Potassium imaging of the K + drop in experimental focal ischemia

Brain tissue [K + ] ([K + ] br ) drops from 64% to 21% of normal 3–4 h after experimental stroke (Jones et al, Soc Neurosci Abstract Viewer, 2003, Abstract 309.3), as shown using directed micropunch sampling (Hu et al, Brain Res, 2000, 868: ), and is hypothesized to represent an initial defect in the blood‐brain barrier. Here we propose that K + histochemical analysis can monitor this drop with increased spatial resolution. Experimental focal ischemia was induced by middle cerebral artery transection and bilateral common carotid artery occlusion in 3 Sprague‐Dawley rats anesthetized with isoflurane and N 2 O with arterial pressure and blood gas monitoring for either 2.5 or 5 h. Physiological parameters were stable and within normal limits. Brains were mounted in a cryostat, sectioned at 40 μm for K + histochemistry, and punched (0.5 x 2 mm), using reflective change (Kharlamov et al, J Neurosci Methods, 2001, 111: 67) to identify ischemic regions, for flame photometry analysis of [K + ] br . After K + staining of the dried sections using sodium cobaltinitrite (Mies et al, Ann Neurol, 1984, 16: 232), digitization, and alignment with the punch positions (MCID system, InterFocus Imaging Ltd.), optical densities in ischemic and homotopic cortical regions were used to express the relative ischemic region [K + ] br in % of normal. K + staining showed [K + ] br decreases to 59% and 21% at 2.5 and 5 h, respectively, which compare favorably with the 55% and 34% observed with flame photometry from the same ischemic core regions, and with our previous results. Changes in non‐ischemic regions were not noted for both K + staining (101%) and flame photometry (110 mM). K + staining allowed visualization of greater K + decreases at the rostral ischemic border at both time points. Support: NIH NS30839.