Breakdown of Calcium Homeostasis in Relation to Tissue Depolarization: Comparison between Gray and White Matter Ischemia

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Abstract
In vitro studies suggest that ischemic injury of cerebral white matter is mediated by nonsynaptic cellular mechanisms, such as Ca2+ entry into axons through reversal of the Na+-Ca2+ exchanger. The authors investigated extracellular Ca2+ concentration in relation to tissue depolarization (direct current potential) in vivo using ion-selective electrodes in cortical gray and subcortical white matter of α-chloralose-anesthetized cats during 120 minutes of global cerebral ischemia. On induction of ischemia, regional CBF, as measured by hydrogen clearance, ceased. The direct current potential decreased rapidly within minutes in gray matter and with little time delay in white matter. Extracellular Ca2+ concentration decreased just as quickly in gray matter. In white matter, in contrast, extracellular Ca2+ increased in the first 20 to 30 minutes, and a delayed and much slower decline, compared with gray matter, was observed thereafter, reaching a minimal level only about 60 minutes after occlusion. Our results suggest that smaller and delayed transmembrane shifts of Ca2+ are correlates of delayed ischemic membrane dysfunction in central white matter tracts, which may be explained by a lack of synaptic mechanisms.