Extracellular Protons Inhibit the Activity of Inward- Rectifying Potassium Channels in the Motor Cells ofSamanea samanPulvini

The intermittent influx of K+ into motor cells in motor organs (pulvini) is essential to the rhythmic movement of leaves and leaflets in various plants, but in contrast to the K+ influx channels in guard cells, those in pulvinar motor cells have not yet been characterized. We analyzed these channels in the plasma membrane of pulvinar cell protoplasts of the nyctinastic legume Samanea saman using the patch-clamp technique. Inward, hyperpolarization-activated currents were separated into two types: time dependent and instantaneous. These were attributed, respectively, to K+ -selective and distinctly voltage-dependent K(H) channels and to cation-selective voltage-independent leak channels. The pulvinar K(H) channels were inhibited by external acidification (pH 7.8-5), in contrast to their acidification-promoted counterparts in guard cells. The inhibitory pH effect was resolved into a reversible decline of the maximum conductance and an irreversible shift of the voltage dependence of K(H) channel gating. The leak appeared acidification insensitive. External Cs (10 mM in 200 mM external K+) blocked both current types almost completely, but external tetraethylammonium (10 mM in 200 mM external K+) did not. Although these results do not link these two channel types unequivocally, both likely serve as K+ influx pathways into swelling pulvinar motor cells. Our results emphasize the importance of studying multiple model systems.