The growth rate of GH-producing rat pituitary tumor cells (GC cells) is dependent on thyroid hormone. Previous studies have shown that GC cells can be partially synchronized in the DNA synthesis phase (S-phase) of the cell cycle by a 25-h incubation with 2 mm thymidine. Measurements in GC cells partially synchronized in S-phase showed a significant increase in cellular thyroid hormone nuclear receptor appearance and disappearance in thymidine-treated GC cells using dense amino acid labeling and subsequent sucrose gradient ultracentrifugation. In comparison with GC cells in asynchronous culture, thymidine treatment causes a steady state increase in thyroid hormone nuclear receptors which results from an increase in the receptor appearance rate without an effect on the disappearance rate. Thus, position in the cell cycle appears to be another factor that influences the appearance rate of nuclear thyroid hormone receptors.
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