Estrogen-Stimulated Uptake of α1-Protease Inhibitor and Other Plasma Proteins by the Mouse Uterus*

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Abstract
Administration of a single low dose of estradiol to the immature female mouse resulted in a rapid increase in uterine trypsin inhibitory capacity. The increase was apparent within 1 h, reached a maximum in 3–4 h, and returned to base line after 18 h. No corresponding increases were observed in liver or heart. The properties of the uterine inhibitor were found to be essentially the same as those of plasma α1-protease inhibitor (α1-PI). When 125I-labeled mouse plasma α1-PI was given iv to immature mice the administration of estradiol caused a specific stimulation of uterine uptake of the labeled protein that closely matched the estrogen-stimulated increase in uterine trypsin inhibitory capacity. Half-maximal stimulation of uptake occurred at a dose of 0.15 μg estradiol/animal. After 3 h 97% of the labeled α1-PI taken up by the uterus was in the soluble (105,000 × g) fraction of which 50% was in the lumen. Estradiol also stimulated the uptake of soybean trypsin inhibitor, bovine serum albumin, porcine fibrinogen, and human α1-macroglobulin. Under conditions where puromycin blocked the synthesis of an estrogen-stimulated uterine-specific hydrolase, puromycin had no effect on the estrogen-stimulated uptake of either mouse α1-PI or mouse albumin. These results suggest that the estrogenstimulated uptake of plasma proteins by the uterus does not require new protein synthesis.