[The effect of light, temperature, and external medium upon the electrical behaviour of Acetabularia crenulata].
1. In sea water at 25°C cells of Acetabularia crenulata exhibit a resting potential (RP) of-170 mV between cytoplasm and external medium. At temperatures below 10°C, or upon addition of 10(-3)m dinitrophenol in darkness, the cell shows a second steady potential (RP') of about-70 mV. Among the cations of sea water, i.e. K(+), Na(+), Mg(++), only K(+) was found to affect RP and RP'. If the ionic strength of the medium is reduced by addition of isotonic mannitol solution, RP decreases, while RP' is not influenced. RP' is explained as a potassium diffusion potential, while for the existence of RP an electrogenic chloride pump is inferred which is driven by ATP of the photo- or oxidative phosphorylation (system X).-2. Starting from RP', the current-voltage relationship consists of two linear portions for inward (R e ) and outward current (R a ), respectively, merging at RP' (Fig. 3). Presumably they represent potassium conductances. For a given cell, the expression RT/F ln R e /R a yields a value which fits the RP' of the cell (Fig. 20).-3. Starting from RP, a N-shaped current-voltage relationship was obtained for depolarisation (Fig. 3). The deviation from the potassium conductance is supposed to be due to the shunt of the potassium channel and the system X (voltage-dependent resistance). An electric circuit diagram was derived from voltage and current clamp experiments (Fig. 21); the elements of the circuit were tentatively analogized with cell functions.-4. Action potentials of about 120 mV, lasting from 30 to 300 sec may arise spontaneously. They can be triggered by lowering the temperature or depolarisation (voltage clamp, current clamp, light-off-cf. Figs. 2,11). The mechanism of the action potential can be derived from the properties of the chloride pump. Action currents were recorded upon different depolarizing steps by voltage clamp to yield current-coltage curves at different times after stimulation (Fig. 13).-5. Pulses of white light shift the potential off RP': light-on elicits a small depolarisation, light-off a large transient hyperpolarisation. The primary event of this response is a change of current (Fig. 19), the voltage change being its consequence. This result is interpreted on the basis of the circuit diagram.
Institut für Biologie, Abteilung Biophysik, der Universität Tübingen, Auf der Morgenstelle 1, D-7400, Tübingen, Deutschland.
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Gradmann