Contraceptive steroid binding to the human uterine progesterone-receptor

Using an equilibrium dialysis method, the competition was investigated between tritated-progesterone and a range of synthetic steroids for binding to 20,000 g. supernatants of human uterine endometrium and myometrium. Both types of uterine tissue showed similar patterns of progestogen binding and probably contain similar, or even identical, progesterone receptors. The gonane progestogen norgestrel binds strongly to the uterine receptors, but only the d-isomer is active. All three pregnane progestogens used in this study (chlormadinone acetate, medroxprogesterone acetate, megestrol acetate) also showed significant binding to receptors, but for the estrane progestogens, norethisterone was the only compound to show a high binding capacity. Other estranes (ethynodiol diacetate, lynestrenol, norethisterone acetate, norethynodrel) showed insignificant binding to the receptors and probably require prior metabolic activation to norethisterone before they can induce progestogenic effects.