The role of transmembrane pH gradients in the lactic acid induced swelling of astrocytes

The swelling of astrocytes is an important component of the morbidity and mortality associated with ischemic brain trauma. In the ischemic brain, lactic acid levels rise dramatically with a concomitant acidification of the extracellular fluid. In this study we have measured the effects of elevated extracellular lactate and reduced extracellular pH (pHo) on astrocyte volume using the human astrocyte-derived cell line UC-11MG. Neither elevated lactate nor reduced pHo alone increased cell volume, but swelling of about 25% was measured when the cells were exposed simultaneously to 20 mM lactic acid and a reduced pHo of 6. The swelling was correlated with an approximately 4-fold increase in intracellular lactate as pHo was decreased from 8.0 to 6.0. As pHo was decreased intracellular pH also decreased, but much more slowly so that at acidic extracellular pH there was an inwardly directed proton gradient. The measured intracellular lactate concentrations closely followed the theoretical levels predicted by a model in which lactate transport is coupled to the inwardly directed proton gradient. Kinetic studies indicated that lactate transport is saturable with a Km of 3.8 mM, consistent with the model for facilitated cotransport of lactate with a proton or exchange of lactate for a hydroxyl ion. These data suggest that an important mechanism of postischemic astrocytic swelling is a proton driven, active accumulation of lactate to levels that result in a significant osmotic gradient of lactate at acidic pH.