Anti-Progestational Activity of Estrogens in Rabbit Endometrium ∗ †

Summary

1) The anti-progestational activity of 4 estrogens—estrone, estradiol, estriol, and stilbestrol—administered subcutaneously along with progesterone into Clauberg rabbits has been demonstrated by estimation of endometrial carbonic anhydrase content and simultaneous measurement of uterine G/M ratio. 2) All these estrogens inhibit the effect of progesterone on carbonic anhydrase content and G/M ratio of the endometrium. Intensity of inhibition depends upon dosage. 3) The anti-progestational activities of these estrogens are approximately the same. 4) When administered alone, these estrogens produce no significant change in either carbonic anhydrase titers or G/M ratios of the endometrium. 5) The linear relationship with negative slope which is obtained between the logarithmic dose of the estrogens and the endometrial carbonic anhydrase content of the progesterone-treated Clauberg rabbit suggests the usefulness of the carbonic anhydrase method as an assay procedure for anti-progestational activity. 6) Comparison of the log-dose-response curves of progesterone with and without estradiol indicates that estrogen may depress reactivity of the endometrium to progesterone rather than neutralize or inactivate progesterone in the body.