Extracellular pH, Potassium, and Calcium Activities in Progressive Ischaemia of Rat Cortex

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Abstract
We measured the relationships between changes in extracellular pH (pHe), potassium (Ke), and calcium (Cae) activities and DC potential (DCe) in progressive ischaemia of rat cerebral cortex. pHe and Ke, or Cae and Ke, were measured at the same point simultaneously, using triple-barrelled, double-ion-sensitive microelectrodes. Ischaemia was produced using bilateral carotid artery occlusion and hypotension in rats under 50% N2O-0.4% halothane anaesthesia. Unilateral carotid artery occlusion did not affect blood flow, but bilateral occlusion reduced flow to ∼40% of normal. Autoregulation of blood pressure (BP) changes was lost after bilateral occlusion, and so progressive hypotension produced a linear decrease in flow. pHe began to decrease at high levels of flow (30–35 ml 100 g−1 min−1) and showed stepwise acidotic shifs with reductions in BP. Ke was affected at flows of ∼15 ml 100 g−1 min −1, during which time it was critically dependent on BP. When Ke reached 6 m M, it increased rapidly to 40 m M and was associated with a negative shift in DCe. When Ke reached ∼10 m M, Cae decreased rapidly to ∼0.1 m M. pHe had reached 6.87 when Ke increased rapidly and showed a transient alkalotic shift of ∼0.14 units at that time. Possible mechanisms for the sequence of ion changes described are discussed.