Uridine transport and RNA synthesis in growing and in density‐inhibited animal cells

Both chick embryo fibroblasts and mouse 3T3 cells reduce the rate at which they incorporate H 3 uridine into RNA as their growth becomes inhibited at high cell density. This reduction occurs as a function of the cell population density, and with chick embryo cells (in contrast to 3T3 cells) it is not accompanied by significant medium alterations. This indicates the importance of the cell population density in the control of cellular metabolism. The decline in H 3 uridine incorporation is paralleled by a decline in the rate of uptake of the isotope into the acid‐soluble pool, suggesting that decreased entry of H 3 uridine into the cell, rather than a decreased rate of RNA synthesis, is responsible for the reduced rate of incorporation into RNA of density‐inhibited cells. This suggestion was confirmed by finding that when the restriction on uridine uptake was overcome by increasing the concentration of uridine in the medium, the density‐dependent inhibition of uridine incorporation was largely reversed. We conclude that, even though the rate of H 3 uridine incorporation into RNA is reduced three‐ to five‐fold in density‐inhibited cells, the rate of synthesis of pulse‐labeled RNA continues at 70 to 85% of the rapidly‐growing rate.