Abstract
We have previously characterized the "RCA" channel (root Ca2+ channel), a voltage-dependent, Ca2+-permeable channel found in plasma membrane-enriched vesicles from wheat roots incorporated into artificial planar lipid bilayers. Earlier work indicated that this channel was insensitive to 1,4-dihydropyridines (DHPs, such as nifedipine and 202-791). However, the present study shows that this channel is sensitive to DHPs, but only with submillimolar Ca2+, when the probability of channel opening is reduced, with flickery closures becoming increasingly evident as Ca2+ activity decreases. Under these ionic conditions, addition of nanomolar concentrations of (+) 202-791 or nifedipine caused an increase in both the probability of channel opening and the unitary conductance. It is proposed that there is a competitive interaction between Ca2+ and DHPs at one of the Ca2+-binding sites involved in Ca2+ permeation and that binding of a DHP to one of the Ca2+-permeation sites facilitates movement of other calcium ions through the channel. The present study shows that higher plant Ca2+-permeable channels can be greatly affected by very low concentrations of DHPs and that channel sensitivity may vary with the ionic conditions of the experiment. The results also indicate interesting structural and functional differences between plant and animal Ca2+-permeable channels.