Control of the uptake of amino acids by serum chick embryo cells, untransformed or transformed rous sarcoma virus.

Forty to fifty minutes after removal of serum, the net total uptake of amino acids in growing secondary cultures of normal or virus-transformed chick embryo cells, stopped or proceeded only at a highly reduced rate. In both normal and transformed cells, the initial (0-40 min) rate of the above uptake was the same in the absence of serum as in its presence. The initial rate of the total uptake of amino acids in growing transformed cells was about the same as in growing normal cells. Neither in the normal nor in the transformed cells was the rate of the total uptake of amino acids reduced by cell confluence alone. In highly dense, hyperconfluent cultures of normal cells in which cell growth was arrested, the rate of uptake in the absence or in the presence of serum was four- to fivefold lower than the rate obtained in growing normal cells under similar conditions; in the absence of serum, the net uptake stopped after 40 min in the hyperconfluent cultures as well. It appears that cells growing in tissue culture require a serum factor for maintenance of the required high rates of uptake of amino acids and that the inhibition of growth at high cell densities is a result of depletion of this factor from serum, or the inability of the cells in a dense culture to respond to the factor. A serum factor is apparently also required for maintenance of the reduced rates of uptake of amino acids observed in hyperconfluent cultures.