Incubation with T3 results in a dose-dependent increase in growth rate of cultured GC cells, a GH-producing rat pituitary tumor cell line. The T3-induced increase in growth rate results mainly from shortening of the G1 period from 79.4 +/- 4.3 (SD) h in cells grown in T3-depleted medium (-T3) to 10.0 +/- 0.9 h. This effect can also be demonstrated in synchronized populations. Addition of T3 (0.3 nM) to cells synchronized in early G1 in the absence of T3 shortened the G1 period, estimated from graphic data, from more than 40-50 h to 13.4 +/- 2.1 h (n = 7). To determine the mechanism of this T3 effect, GC cells were grown in Dulbecco's modified Eagle's medium containing 10% serum plus or minus T3 (0.3 nM) and synchronized at the beginning of the G1 period by mitotic selection. Mitotic cells (85-100%), obtained by controlled mechanical shaking, were isolated by centrifugation and replated. The end of G1 was determined by the onset of DNA synthesis with [3H]thymidine as assessed by autoradiography (percent labeled nuclei). L-T3-induced shortening of G1 was detectable at 0.05 nM T3, half-maximal at physiological T3 (0.17 nM), and maximal between 0.3 nM and 1.0 nM T3. Addition of cycloheximide, 0.025 microgram/ml or 1.0 microgram/ml, decreased protein synthesis by 50% and 90%, respectively, and attenuated the T3 effect on G1 by 80-90%. The attenuation of the T3 effect on G1 by cycloheximide at a dose which inhibited protein synthesis suggests that T3-induced shortening of G1 may require new protein synthesis. Since glucocorticoids decrease the effect of T3 on induction of alpha-aminoisobutyric acid transport, their effect on T3-induced shortening of G1 was determined in G1-synchronized GC cells and in asynchronous cultures. Cortisol, 100 nM, significantly decreased the growth rate of asynchronous GC cells and attenuated the effect of T3 in G1-synchronized cells. Finally, T4 also decreased the length of G1 in a dose-dependent manner with a half-maximal effect at 40.0 nM. The half-maximal effect of T4 occurred at a nuclear iodothyronine concentration that was comparable to that achieved in incubations with 0.17 nM T3 (half-maximal dose). Thus, half-maximal shortening of G1 in synchronized GC cell cultures occurred at iodothyronine concentrations required for half-maximal occupancy of nuclear T3 receptors and for half-maximal induction of GH synthesis, growth rate, alpha-aminoisobutyric acid uptake, and depletion of the nuclear T3 receptor.(ABSTRACT TRUNCATED AT 400 WORDS)