Na⁺-gradient-stimulated AIB transport in membrane vesicles from Ehrlich ascites cells

Summary

Plasma membrane vesicles derived from Ehrlich ascites cells can accumulate 2-aminoisobutyric acid (AIB) twofold, in the absence of ion gradients or potential differences. In addition, AIB uptake is stimulated specifically by the presence of a Na⁺ chemical gradient (high Na⁺ outside). The nature of the counterion (e.g., K⁺, Li⁺, Cs⁺, or ethanolamine) inside the vesicle does not affect the qualitative response although quantitative differences are observed. The level of AIB present in the vesicle decreases as the Na⁺-gradient is dissipated. Gramicidin, which increases the rate of ion gradient dissipation, inhibits the gradient-stimulated uptake. Valinomycin stimulates AIB uptake when Na⁺ is present outside the vesicles and K⁺ is inside, probably by producing a diffusion potential which increases the electrochemical potential difference for Na⁺. As the Na⁺-gradient dissipates, AIB accumulation exceeds that predicted from 100% transfer of the energy from the Na⁺ chemical gradient if a 1∶1 relationship between amino acid and Na⁺ coupling exists. It is possible that a diffusion potential adds to the chemical gradient for Na⁺ making the electrochemical potential difference for Na⁺ adequate to energize AIB accumulation. Ouabain inhibits gradient-stimulated AIB uptake without measurable effects on the ion distributions, thus showing a direct action of ouabain on amino acid transport.