THERMOSENSITIZATION AND MODIFICATION OF CYTOSOLIC CALCIUM-CONCENTRATION BY VERAPAMIL AND DILTIAZEM IN MOUSE MAMMARY-CARCINOMA CELLS

Purpose: The cardinal role of Ca2+ signaling in the development of heat damage at 42 degrees C and 44 degrees C and the effect of verapamil acid diltiazem on the change of cytosolic Ca2+ concentration, [Ca2+](c) were investigated. Methods and Materials: Hyperthermic treatment was performed by immersing the tubes containing the mouse mammary carcinoma FM3A tells in a water-bath set at 42.0 degrees C or 44.0 degrees C. The [Ca2+](c) of a single cell after hyperthermia was monitored by a digital image analyzing technique using Fura-2. Influx of Ca2+ was examined by the measurement of the radioactivity of Ca-45(2+) incorporated into the tells for 60 min during or after hyperthermia. Results: While the [Ca2+](c) of the cells treated at 37 degrees C for 60 min was less than 230 nM, the percentage of the cells showing more than 230 nM increased after 42 degrees C hyperthermia and that of the cells showing more than 300 nM increased after 44 degrees C hyperthermia. When cellular uptake of Ca-45(2+) during and after hyperthermia was examined, the increase in Ca-45(2+) uptake was observed only for 44 degrees C hyperthermia. Verapamil or diltiazem (100 mu M) enhanced a delay of cell growth, an increase in [Ca2+](c) and an increase in Ca-45(2+) influx for 42 degrees C and 44 degrees C hyperthermia. Conclusion: These results are compatible with the view that the increase in [Ca2+](c) after 42 degrees C hyperthermia results from intracellular release from calcium store sites whereas the rise in [Ca2+](c) after 44 degrees C hyperthermia is mainly due to entry of extracellular Ca2+. Verapamil or diltiazem combined with hyperthermia increased [Ca2+](c), which may play a cardinal role in thermosensitization by these agents.