Abstract
Ca2+, Mg2+, and K+ activities in red beet (Beta vulgaris L.) vacuoles were evaluated using conventional ion-selective microelectrodes and, in the case of Ca2+, by non-invasive ion flux measurements (MIFE) as well. The mean vacuolar Ca2+ activity was ∼0.2 mM. Modulation of the slow vacuolar (SV) channel voltage dependence by Ca2+ in the absence and presence of other cations at their physiological concentrations was studied by patch-clamp in excised tonoplast patches. Lowering pH at the vacuolar side from 7.5 to 5.5 (at zero vacuolar Ca2+) did not affect the channel voltage dependence, but abolished sensitivity to luminal Ca2+ within a physiological range of concentrations (0.1-1.0 mM). Aggregation of the physiological vacuolar Na + (60 mM) and Mg2+ (8 mM) concentrations also results in the SV channel becoming almost insensitive to vacuolar Ca2+ variation in a range from nanomoles to 0.1 mM. At physiological cation concentrations at the vacuolar side, cytosolic Ca2+ activates the SV channel in a voltage-independent manner with Kd=0.7-1.5 μM. Comparison of the vacuolar Ca2+ fluxes measured by both the MIFE technique and from estimating the SV channel activity in attached patches, suggests that, at resting membrane potentials, even at elevated (20 μM) cytosolic Ca 2+, only 0.5% of SV channels are open. This mediates a Ca 2+ release of only a few pA per vacuole (∼0.1 pA per single SV channel). Overall, our data suggest that the release of Ca2+ through SV channels makes little contribution to a global cytosolic Ca2+ signal.
Original language | English |
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Pages (from-to) | 3845-3855 |
Number of pages | 11 |
Journal | Journal of Experimental Botany |
Volume | 59 |
Issue number | 14 |
DOIs | |
Publication status | Published - Oct 2008 |
Externally published | Yes |