Dynamic Studies on the Selective Uptake of [3 H] -Estradiol by the Fetal Uterus and Other Fetal Organs of Guinea Pig

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Abstract
Dynamic aspects of the selective retention of [3H]-estradiol in the fetal uterus and other fetal organs of guinea pig after in vivo administration of the hormone to the fetus are presented. The fetal uterus shows a significant retention of radioactivity with values that are 3.5-12 times higher than in other fetal organs. The uterus, lung, kidney and intestine show an increase in radioactivity uptake with time, with maximal values 0.5-1 h after the injection; the heart, brain, adrenal and liver show their greatest uptake at 10 min after the injection, with decreasing values thereafter. The analysis of the radioactive material shows that unmetabolized [3H]-estradiol is predominant in the uterus 6 h after administration of [3H]-estradiol to the fetus (94% of the total radioactivity in this organ) although a relatively significant amount of [3H]-estrone is found, especially in the cytosol fraction, at the early intervals after [3H]-estradiol administration. Important quantities of [3H]-estrone are present in the lung (12-45%) and brain (27-65%). In the liver and intestine, the greater part of the radioactive material are metabolic products of [3H]-estradiol (estrone, and estrone- and estradiol sulfates). Most of the radioactive material in the plasma are estrogen sulfates (53-69%), particularly estrone sulfate. An intense sulfokinase activity is observed in the fetal adrenal and liver. In the fetal uterus 10 min after [3H]-estradiol injection most of the radioactivity is localized in the cytosol fraction (59% of the total radioactivity of the organ), but at 6 h the nuclear radioactivity represents 68%. This nuclear radioactivity is mainly unmetabolized [3H]-estradiol (97% of the total radioactivity of this fraction) which remains at a high concentration 6 h after administration. The concentration of [3H]-estrone in this tissue diminishes very rapidly. The selective uptake and long retention of [3H]-estradiol in the fetal uterus of guinea pig can be related to the presence of the large quantity of estradiol receptors found in this organ.