Effects of procaine on the intracellular pH of Chinese hamster ovary cells heated at 42.0 or 45.0 degrees C

The local anesthetic procaine greatly sensitizes cells to hyperthermia. Though it is generally accepted that procaine is a membrane-active agent that increases membrane fluidity in cells, the mechanism by which it potentiates heat killing is unknown. In this paper we report changes in intracellular pH (pHi) of Chinese hamster ovary (CHO) cells heated at 42.0 or 45.0 degrees C in the presence of procaine. The pHi was measured with flow cytometry using the dye 1,4-diacetoxy-2,3-dicyanobenzene (ADB). Studies were carried out using cells grown at normal pH (7.3) or cells placed in low-pH (6.6) medium 4 h prior to and during heating (acute low-pH treatment). Low-pH-adapted cells (PHV2), which were obtained previously by continuous culture in pH 6.6 medium, were also used. Normal cells heated in the presence of procaine at pH 7.3 underwent a large decrease in pHi compared to cells heated without procaine. Procaine had little additional effect on the intracellular pH of cells in medium with a pH of 6.6 for 4 h before and during 30 min of heating. PHV2 cells exposed to chronic low-pH conditions were resistant to acidification when heated with or without procaine. The surviving fraction of cells heated with procaine was significantly lower under all pH conditions than that of cells heated without procaine. Cells heated at 42.0 degrees C with procaine also became greatly acidified and their survival was reduced. These data suggest that the reduction in pHi caused by procaine may be part of the mechanism of heat sensitization, but cannot account for it entirely. Furthermore, the degree of procaine sensitization and intracellular acidification is dependent on the extracellular pH, with a larger effect occurring at pH 7.3 than at pH 6.6.