Phosphate Transport, Nucleotide Pools, and Ribonucleic Acid Synthesis in Growing and in Density-inhibited 3T3 Cells

Abstract

Comparing growing and density-inhibited cultures of mouse 3T3 cells, we have determined that when cells became density-inhibited: 1. The sizes of the ribonucleotide pools and of the inorganic and organic phosphate pools dropped only slightly, if at all. 2. The sizes of the deoxyribonucleotide pools dropped several-fold. 3. The rate of 32PO4 transport across the cell membrane declined 5-fold. 4. There was no reduction in the rate at which 32PO4 labeled the ATP pool beyond the initial inhibition of the rate of 32PO4 entry into the cell. This indicates that the rate of ATP turnover was approximately the same in growing and in density-inhibited cells. 5. Similarly, the rate at which 32PO4 labeled RNA of density-inhibited cells was reduced only slightly more than was the rate of labeling of the ribonucleoside monophosphates, indicating that much of the decreased incorporation of 32PO4 into RNA could be accounted for by the decreased rate of labeling of the precursor pools. Taking into account the differences in specific activity of the ribonucleoside monophosphates, we calculate that the rate of synthesis of pulse-labeled RNA in density-inhibited cells during a 3-hour pulse was approximately 75% of the rapidly growing rate. We conclude that the density-dependent inhibition of growth is not accompanied by gross, nonspecific reductions in the amount of available energy or in the rate of synthesis of pulse-labeled RNA, but is accompanied by changes in the rate of 32PO4 transport and in the sizes of the deoxyribonucleotide pools.