Cytosol and Nuclear Compartmentalization of Progesterone Receptors of the Rat Uterus*

Abstract

Exchange of [³H]progesterone for progesterone bound in vivo to nuclear progesterone receptors (R_n)of rat uterus was accomplished by overnight incubation at 4 C. The R_nwere similar to cytosol progesterone receptors (R_c) in specificity, N-ethylmaleimide sensitivity, and Kd. Uterine Re and R_nwere measured after progesterone injection in adult rats ovariectomized on day 0, primed with 1.0 μg 17β-estradiol (E₂) sc on days 3 and 4, injected with 500 μg progesterone (0.5 ml 30% ethanol in saline sc) on day 5, and decapitated (0830–0930 h) at various times thereafter. Progesterone injection resulted in the following significant changes: R_naccumulation of 1 h and then retuR_nto control levels by 9–72 h; and Re depletion at 1 h, partial increase by 9–12 h, and secondary decrease to a minimum at 24–72 h. Vehicle injection resulted in no changes in R_n(nor in R_c depletion at 1 h), but R_c was decreased 25% by 12 h to the level of R_c 12 h postprogesterone. Thereafter, R_c levels in the two injection groups were similar. R_c in uninjected rats remained significantly higher at 24 h than in either injection group. Adrenalectomized-ovariectomized rats (day 0, as above) showed significantly decreased R_c 24 h after progesterone (5.0 mg sc) injection but not after corticosterone or vehicle. Thus, in adrenal-intact rats, vehicle injection seems to result in adrenal steroid release (including progesterone) and subsequent chronic (i.e. no immediate translocation) effects on uterine R_c. In adrenal-intact rats, 500 μg or 5 mg progesterone (but not the vehicle alone) inhibited E₂-induced uterine ballooning. The progesterone-induced depression of R_c at 24 h was overcome/inhibited by E₂ injection 12 h postprogesterone, eliminating the possibility of an irreversible block in R_c synthesis. These data indicate that progesterone injection in the rat results in three phases of response by rat uterine progesterone receptors: 1) nuclear translocation, 2) a transient cytosol replenishment, and 3) apparent depression of R_c.