A transient outward-rectifying K+ channel current down-regulated by cytosolic Ca2+ in Arabidopsis thaliana guard cells

Sustained (noninactivating) outward-rectifying K+ channel currents have been identified in a variety of plant cell types and species. Here, in Arabidopsis thaliana guard cells, in addition to these sustained K+ currents, an inactivating outward-rectifying K+ current was characterized (plant A-type current: I-AP). I-AP activated rapidly with a time constant of 165 ms and inactivated slowly with a time constant of 7.2 sec at +40 mV. I-AP was enhanced by increasing the duration (from 0 to 20 sec) and degree (from +20 to -100 mV) of prepulse hyperpolarization. Ionic substitution and relaxation (tail) current recordings showed that outward IAP was mainly carried by Kf ions. In contrast to the sustained outward-rectifying K+ currents, cytosolic alkaline pH was found to inhibit I-AP and extracellular K+ was required for I-AP activity. Furthermore, increasing cytosolic free Ca2+ in the physiological range strongly inhibited I-AP activity with a half inhibitory concentration of approximate to 94 nM. We present a detailed characterization of an inactivating K+ current in a higher plant cell. Regulation of I-AP by diverse factors including membrane potential, cytosolic Ca2+ and pH, and extracellular K+ and Ca2+ implies that the inactivating I-AP described here may have important functions during transient depolarizations found in guard cells, and in integrated signal transduction processes during stomatal movements.