Auxin carriers in Cucurbita vesicles: I. Imposed perturbations of transmembrane pH and electrical potential gradients characterised by radioactive probes

The behaviour of radioactive probes of transmembrane pH gradients ([14C]butyric acid) and electrical potential gradients (86Rb+ and S14CN-) has been characterised in membrane vesicles from Cucurbita pepo L. hypocotyls subjected to changing gradients and ionophore-mediated changes in the conductivity of K+ and H+ ions. The vesicles are sealed in the presence of 25 mM KCl. The K+ ionophore valinomycin changes the distribution of 86Rb+ and of S14CN- in opposite directions, which depend on the external K+ concentration. The accumulation of [14C]butyric acid by the vesicles and its association with the membranous phase are enhanced by increasing acidity, as expected for this lipophilic weak acid (pKa=4.8). Uptake of S14CN- increases and valinomycin-stimulated 86Rb+ uptake decreases when the external pH is lowered, indicating a strongly pH-dependent H+ diffusion potential.