Stimulation of Action Potentials with Radiation in Single Cells of Nitella Gracilis

Summary All-or-none action potentials from single algal cells, Nitella gracilis, were stimulated with ionization radiation. Short pulses (0·1 to 2·0 sec or more) of α-particles (4/2He nuclei) were restricted to a 13 per cent segment of mature, internodal cells. Radiation intercepted cells between stimulating and recording reversible electrodes. Individual pulses of α-particles were presented to each cell to test if they could cause excitation. The electric threshold for excitation was routinely determined prior to and after radiation. Initiation of action potentials required about 1 to 2 × 105 rads/sec of α-particles, although dose-rates from 0·05 to 2·00 Mrads/sec were available. The amount of radiation necessary to trigger an action potential depended upon the radiation previously absorbed by each cell. The radiation threshold (dose and dose-rate) for excitation steadily increased as a function of the accumulated dose. Electric shocks, applied to a non-irradiated segment of each cell, revealed that the electric threshold for excitation remained unchanged until the segment being irradiated absorbed 20-23 Mrads of α-particles. Propagated action potentials were blocked when cells absorbed 28 Mrads (average) of α-particles delivered in fractionated doses. Nitella cells ruptured when given a single exposure of 4 to 5 Mrads of radiation. Rotational streaming of endoplasm of Nitella cells was observed by closed-circuit T.V. Cessation of cyclosis was induced with less radiation than required to initiate excitation.