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

The electrical properties of the vacuolar membrane of the primitive gr een alga Eremosphaera virdis were investigated using the patch-clamp t echnique. In whole vacuole measurements two types of transport systems with long activation time-constants were identified. The first, showi ng marked outward rectification, was activated by an increase in the c ytosolic 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 c urrent component was almost insensitive to changes in cytosolic pH and was blocked by the potassium channel inhibitor TEA. In addition to th ese slowly activating current components, the vacuolar membrane contai ned at least two further transport systems, responsible for an instant aneous current. These two current components were distinguished by the ir different sensitivity to protons, cytosolic calcium, and TEA. Compa ring these electrical properties to those observed in vacuoles of high er plants or in cytoplasmic droplets from characean algae, respectivel y, it seems that Eremosphaera is intermediate, corresponding to the sy stematic position of this simple green alga.