HEAT-SHOCK INCREASES CYTOSOLIC FREE CA2+ CONCENTRATION VIA NA+-CA2+ EXCHANGE IN HUMAN EPIDERMOID-A-431 CELLS

This study characterized cytosolic free Ca2+ concentration ([Ca2+]i) in normal and thermally injured human epidermoid A 431 cells. The resting [Ca2+]i in normal cells at 37-degrees-C was 87 +/- 5 nM (n = 105). When cells were subjected to hyperthermia (40-50-degrees-C), [Ca2+]i increased in a temperature- and time-dependent manner. The maximal increase in cells exposed to 45-degrees-C was observed at 20 min; [Ca2+] i returned to normal within 1 h. The heat-induced [Ca2+]i increase depended on the presence of external Ca2+. La3+ and Cd2+ but not Co2+, verapamil, or nifedipine attenuated the heat-induced [Ca2+]i increase. TMB-8 partially blocked the increase in [Ca2+]i but pertussis toxin and cholera toxin pretreatment did not. The magnitude of the heat-induced [Ca2+]i increase or Ca-45(2+) uptake depended on the presence of extracellular Na+. Heat treatment reduced the apparent Michaelis constant for external Ca2+ from 490 +/- 91 to 210 +/- 60-mu-M, whereas the maximal velocity remained the same. The intracellular Na+ concentration decreased 62.5% after heating. The heat-induced [Ca2+]i increase was completely blocked by amiloride (5-mu-M) and 5'-(N,N-dimethyl)-amiloride (1-mu-M). These results suggest heat activates the Na+-Ca2+ exchange system so as to increase [Ca2+]i and reduce [Na+]i.