Sex Steroids in Plasma and Reproductive Tissues of the Female Guinea Pig

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Abstract
Progesterone and estradiol-17β concentrations were determined in peripheral plasma, uterine, ovarian and placental tissues in nonpregnant (diestrous) and pregnant (mid- and fullterm) guinea pigs. Plasma progesterone concentration increased 56-fold at midterm but decreased at term to 28-fold of that in the diestrous animals. Although plasma estradiol increased during pregnancy, the changes compared with progesterone were minor. There was no significant change in uterine progesterone concentration during pregnancy. Uterine estradiol concentrations nearly doubled at midpregnancy but decreased again at term to a value similar to that at diestrous. In the nonpregnant animals, the progesterone concentrations in ovaries containing corpora lutes were not significantly different than those in the pregnant animals and these concentrations were ∼4 times greater than those in ovaries without corpora lutea. Ovarian estradiol concentration in diestrous animals was ∼10 times that of the uterine concentration. It increased 5-fold at midterm and 15-fold at term. Placental progesterone and estradiol concentrations were much lower than the ovarian concentrations of these steroids. The placental estradiol concentration was even lower than the uterine estradiol concentration whereas in the case of progesterone, the concentrations were 2–3 times higher in the placenta than in the uterus. The very low levels of progesterone in guinea pig uterus and placenta might be explained by the secretion in the plasma of progesterone binding globulin. However, the reason for the concentration of ovarian progesterone remaining very high is not clear.