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 SeO₄(²⁻) (2μM) 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±2.5pS, 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+/SeO₄(²⁻) 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.