Electrochemical Potential Gradients of H+, K+, Ca2+, and Cl- across the Tonoplast of the Green Alga Eremosphaera Viridis

Abstract

Using ion-selective microelectrodes, we measured the activity of H+, K+, CaZ+, and CI-and the electrical potential both in the vacuole and in the cytoplasm of the unicellular green alga €remo-sphaera viridis to obtain comparable values of the named parameters from the same object under identical conditions. l h e cytosol had a pH of 7.3, and activities of the other ions were 130 mM K+, 160 nM Ca2+, and 2.2 mM CI-. W e observed only small and transient light-dependent changes of the cytosolic ca*+ activity. The vacuolar Kf activity did not differ significantly from the cytosolic one. l h e Ca2+ activity inside the vacuole was approximately 200 ~LM, the pH was 5.0, and the CI-activity was 6.2 mM. l h e concentrations of K+, Ca*+, and CI-in cell extracts were measured by induction-coupled plasma spectroscopy and anion chromatography. l h i s confirmed the vacuolar activities for K+ and CI-obtained with ion-selective microelectrodes and indicated that approximately 60% of the vacuolar Ca2+ was buffered. The tonoplast potential was vanishingly low ( 5 2 2 mV). There was no detectable electrochemical potential gradient for K+ across the tonoplast, but there was, however, an obvious electrochemical potential gradient for CI-(-26 mV), indicating an active accumulation of CI-inside the vacuole.

Vacuolar Na + /H + antiporter cation selectivity is regulated by calmodulin from within the vacuole in a Ca 2+ - and pH-dependent manner

“…Most of the vacuolar Ca 2ϩ ions are bound to organic acids and several types of low-affinity Ca 2ϩ -binding proteins (32,33). Although the concentration of free Ca 2ϩ in higher plant vacuoles is still unknown, the concentration of vacuolar free Ca 2ϩ has been estimated to be in the micromolar range (34,35) in algae cells and between 1-2 mM in higher plant cells (36). The ability of AtCaM15 to bind Ca 2ϩ in the micromolar range would suggest that AtCaM15 could play a role as a vacuolar high-affinity Ca 2ϩ -binding protein and that, at the basal intravacuolar Ca 2ϩ concentrations, Ca 2ϩ would be constitutively bound to AtCaM15 in the cells.…”

Fast‐activating cation channel in barley mesophyll vacuoles. Inhibition by calcium

“…Published values for vacuolar free Ca 2+ concentrations range from 0.2 mM to 2.3 mM (Bethmann et aL, 1995;Felle, 1988;Miller and Sanders, 1987). Interestingly, in barley mesophyll cells, the vacuolar Ca 2+ concentration is well regulated and kept constant at 1 to 2 mM when the Ca 2+ concentration in the culture medium is changed 100-fold, from 0.3 to 30 mM (Dietz et aL, 1992).…”

“…Published data sets that include tonoplast potentials and K + gradients seem to corroborate this view. In those cases where the measured tonoplast potential is vanishingly small, vacuolar and cytosolic K ÷ concentrations do not differ significantly (Bethmann et al, 1995;Trebacz etaL, 1994). In those cases where negative tonoplast potentials were measured, ranging from -18 to -27 mV, the vacuolar K ÷ concentration is significantly smaller than the cytosolic one, and in each case the resulting driving force of the K + gradient (+22 to +24 mV) nearly completely balances the electrical driving force (Raven, 1967;Rona et aL, 1982;Vorobiev, 1967).…”

Characteristics of Anion Channels in the Tonoplast of the Liverwort Conocephalum conicum

“…10). Concentrations of 5 Â 10 À5 and 2 Â 10 À3 M free Ca 2þ in the pipet were taken to reflect the extreme values reported for vacuolar free Ca 2þ (Felle 1988, Bethmann et al 1995. In the bath, 0 ($10 À9 ), 2 Â 10 À7 , 2Â 10 À6 , 2Â 10 À5 , 2Â 10 À4 and 2 Â 10 À3 M free Ca 2þ were perfused to test different cytosolic Ca 2þ levels.…”