Concentration of radioiodide by the uterus of the rat and the relationship to blastocyst implantation

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Abstract
When pregnant rats are ovariectomized early in pregnancy and treated with progesterone a raised uterus-plasma concentration ratio (U/P ratio) for radio-iodide is observed at the usual stage of pregnancy (days 3, 4 and 5) but is not followed by implantation. When delayed implantation is induced by the administration of estradiol it is not preceded by a rise in the U/P ratio for I131. Similar findings are obtained when implantation is delayed by concurrent lactation in rats mated at the post-partum estrus. Ovariectomy early in pregnancy without progesterone treatment prevents the rise in U/P and oviduct-plasma (O/P) ratio for Il31 that normally occurs. Administration of progesterone to intact or ovariectomized nonpregnant rats produces a raised U/P and O/P ratio for I131; estrogen may inhibit tills response. The simplest explanation of these results is a direct action of progesterone on the uterus. By gross dissection it was possible to show that the region of high iodide concentration in the rat uterus is the endometrlum. No evidence of a concentration of iodide in the uterus or oviduct of the pregnant rabbit was obtained.