EFFECTS OF NPPB AND NIFLUMIC ACID ON OUTWARD K- CURRENTS ACROSS THE PLASMA-MEMBRANE OF WHEAT ROOT PROTOPLASTS( AND CL)

We have tested the effects of the reported Cl- channel blockers NPPB ( 5-nitro-2-(3-phenylpropylamino)benzoic acid) and niflumic acid (triflu oromethyl-3-phenylamino-2-nicotinic acid) on the slow-activating K+ (I -K,I-out) and fast-activating Cl- (I-Cl,I-out) Outward rectifying curr ents across the plasma membrane of wheat root protoplasts. Both compou nds display a greater potency against I-K,I-out (half maximal inhibiti on (IC50) values of 8.7 mu M for NPPB and 5.3 mu M for niflumic acid w ith [K+](0) = 10 mM) than against I-Cl,I-out (IC50 values of 150 and 1 00 mu M respectively). Single-channel studies using detached patches s how that these blockers reduce the percentage of time that I-K,I-out c hannels are in their open state. The data show that caution is necessa ry when using these compounds on intact cells to ascribe physiological functions to anion channels in plant membranes.