Characterization of Passive Ion Transport in Plasma Membrane Vesicles of Oat Roots

The passive influx and efflux of inorganic ions across plasma membrane vesicles purified from extracts of Avena sativa roots were investigated. Uptake was measured by incubating the vesicles in a radioisotope for various times. The "loaded" vesicles were separated from the external solution by gel filtration. Efflux was measured by dialyzing the preloaded vesicles.Ion transport was differentiated from superficial ion binding by (a) the time course of association of radioisotope with the vesicles; (b) the rate of loss of radioisotope from the vesicles; (c) the linear increase in isotope associated with the vesicles as the external concentration was increased; (d) the enhanced loss of radioisotope from the vesicles induced by Triton X-100; and (e) the low amount of isotope associated with the vesicles at low temperatures.The plasma membrane vesicles were differentially permeable to the alkali cations with the order of decreasing permeation being K(+) > Rb(+) > Cs(+) > Na(+) > Li(+). The relative transport of Rb(+), Na(+), and Cl(-) across the plasma membrane vesicles was about 1.0:0.50:0.18. The permeability coefficient (P) for Rb(+) was estimated to be 0.29 +/- 0.15 x 10(-8) cm/sec.ATP (and ADP) decreased the passive uptake of Rb(+) into the vesicles, however, this effect did not appear to be related to the ATPase of the plasma membrane.