Binding of progesterone by rat myometrium during pregnancy and by human myometrium in late pregnancy

Abstract

The binding of progesterone by the cytosol fraction of rat myometrium during pregnancy and by the cytosol and 20,000g supernatant fraction of human myometrium in late pregnancy has been determined by a competitive protein-binding method and by equilibrium dialysis. In rat myometrium two kinds of progesterone binding components were observed: a non-specific binder resembling corticosteroid-binding globulin (CBG) and a progesterone-specific receptor. The relative amount of the specific receptor reaches a min. in mid-pregnancy and a max. before parturition, at a time when the non-specific binder is below the limit of detectability. The CBG-like binder reappears at least 2 days post partum. The total binding-site concn. of progesterone in the cytosol of rat myometrium during pregnancy was 1.1–6.0 pmol/mg of cytosol protein, being highest in mid-pregnancy and lowest during the last third of pregnancy and remaining so until 2 days post partum. The association constant of the specific progester-one receptor was ∼ 1.0 × 10 8 M −1. In human myometrium of late pregnancy a progesterone-binding component of high affinity and limited capacity was found. The binder was not specific for progesterone. It resembled the CBG-like binder of rat myometrium. The association constant was ∼ 1.0 × 10 9 M −1. The binding-site concn. for progesterone in the cytosol of human myometrium during late pregnancy was 2.3 ± 0.3 pmol/mg of cytosol protein in myometrium samples obtained from elective caesarian sections and 0.6 and 0.9 pmol/mg of cytosol protein in two samples obtained during labour.