The Mechanism of 17β-Estradiol Uptake into Prolactin-Producing Rat Pituitary Cells (GH3Cells) in Culture*

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Abstract
Estrogens stimulate PRL [prolactin] synthesis in cultured GH3 cells, which are clonal strains derived from the rat pituitary gland. This model system was used to study the mechanism by which 17.beta.-estradiol (E2) enters target cells. The cellular uptake of [3H]E2 was rapid at 37.degree. C and reached half-maximal values within 10-15 s. Maximal uptake was observed in < 2 min at 37.degree. C. The initial rates of E2 uptake were a linear function of the extracellular hormone concentration. The uptake of [3H]E2 in intact cells and the binding to cytosol studied at different temperatures resulted in linear Arrhenius plots, and the energies of activation were 39.0 and 33.5 kJ mol-1 degree-1, respectively. Purified GH3 cells membrane fractions which showed specific binding sites for [3H]TRH, displayed the same maximal binding of [3H]E2 in the absence or presence of cold hormone. The amount of membrane-associated [3H]E2 increased linearly with temperature and extracellular hormone concentration. The effect of temperature on binding of E2 to membrane fractions occurred gradually without phase transitions and was not saturable. The mechanism by which E2 is taken up by target cells at physiological temperature involves instantaneous dissolution in the cell membrane from where it diffuses passively into the cell. E2 binds thereafter to specific receptors in an energy-dependent step.