CALCIUM-CHANNEL ANTAGONISTS INDUCE DIRECT INHIBITION OF THE OUTWARD RECTIFYING POTASSIUM CHANNEL IN TOBACCO PROTOPLASTS

Verapamil, nifedipine and bepridil, three antagonists of L-type calciu m channels in animal cells, were shown to induce direct inhibition of outward rectifying potassium current in Nicotiana tabacum cv. Xanthi p rotoplasts with IC50 of 5 mu M, 5 mu M and 1 mu M, respectively. In th e outside-out configuration, verapamil reduced the open probability of the ion channel responsible for the outward rectifying potassium cond uctance. Verapamil also blocked the outward rectifying potassium condu ctance in protoplasts from the N. tabacum cv. Bright Yellow cell line. Thus, studies using these molecules to demonstrate the involvement of calcium channels in plant physiological responses should be regarded with caution.