VACUOLAR ION CURRENTS IN THE PRIMITIVE GREEN-ALGA EREMOSPHAERA-VIRIDIS - THE ELECTRICAL-PROPERTIES ARE SUGGESTIVE OF BOTH THE CHARACEAE AND HIGHER-PLANTS

The electrical properties of the vacuolar membrane of the primitive green alga Eremosphaera virdis were investigated using the patch-clamp technique. In whole vacuole measurements two types of transport systems with long activation time-constants were identified. The first, showing marked outward rectification, was activated by an increase in the cytosolic calcium concentration. Furthermore, it displayed sensitivity to micromolar concentrations of the anion channel blocker Zn2+ and to acidification of the cytosol. In contrast, the second time-activated current component was almost insensitive to changes in cytosolic pH and was blocked by the potassium channel inhibitor TEA. In addition to these slowly activating current components, the vacuolar membrane contained at least two further transport systems, responsible for an instantaneous current. These two current components were distinguished by their different sensitivity to protons, cytosolic calcium, and TEA. Comparing these electrical properties to those observed in vacuoles of higher plants or in cytoplasmic droplets from characean algae, respectively, it seems that Eremosphaera is intermediate, corresponding to the systematic position of this simple green alga.