Characterization of thyroid hormone stimulation of uridine uptake by rat pituitary tumor cells

T3 caused a dose-related increase in the rate of [3H]uridine uptake into GH4C1 rat pituitary tumor cells. T3 increased uridine uptake to 130-180% of the control value, with a half-maximal effect at approximately 1 nM. T3 exerted a half-maximal effect at 1 h and a maximal effect at 2 h. In contrast, epidermal growth factor also increased uridine uptake by 75%, with an ED50 of 0.6 ng/ml (0.1 nM), but a half-maximal response required 4 min and a maximal effect required 20 min. T3 increased the rate of uptake at all uridine concentrations from 30 nM to 130 microM. Equilibrium binding of [125I]T3 to nuclear receptors required from 15 min at 50 nM [125I]T3 to 1 h at 0.5 nM, indicating that occupancy of nuclear receptors precedes maximal stimulation of uridine uptake. T3 did not stimulate the rate of uridine uptake at 20 C, when binding to nuclear receptors does not occur. Various thyroid hormones caused an increase in uridine uptake, with the rank order of potency 3,3',5-triiodothyroacetic acid greater than T3 greater than L-T4 greater than D-T4 approximately equal to 3,3',5,5'-tetraiodothyroacetic acid; rT3 was inactive. This order parallels the affinities of these compounds for nuclear thyroid hormone receptors.