Abstract 386 Poster Session III, Monday, 5/3 (poster 165)

Oxidant stress plays a major role in the pathogenesis of hypoxic-ischemic encephalopathies including PV leukomalacia (PVL) in immature subjects. A circulatory component contributes to PVL. Isoprostanes (in particular 8-iso-PGF2α) are abundant peroxidation products which cause vasoconstriction, but ontogeny and mode of actions of these agents are unknown. We hypothesized that 8-iso-PGF2α causes greater constriction of PV microvessels (30-50 µm) of fetus (∼ 2 3 gestation) and newborn (1-2 days old) than of adult (5-7 months old) pig, and investigated its mechanism of action. 8-Iso-PGF2α caused equivalent constriction (by video-imaging; EC50=10-50 nM) of PV microvessels of fetus and newborn, which was ∼3-fold greater than in adult; vasoconstriction was 2-3-fold greater in PV area than in cortex, particularly in younger subjects. In contrast, cyclooxygenase (COX)-derived PGF2α was more effective in adults. Because we found isoprostanes to act via TXA2 formation, the contribution of the latter during development was investigated. 8-Iso-PGF2α evoked ∼2.5-fold more TXA2 formation in fetus and newborn than in adult PV region; this was associated with higher COX activity and expression of immunoreactive TXA2 synthase in fetus and newborn. 8-Iso-PGF2α-induced constriction was nearly abolished by TXA2 synthase and receptor blockers (CGS12970 and L670596); vasoconstriction elicited by TXA2 mimetic U46619 was slightly greater (1¼-1½ fold) in younger than adult subjects. Mechanisms of TXA2 formation by 8-iso-PGF2α was studied on white matter vascular and parenchymal cells of fetus. 8-Iso-PGF2α stimulated comparable TXA2 generation in endothelial and astroglial (major component) cells, but negligibly in smooth muscle and oligodendrocytes. In endothelial cells, 8-iso-PGF2α-induced formation of TXA2 (and Ca++ transients [using fura 2-AM]) were blocked by Ca++ chelator EGTA and non-voltage gated channel inhibitor SK&F96365. In astrocytes, TXA2 synthesis (and Ca++ transients) by 8-iso-PGF2α were only blocked by principal neural N-type voltage gated Ca++ channel inhibitor ω-conotoxin and by EGTA. Hence, 8-iso-PGF2α stimulates TXA2 formation by enhancing entry of Ca++ via non-voltage and N-type voltage gated Ca++ channels in endothelial and astroglial cells respectively. In conclusion, 8-iso-PGF2α causes more pronounced vasoconstriction in fetus and newborn than in adult brain PV region due to greater TXA2 formation; resulting hemodynamic compromise may contribute to the increased vulnerability of the PV brain areas to oxidant stress-induced injury, which may be attenuated by TXA2 synthase/receptor blockers.

Funded by the Hospital for Sick Children Foundation.