DIRECT EFFECTS OF CA2+-CHANNEL BLOCKERS ON PLASMA-MEMBRANE CATION CHANNELS OF AMARANTHUS-TRICOLOR PROTOPLASTS

Ca2+-channel blockers at concentrations greater than 1 mmol m-3, directly affect the activity of K+-selective channels in the plasma membrane of Amaranthus tricolor protoplasts. These effects are not mediated by the blockade of Ca2+ channels. Blockers tested included 1,4-dihydropyridines (nifedipine, nicardipine), verapamil, bepridil, Gd3+ and La3+, applied to whole-cell and detached outside-out patches of plasma membrane at concentrations from 50 mumol m-3 to 100 mmol m-3. For certain experiments the concentration of Ca2+ on the cytoplasmic side of the plasma membrane ([Ca2+]cyt) was buffered at either 50 mumol m-3 or 500 mumol m-3. The principal currents observed in whole-cells flowed through cation outward rectifier (OR) channels. Each blocker caused an immediate reduction of time-dependent outward currents at doses down to 1 mmol m-3 and produced a different, reversible, kinetic block of the outward current, independent of the level of [Ca2+]cyt. Verapamil also activated a sustained inward cation current at negative p.d. The same effects were found with individual channels in detached outside-out patches. Conductance and selectivity of the cation OR channels were unchanged by the drugs. [Ca2+]ext was varied over a range from 0 to 10 mol m-3. Progressively lower [Ca2+]ext, increasingly enhanced the maximum amplitude of the time-dependent currents. Time-constants for decay of inward tail currents were increased at low [Ca2+]ext. These effects were rapidly reversible. Although there was no evidence that the cation ORs in plasma membrane of Amaranthus tricolor were dependent on [Ca2+]cyt for their activation, they were sensitive to the concentration of free Ca2+ in the extracellular medium.