The behaviour of radioactive probes of transmembrane pH gradients (14Cbutyric acid) and electrical potential gradients (86Rb+and S14CN-) has been characterised in membrane vesicles fromCucurbita pepoL. 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 of86Rb+and of S14CN-in opposite directions, which depend on the external K+concentration. The accumulation of 14Cbutyric 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-stimulated86Rb+uptake decreases when the external pH is lowered, indicating a strongly pH-dependent H+diffusion potential.
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