYA (n = 5). *P < 0.001 from low O₂ ventilation baseline. B: percent change in large and small pial arteriolar diameter during hyperventilation. Constrictor responses were not significantly different between vehicle and 17-ODYA groups for large vessels (P = 0.37; 1-β = 0.55 for group difference of 20% and α of 0.05) and for small vessels (P = 0.07; 1-β = 0.62 for group difference of 20% and α of 0.05).

Fig. 5.

Amount of 20-HETE generated in homogenates of fetal sheep cerebral arteries during 45-min incubation with arachidonic acid under ambient conditions of <2% and 21% O₂ conditions (±SE; n = 6). *P < 0.05 from low O₂.