Serum‐mediated regulation of amino acid transport in cultured chick embryo fibroblasts

When chicken serum was added to serum‐deprived quiescent cultures of chick embryo fibroblasts the activity of amino acid transport by means of the A system, as measured by α‐aminoisobutyric acid and L‐proline uptake after discrimination of the contribution of interacting systems, increased with time of exposure to serum between 30 and 120 minutes (remaining constantly high thereafter). Under the same conditions, DNA synthesis, as measured by thymidine incorporation, increased abruptly six to eight hours after the addition of serum. Serum‐mediated increases of transport activity by the A system have also been detected with glycine, L‐alanine and L‐serine. Transport activities of systems ASC, L and Ly+ did not change appreciably (or decreased slightly) after the addition of serum. The stimulation of amino acid transport was apparently proportional to the length of exposure to serum; its rate declined progressively with time after withdrawal of serum from the culture medium. Kinetic analysis indicated that stimulation of the activity of transport system A by serum occurred through a mechanism affecting Vmax rather than Km; stimulation was prevented by inhibitors of protein synthesis. Our results indicate that the A transport system is the only system which is regulated by serum in cultured avian fibroblasts. Remarkably, the A transport system appears to be the target on which widely different factors and conditions converge to regulate amino acid transport in eukaryotic cells.