Summary. Single cells of Chara braunii and Nitella flexiIis were placed in a microstrip exposure apparatus and subjected to isolated bursts of radiofrequency irradiation. Their electrical responses were observed both extra-and intracellularly and found to be in accordance with theoretical predictions. In particular, the cell membrane displays rectifier-like behavior up to a cutoff near 10 MHz; this cutoff implies for the principal current carriers a transit time through the membrane of roughly 50 nsec and a mobility within the membrane approximately onefifth that of potassium in free solution. An electrical response of purely thermal origin was also detected; it was separated from the athermal rectifier response on the basis of rise time and frequency dependence. This is believed to be the first instance in which (i) a biological effect of radio-frequency radiation has had its thermal and athermal components clearly separated and (ii) a primary effect of ion transit time through the membrane has been directly detected.

A giant cell (circa 10 mm long) of Chara braunii or Nitella flexilis was placed in a microstrip exposure apparatus, and the vacuolar potential at one end was monitored with a micropipette while the other end was exposed to pulses of VHF radiation at electric field strengths up to 6250 V/m. With suitable filtering and signal averaging, offsets of the vacuolar potential could be detected in real time and at levels as low as 1 microV. The only effect that has been reproducibly observed in the carrier frequency range 20-300 MHz was the slow ramp-like hyperpolarization previously reported [Pickard and Barsoum, 1981] and tentatively attributed to electromagnetic heating of the system. The slopes of these ramps became more pronounced with increasing frequency and behaved in accordance with theoretical predictions.

Summary. Electrogenic cells of Chara braunii and Nitella flexiliswere placed in a pulse-modulated radio-frequency electric field of up to 6000 V/re. Their vacuolar resting potentials were found to experience submillivolt depolarizing offsets (typically 140 ~tV at 250 kHz) which were relatively independent of temperature, increased linearly with resting potential from a zero near -210 mV, and had a cutoff (putatively due to ion transit times) near 5 MHz.By contrast, nonelectrogenic cells experienced hyperpolarizing offsets (typically 1100 gV at 250 kHz) which increased in magnitude with increasing temperature, were independent of resting potentiaI, and had a transit time cutoff near 10 MHz.The ionic mobilities inferred from these cutoff frequencies are somewhat higher than would be expected for active transport and presumably reflect passive conductance mechanisms which therefore must be presumed different for the electrogenic and nonelectrogenic states.Key words Characeae . electrogenesis 9 ion flux 9 radio-frequency bioeffects 9 rectification . transit time . fusicoccin

Single giant cells of Chara braunii and Nitella flexilis were placed in a microstrip exposure apparatus and subjected to bursts of electromagnetic radiation (carrier frequencies from 200 to 8,200 MHz) at a nominal power level of 100 W/m2. The vacuolar potential was monitored with a micropipette, and offsets as low as 1 microV could be resolved in real time by suitable filtering and signal averaging; under these conditions, no offsets of the vacuolar potential were detected. At much higher power levels (corresponding to greater than or equal to 2 V rms between microstrip and ground plane), the slow hyperpolarizing ramp reported at lower frequencies could be seen but, because of insufficient power, could not be accurately measured. It appeared to decay beyond 500 MHz and to be absent at and above 950 MHz. To investigate reports that snail neurons irradiated for 1 h at 2,450 MHz and approximately 15.5 W/kg developed lowered membrane resistivities, Characean cells were exposed in the microstrip apparatus for 1 h at 2,450 MHz and 230 W/m2; their membrane resistivities were found to be lowered about 18.5%.