Directed sampling for electrolyte analysis and water content of micro-punch samples shows large differences between normal and ischemic rat brain cortex.

Changes in sodium, potassium, and water content in brain tissue are important in the progression of pathology that follows ischemic stroke. Determining these parameters regionally in rodent models of experimental ischemia has been limited because typical tissue weights of more than 35 mg are too large. Identifying ischemic tissue to direct tissue sampling towards ischemic cortex is also represents a difficult generally unresolved area. We suggest that larger differences between normal and ischemic cortex of sodium, potassium, and water content than previously observed can be obtained from directed sampling of 2-mg brain tissue in a model of focal cerebral ischemia. In five rats, the middle cerebral artery and both common carotid arteries were occluded for 4.9+/-0.13 h (mean+/-SEM). Punch-sampling of 1-mm diameter tissue cores for water content (H(2)O%) by the wet-dry method, and [Na(+)] and [K(+)] by flame photometry, was guided by the observation of a subtle change in the surface reflectivity of ischemic cortex of quickly dried, 20-microm frozen brain sections, that was confirmed by MAP2 immunohistochemistry. The ratio of the lesion areas as determined by the reflective change and MAP2 immunoreactivity was 0.96+/-0.03 (n=5). In ischemic cortex H(2)O% was 79.9%+/-0.8%, [Na(+)] was 550+/-25 mEq/kg dry-weight, and [K(+)] 94.2+/-19.2 mEq/kg dry-weight (n=5), all significantly different from the values in border zone cortex, and in cortex contralateral to ischemic cortex and border zone (for all samples n=60, mean wet weight 2.037+/-0.046 mg). Differences between ischemic and normal cortex were 5.4+/-1.1%, 317+/-21 mEq/kg dry-weight, -304+/-27 mEq/kg dry-weight (n=5) for H(2)O%, [Na(+)], and [K(+)]. These differences between ischemic and normal cortex are 1.4-2.5, 1-3.11, and 1.4-3.5 times greater, respectively, than previous results obtained using samples weighing 35 mg or more. These results extend the association of sodium and potassium with ischemic brain edema in the rodent model, and show that these classical measurements can keep pace with the regionality of histochemical and morphological methods.