Hyperthermia Effects on Cytosolic [Ca2+]:Analysis at the Single Cell Level by Digitized Imaging Microscopy and Cell Survival

Digitized video-intensified fluorescence microscopy with the Ca2*- sensitive fluorescent dye fura-2 was used to measure cytosolic free Àa2* (|Ca2*|f) in HT-29 human colon cancer cells. At 37°C, the [Ca2*|, of individual cells ranged between 50 and 150 IIM.with a mean of 120 UM. Raising the temperature to 41°Cfor l h resulted in a slight reversible decrease (10-20%) in the mean |Ca2*|f. At 44°Cfor 1 h, most (>80%) cells exhibited a |Ca2+|f greater than 200 IIM. This heat-induced rise in [Ca2+]rwas not immediate but commenced after a lag time of 30 min. Postincubation at 37°Cfor 2-6 h after heating for l h at 44°Cresulted in a recovery of the basal |< ;r'*[, in some but not all cells. A linear relationship was determined between percentage of cell killing and the number of cells with |Ca2+|(of >200 nM after 37°Cpost-heating incuba tion. Manipulation of extracellular |< ;r'| between 0.1 and 10 HIMduring heating did not modify the heat-induced changes in |Ca2*)f.No significant differences in survival at 37°Cor 44°Cwere observed with cells incubated at 10, 1.0, and 0 [plus 1.0 HIMethylene glycol bis(/3-aminoethyl ether)- A', Ar, A”, V'-tetraacetic acid) mM extracellular Ca2*. The Ca2* channel blockers verapamil and nifedipine did not protect cells from heat treat ment. These results suggest that irreversible heat-induced changes in intracellular Ca2 homeostasis mechanisms may be a critical factor in heat cytotoxicity.