Effects of 42 degrees C hyperthermia on intracellular pH in ovarian carcinoma cells during acute or chronic exposure to low extracellular pH

Purpose: To determine whether intracellular pH (pH(i)) is affected during hyperthermia in substrate-attached cells and whether acute extracellular acidification potentiates the cytotoxicity of hyperthermia via an effect on pH(i), Methods and Materials: The pH(i) was determined in cells attached to extracellular matrix proteins loaded with the fluorescent indicator dye BCECF at 37 degrees C and during 42 degrees C hyperthermia at an extracellular pH (pH,) of 6.7 or 7.3 in cells, Effects on pH(i) during hyperthermia are compared to effects on clonogenic survival after hyperthermia at pH(e), 7.3 and 6.7 of cells grown at pH, 7.3, or of cells grown and monitored at pH, 6.7, Results: The results show that pH(i) values are affected by substrate attachments, Cells attached to extracellular matrix proteins had better signal stability, low dye leakage and evidence of homeostatic regulation of pH(i) during heating, The net decrease in pH(i) in cells grown and assayed at pH(e) = 7.3 during 42 degrees C hyperthermia was 0.28 units and the decrease in low pH adapted cells heated at pH(e), = 6.7 was 0.14 units, Acute acidification from pH(e), = 7.3 to pH, = 6.7 at 37 degrees C caused an initial reduction of 0.5-0.8 unit in pH(i), but a partial recovery followed during the next 60-90 min, Concurrent 42 degrees C hyperthermia caused the same initial reduction in pH(i) in acutely acidified cells, but inhibited the partial recovery that occurred during the next 60-90 min at 37 degrees C, After 4 h at 37 degrees C, the net change in pH(i) in acutely acidified cells was 0.30 pH unit, but at 42 degrees C is 0.63 pH units, The net change in pH(i) correlated inversely with clonogenic survival. Conclusions: Hyperthermia causes a pH(i) reduction in cells which was smaller in magnitude by 50% in low pH adapted cells, Hyperthermia inhibited the partial recovery from acute acidification that was observed at 37 degrees C in substrate attached cells, in parallel with a lower subsequent clonogenic survival, (C) 1997 Elsevier Science Inc.