Extracellular protons inhibit the activity of inward-rectifying potassium channels in the motor cells of Samanea saman pulvini

The intermittent influx of K+ into motor cells in motor organs (pulvini) is essential to the rhythmic movement of leaves and leaflets in various plant s, but in contrast to the K+ influx channels in guard cells, those in pulvi nar motor cells have not yet been characterized. We analyzed these channels in the plasma membrane of pulvinar cell protoplasts of the nyctinastic leg ume Samanea saman using the patch-clamp technique. Inward, hyperpolarizatio n-activated currents were separated into two types: time dependent and inst antaneous. These were attributed, respectively, to K+-selective and distinc tly voltage-dependent K-H channels and to cation-selective voltage-independ ent leak channels. The pulvinar K-H channels were inhibited by external aci dification (pH 7.8-5), in contrast to their acidification-promoted counterp arts in guard cells. The inhibitory pH effect was resolved into a reversibl e decline of the maximum conductance and an irreversible shift of the volta ge dependence of K-H channel gating. The leak appeared acidification insens itive. External Cs (10 mm in 200 mm external K+) blocked both current types almost completely, but external tetraethylammonium (10 mm in 200 mm extern al K') did not. Although these results do not link these two channel types unequivocally, both likely serve as K' influx pathways into swelling pulvin ar motor cells. Our results emphasize the importance of studying multiple m odel systems.