The role of intracellular pH changes in heat sensitization by procaine

Although it has been known for many years that procaine sensitizes cells markedly to hyperthermia, the mechanism by which this occurs is not yet understood. The recent finding in our laboratory that procaine caused an intracellular acidification following heating prompted further studies using carboxy-SNARF-1 to measure the intracellular pH of cells during heating. We found that procaine actually causes an intracellular alkalinization during heating and the intracellular pH is lowered immediately after the extracellular procaine is removed. These results suggest that procaine causes acid loading analogous to ammonium chloride (NH4Cl) loading. Sensitization could not be attributed entirely to this acid shock, since a comparable acid shock with NH4Cl loading following heating caused only a slight sensitization compared to procaine. Heated cells acidified with NH4Cl loading recovered rapidly from the low intracellular pH, whereas the cells acidified with procaine loading did not. Cell sorting demonstrated that the cells which were unable to recover from acidification by procaine had significantly lower survival than those that recovered. Thus, in addition to causing an intracellular acidification following heating, procaine alters cellular processes responsible for recovery from an acid shock.