Direct Effects of Ca2+-Channel Blockers on Plasma Membrane Cation Channels of Amaranthus tricolor Protoplasts

Ca²⁺-channel blockers at concentrations greater than 1 mmolm^–3, directly affect the activity of K ⁺selective channelsin the plasma membrane of Amaranthus tricolor protoplasts. Theseeffects are not mediated by the blockade of Ca²⁺ channels. Blockers tested included 1, 4-dihydropyridines (nifedipine,nicardipine), verapamil, bepridil, Gd³⁺ and La³⁺, applied towhole-cell and detached outside-out patches of plasma membraneat concentrations from 50µmol m^–3 to 100 mmol m^–3.For certain experiments the concentration of Ca²⁺ on the cytoplasmicside of the plasma membrane (Ca²⁺]cyt) was buffered at either50ftmol m^–3 or 500 µmol m^–3. The principal currents observed in whole-cells flowed throughcation outward rectifier (OR) channels. Each blocker causedan immediate reduction of time-dependent outward currents atdoses down to 1 mmol m^–3 and produced a different, reversible,kinetic block of the outward current, independent of the levelof Ca²⁺]cyt. Verapamil also activated a sustained inward cationcurrent at negative p.d. The same effects were found with individualchannels in detached outside-out patches. Conductance and selectivityof the cation OR channels were unchanged by the drugs. Ca²⁺]ex, was varied over a range from 0 to 10 mol m^–3.Progressively lower Ca²⁺]eI, increasingly enhanced the maximumamplitude of the time-dependent currents. Time-constants fordecay of inward tail currents were increased at low Ca²⁺]eit.These effects were rapidly reversible. Although there was noevidence that the cation ORs in plasma membrane of Amaranthustricolor were dependent on Ca²⁺]cyl for their activation, theywere sensitive to the concentration of free Ca²⁺ in the extracellularmedium. Key words: Verapamil, blocker, cation channels, Amaranthus, protoplasts