SPATIAL STABILITY OF EXTRACELLULAR POTASSIUM-ION AND BLOOD-FLOW DISTRIBUTION IN RAT CEREBRAL-CORTEX AFTER PERMANENT MIDDLE CEREBRAL-ARTERY OCCLUSION

Extracellular potassium ion activity ([K+]o) increases precipitously d uring brain ischemia when blood flow falls below threshold values less than approximately 15 mL/100 g/min. This flow threshold for increase of [K+]o occurs also in focal ischemia producing gradient from ischemi c core to adjacent normally perfused brain. In this study we investiga ted the spatial and temporal stability of extracellular potassium ion and blood flow gradients after permanent middle cerebral artery occlus ion (MCAO) in rats. [K+]o and regional CBF were measured, respectively , with K+-sensitive and polarographic hydrogen-sensitive microelectrod es at different cortical locations in the middle cerebral artery distr ibution region. Spatial assessment of [K+]o and regional CBF was condu cted at 30, 90, and 180 minutes after MCAO. [K+]o in the more lateral cortex (core) increased from near 3 mmol/L before MCAO to greater than 50 mmol/L and was associated with now values less than 25% of pre-isc hemic levels. Measurements medial to the core (penumbra) indicated pro gressively decreasing levels of [K+]o and improvement of CBF. There wa s a tendency for [K+]o in penumbral zones to decrease toward normal le vels with time, but there was little dissipation of [K+]o in core regi ons. In contrast, the spatial CRF profile remained remarkably constant fur the entire recording period. Thus, unlike infarction which has be en reported to expand with time after focal ischemia, the spatial [K+] o disturbance tends to contract primarily due to decreasing [K+]o with time in the penumbra. Thus, steady state levels of [K+]o after focal ischemia may not be a valuable predictor of cell viability.