CHARACTERIZATION OF INTRACELLULAR CALCIUM POOLS AND THEIR DESENSITIZATION IN THERMOTOLERANT HUMAN A-431 CELLS

Background: This study characterizes the intracellular Ca2+ pools in n onthermotolerant and thermotolerant human A-431 cells and the reduced cytotoxicity using the inhibitors of Ca2+ mobilizations. Methods: Nont hermotolerant and thermotolerant cells were treated with different Ca2 + mobilizers in the absence of external Ca2+. The cytosolic Ca2+ conce ntration using fura-2 fluorescence probe was measured to identify the presence of intracellular Ca2+ pools. The cytoxicity of the increase i n [Ca2+](i) was studied using the colony forming efficiency assay. Res ults: The resting intracellular Ca2+ concentration ([Ca2+](i)) in the absence of extracellular Ca2+ was 42 +/- 9 nm, determined by fura-2. B radykinin (10 mu mol/L), monensin (200 mu mol/L), and ionomycin (1 mu mol/L) sequentially treated to cells mobilized Ca2+ and increased [Ca2 +](i) by 64 +/- 23, 40 +/- 6, and 59 +/- 21 nm, respectively. The brad ykinin effect was blocked by 5 mu mol/L U-73122 (an inhibitor of inosi tol trisphosphate production); the ionomycin effect was inhibited by i ncreasing intracellular pH (pH(i)) or treatment with 100 mu mol/L ryan odine while the monensin effect was enhanced by increasing pH(i), but was not inhibited by ryanodine. Cells that were made tolerant to letha l temperatures also responded to bradykinin, monensin, and ionomycin, but the magnitude of the response was diminished. Subsequent treatment s with bradykinin, monensin, and ionomycin increased [Ca2+](i) in ther motolerant cells to levels 68 +/- 8, 44 +/- 5, and 45 +/- 5%, respecti vely, of values found in nonthermotolerant cells. Higher concentration s of these agents did not further increase [Ca2+](i). The bradykinin-i nduced increase in inositol trisphosphates in thermotolerant cells was also reduced, which perhaps accounts for the attenuation in Ca2+ mobi lization. Unlike nonthermotolerant cells, the monensin effect was not enhanced when pH(i) was increased. However, the ionomycin effect was s till dependent on pH(i) and was blocked by ryanodine at a higher conce ntration.Conclusions: These results show that there are bradykinin-, m onensin-, and ryanodine-sensitive pools and that thermotolerance atten uates Ca2+ mobilization stimulated by these three agents. Ionomycin at 10 mu mol/L or NaCN at 10 mM for 1 hour demonstrated cytotoxicity. Pr etreatment with 100 mu mol/L ryanodine and/or 5 mu mol/L U-73122 reduc ed cytoxicity produced by either NaCN or ionomycin. These results sugg est that an attenuation of [Ca2+](i) increases can diminish cytotoxici ty