Effect of extracellular potassium on amino acid transport and membrane potential in fetal human fibroblasts.

The distribution ratio of the lipophilic cation tetraphenylphosphonium (TPP+) has been used to estimate the electrical potential difference across the plasma membrane in cultured human fibroblasts. These cells exhibit a membrane potential markedly influenced by the diffusion potential of K+. High extracellular potassium concentrations depolarize human fibroblasts and depress the activity of transport systems A, ASC (both serving for zwitterionic amino acids), X-AG (for anionic amino acids), and y+ (for cationic amino acids). High doses (100 microM) of the K+-ionophore valinomycin hyperpolarize the cells. This condition enhances the activity of systems A, ASC and y+. Transport systems L (for neutral amino acids) and x-C (for anionic amino acids) are insensitive to changes in extracellular K+ or to valinomycin. System X-AG is inhibited by the addition of 100 microM valinomycin, but the effect of the ionophore appears to be potential-independent. These results indicate that: (a) the activity of systems L and x-C is potential-independent and (b) the activity of systems A, ASC, X-AG and y+ is sensitive to alterations of external [K+] associated to changes in membrane potential.