There is now strong evidence that brain extracellular pH is governed on a fast time scale by one or more forms of extracellular carbonic anhydrase (ECA). A principal function of ECA is to regulate the manifestation of alkaline interstitial pH shifts. These pH changes arise within 100 ms, and accompany synchronous neural activity, seizure, spreading depression and brain ischemia. The speed of these pH changes is sufficient to modulate synaptic transmission, through their effect on NMDA receptors. One form of alkalosis does not require bicarbonate, and arises from a net proton flux into cells. ECA functions to rapidly buffer this pH change by catalyzing the hydration of carbon dioxide. A second form of alkalosis arises from the efflux of bicarbonate across GABA-A anion channels. ECA does not buffer this pH change. Rather it acts to generate the alkaline shift by catalyzing the dehydration of carbonic acid. A third role of ECA is to facilitate the transport of lactate. This laboratory has recently demonstrated that surface carbonic anhydrase on isolated astrocytes and neurons is necessary for the influx of lactic acid by the monocarboxylate transport mechanism. Thus, the cotransport of lactate with H+ apparently requires surface CA to supply protons at an adequate rate. The role of ECA in the buffering and generation of alkaline pH shifts has remained obscure, owing to the poor temporal resolution of pH microelectrodes. Using a fluorescein-dextran probe, we have improved the resolution by two orders of magnitude. This optical recording technique will be used to perform the first quantitative analysis of ECA function during the 100 ms rise of an activity-dependent alkaline transient. Experiments will determine whether ECA is the sole and sufficient means of interstitial pH regulation in this period, and will elucidate the magnitude, time course and ECA-dependence of GABAergic alkaline shifts. By capitalizing on the ECA dependence of lactate transport, the surface CA activity on individual astrocytes and neurons will be identified and quantified. Corollary experiments will address the role of ECA in the transport and utilization of lactate in tissue, using rat hippocampal slices in models of aglycemia and hypoxia. These projects will elucidate how ECA functions to regulate interstitial pH and facilitate lactate transport. This research will thereby provide insights into the role of the hydrogen ion as a modulator of brain function, in the normal, as well as the epileptic, ischemic and post-traumatic setting. ? ?
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