RESUMO
The impact of low-dose ionizing radiation on the electrical signalling pattern and membrane properties of the characea Nitellopsis obtusa was examined using conventional glass-microelectrode and voltage-clamp techniques. The giant cell was exposed to a ubiquitous radionuclide of high biological importance - tritium - for low-dose irradiation. Tritium was applied as tritiated water with an activity concentration of 15 kBq L(-1) (an external dose rate that is approximately 0.05 µGy h(-1) above the background radiation level); experiments indicated that this was the lowest effective concentration. Investigating the dynamics of electrical excitation of the plasma membrane (action potential) showed that exposing Characeae to tritium for half an hour prolonged the repolarization phase of the action potential by approximately 35%: the repolarization rate decreased from 39.2 ± 3.1 mV s(-1) to 25.5 ± 1,8 mV s(-1) due to tritium. Voltage-clamp measurements showed that the tritium exposure decreased the Cl(-) efflux and Ca(2+) influx involved in generating an action potential by approximately 27% (Δ = 12.4 ± 1.1 µA cm(-2)) and 64% (Δ = -5.3 ± 0.4 µA cm(-2)), respectively. The measured alterations in the action potential dynamics and in the chloride and calcium ion transport due to the exogenous low-dose tritium exposure provide the basis for predicting possible further impairments of plasma membrane regulatory functions, which subsequently disturb essential physiological processes of the plant cell.
