Tissue Na, K, and Ca changes in regional cerebral ischemia: their measurement and interpretation

A simple and reliable method of quantifying tissue damage is described. This method, based on atomic absorption spectroscopic determinations of Na, K, and Ca concentrations in small brain samples, was applied to the rat middle cerebral artery occlusion model (MCAo). At the infarct site by 24 hours, Na concentration more than doubled, Ca concentration increased by greater than 70%, and K concentration fell nearly 80%; these changes are consistent with a greater than 80% disruption of cells. A remarkable acceleration of ionic shifts occurred between 4 and 6 hours after MCAo. At 4 hours, only 20-30% of the ionic shifts found at 24 hours had occurred; by 6 hours, 80-100% of the ionic shifts found at 24 hours had taken place. Since the measurements reflect ionic movement into and out of the tissue, they are likely to represent irreversible tissue damage. Although blood brain barrier breakdown may have contributed to an increased rate of ionic shifts, large ionic gradients must have been present between the extracellular space and the vascular compartment at 4-6 hours to drive the ionic shifts. Our results suggest an upper time limit of 4 hours for treatments of acute ischemic tissue damage in the rat MCAo model. The methods and analytical approach described may be useful for determining the time window for therapeutic intervention in acute CNS injuries, as well as for evaluating treatment effects.