Astrocytes are highly interactive and reactive brain cells that are essential for brain function. Accordingly, damage to brain will evolve directly from injury to astrocytes or these cells will modify their behavior in response to injury of adjacent cells. Examination of how astrocytes respond to injury from brain ischemia should lead to an improved understanding of the cellular pathogenesis of this malady. This philosophy guides the long term goal of this project: To understand how astrocytes respond to ischemic brain injury and how acid-base fluctuations participate in their response. Astrocytes either degenerate from severe ischemia (i.e. infarction) or they are transformed into reactive species by less lethal reduction in flow. Little physiologic information exists that characterizes these two cell changes or alludes to what causes the alterations. Since H+i and K+i modulate vital activities in eukaryotic cells, are interrelated in astrocytes, and are two principal ionic species regulated by these cells, the interrelation of changes in astrocytic H+i and K+i to ischemic injury will be examined in detail here. Ion-selective microelectrodes for pH, K+, as well as a newly developed triple barrel microelectrode array sensitive to pH & CO32, will be used to correlate the patterns of astrocytic acid- base and K+ change from ischemia that are associated with structural markers for these cells (i.e measurements of the cell body size, degree of DNA replication, intensity and distribution of GFAP staining as well as change in regional brain GFAP content). Degeneration of astrocytes will be studied in vivo using a rodent model of focal infarction since this type of ischemic brain injury is clinically most prevalent. Astrocytes become severely acidotic and lose their processes during infarction from global ischemia, perhaps because of a critical reduction in [HCO3-]i. How astrocytic pHi, [HCO3-]I and morphology change with focal infarction is unknown and will be determined. Astrocytic changes in H+i and K+i associated with reactive astrocytosis will be studied in vivo using global ischemia and correlated to the above structural markers of astrocytosis. Furthermore, the effect of hypothermia, which retards neuronal destruction from global ischemia, on ionic and structural variables associated with reactive astrocytosis will be examined since such destruction is thought to be necessary for astrocytic proliferation. Additionally, since astroglial pHi rises at the onset of reactive astrocytosis, the effect of hypercarbia, which acidifies these cells will be studied. Finally, the in vitro brain slice preparation will be used to determine plasma membrane-based mechanisms which interrelate changes in astrocytic pHi and K+i.
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