CYTOTOXIC HYPERTHERMIA AND CA-2+ HOMEOSTASIS - THE EFFECT OF HEAT ON CA-2+ UPTAKE BY NONMITOCHONDRIAL INTRACELLULAR CA-2+ STORES

The effect of cytotoxic hyperthermia on Ca2+ transport by intracellular, nonmitochondrial Ca2+ stores of the human colon cancer cell line, HT-29, was studied using cells permeabilized with saponin. Saponin treatment permitted equilibration of the cytosol with a defined extracellular medium consisting of an intracellular-like ionic composition, ATP qand ATP-regenerating system, and Ca2+/EGTA buffers to adjust the free [Ca2+]. Under the conditions employed, ATP-dependent Ca2+ uptake in saponin-permeabilized cells was demonstrated to be exclusively due to nonmitochondrial Ca2+ stores, e.g., endoplasmic reticulum or calciosomes. Heat treatment for 120 min at 44.5°C sufficient to kill 80% of the cells inhibited ATP-dependent Ca2+ uptake by 50% in terms of rate and total Ca2+ accumulated. With cells made thermotolerant by either arsenite or heat treatment 24 h prior to challenge heating. ATP-dependent Ca2+ uptake was resistant to a second equivalent heat dose. Efflux of Ca2+ from saponin-permeabilized cells when measured at 37°C was unaffected by a prior heat treatment (44.5°C for 120 min).