[Increased plasma membrane permeability for Ca2+ in radiation-induced thymocyte apoptosis]

Parameters of the Ca2+ permeability (the 45Ca influx rate in the presence of orthovanadate blocking the Ca(2+)-ATPhase, and the initial rate of the 45Ca uptake) and the DNA fragmentation were determined in rat thymocytes after gamma-irradiation-induced (with a dose of 5 Gy) apoptosis. Within the first 30-90 min after irradiation, the 45Ca influx rate that is characteristic of the membrane passive permeability remained unchanged. The initial rate of the 45Ca uptake that is characteristic of the Ca2+ exchange almost doubled. In 90-180 min, the rate of the 45Ca influx in the irradiated cells increased 1.5 to 2.0 times. An increase in the DNA fragmentation was observed in 90-120 min after irradiation of thymocytes; in 3 h, it developed up to 50%. Thus, modification of the membrane Ca2+ permeability in the irradiated thymocytes precedes a stage of initiation of the DNA degradation. A degree of disturbance of the membrane passive permeability increases as the thymocyte apoptosis progresses. The obtained data suggest that disturbance of the passive Ca2+ permeability and the intracellular Ca2+ homeostasis are responsible for the apoptosis of lethally irradiated thymocytes.