Comparison of the characteristics and of the hormonal control of endometrial and myometrial progesterone receptors.

The characteristics of endometrial and myometrial progesterone receptor of guinea-pig were compared. Affinity for progesterone, hormone specificity, sedimentation properties (in oestrogen-primed animals), inhibition of binding by p-hydroxymercuribenzoate were found to be identical in both tissue. Differences were, however, observed in the hormonal control of the concentration of receptor. In all the situations studied, the concentration of receptors was higher in the endometrium than in the myometrium. In guinea-pigs ovariectomized at dioestrus the concentration was 3500 binding sites per diploid genome in the myometrium and 20300 in the endometrium; 1-3 days after oestradiol injection, this concentration was raised to 46000-38000 and 65000-83000 binding sites respectively. Thus the absolute rise was similar in both tissues but the relative increase was about 15-fold in the myometrium and only three- to fourfold in the endometrium. After injection of 2 mg progesterone, the concentration of receptor previously induced by oestrogen returned to very low values similar to those observed in non-hormonally treated controls. This difference between endometrial and myometrial receptors could be due either to a faster turnover of the latter or to the existence of a stable non-hormonally controlled population of receptors, present only in the endometrium.