pH, K+, and PO2 of the extracellular space during ischaemia of primate cerebral cortex.

pH and K+ from the extracellular space, PO2, and CBF have been measured in the same region during progressive ischaemia of primate cerebral cortex. As blood flow was reduced, the other changes had the following sequence. PO2 fell rapidly to 30% of control levels at regional CBF (rCBF) of 30 ml 100 g-1 min-1. As CBF was further reduced, PO2 continued to fall. pH remained stable until around 20 ml 100 g-1 min-1, below which pH fell rapidly, with an exponential increase in H+ concentration. K+ showed the well-known relationship to CBF, remaining normal until around 10 ml 100 g-1 min-1, below which K+ rose rapidly. pHe and log K+ were lin-early related and confirmed that pH fell by 0.3 U before K+ rose significantly, and fell by 0.6 U before the massive rise in K+. The mechanisms involved in this sequence of events and the role of pH changes in the development of the so-called "ischaemic penumbra" are discussed.