The regulation of amino acid transport across the cell membrane by adaptive mechanisms has been studied in cultured chick embryo fibroblasts. Time-dependent changes of transport activity by the A system (a Na+-dependent agency with affinity for a discrete group of neutral amino acids), as a function of the composition in amino acids of the culture medium, have been evaluated by measurements of 14C-labelled l-proline uptake under conditions approaching initial entry rates. Reults and conclusions based on the adopted experimental procedures include the following: 1.(1) Transport of l-proline in cultured avian fibroblasts is an inverse function of the concentration of amino acid substrates of system A in the medium used for cell culturing before uptake assay. 2.(2) Cells grown in media containing amino acids that are substrates for system A (repressive conditions) exhibit a marked increase of l-proline uptake upon incubation in media devoid of these amino acids (derepressive conditions). 3.(3) Cells grown in media containing amino acids which are not typical substrates of system A (derepresive conditions) undergo a definite decrease of l-proline uptake upon incubation in media supplemented with amino acid substrates of this agency (repressive conditions). 4.(4) The adaptive increases in the transport of l-proline observed when ‘repressed’ cells are incubated under depressive conditions, are abolished in the presence of cycloheximide. 5.(5) The results presented suggest that adaptive regulation of amino acid transport by system A is an intrinsic property of the cells and has a normal function in vivo. Its occurrence under culture conditions demands that one must consider the actual composition of amino acids in the growth medium when investigating amino acid transport in cultured cells.
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