Subcellular Distribution and Cytosolic Receptors of Progesterone and of Estradiol-17β in the Rabbit Myometrium: Effect of Progesterone Treatment1

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Abstract
The distribution and cytoplasmic receptors of progesterone (P) and of estradiol-17β (E2) in rabbit myometrium were studied before and after P treatment. Cytoplasmic P and E2 receptors in the myometrium of estrogenized rabbits decreased 24 h after a single injection of P. However, P concentration in the intact tissue as well as in the cytosol, as determined by radioimmunoassay, increased significantly. The increase in P concentration in other fractions (nuclear, mitochondrial + microsomal) was not significant. There was no change with progesterone treatment in the subcellular distribution of protein, but a marked change in the distribution of P occurred. An insignificant decrease in tissue E2 but a significant decrease in cytosolic E2 concentration was observed after P treatment. Concentrations of nuclear progesterone receptors (extracted with 0.4 M KCl) in the control and P treated rabbits were similar. Neither stripping of cytoplasm of the endogenous P nor its loading with P in vitro led to any increase or decrease in P receptors. Determination by radioimmunoassay of P concentration in the loaded and in the stripped cytoplasm showed that these procedures were highly effective. These data indicate that the occupation of the receptors and the subsequent influence on the receptor determination in vitro cannot account for the reduction of P receptors unless one postulates that, when P binds endogenously to its receptor, it does not exchange as readily as when bound under in vitro conditions. It also seems unlikely that the reduction of cytoplasmic P receptors was due simply to their translocation in the nucleus.