Changes in Extracellular Calcium Activity in Cerebral Ischaemia

Changes in extracellular ion activities were measured during partial ischaemia of the cerebral cortex of primates anaesthetised with α-chloralose. Triple-barrelled, double-ion-sensitive microelectrodes were used to measure the extracellular potassium (K e ) and calcium (Ca e ) activity at the same point simultaneously. The ion changes were related to local cerebral blood flow, and it was shown that at a blood flow of approximately 10 ml 100 g −1 min −1 , there is a threshold below which ion homeostasis is disturbed. This is associated with a dramatic rise in K e and fall in Ca e . Ca e falls from a normal value of 1.31 ± 0.1 mm to approximately 0.28 mm in densely ischaemic tissue. In ischaemia, K e reaches 13.4 ± 3.8 mm before Ca e begins to fall. The fall in Ca e , although related to reduced blood flow, is closely associated with and follows the rise in K e . The change in Ca e is probably due to an increase in membrane permeability, as a result of either depolarisation or a critical lowering of cellular energy reserves.