Influence of rate of heating on thermosensitivity of L1210 leukemia: membrane lipids and Mr 70,000 heat shock protein.

We examined the effect of rate of temperature rise on the thermosensitivity of a murine lymphoblastic leukemia. L1210 cells suspended in RPMI 1630 medium:5% fetal bovine serum at pH 7.4 were heated from 37 degrees C-42 degrees C, or 44 degrees C over variable times (immediately, 30, 60, 120, 180 min) in a circulating water bath controlled by an electronic temperature programmer. Survival of the cells using a soft agar clonogenic assay was plotted against the time at final temperature so that a Do (min of heat required to reduce survival by 63% on the exponential portion of the survival curve) could be calculated as an estimate of thermosensitivity. Cells heated from 37 degrees C-42 degrees C over a time period of 30 min (10 degrees C/h) were less thermosensitive (Do 62.7 +/- 12.5 min) as compared to those exposed immediately to 42 degrees C (Do 38.5 +/- 2.2 min). Cells heated over a period of 180 min (1.6 degrees C/h) showed almost no death even after 4 h at 42 degrees C. Thermosensitivity of cells heated to several other high temperatures was also a function of rate of heating. This relative thermal resistance induced by slow heating was not a result of a change in membrane cholesterol content or fatty acid composition. Similarly, there was no difference between cells heated at slow and fast rates in cell cycle distribution or in cellular protein concentration. The major heat shock protein of Mr 70,000, which was induced by immediate heating, was not synthesized at the same high rate 1-12 h after heat treatment by the cells made thermotolerant with slow heating. We conclude that the thermosensitivity of this neoplastic cell can be altered considerably by the rate of heating. This alteration is not due to a change in membrane lipids. Furthermore, the heat shock protein at Mr 70,000 which was synthesized after immediate heating could not be demonstrated in the gradually heated L1210 leukemia cells.