ldentification of High-Affinity Slow Anion Channel Blockers

Slow anion channels in the plasma membrane of guard cells have been suggested to constitute an important control mechanism for long-term ion efflux, which produces stomatal closing. ldentification of pharmacological blockers of these slow anion channels is instrumental for understanding plant anion channel function and structure. Patch clamp studies were performed on guard cell protoplasts to identify specific extracellular inhibitors of slow anion channels. Extracellular application of the anion channel blockers NPPB and IAA-94 produced a strong inhibition of slow anion channels in the physiological voltage range with half inhibition constants (KlI2) of 7 and 10 IAM, respectively. Single slow anion channels that had a high open probability at depolarized potentials were identified. Anion channels had a main conductance state of 33 f 8 pS and were inhibited by IAA-94. DIDS, which has been shown to be a potent blocker of rapid anion channels in guard cells (Kl12 = 0.2 pM), blocked less than 20% of peak slow anion currents at extracellular or cytosolic concentrations of 100 pM. The pharmacological properties of slow anion channels described here differ from those recently described for rapid anion channels in guard cells, fortifying the finding that two highly distinct types or modes of voltageand second messenger-dependent anion channel currents coexist in the guard cell plasma membrane. Bioassays using anion channel blockers provide evidence that slow anion channel currents play a substantial role in the regulation of stomatal closing. Interestingly, slow anion channels may also function as a negative regulator during stomatal opening under the experimental conditions applied here. The identification of specific blockers of slow anion channels reported here permits detailed studies of cell biological functions, modulation, and structural components of slow anion channels in guard cells and other higher plant cells.