Modification of Rat Thymocyte Membrane Properties by Hyperthermia and Ionizing Radiation

Summary

Thymocytes are one of the most widely used cell models for the study of radiation-induced interphase death. This cell-type was chosen for the study of hyperthermic and radiation effects on two membrane-related processes implicated in the interphase death of cells: Na⁺-dependent 2-aminoisobutyric acid (AIB) transport and cyclic 3′–5′ adenosine monophosphate formation.

The response of AIB transport to heat is dose-dependent, but the biphasic thermal response curve (AIB uptake versus time) differs from the sigmoidal radiation response curve. Heating thymocytes for 20–30 min at 43°C stimulates AIB uptake. Additional heating at 43°C, however, markedly reduces AIB uptake. Despite the immediate stimulating effect of heat (30 min at 43°C), the thymocyte has already developed irrepairable impairments, as demonstrated by the fractionated heating experiments. The heat-induced impairment of AIB uptake is mainly on the Na⁺-dependent component of neutral amino-acid transport, affecting primarily the maximal rate of uptake, i.e. V_max.

Additional evidence for heat-induced plasma membrane damage is the alteration in cAMP levels. Heating thymocytes for 30 min or longer at 43°C causes a massive rise in cAMP level within the cell. This differs from thymocytes exposed to radiation where no rise in cAMP is observed.