Ligand-receptor dissociation: a potential mechanism for the attenuation of estrogen action in the juvenile rabbit uterus.

ABSTRACT

Estradiol binding kinetics and receptor activation were investigated using cytosol estrogen receptor from adult rabbit uterine endometrium and from the undifferentiated uteri of 2-week-old rabbits. The cytosol estrogen receptor from juvenile compared to that from adult rabbit uteri was lower (P less than 0.01) in concentration, was associated with reduced (P less than 0.01) titers of serum estradiol, and had a lower affinity for estradiol (Ka = 10(7) M-1). The equilibrium association constant (Ka) for the estrogen receptor from juvenile uteri was reduced by an increase in the dissociation rate constant (kd), as measured by [3H]E2 dissociation from the receptor. Enhanced steroid-receptor dissociation in juvenile uteri was correlated with a reduced rate (P less than 0.01) of receptor activation, as measured by the binding of steroid-receptor complex to DNA-cellulose. Because receptor activation was limited at elevated temperature (30 C), activation studies were performed at low temperature (0 C), and under these optimum conditions, the change in binding kinetics observed in the juvenile was correlated with a reduced rate of receptor activation. Equilibrium binding of [3H]E2 to the estrogen receptor exhibited positive cooperativity, as indicated by Hill coefficients of 3.39 +/- 0.12 and 3.44 +/- 0.11 for juveniles and adults, respectively. The ratio of bound to free steroid was decreased in cytosol from juvenile compared to adult uteri. Collectively, these results support the hypothesis that the increased off rate and decreased activation rate of estrogen receptor in immature rabbit uteri may represent a mechanism for the attenuation of estrogen action before sexual maturation.