The effect of cell population density on nutrient uptake and cell metabolism: a comparative study of human diploid and heteroploid cell lines

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Abstract
The possibility that contact inhibition of growth in cultures of human diploid cells is influenced by the effects of cell crowding on nutrient uptake by the cells was investigated. Two human lung cell lines were compared, the diploid line MRC-5 and the heteroploid line L-132. In pre-confluent cultures the ability of these 2 cell types to accumulate amino acids was very similar. Post-confluent L-132 cells showed very little change from the pre-confluent cultures but the ability of MRC-5 cells in post-confluent cultures was greatly reduced. The intracellular concentrations of various amino acids necessary to achieve the maximum rate of protein synthesis were found. These values were identical for sparse and crowded cultures but due to the reduced uptake ability of crowded MRC-5 cells a far higher external amino acid concentration was required in post-confluent cultures. This meant that although amino acids did not become growth-limiting until over 80 % utilized in pre-confluent cultures, in post-confluent cultures they became growth-limiting when only 50 % utilized. Although protein synthesis was significantly affected by extracellular amino acid concentration and cell crowding, thus contributing towards the effect of contact inhibition of growth, DNA synthesis was shown to be the major metabolic function in contact inhibition. Increased cell density had a very inhibitory effect on DNA synthesis in MRC-5 cultures, but not in L-132 cultures, and this was unaffected by extracellular amino acid and glucose concentration.