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 w ere used to study membrane potential (Delta Psi)-dependent Ca2+ transp ort. Membrane potentials were imposed on the vesicles using either Kconcentration gradients and valinomycin or SCN- concentration gradient s, and the size of the imposed Delta Psi was measured with [C-14]tetra phenylphosphonium. Uptake of Ca-45(2+) into the vesicles was stimulate d by inside-negative Delta Psi. The rate of transport increased to a m aximum at a Delta Psi of about -80 mV and then declined at more negati ve Delta Psi. When extravesicular Ca2+ concentration was varied, uptak e was maximal in the range 100-200 mu M Ca2+. Neither dihydropyridine nor phenylalkylamine Ca2+ channel blockers had any effect on Ca2+ upta ke but 30 mu M ruthenium red was completely inhibitory with half maxim al inhibition at 10-15 mu M ruthenium red. Calcium transport was also inhibited by inorganic cations. Zn2+, Gd3+ and Mg2+ inhibited by a max imum of 30% while La3+, Nd3+ and Mn2+ inhibited by 70%. The inhibitory effects of La3+ and Gd3+ were additive. Lanthanum-insensitive Ca2+ ti ve Ca2+ transport was totally inhibited by 80 mu M Gd3+ and showed max imum activity at a Delta Psi of -80 mV, with less uptake at both highe r and lower Delta Psi. Lanthanum and Gd3+ also inhibited Ca2+ uptake i nto protoplasts isolated from Zea roots and their individual and combi ned effects were similar in extent to those observed with plasma membr ane vesicles. It is concluded that maize root plasma membrane contains two Ca2+-permeable channels that can be distinguished by their suscep tibility to inhibition by La3+ and Gd3+. Both are inhibited by rutheni um red but not by other organic Ca2+ channel blockers.