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 importa nt in the progression of pathology that follows ischemic stroke. Determinin g these parameters regionally in rodent models of experimental ischemia has been limited because typical tissue weights of more than 35 mg an too larg e. Identifying ischemic tissue to direct tissue sampling towards ischemic c ortex is also represents a difficult generally unresolved area. We suggest that larger differences between normal and ischemic cortex of sodium, potas sium, and water content than previously observed can be obtained from direc ted 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 wer e occluded for 4.9+/-0.13 h (mean+/-SEM). Punch-sampling of 1-mm diameter t issue cores for water content (H2O%) by the wet-dry method, and [Na+] and [ K+] by flame photometry, was guided by the observation of a subtle change i n the surface reflectivity of ischemic cortex of quickly dried, 20-mu m fro zen brain sections, that was confirmed by MAP?, immunohistochemistry. The r atio of the lesion areas as determined by the reflective change and MAP2 im munoreactivity was 0.96+/-0.03 (n = 5). In ischemic cortex H2O% was 79.9%+/ -0.8%, [Na+] was 550+/-25 mEq/kg dry-weight, and [K+] 94.2+/-19.2 mEq/kg dr y-weight (n = 5), all significantly different from the values in border zon e cortex, and in cortex contralateral to ischemic cortex and border zone (f or all samples n = 60, mean wet weight 2.037+/-0.036 mg). Differences betwe en ischemic and normal cortex were 5.4+/-1.1%, 317+/-21 mEq/kg dry-weight, -304+/-27 mEq/kg dry-weight (n = 5) for H2O%, [Na+], and [K+]. These differ ences between ischemic and normal cortex are 1.4-2.5, 1-3.11, and 1.4-3.5 t imes greater, respectively, than previous results obtained using samples we ighing 35 mg or more. These results extend the association of sodium and po tassium 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. (C) 2000 Elsevier Science B.V. All rights reserv ed.