Tissue PCO2 in brain ischemia related to lactate content in normo- and hypercapnic rats

The amount of lactate formed during ischemia determines the rise in tissue PCO2 (PtCO2). Conflicting results exist on the relationship between lactate and PtCO2. The objective of this study was to settle this issue. We varied the preischemic plasma glucose concentration of normo- and hypercapnic rats, assessed tissue lactate and total CO2 contents, and determined the PCO2/lactate relationship over the lactate range 2-40 mmol kg-1. The results showed that whatever the equilibration time, the PCO2/lactate relationship was linear. The results obtained could be reproduced by a theoretical buffer system that mimics the buffering behavior of intracellular fluid. Our results bear on the question of whether compartmentation of H+ occurs during ischemia, with glial cells becoming more acid than neurons. A discontinuous PCO2/lactate relationship, with a constant PCO2 above a certain lactate content, would support this contention. Since our results demonstrate a linear relationship between lactate and PCO2 over the lactate range 2-40 mmol kg-1, they considerably weaken any argument for gross compartmentation of H+.