The effects of serum oestrogen-binding components on the unbound oestradiol-17 fraction in the serum of developing female rats and on inhibition of [3H]oestradiol uptake by uterine tissue in vitro

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Abstract
Total estradiol [E2] concentrations in the serum of young female rats were high and decreased after about 21 days of age. High affinity serum E2-binding components (EBP), however, fell steadily from 5 to 23 days of age while the unbound E-17.beta. fraction, which was low early in development, increased between 21-28 days of age. Injection (i.v.) of immature (EBP-rich) E2-free serum into 21 day old female rats led to a decrease in the unbound E2 fraction and an increase in serum FSH [follicle stimulating hormone] concentrations. Incubation of uterine tissue with [3H]E2, with or without the addition of diethylstilboestrol (DES), showed the rate and degree of total and DES-suppressible uptake of [3H]E2 to be greatest from buffer, less from adult (EBP-poor) serum and negligible from immature (EBP-rich) serum; moreover, there was a positive correlation between the degree of uptake of [3H]E2 and the unbound fraction of [3H]E2 in the incubate. At least in young rats, E2 activity depends more on the availability of free E2 than on its total plasma concentrations.