Cytosol and nuclear levels of thymic progesterone receptors in pregnant, pseudopregnant and steroid-treated rats

Cytosol and nuclear levels of progesterone receptors in rat thymus and uterus were compared in mature Sprague-Dawley rats under a variety of physiological (non-oestrus, oestrus, pregnancy) and experimental circumstances (pseudopregnancy, oestrogen administration, oestrogen plus progesterone administration). Cytosols and nuclear extracts were charcoal treated to remove endogenous steroids, and incubated overnight at 4 degrees C with the synthetic progestin 3H]R5020 +/- greater than 100-fold R5020, in the presence of greater than 200-fold RU26988 (a highly specific synthetic glucocorticoid) to exclude tracer from glucocorticoid receptors. In the various states examined, levels of progesterone receptors in the uterus were an order of magnitude higher (350-1300 fmol/mg protein) than in the thymus (40-140 fmol/mg protein). In contrast, considerable parallelism between uterus and thymus was seen in terms of the effects of the various manipulations upon tissue levels of progesterone receptors, and of their distribution between cytosol and nuclear compartments.