Estrogen Receptors in Rat Uterine Cell Cultures: Effects of Medium on Receptor Concentration*

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Abstract
Both the estrogen responsiveness and binding capacity of cultured rats uterine cells were decreased dramatically when the medium was not changed at 24 h intervals. Treatment of cells for 24 h with 1 nM 17.beta.-estradiol in fresh medium led to a 3-fold increase in progesterone receptor concentration, but without fresh medium, no increase in progesterone receptors was observed. When the medium was changed on cells with a low estrogen-binding capacity (depleted cells), a 6- to 10-fold increase in estrogen-binding capacity (to in vivo levels) occurred within 24 h (fed cells) and total protein was increased 2-fold. The high and low affinity binding characteristics of fed and depleted cells were identical. Recovery of the estrogen-binding capacity of depleted cells was relatively slow, increasing after a 6 h lag and reaching maximal levels by 24 h. While 6 h of 10-5 M cycloheximide treatment (protein synthesis inhibited > 95%) had little effect on control estrogen binding levels, it completely inhibited the increase in the estrogen-binding capacity induced by changing the medium on depleted cells. Estrogen-binding activity apparently can be varied in cultured rat uterine cells by changing medium conditions. These changes may be due to differences in receptor protein levels and not to a receptor activation-inactivation phenomenon.