MEMBRANE POTENTIAL-DEPENDENT CALCIUM-TRANSPORT IN RIGHT-SIDE-OUT PLASMA-MEMBRANE VESICLES FROM ZEA-MAYS L ROOTS

Right-side-out plasma membrane Vesicles isolated from Zea mays roots were used to study membrane potential (Delta Psi)-dependent Ca2+ transport. Membrane potentials were imposed on the vesicles using either K+ concentration gradients and valinomycin or SCN- concentration gradients, and the size of the imposed Delta Psi was measured with [C-14]tetraphenylphosphonium. Uptake of Ca-45(2+) into the vesicles was stimulated by inside-negative Delta Psi. The rate of transport increased to a maximum at a Delta Psi of about -80 mV and then declined at more negative Delta Psi. When extravesicular Ca2+ concentration was varied, uptake was maximal in the range 100-200 mu M Ca2+. Neither dihydropyridine nor phenylalkylamine Ca2+ channel blockers had any effect on Ca2+ uptake but 30 mu M ruthenium red was completely inhibitory with half maximal inhibition at 10-15 mu M ruthenium red. Calcium transport was also inhibited by inorganic cations. Zn2+, Gd3+ and Mg2+ inhibited by a maximum of 30% while La3+, Nd3+ and Mn2+ inhibited by 70%. The inhibitory effects of La3+ and Gd3+ were additive. Lanthanum-insensitive Ca2+ tive Ca2+ transport was totally inhibited by 80 mu M Gd3+ and showed maximum activity at a Delta Psi of -80 mV, with less uptake at both higher and lower Delta Psi. Lanthanum and Gd3+ also inhibited Ca2+ uptake into protoplasts isolated from Zea roots and their individual and combined effects were similar in extent to those observed with plasma membrane vesicles. It is concluded that maize root plasma membrane contains two Ca2+-permeable channels that can be distinguished by their susceptibility to inhibition by La3+ and Gd3+. Both are inhibited by ruthenium red but not by other organic Ca2+ channel blockers.