Bimodal effects of cellular amino acids on Na+-dependent amino acid transport in Ehrlich cells.

Cells depleted of amino acids show lower rates of glycine or aminoisobutyric acid uptake than do freshly isolated cells. In the amino acid-depleted cells, addition of valinomycin stimulates amino acid influx at least to the level observed in freshly isolated cells. In cells containing high levels of cellular amino acids, valinomycin has little effect on influx of amino acids. It is concluded that the transport of amino acids in freshly isolated cells is elevated compared to depleted cells because the cells are hyperpolarized by the continuous loss of cellular amino acids during the transport assay. During this hyperpolarization by amino acid loss, transport of amino acids is not further stimulated by valinomycin at low external [K+] (10 mM +/- 5 mM). With the exception of preloading with glycine, cells preloaded with a single amino acid to a concentration greater than 20 mM show reduced rates of glycine and aminoisobutyric acid influx at early times (less than 15 min) compared to amino acid-depleted cells. The reduction of influx is transient and by 30 min, influx is greater in preloaded than in amino acid-depleted cells. Knowing that increases and decreases in the membrane potential are achieved by using varying external [K+] in the presence of valinomycin and propranolol, and using amino acid-depleted cells, it can be shown that an increased membrane potential increases the V for glycine and aminoisobutyric acid influx. A decrease in the potential difference results in a decreased V. Changes in Km also occur when the membrane potential is varied.