Calcium Activation of Potassium Channels in the Plasmalemma of Eremosphaera viridis

The role of cytoplasmic calcium activity in activation of K⁺-channelsin the unicellular green alga Eremosphaera viridis has beenstudied. As reported previously, after a ‘light off’signal a voltage independent opening of K⁺-channels in the plasmalemmais observed. This effect is indicated by a transient polarization(TP) with a simultaneous increase of the membrane conductance.TPs can also be triggered by different treatments, which allowinvestigations within a ‘short-circuited’ signalchain. (i) After incubation with EGTA a single extended TP canbe released by a sudden increase of the external calcium concentration.The Ca²⁺-channel inhibitors nifedipine (10 ^–2 mol m^–3)and verapamil (5 ? 10^–2 mol m^–3) suppress the releaseof this TP. (ii) In the presence of external calcium the additionof the ionophore A23187 GenBank] (10–3 mol m^–3) causes anextremely prolonged TP. (iii) Low external concentrations ofbarium (10^–2 mol m^–3) induce repetitive TPs in thepresence of external calcium. In this case the Ca²⁺-channelinhibitors are less effective. (iv) Strontium (0.1–1.0mol m^–3) is able to trigger repetitive TPs even withoutexternal calcium. Whereas barium may stimulate a calcium influx,strontium can serve as a substitute for calcium to induce anopening of K⁺-channels. These results indicate strongly a Ca²⁺-dependentand voltage-independent activation of K⁺-channels in the plasmalemmaof Eremosphaera. The participation of cytoplasmic calcium inthe signal transduction chain after a ‘light off’signal is discussed. Key words: Ca²⁺-dependent K⁺-channels, Ca²⁺-channel effectors, A23187, transient membrane potential, Eremosphaera