SENSITIZATION TO HYPERTHERMIA BY INTRACELLULAR ACIDIFICATION OF C6 GLIOMA-CELLS

Hyperthermia has been introduced as a new modality of treatment for gl ioma. In these experiments, the cytotoxicity of hyperthermia in C6 gli oma cells was enhanced by increasing the intracellular acidity with am iloride and/or 4,4'-diisothiocyanatostilbene-2,2' disulfonic acid (DID S). Intracellular pH (pHi) is regulated mainly by Na+/H+ and HCO3-/Cl- antiports through the cell membrane, and amiloride acts on the former , DIDS on the latter to lower pHi. The cellular thermosensitivity to c linically achievable brain hyperthermia at 42 degrees C was enhanced b y 0.5 mM amiloride (Na+/H+ antiport inhibitor). T-0 values (T-0 = the heating period required to reduce experimental survival rate by 1/e) a t 42 degrees C without and with amiloride was 192 and 81 min, respecti vely. The addition of DIDS (HCO3-/Cl- antiport inhibitor) further enha nced. T-0 value was 25 min. Fluorophotometric measurement of pHi was e mployed using the pH sensitive dye, bis(carboxyethyl)carboxyfluorescei n, which is trapped in viable cells. The average pHi in control C6 gli oma cells in pH 7.2 media was 7.21. In the untreated cells heated at 4 2 degrees C for 1 hour, the pHi was 7.12. The pHi of the cells heated in the presence of amiloride was decreased to 6.83. The pHi was furthe r lowered to 6.67 by the treatment with amiloride in combination with DIDS for 2 hours. Hyperthermia with amiloride and DIDS may be a more e ffective treatment for malignant gliomas.