Triiodothyronine stimulates growth of cultured GC cells by action early in the G1 period.

T3 regulates the proliferation rate of GC cells, a rat GH-secreting pituitary tumor cell line, by modulating the duration of the G1 period. In GC cells which were synchronized at the beginning of G1 by mitotic selection, the G1 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). G1 was significantly prolonged in synchronized GC cells, previously maintained in the absence of T3 for more than 1 cycle time. G1 duration decreased when cells were exposed to T3 only during a discrete 4-6 h interval early in G1. Thus, T3 appeared to stimulate progression through the G1 period by action early in G1.