Calcium channels in higher plant cells: selectivity, regulation and pharmacology

Abstract

Rapid influx of Ca²⁺ into the cytosol from extracellular pools or intracellular stores via ion channels can have wide-ranging physiological consequences. In addition, influx of Ca²⁺ across the plasma membrane is necessary for the large net accumulation of Ca²⁺ essential for cellular integrity. In this paper, the properties of Ca²⁺ channels in various plant membranes are reviewed, and compared with new results on the Ca²⁺ channel from the plasma membrane of wheat roots (rca channel) described originally by Piñeros and Tester (1995). The rca channel has been studied at the single channel level by incorporation of plasma membrane-enriched vesicles into planar lipid bilayers. It has a high affinity for Ca²⁺ permeation (K_m = 99 µM) and a maximal conductance of 30 pS. It is highly selective for Ca²⁺ over Cl⁻, but allows the movement both of other divalent cations (with a conductivity sequence: Ba²⁺ > Sr²⁺ > Ca²⁺ >Mg²⁺ > Mn²⁺) and of monovalent cations. The affinity for K⁺ permeation was 6 mM, and the maximal conductance was 164 pS. The permeability ratio, P_Ca²⁺/P_K⁺ ranged from 17 to 41, decreasing with increasing extracellular Ca²⁺. With physiologically reasonable membrane potentials and ionic conditions, the channel will catalyse Ca²⁺ influx. At normal resting potentials (negative of about −135 mV) the channel remains largely closed, but activates rapidly upon depolarization. It is insensitive to ABA and Ins 1,4,5-P₃, but the voltage-dependence for activation was shifted to more negative potentials upon addition of cytosolic ATP. The channel was inhibited by a range of trivalent cations (La₃₊, Al₃₊ and Gd₃₊) and by some organic Ca2+ channel effectors (verapamil, diltiazem, ruthenium red), although it was insensitive to bepridil and 1,4 dihydropyridines [nifedipine and (+) and (−) 202–791], at least in the conditions described here. The properties of this channel are compared with those of other plant and animal Ca²⁺ channels, and are shown to be consistent with its proposed physiological role of divalent cation uptake into roots.