Role of extracellular calcium in the hyperthermic killing of CHL V79 cells

Two major questions are addressed by this study: Can an influx of calcium ion sensitize CHL V79 cells to hyperthermia, and, if so, does this occur during heating and does it play a crucial role in cell death? V79 cells are sensitized to hyperthermia by the calcium ionophore A23187 which also induces an influx of calcium at both 37 and 43 degrees C. Sensitization is at least partially dependent on the presence of extracellular calcium. In the absence of A23187, survival is independent of calcium concentration (from 0 to 25 mM) during heating, which differs from the behavior of hepatocytes which are sensitized to hyperthermia by 15 mM CaCl2. Calcium influx, as assayed by uptake of 45Ca measured after washing in LaCl3, is detectable in 3 mM CaCl2 only after 30 min at 45 degrees C, an exposure which reduces reproductive survival to less than 0.1%. Calcium uptake reaches 6 nmol/10(6) cells after 180 min at 45 degrees C. This is not due to a general loss of membrane permeability since there is no trypan blue staining during this time. In 15 mM CaCl2, influx occurs earlier (15 min) but still succeeds the loss of reproductive survival which is less than 1% at this time. Uptake is much higher in 15 mM CaCl2, reaching 10 nmol/10(6) cells by 30 min and 25 nmol/10(6) cells at 180 min, but the temporal pattern of uptake does not correlate with loss of reproductive survival. Thus, although A23187 sensitizes V79 cells to hyperthermia, probably by increased influx of calcium ion, and increased influx occurs during exposure to 45 degrees C, influx is not a crucial early event in the killing of V79 cells. This does not eliminate the possibility of intracellular calcium redistribution during hyperthermia.