Post-hypoxic neurologic damage during the perinatal period constitutes a common problem of major clinical significance. Following hypoxic insults, there is a 20 to 30% risk for significant neurological and developmental disorders including cerebral palsy, mental retardation, epilepsy, and intraventricular hemorrhage. Although the reasons for this pronounced vulnerability remain unclear, recent evidence suggests that it is related to an inability to adequately regulate cerebral perfusion during hypoxia. Thus, the present studies were designed to enhance understanding of the mechanisms which govern the newborn cerebral circulation during hypoxia. All experiments will be conducted using isolated segments of common carotid, basilar, posterior communicating, and rostral choroidal arteries taken from 3-7 day old lambs and adult sheep. Three main hypotheses will be examined. First, experiments will be conducted to test the hypothesis that the vascular endothelium makes a greater contribution to hypoxic relaxation in adult than in newborn cerebral arteries. Because previous studies by the applicant suggest that the endothelium of adult cerebral arteries plays an important role in hypoxia relaxation, functional immaturity of newborn cerebral arterial endothelium might compromise oxygen delivery during hypoxia. To explore this possibility, contractile dose-response characteristics of acetylcholine (endothelium-dependent), nitroglycerine (endothelium-independent), and oxygen tension will be examined in intact and endothelium denuded cerebral arteries of lambs and sheep. Endothelial morphology will also be examined using scanning electron microscopy. Together, these studies will enable evaluation of the effects of maturation and endothelium on hypoxic cerebrovascular responses. Second, experiments will be performed to test the hypothesis that hypoxic vasodilation is due to increased synthesis of cGMP by mechanisms which are more prominent in adult than in newborn cerebral arteries. Data from the applicant's laboratory suggest that hypoxia increases cGMP in adult arteries. Because increased levels o cGMP have been associated with vasodilation, an inability to produce cGMP in newborn arteries might compromise oxygen delivery during hypoxia. To test this hypothesis, vascular cGMP and cAMP levels will be measured via radioimmunoassay in intact and denuded arteries at rest and during vasodilation produced by acetylcholine, nitroglycerine and hypoxia. The effects of inhibitors of guanylate cyclase (methylene blue) and cGMP-specific phosphodiesterase (M&B 22,948) will also be examined. Third, experiments will be conducted to test the hypothesis that hypoxia produces vasodilation by inhibiting calcium uptake, and that this mechanism is more effective in adult than in newborn arteries. Because previous studies suggest that hypoxia inhibits calcium uptake in adult arteries, an inability of hypoxia to inhibit calcium uptake in newborn cerebral arteries might compromise vasodilation and oxygen delivery. To explore this idea, contractile tensions and calcium uptakes will be measured simultaneously in resting and potassium contracted intact and denuded lamb and sheep cerebral arteries during normoxic and hypoxic conditions. The idea that hypoxia attenuates the calcium sensitivity of the contractile proteins will also be explored by examining dose-response relations between contractile tension and extracellular calcium concentration under normoxic and hypoxic conditions. Together, these studies of endothelium, cyclic nucleotide, and calcium uptake mechanisms will significantly enhance understanding of the processes governing hypoxic cerebral vasodilation in newborns.
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