T3 regulates the proliferation rate of GC cells, a rat GH-secreting pituitary tumor cell line, by modulating the duration of the Gl period. In GC cells which were synchronized at the beginning of Gl by mitotic selection, the Gl period was 11.7 ± (SE)2.1 h, in agreement with the value we obtained in asynchronous cells by other methods (10.0 + 0.9 h). Gl was significantly prolonged in synchronized GC cells, previously maintained in the absence of T3 for more than 1 cycle time. Gl duration decreased when cells were exposed to T3 only during a discrete 4-6 h interval early in Gl. Thus, T3 appeared to stimulate progression through the Gl period by action early in Gl.
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