Cadmium and selenium modulate slow vacuolar channels in rape (Brassica napus) vacuoles

Currents flowing through slow vacuolar SV channels of rape (Brassica napus) growing on media supplemented with Cd2+ (400 μM), and/or SeO42− (2 μM) were examined. The aim of the study was to investigate the role of Cd2+ in modulation of SV channel activity and to determine whether Se reverses the effect of cadmium. Vacuoles were isolated using a quick surgical method to avoid application of any cell wall-degrading enzymes. Vacuoles of rape exhibited typical SV channel activity with slow activation at positive potentials and strong rectification into the vacuolar lumen. Single-channel conductance in cytoplasm-side-out tonoplast patches ranged between 68.8 ± 1.9 pS in the control, 80.1 ± 2.5 pS, in Cd2+-, 74.2 ± 2.4 pS in Cd2+/selenate-, and 80.1 ± 1.8 pS in selenate-pretreated plants. The lack of a clear tendency was likely an effect of equilibration of the pipette solution (without Cd2+/SeO42−) with that of the luminal side of the vesicles. In the vacuole-attached configuration, in which natural vacuolar solution was not exchanged, there was a significant reduction in single-channel conductance in the Cd2+- (40.3 ± 2.8 pS), Cd2+/selenate- (47.1 ± 2.8 pS) and selenate-pretreated (42.3 ± 1.4 pS) plants, compared to the control (60.2 ± 1.7 pS). The reduction in single-channel conductance only partially explained the significant decline in the densities of ion current flowing through the vacuolar membrane in the whole-vacuole configuration in the plants growing on Cd2+ and Cd2+/selenate media. Thus, Cd2+ accumulation in the vacuole reduced the number of active SV channels from 0.28 ± 0.05 μm−2 in the control to 0.021 ± 0.005 and 0.039 ± 0.004 μm−2 in Cd2+ and Cd2+/selenate-pretreated plants, respectively.