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 ([(14)C]butyric acid) and electrical potential gradients ((86)Rb(+) and S(14)CN(-)) 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 (86)Rb(+) and of S(14)CN(-) in opposite directions, which depend on the external K(+) concentration. The accumulation of [(14)C]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 S(14)CN(-) increases and valinomycin-stimulated (86)Rb(+) uptake decreases when the external pH is lowered, indicating a strongly pH-dependent H(+) diffusion potential.