Tissue lactate content and tissuePCO₂in complete brain ischaemia: implications for compartmentation of H⁺

It has recently been proposed by Kraig et al. that H+ is compartmentalized during complete ischaemia, with glial cells of hyperglycemic animals becoming excessively acidic (pH approximately 5.2), compared to neurons (pH approximately 6.2). In part, the evidence for that is a discontinuous delta PCO2/delta lactate relationship, with PCO2 precipitously increasing to a constant value at tissue lactate contents exceeding 20 mmol kg-1. Since the previous study was performed in hypercapnic animals, and encompassed only a small number of animals, we studied the relationship between ischaemic tissue lactate content and tissue PCO2 over the lactate range 8-49 mmol kg-1. Tissue glucose content was varied by glucose injection and ischaemia was induced by decapitation. Tissue PCO2 was measured in vitro at 37 degrees C on excised tissue samples. The results showed a continuous rise in tissue PCO2 over the entire range of lactate concentrations measured, with maximal values around 300 mm Hg. The results thus fail to reveal a discontinuous delta PCO2/delta lactate relationship, with PCO2 increasing to a constant value. The implication of the results for the hypothesis of pH compartmentation is discussed.