Calcium inhibits dihydropyridine-stimulated increases in opening and unitary conductance of a plant Ca²⁺ channel

We have previously characterized the "RCA" channel (root Ca ²⁺ 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 Ca ²⁺ 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 Ca ²⁺-permeable channels.