[Increase in Ca(2+)-permeability of the plasma membrane during radiation-induced apoptosis of thymocytes]

The parameters of Ca(2+)-permeability (rate of Ca2+ influx in the presence of Ca(2+)-ATPase blocker vanadate and initial rate of Ca2+ uptake) were determined in the rat thymocytes after induction of apoptosis in vitro by gamma (gamma-irradiation at 5 Gy. There were no changes in the rate of Ca(2+)-influx within 30 to 909 min after irradiation, thus suggesting passive membrane permeability, while the initial rate of Ca(2+)-uptake, which characterizes Ca(2+)-exchange, approximately doubled in the irradiated cells. Thereafter, the initial rate of Ca(2+)-uptake in the intact and irradiated thymocytes became equal, while the rate of Ca(2+)-influx in the irradiated cells increased reliably 1.5-2 times within 90-180 min. Modification of membrane permeability for Ca2+ in the irradiated thymocytes precedes initiation of DNA degradation, and passive permeability is increasingly disturbed as apoptosis progresses. These data suggest that disturbances in Ca2+ passive permeability and intracellular Ca2+ homeostasis are an apoptosis trigger in lethally irradiated thymocytes.