AbstractHuman dermal fibroblasts arrested in G0by maintenance in medium supplemented with 0.1 serum were not restimulated to divide when fresh medium containing 10 dialyzed serum but lacking group B amino acids (cystine, isoleucine, lysine, phenylalanine and tyrosine) was added. Unlike rodent cells, the addition of fresh serum‐supplemented medium lacking only isoleucine did not cause a growth arrest. The amino acid sensitive growth arrest in human fibroblasts was dependent both on presynchronization in G0as well as a prestarvation for amino acids prior to stimulation with high serum.When cells were restimulated in the absnece of amino acids, they arrested predominantly in G1, although a small percentage of cells entered early S phase. When medium containing a complete complement of amino acids was then added, cells initiated DNA synthesis following a minimum lag of 2–3 hr. Growth arrested cells initiated DNA synthesis even when complete unsupplemented medium was added, although the addition of high concentrations of insulin or 10 serum increased the rate of en
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