Synergistic action of calcium-ionophores and hyperthermia is best interpreted as thermal enhancement of calcium toxicity.

It has been shown that no relation exists between [Ca2+]i and hyperthermic cell killing, although heat-induced increase of [Ca2+]i can be observed in some cell lines. When ionophores are used, dose-dependent rises in [Ca2+]i may be found. Beyond a certain threshold of ionophore-induced increases in [Ca2+]i, cells may be killed. Different threshold levels of [Ca2+]i exist in different cell lines. Hyperthermia can act synergistically with calcium ionophores to potentiate cell killing. Since there is no causal relation between [Ca2+]i and heat toxicity, this synergism can be explained as heat enhanced Ca2+ toxicity. In the current report, it is shown that both ionophore-induced Ca2+ toxicity (37 degrees C) and its potentiation by heat are dependent on extracellular calcium and related to sustained increases in [Ca2+]i. With ionomycin concentrations up to 15 microM, no increase in [Ca2+]i was seen in cells maintained in medium without Ca2+. Ionomycin effects on intracellular compartments were absent, and the drug seemed to act solely on the level of the plasmamembrane. Also, the synergism of heat and ionomycin appeared to act at the plasmamembrane, because depletion of extracellular calcium completely abolished this synergistic effect. The data presented are also discussed in the light of controversies existing in the literature for the role of calcium in hyperthermic cell killing.