Characterization of a nitrate-permeable channel able to mediate sustained anion efflux in hypocotyl cells from Arabidopsis thaliana

We have characterized a new anionic current in Arabidopsis hypocotyl cells. This current, activated by membrane depolarization, has slow activation and deactivation kinetics in the 10 sec range. It presents many distinct properties from the rapid-type anion current already described on the same membrane. The slow-type channel is highly permeable to nitrate with a P-NO3(-)/P-Cl(-) close to 20, but totally impermeable to sulphate. Activation of the channel requires cytosolic ATP and the slow current is partially inhibited by staurosporin, suggesting that channel regulation involves protein phosphorylation. The slow anion channel displays a unique pharmacological profile different from that of the rapid channel: the slow channel is inhibited by DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid) with an IC50 of 26 mu M. The slow and rapid anion channels are probably dedicated to specific functions: the first is able to mediate sustained anion efflux, while the second is a good candidate to be involved in fast electrical signalling.