Effects of NPPB and niflumic acid on outward K+ and Cl- currents across the plasma membrane of wheat root protoplasts

We have tested the effects of the reported Cl- channel blockers NPPB (5-nitro-2-(3-phenylpropylamino)benzoic acid) and niflumic acid (trifluoromethyl-3-phenylamino-2-nicotinic acid) on the slow-activating K+ (I-K,I-out) and fast-activating Cl- (I-Cl,I-out) Outward rectifying currents across the plasma membrane of wheat root protoplasts. Both compounds display a greater potency against I-K,I-out (half maximal inhibition (IC50) values of 8.7 mu M for NPPB and 5.3 mu M for niflumic acid with [K+](0) = 10 mM) than against I-Cl,I-out (IC50 values of 150 and 100 mu M respectively). Single-channel studies using detached patches show that these blockers reduce the percentage of time that I-K,I-out channels are in their open state. The data show that caution is necessary when using these compounds on intact cells to ascribe physiological functions to anion channels in plant membranes.