Progesterone and 20α-Hydroxypregn-4-en-3-one in Plasma, Ovaries and Uteri During Pregnancy in the Rat1

TLDR: Evidence is presented which supports the conclusion that uterine tissue is capable of concentrating pro-gesterone above peripheral blood levels throughout early and midpregnancy, and increased uterine motility, resulting eventually in parturition, apparently follows a reduction in uterus proges-terone concentration.

Abstract: Progesterone and 20α-OH-P levels were measured by double isotope assay in pregnant rat peripheral plasma, ovaries, uteri and placentae at selected times extending from the 2nd day of pregnancy until term. Evidence is presented which supports the conclusion that uterine tissue is capable of concentrating pro-gesterone above peripheral blood levels throughout early and midpregnancy. High intrauterine concentrations of progesterone relate quantita-tively to biological events associated with hormon al function, e.g., nidation, decidua formation and myometrial quiescence. Increased uterine motility, resulting eventually in parturition, apparently follows a reduction in uterine proges-terone concentration. These correlations cannot be made with the closely related progesterone metabolite, 20α-OH-P. This is likely due, in part at least, to its more rapid clearance from systemic compartments. (Endocrinology 87: 43, 1970)